Circuitos de Audio

WHATEVER their particular area ofinterest, most electronics enthusi-asts encounter the need to amplify

and reproduce audio signals. The finalstages of radio receivers, intercom units,security and surveillance installations, orjust a hankering for a big sound from aWalkman or portable CD player, allinvolve audio amplification and a speakersystem. And the amplification usually goeshand-in-hand with some form of signalprocessing.

Music reproduction calls for a widefrequency response and tone-control cir-cuitry. Speech communication, especially

under difficult conditions, is greatlyclarified if the frequency response iscurtailed.

This short series of articles describessimple, but effective, ways of meetingthese different requirements. Although thecircuits are capable of a good standard ofreproduction, they will particularly interestthe constructor who looks for plenty ofperformance per pound or dollar.

We begin with the power amplifier. Sixalternatives are given and, with the require-ment of maximum performance for mini-mum cost and effort, they are all based onwidely available integrated circuits (i.c.s):

Before considering the various circuits,

it is worthwhile to reflect on the amount ofpower actually needed.

Clear reproduction in Walkman typeearphones of reasonable sensitivity canbe achieved with a miserly milliwatt(0·001W). When listening to talk pro-grammes in a quiet domestic setting, thepower fed to the speaker will hoveraround 50mW (0·05W), and this is amplefor the operator of a communicationsreceiver whose ears are closer to thesound source.

During the valve era, ten watts wasconsidered adequate for the realisticreproduction of orchestral music, andsome experts suggested a figure as low asfive. One watt of power delivered to a

340 Everyday Practical Electronics, May 2002

Part 1 – Introduction and Power Amplifiers

LM386N-1 TDA7052 TBA820M LM380N TDA2003.

Collection of simple i.c. power amplifiermodules (left-to-right, top-to-bottom). Single TDA2003 Amp Twin TDA2003 Amp TDA7052 Amp TBA820M Amp LM380N Amp LM386N-1 Amp

reasonably efficient speaker will produce aloud sound, a continuous five watts isbecoming deafening, and ten will rattlewindows. This assumes single channel, ormono, reproduction in a normal livingroom. The impact is, of course, greaterwith a stereo system.

This is at odds with the high power rat-ings of many of the quality amplifiers cur-rently advertised. The desire for a bigreserve of power, the low efficiency ofsome modern speakers, and different waysof measuring output, may account for thedifference.

Output is variously rated as musicpower, sustained music, speech and music,and sinewave power. To add to the confu-sion, the figures are quoted at different dis-tortion levels. The standard most oftenused when valves were commonplace, andthe one adopted in this article, is the r.m.s.(root mean square) value of a continuoussinewave. This gives the lowest rating andis the most realistic expression of theamplifier’s ability to deliver power into aload (speaker).

An increase in output power is reflectedas much, if not more, in the cost of thepower supply as it is in the actual amplifi-er. Because the theme of this article isgood performance at modest cost, the mostpowerful amplifier described is rated at12·5W r.m.s.

Manufacturers of power-amplifier inte-

grated circuits and modestly priced hi-fisystems (which invariably incorporatedevices of this kind) usually rate the max-imum power output at 10 per cent distor-tion. At this level there is a very noticeableroughness to the sound and clipping of thewaveform on loud passages.

The power output levels quoted herehave been measured just before the onsetof clipping or any noticeable distortion ofthe output waveform. They are somewhatlower than the figures quoted by the i.c.manufacturers, but they do represent thehighest output, free from audible distor-tion, that the device can deliver for a par-ticular supply voltage and load.

Modern power amplifier i.c.s have a

very low noise level. Manufacturers usual-ly define this internally generated electri-cal noise as an equivalent signal voltage atthe input, but this doesn’t give the averageexperimenter an immediate impression ofits audible effect.

Accordingly, the devices described herewere tested by disconnecting the signalsource, turning the input or volume controlto maximum, and then listening to the out-put on a pair of sensitive, Walkman typeearphones.

In all cases the noise was no more thanbarely audible. The two devices which canbe configured for high gain (LM386N andTBA820M) did produce a faint, but audi-ble, hiss when the gain was set at maxi-mum. The hiss was also noticeable with aloudspeaker connected.

However, when the gain preset wasturned back a little, these i.c.s became assilent as the rest. Some constructors mayneed the highest possible gain, and detailswill be given later of measures which canbe taken to eliminate the noise.

overloads them, causing distortion andloss of clarity.

Indeed, if the amplifier is being usedprimarily for speech, clarity can be muchimproved by rolling-off the frequencyresponse below 300Hz, and an even lowervalue of coupling capacitor, say 100F oreven 47F, would be of benefit. Readersseeking quality music reproduction at lowpower, via a speaker of reasonable size,should increase the coupling capacitor tosay 1000F.

This relationship between couplingcapacitors and frequency response will beconsidered more fully in the next article.

The integrated circuits covered here are

electrically robust but they are by nomeans indestructible. The TDA7052,LM380, and the TDA2003 incorporateprotection against overload andoutput short ciruits: theother devices do not.

H o w e v e r ,even with this pro-

tection, excessive sup-ply voltage will result in

immediate failure, and shortingthe output when the amplifier is

being driven hard and/or when the sup-ply voltage is close to the operational max-imum will quickly ruin the device.

Suitable mains power supplies will be

covered in a later article, but it should bementioned now that, off-load, d.c. outputvoltages rise to 1·4 times the a.c. voltagedelivered by transformer secondaries.When using unregulated mains power sup-plies care should, therefore, be taken toensure that the off-load voltage is alwaysless than the maximum safe working volt-age of the amplifier. Never connect aworking power supply to an amplifierwithout first checking its output voltage.

The electrical characteristics of the

various devices are tabulated alongside thecircuit diagrams (except one) for easyreference. Power output figures are based onmeasurements taken on a single, randomlypurchased sample. For reasons alreadygiven, they are somewhat lower than thefigures quoted by the manufacturers.

Recommendations are made regarding thespeaker impedances to use with various sup-ply voltages in order to keep the dissipation ofthe devices within reasonable limits.

The input resistance, maximum voltageratings, and frequency response details arethose supplied by the manufacturers.

Everyday Practical Electronics, May 2002 341

Provided a few basic precautions are

observed, the amplifiers are all uncondition-ally stable. Most i.c.s of this kind have aground connection for the input circuitry anda separate ground pin for the output stage.

The printed circuit board (p.c.b.) layoutshave been designed to maintain this isola-tion, and care should be taken to ground thesignal inputs and connect the negative powersupply lead to the designated points on theboard. Failure to do this could result in“motor boating’’ (low frequency instability).

Input leads should be screened to avoidmains hum and radio frequency (r.f.) sig-nal pick up. Speaker leads should be twist-ed together to minimise external fields.Input and output leads should be spaced asfar apart as possible: this is particularlyimportant when the LM386N andTBA820M are set for high gain.

All of the circuits include high and low fre-quency bypass capacitors across the supplyrails. The former minimise the possibility ofr.f. oscillation: the latter avoid low frequencyinstability when long power supply leads areused, or when batteries are ageing.

The bandwidth of the amplifiersextends into the r.f. spectrum, andthis makes the devices vulnerable

to r.f. interference. Some of the i.c.s pro-vide for the connection of an externalcapacitor in a negative feedback loop to“roll-off’’ the high frequency response.Selecting an appropriate value for thiscomponent will help to make the deviceimmune.

The problem of r.f. pick up invariablymanifests itself when a high value (morethan 10 kilohms) input potentiometer(VR1) is used to match the amplifier to theimpedance of a signal source. If the poten-tiometer or volume control must have ahigh resistance, connecting a 1nF or, atmost, 10nF capacitor across its track willshunt unwanted r.f. to ground.

The low frequency response of three ofthe lower powered amplifiers has beencurtailed a little by fitting a 220F speak-er coupling capacitor. Amplifiers of thiskind are invariably used with small, inex-pensive speakers which are incapable ofproducing an audible output at frequen-cies below 150Hz or so. Feeding low fre-quencies to speakers of this kind only

Twin TDA2003 “b

ridge’’ p

ower amplifie

r.

In use, there is little to distinguishbetween the four, low powered amplifiers,all perform well. There are, however, dif-ferences which make one device more suit-able than another for a particularapplication.

Low current consumption is importantwhen equipment is powered from dry bat-teries. Quiescent current drawn by thesmall amplifiers is in the region of 6mA(13mA for the LM380).

In the case of the LM386N, TBA820Mand LM380, current rises to around 120mAwhen 500mW is being delivered into an 8ohm load. Current consumed by theTDA7052 is approximately 220mA, oralmost double, under these conditions.

In all cases, the signal input pin hasbeen connected to the slider (moving con-tact) of the Volume control potentiometer(via a blocking capacitor in the case of theTDA2003). This minimises hum andnoise and ensures that a more or less con-stant impedance is presented to the signal

source. Potentiometers of 4700 ohms or10 kilohms (10k) are usual, but the valuecan be increased to 100k to raise inputimpedance.

This will, however, make the circuitsmore vulnerable to mains hum, r.f. interfer-ence and instability, and the value shouldbe kept as low as the signal source imped-ance permits. This applies particularly tothe TDA7052, where the value of theVolume control should, if possible, be nomore than 10k. Earlier comments regard-ing stability are of relevance here.


A circuit diagram for a simple amplifier using the low-voltageLM386N-1 power amplifier i.c. is shown in Fig.1. Also shown arethe general performance and electrical characteristics of thecircuit.

Blocking capacitor C1 prevents any disturbance of the d.c. con-ditions in the signal source and potentiometer VR1 (the Volumecontrol) sets the input level. The manufacturers of the chip,National Semiconductor, suggest an input network to roll-off highfrequencies and resistor R1 and capacitor C2 perform thisfunction.

The unused non-inverting input (pin 3) is grounded to avoidinstability when gain is set high. Capacitors C3 and C4, connectedacross the supply rails, prevent low and high frequency instability.

An internal negative feedback path can be accessed via pin 1

and pin 8. Bypass capacitor C5 reduces the feedback and increas-es the gain of the chip from 23 to 170 times (as measured: sampleswill vary). Preset potentiometer VR2 (wired as a variable resistor)controls the bypassing effect of C5 and enables the gain to be setwithin these limits.

Bypass capacitor C6 makes the device more immune to supplyline ripple, and C8 couples the output to the speaker LS1. TheZobel network, formed by resistor R2 and capacitor C7, ensuresthat the speaker always presents a resistive load to the amplifier.Without these components there is a risk of high level transientscausing damage to the output transistors.

Tabulated power output levels for various supply voltagesand speaker impedances are included below the circuitdiagram. Sustained operation at more than 300mW is notrecommended.

The printed circuit board component layout, wiring details and

full-size copper foil master pattern are shown in Fig.2. This boardis available from the EPE PCB Service, code 343 (LM386N-1).

Completed LM386N-1 circuit board.

C14 7µ

100nC4220µ

C3

8ΩLS1

+ +3V TO 12V

0V

SCREEN

10kVR1

220µC8

10ΩR2

47nC7

10µC6

470ΩR1

1nC2

10kVR2

10µC5

2

3

47

58

SIGNALINPUT

VOLUME

+

+

+

+

+

6

LM386N-1IC1

1

1

2

3

4 5

6

7

8SET GAIN SET GAIN

GROUND (0V)

SUPPLY VOLTAGE RIPPLE REJECTION

SUPPLY VOLTAGE V+

SIGNAL OUT

TOP VIEW OF LM386N-1

INPUT +

INPUT

+W

Fig.1. Circuit diagram and pinout details for the LM386N-1Power Amplifier.

LM386N-1 POWER AMPLIFIERR.M.S. power output just before the onset of

waveform clippingSpeaker Supply VoltageImpedanceohms 3V 4·5V 6V 9V 12V

4 60mW 150mW 320mW 500mW – 8 26mW 105mW 200mW 560mW 900mW16 15mW 60mW 110mW 320mW 605mW32 - 35mW 62mW 170mW 330mW

Quiescent current: 6mAInput resistance: 50k ohmsInput sensitivity for 560mW

output (8 ohm load, 9V supply), (a) VR2 set for maximum resistance: 90mV r.m.s. (gain 23)(b) VR2 set for minimum resistance: 12mV r.m.s. (gain 170)

Absolute maximum supply voltage,beyond which damage will occur: 15V

Suggested maximum supply voltage with a 4 ohm speaker 6V

Frequency response up to 300kHz

Philips have adopted a bridge arrange-ment for the TDA7052’s output stage. Thisenables the chip to maintain a good outputat low supply voltages and eliminates theneed for a speaker coupling capacitor.

Gain is fixed internally, no provision ismade for ripple rejection, and there is noZobel network. This reduces the externalcomponent count to the d.c. blockingcapacitor C1, Volume control VR1 and thesupply line bypass capacitors, C2 and C3.The full circuit diagram, together with a

specification guide, for the TDA7052amplifier is shown in Fig.3.

Protection against output short circuitsis built in and the device shuts downwhen the dissipation becomes excessive.This explains the small rise in sus-tainable output when the speaker imped-ance is increased to 16 ohms with a 9Vsupply.

Although usually costing a little morethan the other low-power chips, this is thedevice of choice when the supply voltage

has to be low, a good output is required,and high gain is not important. Currentconsumption for a given output power is,however, almost twice that of the LM386Nand the TBA820M.

The printed circuit board component

layout, wiring details and full-size copperfoil master pattern are shown in Fig.4. Thisboard is available from the EPE PCBService, code 344 (TDA7052).


343

POWER SUPPLY VE

VOLUME

VR1 (FRONT VIEW)

POWER SUPPLY VE+

TO SPEAKER(LS1)

SCREENEDINPUT LEAD

C1

VR2

R1 IC1

C6 C8

R2

C7

C4

C3C5

C2

++

+ +

+

2.0IN (50.8mm)

1.6I

N (

40. 6

mm

)

(0V)

W

Fig.2. Printed circuit board, component layout, full-size cop-per foil master and interwiring for the LM386N-1 Amp.

Approx. CostGuidance Only ££1100..5500

excluding case & speaker

LM386N-1 AMPLIFIERResistors

R1 470 R2 10All 0·25W 5% carbon film

PotentiometersVR1 10k min. rotary carbon, log.VR2 10k enclosed carbon preset

CapacitorsC1 47 radial elect. 25VC2 1n disc ceramicC3, C8 220 radial elect. 25V (2 off)C4 100n disc ceramicC5, C6 10 radial elect. 25V (2 off)C7 47n polyester

SemiconductorIC1 LM386N-1 audio power amp i.c.

MiscellaneousLS1 4 to 32 ohm loudspeaker (see text)

Printed circuit board available from the EPE PCB Service, code343 (LM386N-1); case (optional), size and type to choice; 8-pin d.i.l.socket; multistrand connecting wire; audio screened cable; solderpins; solder etc.

SeeSSHHOOPPTTAALLKKppaaggee

TDA7052 POWER AMPLIFIERR.M.S. Power output just before the onset

of waveform clipping

Speaker Supply VoltageImpedanceOhms 3V 4·5V 6V 9V 12V

4 70mW 500mW 780mW – –8 60mW 455mW 640mW 1W –

16 40mW 235mW 450mW 1·12W –32 24mW 145mW 250mW 600mW 1·26W

Quiescent current 5mAInput resistance 100k ohmsInput sensitivity for 1W

output (8 ohm load, 9V supply) 40mV r.m.s. (gain 70)Absolute maximum supply voltage

beyond which damage will occur 18VSuggested maximum supply voltage:

with a 4 ohm speaker 6Vwith 8 or 16 ohm speakers 9V

Frequency response at the –3dB points 25Hz – 20kHz

C110µ

8ΩLS1

+ +3V TO 12V

0V

4k7VR1

220µC3

100nC2

2

3

68

1SIGNALINPUT

VOLUME

+

+

IC15

TDA7052

1

2

3

4 5

6

7

8

INPUT

SIGNAL OUT

TOP VIEW OF TDA7052

NOT CONNECTED

NOT CONNECTED

INPUT GROUND OUTPUT GROUND

SIGNAL OUT

SCREEN

W

SUPPLY VOLTAGE V+

Fig.3. Circuit diagram and pinout details for the TDA7052Amp. See left for performance guide.

EPE Online

Note that the circuit boards used in EPE Online projects are available from the EPE Online Store at www.epemag.com (also note that the codes for the boards in the online store are prefixed with 7000, so a board with a code of say 256 will appear as 7000256 in the online store).


Approx. CostGuidance Only ££99..5500


TDA7052 AMPLIFIER

CapacitorsC1 10 radial

elect. 25VC2 100n disc

ceramicC3 220 radial

elect. 25V

PotentiometersVR1 4k7 min. rotary carbon,

log.

SemiconductorIC1 TDA7052 power amp i.c.

MiscellaneousLS1 4 to 32 ohm loudspeaker

(see text)

Printed circuit board available from theEPE PCB Service, code 344 (TDA7052);case (optional), size and type to choice; 8-pin d.i.l. socket; multistrand connectingwire; audio screened cable; solder pins;solder etc.


344

POWER SUPPLY VE

VOLUME

VR1 (FRONT VIEW)

POWER SUPPLY VE+

TO SPEAKER

SCREENEDINPUT LEAD

C1

IC1

C3

C2

++

1.2IN (30.5mm)

1.6I

N (

40. 6

mm

)

w

(0V)

(LS1)

Only four componentsare mounted on the

TDA7052 p.c.b.

Fig.4 (below). Component layout,interwiring and full-size copper foil

master for the TDA7052 Amp.

A circuit diagram incorporating the TBA820M audio amp i.c.,

which is manufactured by SGS-Thomson, together with a generalperformance guide, is given in Fig.5. The input arrangements,supply line bypassing, speaker coupling and Zobel network areconventional, and the relevant components can be identified fromprevious circuit descriptions.

Gain can be controlled by shunting an internal negative feedbackloop, which is accessed at pin 2. Preset potentiometer VR2, placed in

C14 7µ

100nC3220µ

C2

8ΩLS1

+ +3V TO 12V

0V

10kVR1

220µC8

1ΩR2

220nC7C4

3

C547µ

100µ

100ΩVR2

22ΩR1

2

4

390pC6

1

57

SIGNALINPUT

VOLUME

+

+

+

+

+

TBA820MIC1

68

1

2

3

4 5

6

7

8SET HIGHFREQUENCY RESPONSE

SET GAIN

INPUT

SUPPLY VOLTAGERIPPLE REJECTION

BOOTSTRAP

SUPPLY VOLTAGE V+

SIGNAL OUT

TOP VIEW OF TBA820M

SCREEN

GROUND (0V)

W

Fig.5. Circuit diagram and pinout details for the TBA820Mpower amplifier. See right for general performance details.

TBA820M POWER AMPLIFIERR.M.S. power output just before the onset

of waveform clippingSpeaker Supply VoltageImpedanceOhms 3V 4·5V 6V 9V 12V

4 10mW 320mW 405mW 980mW –8 20mW 200mW 300mW 680mW 1·1W16 30mW 115mW 180mW 405mW 720mW32 20mW 60mW 90mW 225mW 390mW

Quiescent current 6mAInput resistance 5M ohmsInput sensitivity for 680mW output (8 ohm load, 9V supply):

(a) VR2 set for maximum resistance 56mV r.m.s. (gain 40)(b) VR2 set for minimum resistance 10mV r.m.s. (gain 230)

Absolute maximum supply voltagebeyond which damage will occur 16V

Suggested maximum supply voltage:with a 4 ohm speaker 9Vwith an 8 ohm speaker 12V

High frequency response at –3dB point:with C6 220pF 20kHzwith C6 680pF 7kHz

Completed TBA820Mamplifier module.

EPE Online


series with capacitor C4, controls the shunt-ing effect and, with the sample tested, gaincould be set between 40 and 230.

High frequency response is determinedby capacitor C6. The response at the –3dBpoints for different capacitor values is alsolisted in the table. If desired, the value ofC6 can be increased to reduce the upperfrequency response even more.

In this application, the speaker LS1 is

connected to the positive supply rail as thisreduces the component count (a capacitorand resistor are saved).


layout, wiring details and full-size copperfoil master pattern are shown in Fig.6. Thisboard is available from the EPE PCBService, code 345 (TBA820M).

Approx. CostGuidance Only ££1100


TBA820M AMPLIFIER

ResistorsR1 22R2 1

All 0·25W 5% carbon film

PotentiometersVR1 10k min. rotary carbon,

log.VR2 100 enclosed carbon

preset

CapacitorsC1 47 radial elect. 25VC2, C8 220 radial elect. 25V

(2 off)C3, 100n disc ceramicC4 100 radial elect. 25VC5 47 radial elect. 25VC6 390p ceramicC7 220n polyester

SemiconductorIC1 TBA820M audio power

amp i.c.


(see text)

Printed circuit board available from theEPE PCB Service, code 345 (TBA820M);case (optional), size and type to choice; 8-pin d.i.l. socket; multistrand connectingwire; audio screened cable; solder pins;solder etc.


C1 C6

C3

C4

VR2

C7

C2

C8

C5

IC1R1

R2

+

+

+ +

+

2.4IN (61.0mm)

1.6I

N (

40. 6

mm

)

POWER SUPPLY VE

VOLUME

VR1 (FRONT VIEW) POWER SUPPLY VE+

SCREENEDINPUT LEAD

345

TO SPEAKER(LS1)

(0V)

W

Fig.6. Topside component layout, off-board wiring details and full-size copper foilmaster for the TBA820M Amplifier.

An amplifier circuit diagram incorporating the LM380N audioi.c. is shown in Fig.7. A general specification guide is shownbelow.

The manufacturers, National Semiconductor, have made provi-sion for optional heatsinking via some of the pins, and this makesthe package larger (14-pin). The output is short-circuit proofedand there is dissipation limiting. Gain is fixed.

Again, the purpose of the various components will be evidentfrom earlier descriptions. In this version of the circuit, the signal

LM380N POWER AMPLIFIERR.M.S. power output just before the onset

of waveform clippingSpeaker Supply Voltage ImpedanceOhms 9V 12V 15V 18V

4 400mW 1·12W 1·62W –8 275mW 720mW 1·32W 2·25W

16 137mW 405mW 720mW 1·32W32 68mW 202mW 360mW 765mW

Quiescent current 13mAInput resistance 150k ohmsInput sensitivity for 720mW


beyond which damage will occur 22V

A heatsink should be fitted if the device is to be operated, otherthan intermittently, at output levels in excess of 1W.

Without a heatsink, the suggested maximum supply voltages are:with a 4 ohm speaker 12Vwith an 8 ohm speaker 15V

Frequency response up to 100kHz

Finished LM380Ncircuit board. A “twin’’

heatsink may berequired for this chip


EPE Online


is applied to the inverting input (pin 6) and,to avoid instability, the non-inverting input(pin 2) is grounded (0V).

The manufacturers quote a minimumsupply voltage of 10V. The sample test-ed worked with a 9V supply, but perfor-mance became erratic at lower voltages.Quiescent current, although modest, is

double that of the other low-powerdevices, and this, together with thehigher operating voltage, makes the i.c.more suitable for mains-poweredequipment.

Details of the printed circuit board

component layout, wiring and copper foilmaster are given in Fig.8. This board is alsoavailable from the EPE PCB Service, code346 (LM380N).

Although the board has been keptsmall, as much copper as possible hasbeen retained to afford some heatsinking.

C14 7µ

100nC3220µ

C2

8ΩLS1

+ +9V TO 18V

0V

1

10kVR1

220µC5

2 7ΩR1

100nC6

10µC4

7

8

2

6

3,4,5,10,11,12

SIGNALINPUT

VOLUME

+

+

+

+

LM380NIC1

14

SUPPLY VOLTAGE V+

INPUT +

INPUT

1

2

3

4

5

6

7 8

9

10

11

12

13

14SUPPLY VOLTAGE RIPPLE REJECT

OUTPUT, GROUND(0V) AND HEATSINK

OUTPUT, GROUND(0V) AND HEATSINK

INPUT GROUND (0V)

NOT CONNECTED

NOT CONNECTED

SIGNAL OUT

TOP VIEW OF LM380N

+

SCREEN

W



LM380N AMPLIFIER

ResistorsR1 27 0·25W 5% carbon film

PotentiometersVR1 10k rotary

carbon, log.

CapacitorsC1 47 radial elect. 50VC2, C5 220 radial elect. 50V (2 off)C3, C6 100n disc ceramic (2 off)C4 10 radial elect. 50V

SemiconductorIC1 LM380N audio power amp i.c.

MiscellaneousLS1 4 to 32 ohm loudspeaker (see text)

Printed circuit board available from the EPE PCB Service, code346 (LM380N); case (optional), size and type to choice; 14-pin d.i.l.socket; heatsink (see text); multistrand connecting wire; audioscreened cable; solder pins; solder etc.


Fig.7. Circuit diagram for the LM380N Amplifier.

Produced by SGS-Thomson, the TDA2003 low-cost i.c. is

mainly for use in car radios. Although chips designed specificallyfor “hi-fi’’ amplifiers are available, they usually require highervoltage and/or split rail power supplies. This makes them less easyand more expensive to use.

SIGNALINPUT

VOLUME

C14 7µ

C24 7µ

100nC4

470µC6

39nC5

220µC3

4ΩLS1

220ΩR2

39ΩR1

+ +6V TO 15V

0V

15

2

3

4

10kVR1

2 2ΩR3

1000µC7

1ΩR4

100nC8

TDA2003IC1

+

+

+

+

+

SUPPLY VOLTAGE V+SIGNAL OUT

INPUT +

METAL TAGCONNECTED TOPIN 3 (GROUND)

INPUT

TYPE No.

FRONT VIEW OF TDA2003

+

SCREEN

GROUND (0V)

W

54321

Fig.9. Circuit diagram for a single TDA2003 Amp.

TDA2003 POWER AMPLIFIERR.M.S. power output just before the onset

of waveform clippingSpeaker Supply VoltageImpedanceOhms 9V 12V 15V

2 2·25W 4W 5·75W4 1·28W 2·6W 3·9W8 720mW 1·44W 2·1W

Quiescent current 45mAInput sensitivity for 2.6W


beyond which damage will occur 28VAbsolute maximum operating voltage 18VFrequency response: 40Hz to 15kHz at the –3dB points.The upper frequency limit can be extended by reducing

the value of C5.


Component layout onthe TDA2003 p.c.b.

EPE Online


For those readers who wish to get themost out of the chip, a suitable heatsink forthe LM380 (Fig.7 and Fig.8) can beformed from two, 40mm (15/8in.) lengths

of 25mm × 0·4mm (1in. × 1/64in.) brassstrip. Make two shallow cuts, 5mm (3/16in.)apart, close to the centre, and bend out atag which can be soldered to the relevantpins of the i.c. Thin brass strip can be pur-chased from almost all model shops.

C4

C1

C2C3

C5

C6

R1

+

+

+

+

IC1

346

VOLUME

VR1 (FRONT VIEW)

POWER SUPPLY VE+SCREENEDINPUT LEAD

2.5IN (63.5mm)

1.8I

N (

45. 7

mm

)

POWER SUPPLY VE

HEATSINK

TO SPEAKER(LS1)

(0V)

W

Fig.8. The LM380N printed circuit board component layout, off-board interwiringand full-size copper foil master pattern. Note the heatsinks.

The TDA2003 incorporates short circuitand overload protection, and is extremelyrugged. It will deliver a worthwhile output atmodest supply voltages, and the suitabilityof car batteries as a power source may makeit of particular interest to some readers. The

circuit diagram of a single chip TDA2003audio amplifier is given in Fig.9.

Grounding the input (pin 1) of thisdevice would upset the internal biasingarrangements, so a second blockingcapacitor C2 must be provided. The high

3.4IN (86.4mm)

1.2I

N (

61. 0

mm

)

POWER SUPPLY VEPOWER SUPPLY VE+

347

VOLUME

VR1 (FRONT VIEW)

SCREENEDINPUT LEAD

C4C1

C2

C6

R1

R3

R2

C5 IC1

C7

C3 C8

R4

1

2

3

4

5+

++

+

+

TO SPEAKER(LS1)

W

Fig.10. Printed circuit board component layout, full-size foil master and off-boardwiring for the single TDA2003 Amplifier.

frequency response is set by capacitor C5in conjunction with resistor R1. Theresponse can be extended by reducing thevalue of C5. Supply line ripple rejection isafforded by capacitor C6.

The outputs which can be delivered atvarious supply voltages are tabulated in theaccompanying table. The current drawnfrom a 15V supply when 4W are dissipat-ed into a 4 ohm load is around 500mA.The 2 ohm load is obtained by connectingtwo 4 ohm speakers in parallel.

At these power levels, the device must,of course, be connected to an adequateheatsink, and this is discussed later.

The printed circuit board componentlayout, wiring and full-size copper foilmaster pattern for the single chipTDA2003 amplifier are shown in Fig.10.This board is available from the EPE PCBService, code 347 (TDA2003).

A circuit diagram using two TDA2003

chips in a bridge configuration is shown inFig.11, together with a general perfor-mance guide.

Drawing around 1·7A from a 15V sup-ply, this combination will deliver a clean12·5W into a 4 ohm load. The case for thisbeing adequate for domestic listening hasalready been argued, but individual con-structors will, of course, decide whether ornot it will meet their needs.



TDA2003 AUDIO AMPLIFIER

ResistorsR1 39R2 220R3 22R4 1


PotentiometersVR1 10k rotary carbon, log.

CapacitorsC1, C2 47 radial elect. 50V

(2 off)C3 220 radial elect. 50VC4 100n disc ceramicC5 39n polyesterC6 470 radial elect. 50VC7 1000 radial elect. 50VC8 100n polyester

SemiconductorIC1 TDA2003 audio power

amp i.c.


(see text)

Printed circuit board available from theEPE PCB Service, code 347 (TDA2003);case (optional), size and type to choice;heatsink (see text); audio screened cable;multistrand connecting wire; solder pins;solder etc.



EPE Online


C14 7µ

C24 7µ

100nC3

22µC4

100nC6

100nC5

10µC7

100nC8

C94 7µ

220µC101Ω

R4

4ΩLS1

10ΩR5

470ΩR6

220ΩR2

10ΩR3

1ΩR1

+ +6V TO 15V

0V

SIGNALINPUT

10kVR1

VOLUME

+

+

+

+

++

SCREEN

W

15

2

3

4TDA2003

IC1+ 15

2

3

4TDA2003

IC1+

4.0IN (101.6mm)

1.2I

N (

61. 0

mm

)

POWER SUPPLY VE (0V)

VOLUME

VR1 (FRONT VIEW)

POWER SUPPLY VE+

TO SPEAKER (LS1)

SCREENEDINPUT LEAD

348

+

++

+

+

+1

2

3

4

5C1

C2

C3

R3

R2

C4

R1

C5

R4

R5

C6R6 C7

C9C8

C10

IC1 IC2

1

2

3

4

5

W

TWO TDA2003 BRIDGE CONFIGURATIONPOWER AMP

R.M.S. power output just before the onsetof waveform clipping

Speaker Supply VoltageImpedanceOhms 9V 12V 15V

2 6·25W 10·5W –4 3·78W 8W 12·5W8 2W 5W 8·2W

Quiescent current 80mAInput sensitivity for 8W

output (4 ohm load, 12V supply) 70mV r.m.s. (gain 40)See single TDA2003 for details of absolute maximum ratings.


Twin TDA2003 Amplifiercircuit board componentlayout.

TWIN TDA2003 POWER AMP

ResistorsR1, R4 1 (2 off)R2 220R3, R5 10 (2 off)R6 470


PotentiometersVR1 10k rotary carbon

CapacitorsC1, C2

C9 47 radial elect. 50V(3 off)

C3, C8 100n disc ceramic(2 off)

C4 22 radial elect. 50VC5, C6 100n polyester (2 off)C7 10 radial elect. 50VC10 220 radial elect. 50V

SemiconductorIC1, IC2 TDA2003 audio power

amp i.c. (2 off)


(see text)

Printed circuit board available from theEPE PCB Service, code 348 (TDA2003);case (optional), size and type to choice;heatsink (see text); audio screened cable;multistrand connecting wire; solder pins;




Fig.12 (below). Component layout, off-board inter-wiring and full-size copper foil master for the TwinTDA2003 Amp. You will need a heatsink for thesedevices.

Fig.11 (above).Circuit diagramfor the TwinTDA2003 PowerAmplifier.

EPE Online


The printed circuit board componentlayout, wiring and copper foil master aredetailed in Fig.12. Again, combined or sep-arate heatsinks must be fitted to the inte-grated circuit’s metal tabs. The p.c.b. isobtainable from the EPE PCB Service,code 348 (Twin TDA22003).

A large area metal heatsink is required

for the TDA2003 (Fig.9 and Fig.11).Because the device incorporates overloadprotection, the actual size is not too critical(the i.c. will shut down when it begins tooverheat), but sustained high output willonly be developed if the heatsink is ade-quate. At the very least use 40sq. cm(6·5sq. in.) of 16s.w.g. aluminium per chip,or fit a proprietary heatsink with a thermalresistance not greater than 7°C per watt.

The i.c.s are arranged on the p.c.b. (seeFig.10 and Fig.12) so that they can be bolt-ed to the back of a metal case by theirmetal tabs. A 50mm × 150mm × 200mm(2in. × 6in. × 8in.) aluminium box wouldbe more than adequate as a heatsink.Insulating washers are not required, but asmear of heat transfer compound should beapplied.

Slight differences in the i.c. type num-

bers can cause confusion. The LM386N-1has the lowest power rating of this group ofdevices. The suffixes “N-3’’ and “N-4’’indicate devices rated at 700mW and 1Wrespectively. The suffix “M’ indicates sur-face mounting. Suppliers offering the

LM386 are usually referring to the N-1version.

The TDA7052 is sometimes given thesuffix “A’’. This indicates that the chip con-tains a d.c. volume control and is not suit-able for the circuit described here.

Some suppliers give the LM380 the suf-fix “14’’ to indicate the 2·5W 14-pin ver-sion, and the suffix “8’’ for the 8-pin600mW alternative. When ordering, makeit clear that the 14-pin chip is required.

The suffix “P’’ or “V’’ is sometimesadded by suppliers to the TDA2003 to indi-cate that it is for vertical, and “H’’ for hor-izontal, mounting. There is no electricaldifference, but the p.c.b.s illustrated herehave been designed for vertical chips.

All the amplifiers covered in this part are

assembled on printed circuit boards andconstruction is reasonablystraightforward. The use ofan i.c. holder will permit thesubstitution and checking ofthe low power amplifiers.However, if reliance is to beplaced on the p.c.b. foil forminimal heatsinking of theLM380, the device should besoldered directly in place.Solder pins, inserted at thelead-out points, will simplifyoff-board wiring.

It may help to start con-struction of the chosen cir-cuit board by first placingand soldering the i.c. holder

on the p.c.b. to act as an “orientation’’guide. This should be followed by the lead-off solder pins, and then the smallest com-ponents (resistors) working up to thelargest, electrolytic capacitors and presets.Finally, the lead-off wires (including thescreened input cable), off-board Volumecontrol and loudspeaker should be attachedto the p.c.b.

On completion, check the board for poorsoldered joints or bridged tracks. Check theorientation of the electrolytic capacitorsand the i.c.(s).

If using a mains power supply, makesure the voltage delivered does not exceedthe safe working voltage of the amplifierfor the load impedance being used.

If all is in order, connect the power sup-ply and check the quiescent current con-sumption. Inject a signal and re-check thecurrent drain and supply voltage.

Next Month: Transistor preamplifiers


A Complete range of regulated inverters to power 220V and 240V ACequipment via a car, lorry or boat battery. Due to their high performance(>90%) the inverters generate very little heat. The high stability of theoutput frequency (+/-1%) makes them equally suitable to powersensitive devices.

These inverters generate a modified sine wave, which are considerably superior to the square waves which are produced bymost other inverters. Due to this superior feature they are capable of powering electrical equipment such as TV,s, videos,desktop & notepad computers, microwave ovens, electrical lamps, pumps, battery chargers, etc.Low Battery AlarmThe inverters give an audible warning signal when the battery voltage is lower than 10.5V (21V for the 24V version). The inverterautomatically shuts off when the battery voltage drops below 10V (20V for the 24V version). Fuse protected input circuitry.

A COMPLETE RANGE OF

INVERTERS150W TO 2500W - 12V & 24V

WWWWWW.BKELEC.COM/INVERTERS.HTM.BKELEC.COM/INVERTERS.HTM

Order Code

651.581651.578651.582651.585651.583651.593651.587651.597651.602651.605651.589651.599

Power

150W Continuous150W Continuous300W Continuous300W Continuous600W Continuous600W Continuous1000W Continuous1000W Continuous1500W Continuous1500W Continuous2500W Continuous2500W Continuous

Voltage

12V24V12V24V12V24V12V24V12V24V12V24V

Price

£36.39£36.39£50.64£50.64£101.59£101.59£177.18£177.18£314.52£314.52£490.54£490.54

All prices are inclusive of V.A.T. Carriage £6.00 Per Order

For Full Specifications View our web site at:-B.K. ELECTRONICS UNIT 1, COMET WAY, SOUTHEND-ON-SEA, ESSEX. SS2 6TRTEL.: +44(0)1702-527572 FAX.:+44(0)1702-420243

Many uses include:- * Fetes * Fairgrounds * Airshows * Picnics *Camping * Caravans * Boats * Carnivals * Field Research and * AmateurRadio field days * Powering Desktop & Notepad Computers.

DELIVERY CHARGES ARE £6-00 PER ORDER. OFFICIALORDERS FROM SCHOOLS, COLLEGES, GOVT. BODIES, PLC,SETC. PRICES ARE INCLUSIVE OF V.A.T. SALES COUNTER. VISAAND ACCESS ACCEPTED BY POST, PHONE OR FAX, OR EMAILUS AT [email protected] ALTERNATIVELY SEND CHEQUEOR POSTAL ORDERS MADE PAYABLE TO BK ELECTRONICS.

ILLUSTRATION SHOWN IS 651.583 600W VERSION

REF D4

IF a modest output from one of thesmaller power amplifiers (May ’02) isall that is required, dry batteries repre-

sent a suitable power supply. However,when the output is expected to exceed thehalf-watt level for sustained periods, amains power unit is more appropriate.Savings in the cost of batteries will quick-ly cover expenditure on components.

Compromises, inherent in the design ofloudspeakers, give rise to limitationswhich are normally overcome by the use oftwo or more units and a crossover.

Power supplies, loudspeakers and asso-ciated networks are the topics to be cov-ered this month.

A simple mains power supply compris-

ing a full-wave rectifier and capacitor inputfilter will deliver an off-load voltage ofaround 1·4 times the transformer sec-ondary voltage.

With a secondary rated at 12V a.c., theoff-load d.c. output voltage will, therefore,

be almost 17V. If the power supply outputis close to the maximum safe operatingvoltage of the amplifier i.c., there is a dan-ger that, under no-signal conditions, thedevice will be ruined.

When fully loaded, the d.c. output volt-age will fall to around 14V with an ade-quately rated transformer; lower when thetransformer specification has beenskimped. Voltage will, therefore, be low atthe very moments when the power amplifi-er is being called upon to deliver a highoutput.

These voltage variations are a cause ofdistortion and impair the performance ofthe power amplifier. Moreover, when high-gain preamplifiers or radio tuners are fedfrom the same supply, the variations canalso result in instability, even when sub-stantial decoupling is provided.

These problems can be avoided by regu-

lating the output of the power supply, anda versatile circuit, which can be adapted

for single or stereo pairs of any of theamplifiers described in Part 1 (May ’02), isgiven in Fig.1. The mains voltage isstepped down by transformer T1, and afull-wave bridge rectifier arrangement, D1to D4, produces the d.c. output. Reservoircapacitor C5 reduces supply ripple.

Voltage regulators IC1 and IC2 virtuallyeliminate any voltage swings caused byload variations. The regulators also removeany residual 100Hz ripple on the supplyvoltage rails and permit the use of a lowervalue reservoir capacitor (C5). Low levelelectrical noise, extending into the r.f.spectrum, is present in the output of thei.c.s, and bypass capacitors, C6, C7, C8and C9, shunt this to the 0V rail.

The voltages required by amplifiers,preamplifiers and auxiliary equipment areoften different, and provision is made fortwo regulated outputs. Alternatively, eachoutput can supply a separate channel of astereo system in order to double the currentrating.

The switching action of the rectifierdiodes (D1 to D4) modulates any r.f. (radiofrequencies) present in the mains input.This modulated r.f. can be picked up byradio receivers connected to the supply andit manifests itself as a 100Hz hum whichonly appears when a station is tuned in.Capacitors C1 to C4, connected across thediodes, suppress this interference, which isknown as modulation hum. If radio tunersare to be powered from this circuit, thesecapacitors must be fitted.

Fuse

It is good practice to protect the equip-ment with an internal fuse of the lowestpossible rating. Because of the nature ofthe load, this should be of the anti-surge orslow-blow type, and a component rated atone amp (1A) would be suitable for powersupplies serving the amplifiers describedin this series of articles.

TransformerThe rectified d.c. voltage across the

reservoir capacitor (C5) must be at least3V more than the regulator output when

500 Everyday Practical Electronics, July 2002

Part 3 – Power Supplies, Loudspeakers,Crossover Networks and Filters

Simple i.c. Power Amp.modules (left-to-right, top-to-bottom) from May ’02issue. Single TDA2003Amp Twin TDA2003 Amp TDA7052 Amp

TBA820M Amp LM380NAmp LM386N-1 Amp

maximum current is being drawn from thesupply. Further, the maximum input volt-age to the regulator i.c., which is usually35V for devices with a 2A rating, must notbe exceeded. It is also desirable for thevoltage drop across it to be no more than10V or so, or power dissipation within thechip will be increased and more elaborateheatsinking will be required.

These requirements can best be met ifthe mains transformer secondary voltage is3V more than the regulated d.c. output.

To determine the required current ratingof the secondary winding, add together thedemands of the amplifiers and ancillaryequipment to be connected to the powersupply, and increase this by at least 25 percent to allow for the reactive load present-ed by the reservoir capacitor (C5). Thecurrent requirements of the power ampli-fiers were given in Part 1. For conve-nience, they are repeated here in Table 2.

Manufacturers usually indicate the cur-rent delivering capacity of their mainstransformers by quoting a VA rating. Thisis, of course, the secondary output voltagemultiplied by the maximum current whichthe transformer can supply.

In Europe, mains transformers oftenhave two 115V primary windings and twoidentical secondary windings. The primarywindings must be series or parallel con-nected to suit the local supply voltage, andthe secondary connected to deliver thedesired output. Parallel connecting the sec-ondary will, of course, double the currentavailable. Connect the windings in phaseor the transformer will be short circuited.

series. Maximum current ratings are 5A for12V and 3A for 15V units, but chips rated atmore than 2A can be difficult to obtain.When the current demand exceeds 2A; e.g.when two, bridge-connected, pairs ofTDA2003 audio power amplifier modulesare used in a stereo combination, fit a 2Aregulator to each output of the power supplyand use one for each stereo channel.

Suppressor CapacitorsThe working voltage of capacitors C1 to

C4, connected across the rectifier diodes,should be at least four times the secondaryvoltage of the mains transformer. Bypasscapacitors C6, C7, C8 and C9, should havea working voltage at least 1·5 times thetransformer secondary voltage to protectthem in the event of regulator failure.

Everyday Practical Electronics, July 2002 501

C2100n

D21N4002

D41N4002

D31N4002

D11N4002

C4100n

C3100n

C1100n IC1

IC2

IN

COM

OUT

IN

COM

OUT

C6100n

C7470µ

C8100n

C9470µ

C52200µ

SEE TABLE 1 FORDETAILS OF VOLTAGE

REGULATORS IC1 AND IC2

SEE NOTE

REGULATEDOUTPUT 1

OUTPUT 2REGULATED

L

N

E

1A TIME DELAY(SLOW BLOW)

FUSEFS1

0V

230V

MAINS TRANSFORMERPRIMARY TO SUIT SUPPLY

VOLTAGE. SEE TABLE 1FOR DETAILS OF SECONDARY

T1

EURO STYLEMAINS INLET PLUG

a

a

a

ak

k

k

k

+

+

+

+V

+V

0V

PL1

+SEECOMPONENT

LIST

Fig.1. Circuit diagram for a Dual Output Regulated Power Supply.

Table 1: Component RatingsRegulated Output Transformer Sec. Regulator I.C. C5

V d.c. V r.m.s. (1A max output) Working Voltage

6 9 L7806 259 12 L7809 25

12 15 L7812 3515 18 L7815 35

NOTES:(1) To determine the transformer current rating, add together the current demands of pre and

power amplifiers and any ancillary equipment, then increase the total by at least 25% toallow for the reactive load presented by C5.

(2) A bridge-connected pair of TDA2003 i.c.s with a 4 ohm load will draw 1·7A from a 15Vsupply and the ratings of the rectifiers, regulator and reservoir capacitor must beincreased. Use 1N5401 rectifiers, an L78S15 regulator and a 4700F capacitor for C5(35V working).

(3) For two, bridge-connected pairs of TDA2003 i.c.s in a stereo combination, fit a 10000F(or two 4700F) 35V reservoir capacitor, two L78S15 regulators, (one for each stereochannel) and use P600D rectifiers.

Table 2: Power Amplifier Current RequirementsPower Amp Speaker Imp Supply volts Current drain Power output

I.C. Ohms V d.c. A W

LM386N-1 4 6 0·13 0·32LM386N-1 8 9 0·12 0·56TDA7052 4 6 0·42 0·78TDA7052 8 9 0·39 1TBA820M 4 9 0·23 0·98TBA820M 8 12 0·17 1·1LM380N 4 12 0·23 1·12LM380N 8 15 0·19 1·32TDA2003 4 15 0·5 3·92TDA2003 8 15 0·27 2·1TDA2003 x 2 4 15 1·7 12·5TDA2003 x 2 8 15 0·96 8·2

Current drain and power output measured just before the onset of clipping.

Completed power supply board.

RectifiersWith a capacitor input filter, the rectifiers (D1 to D4) must have a

p.i.v. (peak inverse voltage) rating at least three times the secondaryvoltage of the mains transformer. Their current rating should be at least50 per cent greater than the maximum load on the power supply.

Reservoir CapacitorThe value of the reservoir capacitor, in microfarads (F), should be

at least 2500 times the maximum load current in amps when the supplyis regulated, and double this value when unregulated. The workingvoltage should be at least double the secondary voltage of the mainstransformer.

RegulatorsThe current rating of the voltage regulators (IC1 and IC2) must, of

course, be equal to or greater than the maximum current demand on thepower supply. The maximum input voltage rating (usually 30V to 35V)must be at least 1·5 times the secondary voltage of the mainstransformer.

Regulator i.c.s are available in a range of output voltages suitable for theaudio amplifiers (May’02) and preamplifiers (June’02) described in this



excluding case

POWER SUPPLYCapacitors

C1 to C4 100n ceramic, 100V (4 off)C5 2200radial elect.

(see Table 1)C6, C8 100n ceramic, 50V (2 off)C7, C9 470 radial elect. 50V (2 off)

SemiconductorsD1 to D4 1N4002 rect. diode for 1A max. output (4 off)

1N5401 rect. diode for 3A max. output (4 off)P60D rect. diode for 4A max. output,

limited by regulators (4 off)IC1, IC2 78 series for 1A; 78S series for 2A

maximum output. See Table 1 (2 off)

MiscellaneousT1 mains transformer – see text and Table 1FS1 1A 20mm slow-blow fuse to suit holderPL1 Euro fused mains inlet, chassis mounting,

plug with line socket

Printed circuit board available from the EPE PCBService, code 356 (PSU); metal case, size and type tochoice; multistrand connecting wire; mains cable; aluminiumsheet or proprietary heatsink and heatsink compound; solderpins; nuts, bolts and washers; stand-off pillars (4 off); solderetc.


DDUUAALL OOUUTTPPUUTT RREEGGUULLAATTEEDDPPOOWWEERR SSUUPPPPLLYY

356

a

k

a

k

a

k

a

k

T1

0V

230V

LN

E

FS1

D1

D2

D3

D4 C2

C1

C3

C4

C6

C8

C7

C9

C5

+

++

IC1

IC2

IN

IN

COM

COM

OUT

OUT

SEPARATE EARTH(0V RAIL) RETURNSTO PREAMP, POWERAMP, ETC.

REGULATED V+

REGULATED V+

OUTPUT 1

OUTPUT 2

HEATSINK3.4IN (86.4mm)

1.85IN (47mm)

SOLDER TAG

PL1

1 2 3VIN VOUT

COMMON

TYPENUMBERIC1 AND IC2

CONNECTIONDETAILS

Fig.2. Power Supply printed circuit board, full-size copper master and suggested mains transformer and separatepanel fuseholder interwiring. The 16 s.w.g. aluminium heatsink measures 45mm x 45mm.

Any readers who have no experience of building or commis-

sioning mains-powered equipment are reminded that the volt-ages involved can kill! Anyone who feels unsure of his or herability to complete a project of this kind MUST seek help andguidance from an experienced constructor.

The small components are assembled on the printed circuit board(p.c.b.) as illustrated in Fig.2, together with a full-size copper foilmaster and the interwiring to off-board components. This board isavailable from the EPE PCB Service, code 356.

Commence construction by first soldering in position on the p.c.b.the rectifier diodes and non-electrolytic capacitors. This can be

EPE Online


followed by the larger electrolytic types andthe voltage regulators IC1 and IC2. Finally,you will need to bolt a heatsink to the regu-lators and details of choosing a suitableheatsink will be given shortly. Solder pins,inserted at the lead-out points, simplify thetask of off-board wiring.

Diodes D1 to D4, the reservoir capaci-tor, C5, and the regulators, IC1 and IC2,have to be chosen to suit the voltage andcurrent to be delivered by the power sup-ply. The requirements are summarised inTable 1 and the associated notes. Details ofthe modest current needs of the variouspreamplifiers were given in Part 2 of theseries, and the current demands of thepower amplifiers are scheduled in Table 2.

Dimensions and fixing arrangements formains transformers vary and this heavycomponent should be mounted directlyinto or on the metal equipment case bottomor chassis panel. A Euro-style mains inletplug, with a built-in fuseholder for FS1, isstrongly recommended. You can, of course,use a separate panel-mounting fuseholderif you wish, see Fig.2.

Mains Earth should be connected to anymetal case and to the core and cladding ofthe transformer. (A solder tag bolted underone of the mains transformer mountinglugs makes a good earthing point for themains Earth lead.)

Interwiring details to off-board compo-nents are also shown in Fig.2. Leads con-necting the mains transformer to the inletplug and the p.c.b., and any mains switchwiring, should be tightly twisted to minimiseexternal fields. Keep the transformer at least150mm (6in.) away from signal input wiring.

Toroidal transformers have a smallerexternal field than units with conventionalcores. They are the component of choicewhen the equipment is particularly compactand/or high gain preamplifiers are used.

Unless the current drain is to be very low

(say 20mA or less), the regulator i.c.s mustbe bolted to a heatsink. The 45mm × 45mmsheet of 16s.w.g. aluminium shown on thedrawing (Fig.2) is sufficient for currentdrains up to 1A when the voltage dropacross the regulators is not too extreme.

For larger current loads it is suggestedthat the heatsink be extended and bolted tothe metal case or chassis of the unit toensure adequate heat transfer. Failure toproperly dissipate heat from the regulatorswill result in the devices shutting down.

Once construction has been completed,

check the p.c.b. for poor soldered joints andbridged tracks. Check the orientation of elec-trolytic capacitors, diodes and regulators.

Make sure that the primary windings ofthe mains transformer are connected to suitthe local supply voltage, and that the sec-ondary windings are connected, in phase,to deliver the correct voltage to the powersupply p.c.b. It is a good idea to connectthe transformer to the mains and check thesecondary voltage with a test meter beforelinking it to the p.c.b. Extra care must betaken when carrying out this last task.

Check the voltage across the reservoircapacitor C5, and that the voltages deliv-ered by regulators (IC1 and IC2) are cor-rect before using the supply to power anyequipment.

Loudspeaker (speaker) designers have tomake compromises. Sensitivity, good tran-sient and good high frequency responsecall for a lightweight cone and speech coilassembly. Power handling and an extendedlow frequency response require a large,strong (and heavy) cone and coil.

For good sensitivity, the magnetic fieldcutting the voice coil must be intense.Unfortunately, this increases the imped-ance at the cone’s resonant frequency.However, this impedance rise can be con-trolled by the speaker enclosure, and apowerful magnet is always preferable.

The reproduction of low frequenciesinvolves large cone excursions and the sus-pension must be highly compliant. Highcompliance also lowers the cone’s resonantfrequency, and this extends the speaker’slow frequency response. However, the needto maintain control of the position of thevoice coil in the magnet gap imposes lim-its on how free the suspension can be.

Cone movement for a given sound out-put reduces with increasing speaker sizebut, as we have seen, greater diaphragmmass impairs transient and high-frequencyresponse.

To avoid performance being excessively

degraded by these conflicting require-ments, domestic “hi-fi” systems usuallycombine two or more speakers, each beingdesigned to reproduce part of the audio fre-quency spectrum.

The low frequency unit, or bass speaker,has a comparatively heavy cone and voicecoil with a highly compliant suspension.Clever designers have managed to obtainreasonable results with small speakers, butan extended low frequency response andgood power handling are more easy toachieve with speakers of 200mm (8in.) ormore in diameter.

Mid-range units are sometimes providedwhen the low frequency speaker is large(300mm to 450mm or 12in. to 18in. diam-eter). As one would expect, cones arelighter, the compliance is often stiffer, andthe chassis can form a sealed enclosure.

High-frequency units, or “tweeters”, havea very small diaphragm, which is commonlydome shaped to improve sound dispersal.Units of this kind always have sealed backs.

Whilst moving coil tweeters are the pre-ferred option for hi-fi applications, horn-loaded piezoelectric units are often fitted inthe high power speaker systems used bymusicians. The impedance of these devicesrises, and their power consumption fallsalmost to zero, as the applied frequency islowered. They do not, therefore, require a“crossover unit”, and are easy to connectinto multiple speaker systems.

Loudspeakers intended primarily for

speech reproduction in communicationsequipment have to perform well over arestricted frequency range, usually around300Hz to 3000Hz.

Inexpensive speakers of the type manu-factured for portable receivers are bettersuited for this purpose, and, if space isavailable, a 102mm (4in.) diameter unit is

to be preferred. Clarity will be impaired iflow frequencies are allowed to excite thecone of a speaker of this kind, and mea-sures to prevent this were discussed inPart 1 (May ’02).

Speech coil impedance is usually mea-

sured at around 400Hz. At this frequency,the inductance of the coil has a minimaleffect, and its impedance is only one or twoohms more than its d.c. resistance. As fre-quency rises, the inductance of the speechcoil has a growing impact and impedancemounts steadily.

The movement of the speech coil in themagnetic field induces in it a voltage whichopposes the signal voltage. At the cone’sresonant frequency, very little energy isneeded to sustain it in motion, and itvibrates readily, over larger distances, for acomparatively small power input.

These larger cone excursions generate agreater opposing voltage, or back-e.m.f.,and speech coil impedance, at resonance,increases by as much as a factor of ten overits nominal value. The more powerful themagnetic field, the more dramatic the risein impedance.

Impedance peaking at cone resonance(between 30Hz and 100Hz for low fre-quency speakers), and the gradual rise inimpedance with increasing frequency,makes the response of the speaker non-lin-ear. (The power which can be fed to aspeaker system falls as its impedancerises). Fortunately, the former can be tamedby good enclosure design, and the lattercan be overcome by the use of filter net-works and the addition of a tweeter.

Care must always be taken to ensure thatthe rated impedance of a speaker system isnot too low for the power amplifier. Toolow an impedance will cause excessive dis-sipation in the output transistors and, ifthere is no overload protection circuitry,the power amplifier will be ruined.


Safe supply voltage and speaker imped-ance combinations for the various i.c.power amplifiers were given in Part 1.They are summarised here in Table 2.

When two or more speakers are used to

improve performance, arrangements mustbe made to allocate the audio spectrumbetween them.

The resistance presented by capacitorsto the flow of alternating current decreasesas frequency rises. With inductors, resis-tance increases with rising frequency. Thisfrequency-dependant opposition to currentflow is known as reactance.

Capacitors and inductors can be com-bined in simple networks which utilise thisphenomenon to allocate frequency bandsto different speakers. Circuits and designdata are given in Fig.3 and inductor andcapacitor values for common speakerimpedances, and a range of crossoverfrequencies, are set out in Table 3. Thereactances of standard value capacitors, at

various audio frequencies, were tabulatedin Part Two.

The simple “first order” filters shown in

Fig.3a and Fig.3d are perfectly suitable fordomestic systems rated at up to 15W.

Low frequency roll-off above thecrossover frequency is 6dB per octave andthis may not be sufficient to protect sometweeters when higher powered amplifiers areused. In these cases, the second order filters,shown in Fig.3b and Fig.3e, which producea 12dB roll-off, are safer options.


Table 3: Crossover Network Inductor and Capacitor Values

Crossoverfrequency 500 1000 1500 2000 2500 3000 3500 4000 4500

Hertz

4 ohm Speaker L 1·3 0·63 0·42 0·32 0·25 0·21 0·18 0·16 0·141st Order Filter C 80 40 26 20 16 13 11 10 8

4 ohm Speaker L 1·8 0·9 0·6 0·5 0·35 0·3 0·25 0·22 0·22nd Order Filter C 56 28 18 14 11 9 8 7 6

8 ohm Speaker L 2·6 1·26 0·84 0·64 0·5 0·42 0·36 0·32 0·281st Order Filter C 40 20 13 10 8 6·5 5·5 5 4

8 ohm Speaker L 3·6 1·8 1·2 1 0·7 0·6 0·5 0·44 0·42nd Order Filter C 28 14 9 7 6 4·5 4 3·5 3

Inductance values, L, are given in mH (millihenries).Capacitor values, C, are given in F (microfarads).See text for guidance on rounding figures up or down to nearest standard value.

BASS

TREBLE

C

L

INPUT

A)

FIRST ORDER NETWORKTWO SPEAKER SYSTEM

Make reactance of inductor, L, andcapacitor, C, at the crossover fre-quency, equal to the rated speakerimpedance.

BASS

TREBLE

INPUT C

L

L

C

B)

SECOND ORDER NETWORKTWO SPEAKER SYSTEM

(1) Calculate inductor and capacitorvalues, as for the first order network.

(2) Divide the capacitor values by 1·4and multiply the inductor values by 1·4to obtain the correct values for thesecond order two speaker network.

Inductors, L, are identical.Capacitors, C, are identical.

MID RANGE

C L

INPUT

C)

FIRST ORDER NETWORKMID-PASS ARRANGEMENT

Make the reactance of inductor, L, andcapacitor, C, at the centre frequency ofthe pass band, equal to the rated speak-er impedance.

Assume a band centre frequency of1000Hz when the circuit is being usedas a band-pass filter for speechfrequencies.

MID RANGE

BASS

TREBLE

INPUT C1

L1

C2

L2

D)

FIRST ORDER NETWORK THREE SPEAKER SYSTEM

(1) Make inductor, L1, and capacitor,C1, reactances, at the bass/mid-range cross-over frequency, equal tothe rated speaker impedance.

(2) Make inductor, L2, and capacitor,C2, reactances, at the mid-range/-treble cross-over frequency, equal tothe rated speaker impedance.

MID RANGE

BASS

TREBLE

INPUT C1

L1

L2

L2

C1

L1

C2 C2

E)

SECOND ORDER NETWORKTHREE SPEAKER SYSTEM

(1) Calculate inductor and capacitorvalues as for the first order threespeaker network.

(2) Divide the capacitor values by 1·4and multiply the inductor values by1·4 to obtain the correct values forthe second order three speakernetwork.

Fig.3. Circuit and design data forloudspeaker crossover networks.Inductor and capacitor values forcommon speaker impedancesand crossover frequencies aregiven in Table 3.

Components with the same ref-erence numbers have identicalvalues – i.e. L1 and L2 are twoinductors of the same value; C1and C2 are capacitors of thesame value.

With two-speaker systems the cross-

over frequency is usually between 1kHzand 4·5kHz, and the tweeter manufactur-er’s recommendations should be followed.If the unit is of uncertain origin, adopt acrossover frequency of around 2·5kHz: thiswill normally be satisfactory.

When the bass speaker is large (12 inch-es diameter or more), a crossover at 1kHzor even lower can produce a more even fre-quency response. Suitable tweeters tend tobe rather costly, but an inexpensive alterna-tive will be described later.

Another way of ensuring a more even

response when a large bass speaker isused is to install a third, mid-range unit.Suitable circuits are given in Fig.3d andFig.3e.

The bass/mid-range crossover point isusually around 500Hz with open chassismid-range speakers, and 1000Hz withsealed back units. The mid-range/treblecrossover is generally between 4·5kHzand 6kHz. Again, the recommendationsof the speaker manufacturer should befollowed.

Parallel connected bass speakers must

be wired in phase to avoid cancellation ofthe lower audio frequencies. Use a 1·5Vdry cell to test for phasing on unmarkedspeakers by noting the battery positive con-nection for the outward movement of thecone.

Crossover networks introduce phaseshift, but, as frequency increases, phasingbecomes less important. Readers can tryreversing the connections to mid-rangeunits. However, unless they have a veryrefined ear, they are not likely to detect anydifference.

Inductors

Inductors for home-made crossovershave to be hand wound. The amount ofwire, and the resistive losses, can begreatly reduced by winding the coils onshort lengths of ferrite aerial rod. Coresaturation problems should not arise atthe power levels encountered in domesticinstallations.

Bobbin construction is illustrated inFig.4. Winding details for the inductor val-ues likely to be encountered are given inTable 4.

The wire should be wound on evenly,and masking tape, applied over each layer,will make the task a little easier.Constructors who have difficulty produc-ing neat windings should increase thediameter of the bobbin ends for the largerinductance coils.

CapacitorsThe bipolar electrolytic capacitors used

in crossover networks are available in alimited range of values. Capacitors of thiskind can be formed by connecting twoordinary electrolytics back-to-back, andthis makes possible the production of non-standard values. The details are given inFig.5. Capacitors rated at 50V working

will be suitable for all of the power ampli-fiers described in Part 1.

The performance of electrolytic capaci-tors can become uncertain at high audiofrequencies, and the best crossover net-works use components with a paper, poly-ester or polypropylene dielectric.

TolerancesVariations in the composition of ferrite

rod will affect the tabulated inductor valuesshown in Table 4 by plus or minus 10 percent or so. Bipolar electrolytics, whetherpurchased or homemade, have a tolerance,at best, of plus or minus 20 per cent.

Fortunately, loudspeaker crossover net-works are very forgiving, and componentspreads even greater than this produce noaudible difference. When calculated valuesare being rounded up or down, it is prudentto err on the high side with inductors andon the low side with capacitors.

Mention has already been made of the

desirability of restricting the audio band-width of speakers used primarily forspeech communication. An inductor andcapacitor can be combined to produce abandpass effect, and a typical circuit isgiven in Fig.3c.

As a starting point, select the inductorand capacitor values for a centre frequencyof 1000Hz (1kHz). If a more severe atten-uation of frequencies below 300Hz andabove 3000Hz (3kHz) is required, reducethe capacitor and increase the inductorvalue. When using this network with ear-phones, connect both earpieces in parallelto produce an impedance of 16 ohms, andperform the calculations on this basis.

Although extremely simple, this mea-sure will greatly improve the clarity ofspeech, especially when signals are over-laid by received or generated noise withinthe amplifiers.

The circuit diagram for an inexpensive

8ohm Crossover/Filter unit suitable for amulti-purpose workshop speaker is shown


Table 4: Inductance of Ferrite-cored CoilsInduct 0·1 0·2 0·3 0·4 0·5 0·75 1 1·5 2 2·5 3 3·5mHNo. of 45 60 75 90 100 125 150 175 200 225 250 275turns

Use 20 s.w.g. (19 a.w.g.) enamelled copper wire for coils up to 2mH.Use 22 s.w.g. (21 a.w.g.) enamelled copper wire for 2·5mH to 3·5mH coils.See illustration for details of bobbin and core.

Fig.4. Inductor bobbin construction details.

FORMULAE FOR THE REACTANCE OFINDUCTORS AND CAPACITORS

XL = 0·00628 f L ohms

XC = 159000 ohmsf c

wheref is in HertzL is in millihenries (mH)

and C is in microfarads (F)

+ +

C1

C1

C2

C2+ +

Fig.5. Creating a bipolar electrolyticfrom two capacitors.

FORMULAE FOR COMBINING CAPACITORS

Two capacitors in series:

Cx = C1 × C2C1 + C2

Capacitors in parallel:

Cx = C1 + C2 + C3 . . . . .

The working voltage of each capaci-tor should be at least 1·5 times thepeak-to-peak signal voltage developedacross the loudspeaker at maximuminput.

CUT BOBBIN ENDS FROM3mm HARDBOARD(MASONITE IN USA)USING A HOLE SAW INAN ELECTRIC DRILL

GLUE BOBBIN ENDTO PAPER TUBE

13mm

50mm

25mm

CORE 60mm LENGTHOF 9.5mm DIAFERRITE ROD

ROLLED AND GLUEDPAPER TUBE 13mm O.D.



excluding speakers

CROSSOVER/AUDIO FILTERCapacitors

C1 10 bipolar radial elect.50V (Alternatively, two22 standard elect.connected back-to-back– see text and Fig.5)

InductorL1 9·5mm (3/8in.)

dia. ferriterod, length63mm(2½in.); card, hardboardand glue for bobbin.Enamelled copper wire:Crossover only – 100turns 20 s.w.g. (19a.w.g.). Crossover andFilter – 200 turns 22s.w.g. (21 a.w.g.). SeeFig.4 and text

MiscellaneousS1 3-pole 4-way rotary switch

(only two poles used)SK1, SK2 4mm screw terminal

post/socket (2 off)

Printed circuit board available fromthe EPE PCB Service, code 357(Crossover/Filter); multistrand andconnecting wire; plastic control knob;speaker terminals; solder pins; solderetc.


10m

BIPOLARELECTROLYTIC

LS18Ω

8Ω

BASSSPEAKER

LS2 TREBLESPEAKER

FINISH200 TURNS

START

TAP100 TURNS

L1

C1

INPUT FROMAMPLIFIER

S1a

S1b

1

2

3

4

5

6

7

8

1) WIDE RANGE2) LOW PASS3) HIGH PASS4) MID RANGE

S1 POSITIONS

P

P

Fig.6. Circuit diagram for the Loudspeaker Crossover/AudioFilter.

SK1

SK2

INPUTTERMINALS

TREBLE BASS

SPEAKERS

123

4

56

7 89

10

1112

A

B

C

S1

L1

FINISH

START

TAP

C1

1.95IN (49.5mm)

357

Fig.7. Crossover/Audio Filter printed circuit board component layout, interwiring to off-board componentsand full-size copper foil master. The completed crossover is shown in the above photograph.

2·95IN (74·9m

m)

EPE Online


in Fig.6. The first order filter serves as abasic crossover when the speaker is beingused for testing or listening to “hi-fi”equipment.

Switching out the Treble speaker andconnecting the inductor in series with theBass speaker gives a low-pass (top cut)effect. Connecting the capacitor in serieswith the speaker provides a high-pass (basscut) arrangement. With the inductor andcapacitor in series with the speaker,response to speech frequencies is empha-sised, making the unit suitable for use witha communications receiver or for surveil-lance work.

Rotary switch S1 selects the requiredfunction, and the inductor is tapped to pro-vide appropriate values for the crossoverand speech filter.

Construction of the Crossover/Filter

Unit is based on a small single-sided print-ed circuit board (p.c.b.). This board isavailable from the EPE PCB Service, code357.

The topside component layout, full-sizecopper foil master and off-board wiringdetails are illustrated in Fig.7. Again, sol-der pins at the lead-out points will simplifyoff-board wiring. The p.c.b. makes provi-sion for series and parallel combinations ofcapacitors, and a wire link must be inserted

if capacitor C1 is a single, bipolarelectrolytic.

Constructors interested only in “hi-fi”applications can ignore the switchingarrangements and simply connect a 100-turn (0·5mH) inductor and the capacitor asshown in Fig.3a.

Next Month: The final part will dealwith speaker enclosures and include alow-cost, high-performance design

which incorporates this month’sCrossover/Filter unit.

The construction of a simple and inex-pensive oscillator and resonance detec-tor, which can be used to match anyspeaker to an enclosure and optimiseperformance, will also be described.


Low Frequency Oscillator for loud-speaker resonance checking.

Infra-Red AutoswitchAs the Infra-Red Autoswitch project is mains powered, all the components

have been specially selected to fit directly on the small printed circuit board(p.c.b.). If alternative, non-board mounting components, such as the mainstransformer and relay, are used you must take extra care when building andtesting this unit. In this case, it is very important that the p.c.b. and any off-board parts be mounted in its case before testing and that a separate bat-tery supply is used for checking its operation, prior to mains connection.

The special Sharp IS471F infra-red sensor/detector came from RSComponents and carries the order code 564-396. They also supplied thep.c.b. mounting, short-circuit proof, mains transformer with twin 9V 0·027A(0·5VA total) secondaries, code 310-1263. These components can beordered from any bona-fide RS stockists, including some of our advertisers.You can order direct (credit card only) on 01536 444079 or on the web atrswww.com. A post and handling charge will be made.

The 12V d.c. low-profile relay, with 12A 250V a.c. rated single-polechangeover contacts, used in the model was purchased from RapidElectronics ( 01206 751166 or www.rapid electronics.co.uk), code 60-4630. We understand that RS (see above) also stock a similar relay, code198-6933.

The specified low-profile case came from CPC (credit card only), 08701 202530, code EN55028. A post and packing charge is made on allorders under £30. The Autoswitch printed circuit board is available from theEPE PCB Service, code 358 (see page 539).

Teach-In 2002 – Lab 9Once again, it’s only the sensor and semiconductor devices called for

in this month’s Teach-In 2002 Lab Work that will give some readers sourc-ing grief. Starting with the Nemoto NAP-7AU gas sensor/compensatorpair, these were obtained from Maplin ( 0870 264 6000 orwww.maplin.co.uk), code FM87U and are sold as a pair.

We have found two listings for the precision low off-set op.amp typeOP177 and it can be ordered from Rapid Electronics ( 01206 751166 orwww.rapidelectronics.co.uk), code 82-0092, or RS Components (01536 444079 or on the web at rswww.com), code 127-2868. Expect to paya handling and postage charge.

If readers experience any difficulty in finding a local source for the 4093quad 2-input NAND Schmitt trigger (Rapid 83-0420) and the ADC0804 8-bitanalogue-to-digital chip (Maplin QQ00A or RS 411-674) they should contactthe above mentioned companies. The relevant code numbers are shown inbrackets.

The Linear Technology LTC1062CN8 5th order switched capacitor low-pass filter i.c., used in the Anti-aliasing Filter (Lab 9.5), appears to be listedonly by RS (see above), code 633-880.

EPE StylopicA couple of items proved hard to find when tracking down parts for the EPE

StyloPIC project. The National Semiconductor LM13600 transconductanceamplifier i.c.and the SGS-Thompson L272 dual power op.amp i.c.only appearto be listed by RS, codes 304-453 and 635-167 respectively. You can order

them direct from RS (credit card only) on 01536 444079 or on the web atrswww.com. A post and handling charge will be levied.

The above company supplied the Texas TLC7524CN 8-bit digital-to-ana-logue converter chip, code 650-087. It is also currently listed by Rapid (01206 751166 or www.rapidelectronics.co.uk), code 82-0764, but doublecheck it is the 16-pin device being supplied.

For those readers unable to program their own PICs, a ready-pro-grammed PIC16F877-20 microcontroller can be purchased from MagentaElectronics ( 01283 565435 or www.magenta2000.co.uk) for the inclu-sive price of £10 each (overseas add £1 p&p). It is the 20MHz version yourequire. The software is available on a 3·5in. PC-compatible disk (EPE Disk5) from the EPE Editorial Office for the sum of £3 each (UK), to cover admincosts (for overseas charges see page 539). It is also available Free from theEPE web site: ftp://ftp.epemag.wimborne.co.uk/pub/PIC/StyloPIC.

The printed circuit board/keyboard is available from the EPE PCBService, code 359 (see page 539).

Simple Audio Circuits – 3Most of our components advertisers should be able to supply all the parts

needed to construct the circuits in this month’s instalment of the SimpleAudio Circuits. A suitable Bulgin fused Euro-style mains inlet, chassismounting, plug (code MK18U or FT37S) together with an insulation, reartag, protective cover (code JK67X) and line socket (UL16S) is listed byMaplin ( 0870 264 6000 or www.maplin.co.uk). They also list the 6A200V P600D rectifier diode for one version of the Power Supply Unit, codeUK60Q.

If problems are experienced in obtaining a ferrite rod for the Crossoverunit, we understand, from the author, that one is obtainable from JAB, POBox 5774, Birmingham, B44 8PJ (mail order only), and J. Birkett( 01522 520767).You will need to cut the rod down to size (take care, it isbrittle!). These two firms can also supply 50g (2oz) reels of enamelled cop-per wire for the Crossover.

The two printed circuit boards are available from the EPE PCB Service,codes 356 (PSU) and 357 (Crossover) – see page 539.

Rotary Combination LockProbably the most expensive item when purchasing components for the

Rotary Combination Lock project is likely to be the heavy-duty power sole-noid. The one in the model cost about £15 and came from RS ( 01536444079 or rswww.com) and is their 12V d.c. standard pull action, springreturn type, code 250-1303. They also supplied the Omron 12V d.c. ultra-min., p.c.b. mounting relay, code 369-359.

The two printed circuit boards are available from the EPE PCB Service,code 260 (Lock) and 361 (Interface).

PLEASE TAKE NOTEL.E.D. Sequencer (Ingenuity Unlimited) June ’02

Page 406. To prevent the i.c. outputs (IC2, IC3) from adversely affectingeach other, 1N4148 signal diodes should be inserted between each i.c. pinand the respective l.e.d. The anode on the pin and cathode on the l.e.d.World Lamp June ’02

Where it is said that VR1 should be turned clockwise, this should readanti-clockwise, and where anti-clockwise, clockwise.Toolkit TK3

Updated files for V1.2 are now on our FTP site. Only files Disk 1 andDisk 3 are affected.

EPE Online


EPE Online

Note that you can purchase pre-programmed PIC microcontrollers for our PIC projects as described on this “Shop Talk” page. Alternatively, if you wish to program the PIC yourself, you can find the code files by bouncing over to the EPE Online Library (visit www.epemag.com, click in the “Library” link in the top navigation, then on the “Project Code Files” link).

A LTHOUGH the power amplifiersdescribed last month have arespectable amount of gain, some

signals may be too weak to produce anadequate loudspeaker output without addi-tional amplification. They can also be fur-ther weakened by an excessive mismatchbetween signal source and amplifier. Tonecontrols are usually required when musicis being reproduced, and restricting thebandwidth will clarify speech signals,especially under noisy conditions.

These three issues: preamplification,impedance matching and tailoring thefrequency response, are covered in thisarticle.

Impedances

The impedances presented by the inputand output ports of transistor amplifierstages are extremely variable. Load andbias resistors exert a major influence, as dothe gain of the transistor and its emittercurrent. Negative feedback can either raiseor lower impedance and, to further confusethe issue, the load connected across oneport influences the impedance presentedby the other.

The impedance figures quoted are,therefore, intended as no more than aguide when selecting the best circuit for aparticular application.

BiasingTransistor amplifier stages are usually

biased so that the output (collector or emit-ter; drain or source) rests at half the supplyvoltage under no-signal conditions. Thisenables the stage to deliver the greatestpossible signal swing; i.e. the highest out-put, before the onset of clipping.

Transistor gain (hfe), and supply voltage,affect the biasing. However, over a widerange of hfe values (at least 200 to 600),and supply voltages from +9V to +12V, thecircuits described here will deliver a lowdistortion output that is more than

sufficient to fully drive the power ampli-fiers described last month.

Experimenters who require the stages tohave the highest possible signal-handlingcapability for a given supply voltage mayhave to adjust the bias resistors. Guidanceon this is given later.

CascadingThe various preamplifiers, tone controls

and filters can be combined to suit individ-ual requirements. Blocking capacitorshave been provided at the inputs and out-puts, and the units can be used safely withany equipment.

Cascading makes one of these capaci-tors redundant. Similarly, when they areconnected to the power amp described lastmonth, the output blocking capacitor canbe omitted (C1 on the power amplifierp.c.b. duplicates this component).

DecouplingAll of the preamplifier circuits are

decoupled from the power supply by aresistor and capacitor. Failure to includethese components will almost certainlyresult in motor boating (low frequencyinstability).

The main cause of this instability is thewide swing in power amplifier currentdrain: even with small units this can rangefrom 10mA to 150mA. These signal-induced current swings cause variations inthe voltage of dry batteries or badly regu-lated mains power supplies. When highgain preamplifiers share the same supplyrail, the resulting feedback causes low-fre-quency oscillation.

If problems are encountered, increase thevalue of the decoupling resistor, or capacitor,or both, by a factor of ten. A capacitor of2000F or more, connected across a dry bat-tery power supply, will also help to eliminateinstability at high volume levels.

R.F. InterferenceThe single transistor preamplifiers

described here have an extended high

418 Everyday Practical Electronics, June 2002

Part 2 – Preamplifiers, Tone Controls and Filters

Four single-transistor preamplifiers (left-to-right). Low Impedance MediumImpedance High Impedance F.E.T. High Impedance.

frequency response, and problems with r.f.interference may be encountered.Connecting a low value ceramic capacitorbetween the input (emitter or base) and the0V rail will cure the problem, and theaccompanying printed circuit board(p.c.b.) makes provision for this.

In many cases, all that is required is theadditional gain and/or impedance match-ing afforded by a single transistor stage.Four circuits will now be considered.

It is convenient, with simple intercomunits, to make the speaker double up as amicrophone. Voice coil impedance andoutput are very low: a few ohms and lessthan 1mV at a close speaking distance.Transformers are often used to increase theimpedance and voltage of this signal


layout, wiring details and full-size copperfoil master pattern are shown in Fig.2. Thisboard is available from the EPE PCBService, code 349 (Single Trans.).

Before commencing assembly, checkthe component, construction and intercon-nection notes at the end of the article.

Readers wishing to operate the stage

from lower supply voltages should checkthe voltage on the collector (c) of transis-tor TR1 under no-signal conditions. If it ismuch more than half the supply voltage,reduce the value of resistor R3 to increasethe bias current. With 3V on the supplyrail, R3 will need reducing to around 6·8kilohms and, with a 6V supply, its valuewill be in the region of 12k.

Because of its very low input impedance,the circuit of Fig.1 is not prone to capacita-tive hum pick up, and the input lead can be

Everyday Practical Electronics, June 2002 419

source, but a transistor can be made to dothe job just as well.

The “grounded base’’ stage illustratedin Fig.1 has an input impedance ofaround 50 ohms, an output impedanceroughly equal to the collector load resis-tance (R2) of 10 kilohms, and a voltagegain of around 100. Although more com-monly encountered at the front-end of aradio receiver, this configuration is suit-able for matching low source impedancesto the power amplifier and, at the sametime, providing a useful amount ofvoltage gain.

In the circuit diagram for the Low InputImpedance Preamplifier shown in Fig.1,C1 is a d.c. blocking capacitor, R1 and R2are the input and output load resistors, andresistors R3 and R4 bias the transistor. Thebase (b) is grounded at audio frequenciesby capacitor C3.

Supply line decoupling is effected byC4 and R5, and C2 is the output couplingand d.c. blocking capacitor.

SIGNALINPUT

SIGNALOUTPUT

SCREEN SCREEN

bc

e

++

+

+

TR1BC549C

R5100Ω

C1100µ

C347µ

C24 7µ

C4100µ

R210k

R11k

R318k

R42k2

0V

++9V

TO 12V

VOLTAGE GAIN 100 OVER AN hfe SPREAD OF 110 TO 600.CURRENT DRAIN AT 9V SUPPLY 0·75mA.

Fig.1. Circuit diagram for the single-transistor Low InputImpedance Preamplifier.

+

+

+ +

SCREENED SIGNALINPUT LEAD, OR USETIGHTLY TWISTED PAIR.(SEE TEXT) SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V

TO COMMON 0V POINTON POWER SUPPLY P.C.B.

R1R

4

R3 R2 R5

C4C2C1

C3

TR1e

c

b

INPUTOUTPUT

1.7I

N (

43. 2

mm

)

2.35IN (59.7mm)

349

Fig.2. Printedcircuit boardcomponent layout,wiring and full-sizecopper foil masterpattern for the LowInput ImpedancePreamplifier.

!


LOW INPUT IMPEDANCE

ResistorsR1 1kR2 10kR3 18k (see

text)R4 2k2R5 100



(2 off)C2 47 radial elect. 25VC3 47 radial elect. 25V

SemiconductorsTR1 BC549C npn transistor

(or similar – see text)

MiscellaneousPrinted circuit board available from

the EPE PCB Service, code 349 (SingleTrans); audio screened cable; multi-strand connecting wire; input and outputsockets, type to choice; solder pins;solder etc.


Low Input Impedance Preamplifier componentsmounted on the “single’’ p.c.b.

"" !!!!##

EPE Online


tightly twisted flex rather than screenedcable. If r.f. interference problems areencountered, connect a 100nF capacitorbetween the emitter (e) of TR1 and the 0Vrail: provision is made for this on the p.c.b.

Combining this low impedance circuit(Fig.1) with the LM386N-1 or theTBA820M power amplifiers (fully describedin Part 1, last month) will produce a decentintercom unit, but more amplification isneeded for surveillance purposes. Cascadingthe grounded base stage with the mediumimpedance preamplifier described next(Fig.3) is one possible answer.

The input impedance of the singletransistor, common emitter preamplifier

illustrated in Fig.3 is approximately 1500ohms (1·5k), and the output impedanceroughly equal to the value of the load resis-tor, R2; i.e. 4700 ohms (4·7k).

Base bias resistor R1 is connected totransistor TR1 collector (c) rather than thesupply rail. The resulting d.c. negativefeedback makes the biasing more immuneto transistor gain spreads and variations insupply voltage.

Preset potentiometer VR1 acts as theemitter bias resistor. Connecting capaci-tor C2 to the slider (moving contact)enables part of it to be left un-bypassed.This introduces varying levels of negativefeedback and, with the specified transis-tor, the gain of the stage can be setbetween 10 and 160 times to suit differentapplications.

Comment has already been made aboutsupply rail decouplers, R3 and C4, andblocking capacitors, C1 and C3.


layout, wiring details and full-size cop-per foil master pattern are shown inFig.4. This board is available from theEPE PCB Service, code 349 (SingleTrans.).

Before undertaking assembly work, seethe component, construction and inter-connection details at the end of thearticle.

Provision is made for connecting an r.f.bypass capacitor across the input. A 1nF or10nF ceramic component should be ade-quate if problems arise.


SCREEN SCREEN

SIGNALINPUT

SIGNALOUTPUT

bc

e

+

+

+

+

R11M

R24k7

TR1BC549C

C247µ

C14 7µ

C34 7µ

C4100µ

0V

++9V

TO 12V

VR1470Ω

R3100Ω


MEDIUM INPUT IMPEDANCE

ResistorsR1 1MR2 4k7R3 100


PotentiometersVR1 470 enclosed carbon

preset


(2 off)C2 47 radial elect. 25VC4 100 radial elect. 25V




the EPE PCB Service, code 349 (SingleTrans); audio screened cable; multi-strand connecting wire; input and outputsockets, type and size to choice; solderpins; solder etc.


VOLTAGE GAIN WITH VR1 SLIDER AT 0V RAIL, 8 TO 10 OVER AN hfeSPREAD OF 110 TO 600.VOLTAGE GAIN WITH SLIDER AT TR1 EMITTER, 80 TO 600 OVER AN hfeSPREAD OF 110 TO 600.CURRENT DRAIN AT 9V SUPPLY: 1·25mA.

Fig.3. Circuit diagram for the Medium Input ImpedancePreamplifier.

1.7I

N (

43. 2

mm

)

2.35IN (59.7mm)

349

Fig.4. Medium Input Impedance Preamplifier printed circuit boardcomponent layout, wiring and full-size copper foil master.

Medium Input Impedance preamplifier componentsmounted on the “single’’ p.c.b.

+

+

+

+

SCREENED SIGNALINPUT LEAD SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V


R1

R2

C4

C2

C1

C3

TR1e

c

b

R3

VR1

INPUTOUTPUT

EPE Online


Crystal microphones and ceramicgramophone pick-ups (there are still a fewin use) require an amplifier with a highinput impedance, and a stage of this kind isuseful when the damping on a signalsource has to be kept low.

Configuring a bipolar transistor in the emit-ter-follower (common collector) mode resultsin a high input and low output impedance, anda typical High Input Impedance Preamplifiercircuit diagram is shown in Fig.5. The inputimpedance is roughly equal to the gain of thetransistor (hfe) multiplied by the value of theemitter load resistor R2.

This is, however, limited by the biasresistor R1, and the output load, whichshunts the emitter resistor. Nevertheless, ahigh gain transistor will still produce aninput impedance of about 100 kilohms.

Often the low output impedance is thesought after feature, either for matchingpurposes or for avoiding high-frequencylosses and hum pick-up when long screenedcables have to be used. Output impedance is

directly related to the impedance presentedby the signal source, and is usually in theregion of 1000 ohms. The voltage gain ofthe circuit is a little less than unity.

The printed circuit board component lay-

out, wiring details and full-size copper foilmaster pattern for the High Input ImpedancePreamplifier are shown in Fig.6. This boardis the same one used for all the single tran-sistor preamplifiers, and is available from theEPE PCB Service, code 349 (Single Trans.).See the component, construction and inter-connection notes at the end of the article.

High input impedance makes the stagevery vulnerable to hum pick up. Carefulattention must, therefore, be paid to screeningthe input leads and, possibly, the entire unit.

It is possible to obtain higher input

impedances with a bipolar transistor byapplying positive feedback from the emit-ter to the base bias network. This involvesan extra pair of resistors and a capacitor,and an alternative solution, if very high

input impedances are required, is to use afield effect transistor (f.e.t.); a devicewhich tends to introduce less noise ataudio frequencies.

!"!!A circuit diagram for a F.E.T. High Input

Impedance Preamplifier is given in Fig.7.The gate resistor R1 is tapped down to thesource resistors R2/R3 in order to improvebiasing and, hence, signal handling. By thismeans the f.e.t. develops its gate biasacross R2, and R3 drops an additional 3Vor so to fix the voltage on the source ataround half the supply voltage.

Connecting the gate resistor R1 in thisway applies a proportion of the in-phase out-put signal to its lower end, and the resultingpositive feedback, or “bootstrapping’’,increases its effective resistance, and theinput impedance of the circuit, to around6 megohms (6M).

Output impedance is independent of sig-nal source impedance. It is governed by thetransconductance (gain) of the device, andis usually of the order of 500 ohms.


#"


HIGH INPUT IMPEDANCE

ResistorsR1 1MR2 4k7R3 100


CapacitorsC1 100n polyesterC2 10 radial elect. 25VC3 100 radial elect. 25V






##""""""##$$ ""

SIGNALINPUT SIGNAL

OUTPUT

bc

e

+

+

SCREEN SCREEN

R11M

C1100n

R24k7

TR1BC549C

C210µ

R3100Ω

C3100µ

0V

++9V

TO 12V

VOLTAGE GAIN: UNITYCURRENT DRAIN AT 9V SUPPLY: 1·25mA.

Fig.5. High Input Impedance Preampli-fier circuit diagram.

+

+

SCREENED SIGNALINPUT LEAD SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V


R1

C2

C1

C3

TR1e

c

b

R2

R3

INPUTOUTPUT

Fig.6. Printed circuit board component layout, wiring and full-sizecopper foil master for the High Input Impedance Preamplifier.

High Input Impedance Preamplfier circuit board.1.

7IN

(43

. 2m

m)

2.35IN (59.7mm)

349

EPE Online


This is the circuit of choice when a highimpedance source has to be connected to along screened cable; e.g., a capacitor orcrystal microphone. However, f.e.t. charac-teristics vary widely, and readers wishingto use the circuit of Fig.7 should be pre-pared to adjust the value of resistor R3,over the range of 1500 to 4700 ohms, espe-cially when low supply voltages are used,in order to optimise signal handlingcapability.

Details of the printed circuit board

component layout, wiring and copper foilmaster pattern are given in Fig.8. Theboard is the single transistor version andis available from the EPE PCB Service,code 349 (Single Trans).

Before assembly, check the component,construction and interconnection details atthe end of the article.

Amplifiers introduce unwanted noiseand, as gain increases, more care has to betaken to prevent the noise becoming toointrusive. The noise generated by a bipolartransistor can be reduced by operating it ata low collector current, typically between10A and 50A. This technique has beenadopted for the first stage of the directly-coupled, two transistor, Low-NoisePreamplifier shown in Fig.9.

Overall gain is stabilised by negative feed-back applied via preset VR2. With the valueshown, gain is approximately 300. If a 47kpotentiometer is used instead, gain will bereduced to around 150, and it can be takendown to 70 or so with a 22k component.

Rotating the slider (moving contact) ofpreset VR2 causes it to be progressivelybypassed by capacitor C6, increasing the

negative feedback, and reducing gain, athigh frequencies. This feature is useful forreducing noise and for correcting therecording characteristic of long playingrecords. It is usual to incorporate morecomplicated RC networks in the VR2 posi-tion for the latter purpose but, unless thelistener has a very refined ear, there will belittle or no discernible difference.

Operating conditions are stabilised by d.c.negative feedback applied via resistor R5.This, together with the high value collectorload, R3, fixes the collector current of transis-tor TR1 at around 50A with a 12V supply.

Input impedance is around 50k, but theoptimum signal source resistance for low-est noise is between 5k and 10k. This hasinfluenced the value of the input poten-tiometer, VR1.

The purpose of the remaining compo-nents will be evident from earlier circuitdescriptions. However, because of the


HIGH INPUT IMPEDANCE (F.E.T.)

ResistorsR1 2M2R2 1kR3 1k8 (see

text)R4 100


CapacitorsC1 100n polyesterC2 10 radial elect. 25VC3 100 radial elect. 25V

SemiconductorsTR1 2N3819 n-channel field

effect transistor (f.e.t.)




SIGNALINPUT

SIGNALOUTPUT

g

d

s

+

+

SCREENSCREEN

R12M2

C1100n

R31k8

R21k

TR12N3819

C210µ

R4100Ω

C3100µ

0V

++9V

TO 12V

VOLTAGE GAIN: UNITYCURRENT DRAIN AT 9V SUPPLY: 1·75mA.

Fig.7. Alternative circuit diagram for aHigh Input Impedance Preamplifierusing a field effect transistor (f.e.t.).

R4

R3

R2 +

+SCREENED SIGNALINPUT LEAD SCREENED LEAD TO

POWER AMPLIFIER

+ +9V TO 12V


R1

C2

C1

C3

TR1s

g

d

INPUTOUTPUT

Fig.8. Printed circuit board component layout, wiring and full-size copperfoil master for the F.E.T. High Input Impedance Preamplifier.

F.E.T. High Input Impedance Preamplifier p.c.b.


1.7I

N (

43. 2

mm

)

2.35IN (59.7mm)

349

EPE Online


higher gain, the supply line decouplingcapacitor C7 has been increased in value toensure stability.


layout, wiring details and full-size copperfoil master pattern for the Low-NoisePreamplifier are shown in Fig.10. Thisboard is available from the EPE PCBService, code 350 (Dual Trans.).

See the general construction, componentand interconnection guide-lines on the lastpage.

Some readers may wish to use this

circuit with electret microphones whichcontain an internal line-powered f.e.t.amplifier. The load for this remote deviceis provided by resistor R1, and the supplyvoltage is reduced to around 4·5V, which isoptimum for most microphones of thiskind, by resistor R2. Decoupling is bymeans of capacitor C1.

These components (R1, R2 and C2)should only be fitted if an electret micro-phone is used, as the circuit maintains a


excluding microphone

LOW-NOISE PREAMPLIFIER

ResistorsR1* 1kR2* 10kR3, R5 220k (low-

noisemetal filmpreferred) (2 off)

R4 270R6 6k8R7 560R8 100

All 0·25W 5% carbon film, except R3 andR5.*Only required if electret mic. used

PotentiometersVR1 10k enclosed

carbon presetVR2 100k

enclosed carbonpreset

CapacitorsC1*, C4 100 radial elect. 25V

(2 off)C2 47 radial elect. 25VC3 100n polyesterC5, C8 10 radial elect. 25V

(2 off)C6 10n polyesterC7 1000 radial elect. 25V

*Only required if electret mic. used

SemiconductorsTR1, TR2 BC549C npn transistor

(or similar – see text)(2 off)

MiscellaneousPrinted circuit board available from the

EPE PCB Service, code 350 (Dual Trans);audio screened cable; multistrand connect-ing wire; input and output sockets, type tochoice; solder pins; solder etc.


Fig.10. Printed circuit board component layout, wiring and full-size copper foilmaster for the Low-Noise Two-Transistor Preamplifier.

R11k

C1100µ

R210k

C24 7µ

C3100n

VR110k

R4270Ω

R5220k

R3220k

R66k8

R7560Ω

C4100µ

C71000µ

C610n

VR2100k

C510µ

C810µ

R8100Ω

bc

e

bc

e

+ +

++

+

+

SCREEN

SCREEN

SIGNALINPUT

SIGNALOUTPUT

0V

++9V

TO 12V

TR1BC549C

TR2BC549C

**

*

*

SEE TEXT

VOLTAGE GAIN 300 OVER hfe SPREAD OF 450 TO 600. CURRENT DRAIN AT 9V SUPPLY: 1mA.

Fig.9. Circuit diagram for the Low-Noise Preamplifier. Components marked with anasterisk are only needed if an electret microphone is used. Increase the value ofR2 to 18k with 12V supplies.

Completed p.c.b. for the Low-Noise Preamplifier.


SCREENED SIGNALINPUT LEAD

SCREENED LEAD TOPOWER AMPLIFIER


350

3.4IN (87.5mm)

1.8I

N (

46. 5

mm

)

C2 C1

VR1

C3

R1

R2

R3

R6

R4

R5

R7C4

TR1

TR2

C5

C7

C6

C8VR2

R8

e

eb

bc

c

++

+

+

+

+

OUTPUT

+ +9V TO 12V

EPE Online


d.c. voltage on the input which could dis-turb the action of some signal sources.

This circuit, and variations of it, form thebasis of the front-ends of most high qualitypreamplifiers. With the component valuesshown, 3·3mV r.m.s. input will produce a 1Voutput before the onset of clipping.

The noise introduced by the amplifier isabout the same, or a little less, than thatgenerated by the single transistor amplifierset for a gain of 150. The noise level couldbe further reduced by using low-noise,metal film resistors for R3 and R5.

Although inductors are sometimes usedfor “tailoring’’ the frequency response, thekey components in networks which modifyaudio frequency response are normallycapacitors.

The resistance presented by a capacitor tothe flow of alternating current (a.c.) decreas-es as frequency rises. This frequency depen-dant resistance is known as reactance.

Capacitors combined with resistors formfrequency dependant potential dividerswhich can be used to tailor the response.

These RC networks can, of course, onlyattenuate signals. So called “bass boost’’ isobtained by reducing the response of thesystem to the higher audio frequencies.

Table 1 lists the reactances of a range ofstandard capacitor values, at spot frequen-cies, across the audio spectrum. Referringto it, an 0·1F (100nF) capacitor presents aresistance of 5300 ohms at a frequency of300Hz. This rises to 32000 ohms at 50Hzand falls to 320 ohms at 5kHz.

Fitting a blocking capacitor of this valueto an amplifier with an input impedance of5000 ohms will result in signal levels at300Hz being halved. (Capacitor and inputimpedance act as a potential divider). Thisattenuation will increase as the frequencyis lowered, and reduce as frequency israised, at a rate of 6dB per octave.

Fitting low value d.c. blocking capaci-tors to one or more stages will, therefore,roll-off the low frequency response.Capacitors connected from signal lines toground; e.g. across the tracks of volumecontrols, will progressively attenuate highfrequencies. Although simple, these mea-sures can make a significant improvementin clarity and signal-to-noise ratio.

Refer to Table 1 when selecting a capac-itor to give the desired roll-off with a par-ticular input impedance, then refine itsvalue by trial and error.

Capacitors are used to make gain-reduc-

ing negative feedback networks frequencydependant; for example, capacitor C6 inthe two-transistor Low-Noise Preamplifiershown in Fig.9.

Reducing the emitter bypass capacitorC2, in the single transistor preamplifiershown in Fig.3, to 4·7F, will progressive-ly increase feedback, and reduce gain, asfrequency lowers. This is another simple,but effective, way of securing low frequen-cy roll-off.

Some means of continuously varying the

frequency response is desirable whenmusic is being reproduced, and a suitableTone Control circuit diagram is given in

Fig.11. This is the medium impedancetransistor preamplifier illustrated in Fig.3with negative feedback applied, via a fre-quency dependant network, from transistorTR1 collector to base. First published by PJ Baxandall in 1952, the circuit has sincebeen used, with minor variations, in mosthigh quality preamplifiers.

Potentiometers VR1 (Bass), and VR2(Treble), control the impact of capacitors C1,C2 and C3 on the feedback network.Resistors R2 and R3 minimise interactionbetween the controls, and the circuit affords15dB of “boost’’ or cut at 100Hz and 10kHz.


layout, wiring details and full-size copperfoil master pattern are shown in Fig.12.This board is available from the EPE PCBService, code 351 (Tone).

Before undertaking any assembly work,see the general component, construction andinterconnection notes at the end of the article.

When circuits are cascaded, the Tone

Control unit should always be the last inthe chain; i.e. the one connected to thepower amplifier. Most high quality pream-plifiers consist of the two transistor circuitillustrated in Fig.9 followed by this ToneControl circuit.

Reducing bandwidth to around 300Hz to

3kHz greatly improves the clarity of speechsignals, and the practice is adopted by tele-phone companiesaround the world.Limiting the frequencyresponse in this waysignificantly improvesthe signal-to-noiseratio. This is particular-ly desirable with sensi-tive radio equipmentand surveillance sys-tems, where the highlevel of amplificationneeded for the weakestsignals brings with it agood deal of back-ground and equipmentgenerated noise.

For best results,roll-off beyond thepass band should befairly steep: the 6dBper octave afforded bya single RC combina-tion is not sufficient.

The Bandpass Filter circuit diagram shownin Fig.13 cascades three high-pass (low fre-quency cut) sections between transistorsTR1 and TR2, and three low-pass (high fre-quency cut) sections between TR2 and TR3.By this means, a roll-off of 18dB per octaveis achieved above and below the desired fre-quency range.

Filter networks of this kind need to be fedfrom a comparatively low impedance, andfeed into a high impedance. The emitter fol-lower stages, TR2 and TR3, are thus emi-nently suitable, and amplifiers of this kindhave already been discussed. The input stage,transistor TR1, overcomes signal losses, or,with the slider of VR1 at TR1 emitter (e),ensures an overall circuit gain of around 25.

Emitter to base feedback around TR2and TR3, via the RC networks, improvesthe action of the filters. Component valueshave been selected to start the roll-off justwithin the pass band, and the response fallssteeply below 300Hz and above 3kHz.

Two capacitors have to be combined toproduce a difficult-to-obtain value. Toavoid confusion they are shown separatelyon the circuit diagram as C8 and C9.

Details of the printed circuit board com-

ponent layout, wiring and copper foil mas-ter are given in Fig.14. The Bandpass Filterboard is also available from the EPE PCBService, code 352 (Filter).

See component, construction and inter-connection notes before commencingbuilding.


Table 1: Reactance, in Ohms, of standard value capacitorsat stated audio frequencies

Cap. 50 100 200 300 400 500 1 2 3 4 5 10 20F Hz Hz Hz Hz Hz Hz kHZ kHz kHz kHz kHz kHz kHz1000 32 16 08 05 – – – – – – – – –470 68 34 17 11 – – – – – – – – –100 32 16 8 5 4 32 16 – – – – – –47 68 34 17 11 85 68 34 17 11 – – – –10 320 160 80 53 40 32 16 8 53 4 32 16 –4·7 680 340 170 110 85 68 34 17 11 85 68 34 171 3k2 1k6 800 530 400 320 160 80 53 40 32 16 80·47 6k8 3k4 1k7 1k1 850 680 340 170 110 85 68 34 170·1 32k 16k 8k 5k3 4k 3k2 1k6 800 530 400 320 160 800·047 68k 34k 17k 11k 8k5 6k8 3k4 1k7 1k1 850 680 340 1700·01 320k 160k 80k 53k 40k 32k 16k 8k 5k3 4k 3k2 1k6 8000·0047 680k 340k 170k 110k 85k 68k 34k 17k 11k 8k5 6k8 3k4 1k7

Reactance values rounded off

Bandpass Filter (top) and Tone Control p.c.b.s.



excluding case

TONE CONTROL

ResistorsR1, R3,

R4, R6 4k7 (4 off)R2 27kR5 1MR7 470R8 100


PotentiometersVR1, VR2 100k min. rotary carbon,

linear (2 off)

CapacitorsC1, C3 2n2 polyester (2 off)C2 47n polyesterC4, C8 10 radial elect. 25V

(2 off)C5 1 radial elect. 25VC6 47 radial elect. 25VC7 100 radial elect. 25V




the EPE PCB Service, code 351 (Tone);metal case (optional), size and type tochoice – see text; audio screened cable;multistrand connecting wire; input andoutput sockets, type to choice; solderpins; solder etc.


351

C1

C1

C2

C3

C4

C5

C6C7

C8

ANCHOR PIN FORINPUT POTENTIALDIVIDER RESISTORS

2.8IN (71.0mm)

1.44

IN (

36. 5

mm

)

R1

RX

RY

R2

R3

R4

R5

R6

R7

R8

USE THIS RESISTORNETWORK TO ATTENUATEINPUT SIGNAL. (SEE TEXT)





+

+

+

+

+

TR1e

c

b

VR1

VR2

BASS

TREBLE

+ +9V TO 12V

Fig.12. Tone Control printed circuit board component layout, interwiring and full-size copper foil master.The tape and CD player signal input attenuation resistors (see text) are shown in the inset diagram (left).

BASS

TREBLE

R14k7

C12n2

C32n2

R44k7

R227k

R34k7

C51µ

C410µ

R51M

R64k7

R7470Ω

C647µ

C810µ

C7100µ

R8100Ω

C247n

VR1100k

VR2100k

bc

e+

+

+

++

SCREEN

SCREEN

SIGNALINPUT

SIGNALOUTPUT

0V

++9V

TO 12V

TR1BC549C

= BOOST END OF POTENTIOMETERS (VR1, VR2) MOVING CONTACT (SLIDER).VOLTAGE GAIN UNITY WHEN VR1 AND VR2 SET AT MID TRAVEL.BOOST AND CUT ±15dB AT 100Hz AND 10kHz.CURRENT DRAIN AT 9V SUPPLY: 1·25mA.

Fig.11. Circuit diagram for the Tone Control (bass, treble boost and cut).

Tone Control printed circuit board.

EPE Online



BANDPASS FILTER

ResistorsR1, R5, R10 1M (3 off)R2, R6, R11 3k9 (3 off)R3 6k8R4 3k3R7 to R9 12k 1% metal film (3 off)R12 100

All 0·25W 5% carbon film, except R7 to R9

PotentiometersVR1 1k carbon preset

CapacitorsC1, C12 1 radial elect. 25V (2 off)C2 47 radial elect. 25VC3 to C5 10n polyester (5% or

better) (3 off)

C6 15n polyesterC7 22n polyesterC8* 1n polyesterC9* 470p ceramicC10 100n polyesterC11 100 radial elect. 25V

*Combined (parallel) to give 1n5

SemiconductorsTR1 to TR3 BC549C npn transistor

(or similar – see text) (3 off)

MiscellaneousPrinted circuit board available from the

EPE PCB Service, code 352 (Filter);audio screened cable; multistrand con-necting wire; input and output sockets,type to choice; solder pins; solder etc.



Bandpass Filter printed circuit board.

bc

e

bc

eb

c

e

+

+

++

SCREEN

SIGNALINPUT

SIGNALOUTPUT

0V

++9V

TO 12V

TR1BC549C

TR2BC549C

TR3BC549C

R11M

C11µ

C247µ

VR11k

R36k8

R23k9

C310n

C410n

C510n

R43k3

R51M

R712k

R63k9

C615n

C722n

R812k

R912k

C10100n

C81n

C9470p

R113k9

R101M

C121µ

C11100µ

R12100Ω

SCREEN

* *

*SEE TEXT

352



+9V TO +12V


3.84IN (97.5mm)

1.63

IN (

41. 5

mm

)

+

+

+

+

R1

C4

C2

C1 C3

TR1 TR2 TR3e e e

c c c

b b b

VR1

R2

C5

R3

R4

R6

R7

C6

C8 C9

C10

C7 C11

C12

R12

R10

R9

R8

R5

R11

INPUT OUTPUT

Fig.14. Printed circuitboard componentlayout, wiring and full-size copper foil masterfor the BandpassFilter.

VOLTAGE GAIN WITH PASSBAND, UNITY WITH VR1 SLIDER AT 0V RAIL; 25 WITH SLIDER AT TR1 EMITTER END.ROLL-OFF 18dB PER OCTAVE BELOW 30Hz AND ABOVE 3kHz. CURRENT DRAIN AT 9V SUPPLY: 4mA.

Fig.13. Circuit diagram for the Bandpass Filter for speech frequencies (300Hz - 3kHz).

EPE Online


Operational amplifiers (op.amps) are

more commonly used in filters of this kindbut, when the need is simply for a unitygain buffer with a high input and low out-put impedance, the ubiquitous bipolar tran-sistor can be made to serve our purpose justas well.

Radio Receivers

The output from the detector or f.m. dis-criminator in a superhet radio receivershould fully load the power amplifiersdescribed last month. After the usual filter-ing, the signal can be fed directly to thepower amplifier, or via the Tone Controlunit shown in Fig.11 and Fig.12.

MicrophonesThe single transistor preamplifiers

shown in Fig.1 to Fig.8 will provide appro-priate matching and sufficient gain fordynamic (moving coil), electret and crystalmicrophones when they are used for inter-com purposes. (A circuit for line-poweringelectret microphones can be taken fromFig.9). The common emitter circuit givenin Fig.3 should be used with moving coilunits as these present an impedance ofaround 600 ohms.

When electret or dynamic microphonesare deployed for surveillance or “soundcapturing’’ purposes, the two transistor cir-cuit of Fig.9 will ensure a good degree ofsensitivity. Electret microphones have anextended low frequency response. If thisproves troublesome, reduce the value of thed.c. blocking capacitor C2. Try 47nF(0·047F) as a starting point.

Gramophone Pick-upsThe low output of moving-coil pick-ups

necessitates the use of the two transistorpreamplifier detailed in Fig.9. Omit presetVR1 and feed the signal to the base of tran-sistor TR1 via capacitor C3. Low outputceramic pick-ups should be connected viaa 1M (megohm) or 2M2 series resistor topreserve low frequency response.

The F.E.T. Preamplifier circuit illustrat-ed in Fig.7 is more suitable for high outputceramic and crystal pick-ups.

Personal Tape and CD PlayersAn arrangement for extracting the signal

from personal cassette players and headphoneradios is given in Fig.15. The 47 ohm resistorssubstitute for the 32 ohm earpieces, and the470 ohm resistors attenuate the signal.

Preamplification isnot required, but read-ers may wish to usethe Tone Control unitto process the signal.Provision is accord-ingly made, on theTone Control p.c.b.illustrated in Fig.12,for a signal attenuat-ing network; resistorsRx and Ry.

The chosen system

must, of course, beduplicated if stereo operation is required.Tone and Volume controls are usuallyganged, and an additional potentiometer isprovided to balance the gain of the twochannels.

With the simple circuit arrangementshown in Fig.16, the Balance potentiome-ter is connected across the ganged Volumecontrols at the inputs to the two poweramplifiers (VR1 on the power amplifiercircuit diagrams).

All of the components, for this part of

the series, are readily available from a vari-ety of sources. Transistor types are not crit-ical and almost any small-signal npndevice will function in the circuits.

A low-noise, high gain transistor will,however, ensure the best performance, andthe base connections for some alternativetypes are given in Fig.17. With Europeantransistors, the suffix “C’’ indicates thehighest gain grouping.

If possible, use transistors with an hfe ofat least 450 for the input stage of the Low-Noise Preamplifier and for the variousemitter follower stages (where high inputimpedance depends on the use of a highgain device).

All the preamplifiers covered in this

part are assembled on printed circuitboards and construction is reasonablystraightforward. Solder pins, inserted atthe lead-out points, will simplify any off-board wiring. Remember to earth themetal bodies of rotary potentiometers andto use screened audio (mic.) cable for theleads to tone and volume controls to min-imise hum pick-up.

The single transistor preamplifiers alluse the same p.c.b. and wire links arerequired. If units are cascaded, and cou-pling capacitors deleted, remember toinstall wire links to maintain the signalpath.

It may help to start construction by firstplacing and soldering in position the vari-ous wire links on the chosen preamplifierp.c.b. This should be followed by the lead-off solder pins, and then the smallest com-ponents (resistors) working up to thelargest, electrolytic capacitors and presets.Finally, the lead-off wires (including thescreened cables) should be attached to thep.c.b.

On completion, check the orientation ofelectrolytic capacitors and transistors, andexamine the board for poor connectionsand bridged tracks, before connecting thepower supply. The approximate currentdrains are included with the circuitdiagrams.

Overall voltage gain can be in excess of

2000, and care must be taken to avoid humpick-up and instability.

Hum pick-up is of two kinds, capacita-tive and inductive. High impedance circuitsare prone to the former, and low impedanceto the latter. Housing the pre- and poweramplifiers in a metal case will do much tominimise these problems.

If hum increases when a finger isbrought near to the preamplifier, the pick-up is capacitative. It can usually be curedby providing an earthed metal screenaround the input wiring or even the entirepreamplifier board.

All mains and a.c. power leads withinthe metal case of the unit must be tightlytwisted to minimise external fields, and themains transformer should be sited at least150mm (6in) from the input circuitry.Tightly twist power amplifier output leads,and keep them as far away as possible frompreamplifier inputs. Keep all leads as shortas possible.

Run a separate negative power supplyconnection from each of the p.c.b.s to acommon 0V point on the power supplyboard, or to the negative battery terminal.Do not connect one circuit board viaanother to supply negative, or rely uponscreened cable braiding or a metal case toprovide this connection. Make only oneconnection to any metal case, close to thenegative terminal on the power supplyp.c.b.

If all of the above measures have beenadopted and hum problems still persist, trydisconnecting, one by one, the screens ofthe audio cables, at one end only.Reorientating the mains transformer canalso effect a cure.

Next Month: Mains power supplies,loudspeakers and signal filtering will bediscussed.


47Ω

47Ω

470Ω

470Ω

TIP

TIP

RING

RING

SHANK

SHANK

LEFTINPUT

RIGHTINPUT

OV RAIL

PERSONALEARPHONE

JACK

LINK HEREIF MONO INPUT

REQUIRED

Fig.15. Method of connecting a“Walkman’’ tape or CD player.

BC549CBC239C

BC169C2N3711

BC109C

2N3819BF244A

BF245 MPF102

e e

e

b b

b

c c

c

d d dg g gs s s

UNDERSIDE VIEWS

Fig.17. Base connections for suitabletransistors and f.e.t.s.

LEFTCHANNEL

INPUT

RIGHTCHANNEL

INPUT

GANGEDVOLUME

CONTROLS

TO POWERAMP LEFT

TO POWERAMP RIGHT

BALANCECONTROL VR1a

10k

VR1b10k

22k

+

+

Fig.16. Circuit arrangement for a stereo Balance control.

ART and science collide in the designof loudspeaker enclosures and, tran-scending all the conflicting opin-

ions, is the way a vibrating paper cone canreproduce sounds ranging from the humanvoice to a symphony orchestra with vividrealism.

Last month we discussed speakers andcrossover networks. In this final instal-ment, enclosures and the simple test equip-ment needed to optimise performance arecovered.

Sound waves formed by the front of the

speaker cone are out of phase with those atthe back. If the pressure variations can leakaround the cone there will be cancellation,particularly at low frequencies, and soundoutput will be reduced. The primary dutyof the enclosure is, therefore, to preventthis leakage.

Speaker cones have a natural resonantfrequency (just like a guitar string). Thegreater the mass of the cone, and the freerits suspension, then the lower the resonantfrequency.

At resonance, very little energy isrequired to make the cone vibrate vigor-ously. This has electrical drawbacks,which were discussed last month. It is alsoundesirable from an acoustical point ofview for speaker sensitivity to peak sharplyat one frequency.

The second requirement of the enclo-sure is, therefore, to retain a volume of airwhich damps the cone and evens out theresponse of the system.

Ignoring simple open baffles, there are

four basic types of enclosure.

Infinite Baffles.Infinite baffles are no more than sealed

boxes filled with acoustic wadding to

absorb the sound output from the rear ofthe speaker. Air trapped inside the boxdamps the cone, raising its resonant fre-quency by up to an octave (a doubling).Low frequency output falls off rapidlybelow resonance, and special speakerswith high mass, high compliance (very lowresonance) cones are sometimes used tooffset the rise in resonant frequency.

Absorption of the energy delivered bythe rear of the cone, together with the highcone mass, result in an acoustic efficiencyas low as 1 per cent. Our Twin TDA200312·5W Amplifier (8·2W into 8 ohms: seePart One) requires a more efficient speakerthan this if windows are to rattle.

Acoustic LabyrinthAcoustic labyrinth enclosures are, in

effect, a duct one quarter of a wavelengthlong at the speaker’s resonant frequency(e.g., 7ft at 40Hz). Folding the fibreboardor plywood duct into a box shapeproduces a labyrinth, hence the name.Some designers fill the duct with acousticwadding: others just line the interiorsurfaces.

588 Everyday Practical Electronics, August 2002

Part 4 – Loudspeaker Enclosures, TuningOscillator and Resonance Detector

Fig.1. Speech coil impedance in region of resonance.

CURVE A WITH SEALED ENCLOSURE CURVE B WITH VENTED AND TUNED ENCLOSURE

The quarter wavelength air columnimposes the desired heavy damping on thecone at its resonant frequency. As frequen-cy rises through an octave (i.e., towards80Hz in our example) the air columnapproaches half a wavelength. The phaseof the radiation from the rear of the cone isthen inverted, and it emerges from the ductto reinforce that from the front, therebyincreasing output.

Enclosures of this kind are not easy toconstruct or tune to suit different speakers.In our quest for good performance for amodest outlay of cash and effort, this high-ly regarded system has, therefore, to berejected.

HornsLoading the speaker cone with an

expanding column of air in the shape of ahorn results in very high efficiencies; ofthe order of 40 per cent to 50 per cent. Thehorn effects an impedance transfer: high atthe throat and low at the mouth. The result-ing heavy damping on the speaker cone,and the small cone excursions and lowpower input needed for a given sound out-put, greatly reduce distortion.

Many ingenious designs have been pro-duced for folding large, low frequency hornsinto cabinets. However, cost, size, and com-plexity of design and construction removethis system from our consideration.

Bass ReflexBass reflex enclosures, also known as

acoustic phase inverters, are based on thework of a German physicist, HermanLudwig Ferdinand von Helmholtz (1821-1894).

Whilst exploring the nature of sound, heinvestigated the way air resonates insidevented chambers and close to the ventitself. The idea of mounting a loudspeakerin a Helmholtz resonator was patented,about half-a-century later, by A. L. Thuras.

Enclosures of this kind are simple andcheap to construct and tune. Efficiency iscomparatively high: some authorities sug-gest 15 per cent to 20 per cent dependingon the size of the loudspeaker (the biggerthe better).

A reflex enclosure is, therefore, the nat-ural choice when cost and effort are to bekept to a minimum and limited amplifierpower demands good speaker efficiency.

25 per cent and tune to resonance byreducing the vent area or providing a duct.

When reflex enclosures are designed inthis way, the frequency ratio between thetwo smaller resonances formed by tuningshould be not less than 1·5:1 and not morethan 2·4:1.

During the 1960’s, Australians, Neville

Thiele and Richard Small, extended earlierloudspeaker research carried out byAmerican, James Novak.

They were able to show that, for opti-mum performance, enclosure size isdependant upon the relationship betweenthe damping effect of the enclosed air andthe compliance of the cone suspension. If,when the enclosure vent is sealed, the fre-quency of the single resonant peak is 1·5 to1·6 times the free-air resonant frequencyof the cone, the relationship is correct.

Thiele and Small described an experi-mental method for determining suspensioncompliance, and produced formulae relat-ing this, and other speaker properties, toenclosure size and vent area. Known as theThiele-Small parameters, these speakercharacteristics are now published by anumber of manufacturers.

Everyday Practical Electronics, August 2002 589

A bass reflex enclosure is no more than

a box with a small opening known as the“vent” or “port”. The mass of air withinthe box is tuned, by the vent, to resonate atthe same frequency as the speaker cone.This imposes heavy damping and results intwo smaller resonances, one of lower andone of higher frequency than the unventedcone resonance.

Speaker output falls off rapidly belowresonance, and the development of thelower frequency peak extends the speak-er’s bass response by almost an octave.Phase inversion takes place over most ofthe low frequency range, and output fromthe vent augments that from the front ofthe cone (the operation of the system iscomplex, and phase inversion does notoccur at all frequencies).

Output falls off very rapidly below thelower peak but, in a well designed system,this will be in a region where there is littleor no signal content.

The damping effect of the vented enclo-sure is displayed graphically in Fig.1. Aplot of speech coil voltage againstfrequency, it represents variations inimpedance which are intimately related toresonances in the system. The single reso-nant peak (curve A) developed when thevent is sealed contrasts with the two lowerpeaks (curve B) which form when the ventis opened. Correct tuning is indicatedwhen the peaks are of equal magnitude (asis the case here).

Traditionally, designers matched enclo-

sure resonance to the free-air resonance ofthe speaker cone on the basis of vent areabeing equal to effective cone area. Thisoptimised low-frequency reinforcement bythe vent but resulted in large enclosures.

Readers who like to build on a grandscale might find the formulae in Table 1helpful. Much simplified, they relatespeaker size and cone resonance to enclo-sure volume. The relevant speaker parame-ters are listed in Table 2.

Enclosures as large as this tune verybroadly, and sizeable variations in ventarea have only a modest effect on perfor-mance. As we shall see, enclosures can betoo big, and it would be prudent to reducethe volume given by the formulae by, say,

TABLE 1: TRADITIONAL ENCLOSURE DESIGNFormulae relating enclosure volume to speaker cone size and

resonant frequency

f res Hz 40 50 60 70 80 90 100 110Vol cu ft 3R 2R 1·4R 1R 0·8R 0·6R 0·5R 0·4R

Notes:(1) F res is the free air resonant frequency of the cone, in Hertz.

Vol is the internal volume of the enclosure in cubic feet.R is the effective radius of the speaker cone in inches

(see Table 2).(2) These formulae are derived from traditional design proce-

dures. Calculations in accordance with current practice, whichrelates cone compliance to enclosed air compliance, usuallyresult in a smaller enclosure (see text).

(3) Although much simplified, the formulae will produce sufficient-ly accurate results (as size increases towards this maximum,tuning becomes less and less critical).

(4) Formulae are based on enclosure port area being equal to theeffective cone area. See Table 2 for details of effective coneareas.

TABLE 2: LOUDSPEAKER DATA

Speaker Diameter (inches) 8 10 12 15 18Effective cone radius R in. 3 3·75 4·75 6 7·5Effective cone area sq. in. 28 44 71 113 177

Crossover/Audio Filter selection switchand amplifier input terminals.

D. B. Keel subsequently adapted the for-mulae for processing on a pocket calcula-tor, but the procedure is still complicated.Readers with a mathematical turn of mindwho want to optimise their enclosures inthis way are urged to study the extensiveliterature on the subject.

Theile-Small parameters are not usually

available for the low cost, but often reason-able quality, speakers of Far Eastern origin(or for speakers in spares boxes). Even ifthey were, it is likely that many readerscouldn’t face the tedium of thecalculations.

An alternative approach is to make anenclosure of manageable dimensions, hav-ing regard to the size of speaker, and thentune it to optimise performance.

Quite small enclosures can be tuned tofrequencies in the 50Hz to 100Hz range.However, as volume is reduced vent areahas to be reduced to secure resonance at aparticular frequency.

Eventually, a point is reached when ventoutput is negligible and the enclosure isperforming almost like a sealed box.Moreover, as size is reduced, the smaller,“stiffer” volume of air increases dampingon the cone and its resonant frequency risesunacceptably.

The resonant frequency of a given ventand enclosure combination can be loweredby forming a duct or pipe behind the vent.The longer the duct the lower the resonantfrequency. Although this involves moreconstructional effort, it does allow a rea-sonable vent area to be maintained whenenclosure volume is small.

Speaker units were discussed last month,

and it was clear that an extended and power-ful low-frequency response becomes easierto achieve as speaker size is increased. It wassuggested that speaker size ought not to beless than 8in, and this is especially true whenan inexpensive unit is to be fitted.

Readers may wish to use even largerspeakers for the advantages they offer:some highly regarded studio monitorscomprise a 15in bass unit in a 5 cubic footreflex enclosure.

Cabinet dimensions should not be exactmultiples of one another, and some expertsmaintain that deep enclosures perform bet-ter than shallow ones. Greater depth alsopermits a longer duct.

Chamfers, formed around the enclosurefront and reaching almost to the speakeraperture, are said to improve clarity at lowfrequencies, but this makes constructiondifficult. Keeping the front panel as narrowas possible is probably the best we can doto achieve this objective.

The vent can be any shape provided itssmallest dimension is not less than oneinch. Circular vents can be ducted with alength of cardboard tube, but some buildersmay find rectangular openings and box-form ducts easier to fabricate.

The above requirements, together with

the desirability of a reasonable vent areaand the obvious influence of speaker diam-eter, tend to determine the smallest accept-able enclosure size. Suggested internaldimensions to suit standard speakers arelisted in Table 3 and the general make-upof the enclosure is shown in Fig.2.

The enclosures for the 15in and 18in unitsare rather deep,and the speakeraperture and ventopening could beformed on the facewith the largerdimension ifdesired (these cab-inets are largeenough for thecone to still be anadequate distancefrom what wouldthen be the back).

Whilst thewidth of the frontis determined bythe speaker chas-sis and cannot bereduced much, theother dimensionscan be changed tosuit materials thatare to hand or aparticular spacein a room. When making changes, try notto reduce the volume by more than 10 percent or so (especially with the 8in. and10in. units); and try to avoid dimensioncombinations that are exact multiples.

One of the best materials for cabinet

construction, acoustically speaking, ismedium density fibreboard (MDF). Thismaterial is reasonably heavy, easy to work,has a desirable “dead” quality and is inex-pensive. Chipboard, blockboard and ply-wood are also perfectly acceptable.

Enclosures for the 8in., 10in. and 12in.speakers should be formed from 13mm(1/2in.) thick sheet with 19mm (3/4in.)square glued and screwed softwood cornerfillets. The two larger enclosures require19mm (3/4in.) material and 25mm (1in.)square fillets. One or two lengths of 25mmsquare softwood should be fixed across thelarger enclosures, from side-to-side, nearmid panel, to inhibit vibrations.

The construction must be air-tight. Ifany of the joints are less than perfect, apply

liberal quantities of adhesive to fill thegaps. Use plastic foam draught excluder toseal the access panel.

Ducts need not be as rigid as the enclo-

sures, and hardboard (Masonite in theUSA) or very thick cardboard are suitablematerials. Circular ducts can be formed byapplying paste to a long strip of paper orthin card and winding it around a food orpaint container until a thickness of 3mm(1/8in.) or so has been built up.

Slide the duct from the former and placeit somewhere warm for the paste to dry. Itis not too difficult to combine two pipes toform an adjustable, telescopic duct.

Tweeters can be mounted axially in front

of the bass speaker to avoid the need foranother hole in the cabinet. Small hooksand eyes and the kind of springy wire usedfor hanging net curtains are ideal for thispurpose.

If the wires are cut short to provide a lit-tle tension the speaker will be held firmlyin place. Strong rubber bands could beused, but these may perish over time.

Bass reflex cabinets are resonators andacoustic treatment should be applied spar-ingly. The rear and top of the enclosureshould, however, be lined with about 50mm(2in.) of cellulose wadding to prevent thereflection of mid-frequency sounds whichcould otherwise escape through the speakercone and impair clarity.

Cellulose wadding can be obtained fromupholsterers and craft shops (it is used forstuffing soft toys).

The accompanying photographs show an

enclosure for an 8in. speaker, constructedin accordance with the earlier guidelines,and incorporating the crossover and audiofilter unit described last month. It is intend-ed for workshop use, and this is reflected inthe style and type of finish. Constructorswanting “hi-fi” speakers will have theirown ideas for giving the units a moredomestic appearance.

The surface mounted grille is of the typefitted to musician’s speakers. The bezelaround the vent opening is formed from


Using cutdown curtain wire, hooks and eyes to suspend thetreble speaker over the bass speaker.

hardboard and nylon mesh is used as ascreen. Bezel and mesh are spray finishedmatt black.

Photographs of the tweeter mountingwere taken before the suspension wireswere painted black to conceal them behindthe grille. Car spray paints were used todecorate the cabinet, and the hard, smoothsurface of the MDF makes it easy to obtaina good finish (spraying should be under-taken outdoors or where there is plentyof ventillation). Rub-down lettering,protected by varnish, is used for the panelannotations.

Manufactured in the Far East, the bass

speaker used in the model is an inexpen-sive 8in. diameter unit with a rolled sur-round. Speakers of this kind are widelyretailed and cost between £8 and £15($12 and $22).

A compliant suspension and robust conegive these units a free-air resonance in theregion of 60Hz. Speakers with a free-airresonance much higher than 70Hz shouldbe avoided if possible.

Suitable tweeters are readily available ata fairly reasonable cost. The paper-conedunit mounted in the prototype is a cheapsurplus component.

It is sometimes desirable to adopt across-over frequency around 500Hz whenlarge (15in. or 18in.) bass speakers areused. Suitable tweeters can be expensive,and experimentally minded readers maycare to try one of the cheap Mylar conespeakers intended for alarm systems. Theclaimed frequency response extends up to20kHz, and a 3in. or larger unit shouldcope with the lower cross-over frequency.

Chassis perforations should be coveredwith several layers of sticky tape to preventinteraction with the bass speaker.Alternatively, isolate the tweeter by mount-ing it inside a small box formed within themain enclosure. Fill the box with cellulosewadding. A 3in. diameter Mylar conespeaker performed better than the purpose-made tweeter mentioned above.


LOUDSPEAKERENCLOSURE . . . YOU WILL NEEDBass Speaker: 8in. diameter, 8 ohms impedance, preferablywith a free-air resonance below 70Hz (most speakers with arolled surround will meet this requirement).

Moving coil treble unit, 8ohms impedance (see text).

Sheet of MDF, 1200mm x 600mm x 13mm (4ft x 2ft x 1/2in.)thick; softwood corner fillets 4m × 19mm square (13ft of 3/4in.square); glue and screws.

Speaker and vent grilles; material for any duct (see text);draught excluding strip; springy curtain wire and small hooksfor mounting tweeter unit; finishing materials etc.

The parts list for the crossover unit was included with Part 3,last month.

TABLE 3: RECOMMENDED MINIMUM ENCLOSUREDIMENSIONS

Speaker Diameter 8 10 12 15 18Width A 9·5 11·5 13·5 17 20Height B 15 18 21 27 33Depth C 12 14·5 17 21 24Speaker Aperturediameter D 7 9 11 13·75 16·5Vent diameter E 4 5 6 7 8Vent area sq. in. 12·5 19·5 28 38 50Minimumdistance F 3 4 5 7 8EnclosureVolume (cu. in.) 1710 3002 4820 9639 15840EnclosureVolume (cu. ft.) 1 1·75 2·75 5·5 9

Notes:(1) All dimensions are in inches unless otherwise stated.(2) Enclosure volumes expressed in cubic feet are approximate.(3) Enclosures produced to these dimensions must be tuned for

optimum performance (see text).

Fig.2. Front and side elevations showing the speaker and vent apertures.Recommended enclosure dimensions are listed in Table 3 above.

Lining the rear of the cabinet withsound-absorbent wadding.

Main speaker and crossover filter (lastmonth) mounted on the rear of theenclosure front panel.

E

D

A C

B

F

In order to tune our enclosure we needsome means of exciting and detectingresonances.

A simple Low Frequency Oscillator cir-cuit diagram is shown in Fig.3, where IC1,a 741 op.amp, provides the necessary gain.A Wien bridge network, formed by C1, C2,R1, R2 and VR1a and VR1b, controls thephase of the positive feedback from IC1output (pin 6) to the non-inverting input(pin 3). Potentiometer VR1 sets the fre-quency of oscillation.

Negative feedback, from the output tothe inverting input (pin 2), determines thegain, thereby controlling the level of posi-tive feedback. Gain should be as low aspossible consistent with reliable oscillationover the full swing of Frequency controlVR1. Negative feedback increases, andgain reduces, as the slider (moving contact)

of preset potentiometer VR2 is rotatedtowards resistor R3.

The stabilising circuit usually incorpo-rated into the negative feedback loop hasbeen omitted in the interests of simplicity.Despite this, signal amplitude is constantover the frequency range and waveform isgood when VR2 is correctly set.

Most of the oscillator components are

assembled on a small single-sided printedcircuit board (p.c.b.). This board is avail-able from the EPE PCB Service, code 364.

The topside component layout, inter-wiring and full-size underside copper foilmaster pattern for the Low FrequencyOscillator board are shown in Fig.4. Solderpins, inserted at the lead-out points,

simplify off-board wiring, and a holder forIC1 facilitates substitution checking.


B19V

B29V

FREQUENCY

S1b

S1a

ON/OFF

+3

2

7

4

6IC1741

VR1a10k

VR1b10k

R12k7

R22k7

C2470n

C1470n

R3820Ω

R4390Ω

VR2100Ω

OUTPUT

SK1

0V

+9V

9V

Fig.3. Circuit diagram for a simple Low Frequency Oscillatorfor loudspeaker resonance checking.

VR1a

VR1b

FREQUENCY

TO B1

TO B2

+9V

9V

S1a S1b

SK1

OUTPUT

PHONOSOCKET

ON/OFF

TO B1 VE

TO B2 VE+

R2 R4

C1

C2

VR2

R1

R3IC1

1.3IN (33.0mm)

2.3IN (58.4mm)

364

Fig.4. Low Frequency Oscillator printed circuit board component layout, interwiringto off-board components and full-size underside copper foil master pattern.


excluding batts.

OSCILLATORResistors

R1, R2 2k7 (2 off)R3 820R4 390


PotentiometersVR1 10k dual-ganged rotary

carbon, lin.VR2 100 enclosed carbon

preset

CapacitorsC1, C2 470n polyester layer, 5%

tolerance desirable (2 off)

SemiconductorsIC1 741 gen. purpose op.amp


the EPE PCB Service, code 364; smallplastic case, size and type to choice;PP3 batteries and holders; pointed con-trol knob; 8-pin i.c. holder; solder pins;multistrand connecting wire.


Component layout on the completed circuit board.

EPE Online


Potentiometer VR1, On/Off switch S1,the p.c.b. and the batteries can be housed ina small plastic box. The compact internallayout inside the prototype unit is shown inthe photographs.

It is not necessary to know the precisefrequency to tune the enclosure, but anapproximate idea is useful. Component

tolerances will affect calibration, but the orig-inal dial should provide an approximate guideto the frequency control settings on otherunits. It is reproduced, full-size, in Fig.5.


Fig.5. Full-size front panel dial as usedin the prototype Low FrequencyOscillator.

Some test meters, set to the lowest a.c.range, could be used to monitor the voltagedeveloped across the speech coil.However, unless the meter is sensitive, thesound level from the speaker under testwould be distressingly loud. Further, a

resistor has to be wired in series with thespeech coil to facilitate the test. This couldmake it difficult for the amplifier to deliv-er sufficient output to produce a reading onan insensitive meter.

Greater sensitivity can be achieved byrectifying the signaland measuring theresultant d.c. on thelowest testmeterrange. A suitablel o u d s p e a k e rResonance Detectorcircuit is given inFig.6, where diodesD1 and D2 are con-figured as a voltage

doubler delivering almost the peak-to-peakvalue of the signal.

When the Resonance Detector unit isconnected to a high impedance digitalmeter, reservoir capacitor C2 slows theresponse to voltage changes, and resistorR2 is included to reduce the delay.

Series resistor R1 increases the imped-ance of the signal source and magnifies theeffect of changes in the impedance of thespeech coil. The values of electrolyticcapacitors C1 and C2 have been chosen tosuit the frequencies involved.

All the components for the Resonance

Detector are assembled on a small printed

INPUT FROMAMPLIFIER

TOSPEAKER

TO DIGITALOR MOVINGCOIL METER(2V RANGE)

+

+R1

47ΩC11µ

D2OA47

D1OA47

C21µ

R2220k

a

a

k

k

+

Fig.6. Circuit diagram for the loudspeaker Resonance Detector.

TO POWERAMPLIFIER

TESTMETER SETTO 2V D.C. RANGE

SPEAKER

R1

C1D1

D2

C2

R2

a

a

k

k+

+

1.1IN (27.9mm)

2.15IN (54.6mm)

365

+

+

Fig.7. Printed circuit board component layout, interwiring details and full-sizeunderside copper foil master for the loudspeaker Reasonance Detector.


excluding speakers

RESONANCE DETECTORResistors

R1 47R2 220k


CapacitorsC1, C2 1 radial elect. 25V (2 off)

SemiconductorsD1, D2 OA47 or OA90 germanium

diode (1N914 silicon iflower sensitivity can betolerated – see text)(2 off)


the EPE PCB Service, code 365; multi-strand connecting wire; solder pins; sol-der, etc.


40

35

30

26 126 120100

9080

70

60

55

50

45

Hz

Packing the LowFrequency Oscillatorcomponents on the rear ofthe small plastic box lid.

EPE Online


circuit board (p.c.b.). This board is avail-able from the EPE PCB Service, code365.

The p.c.b. component layout, wiringand full-size underside copper foilmaster pattern details are illustrated inFig.7. Construction is very straight-forward and only the polarity of thecapacitors and diodes needs specialattention. Also, germanium signaldiodes, D1 and D2, can be damagedby excessive heat and it is prudent toleave a good lead length and apply a heatshunt when soldering.

No difficulty should be encountered

obtaining any of the materials and compo-nents needed for the construction of theloudspeaker enclosure and the setting upequipment. Details of the cross-over unitwere given last month.

Silicon diodes (type 1N914) can be usedin place of the germanium devices in thevoltage doubling rectifier circuit of theResonance Detector. The higher knee volt-age (0·6V instead of around 0·2V) reducessensitivity, but they will still reveal the res-onance peaks when the sound from thespeaker is not too loud, and this is the mainrequirement.

The free-air resonance of the bass speak-

er should be checked before embarking onthe construction of the enclosure. To dothis, wire up the test circuit shown in Fig.8.Details of the connections to theResonance Detector are given in Fig.7. TheOscillator output is in the region of 4·5Vr.m.s., and the 10 kilohm input attenuatorpotentiometer will have to be turned welldown.

Hold the speaker, by the magnet, wellaway from other objects and sweep theOscillator until the voltage across thespeech coil peaks. The rise will be suddenand dramatic. Note the reading on theOscillator dial. If an extended low frequen-cy response is important, it ought not to bemore than 70Hz.

With the speaker now in the enclosure,

connect it to the test circuit shown in Fig.7(directly, not via the crossover). Seal thevent, sweep the oscillator and note the

frequency and magnitude of the peak. Itwill now be at a higher frequency than thefree-air resonance.

Open the vent and sweep the oscillator,again noting the frequency and magnitudeof the peaks. If the tuning is correct (mostunlikely), two peaks of equal magnitudewill be revealed on either side of the origi-nal, vent-sealed peak.

If the higher frequency peak is of greatermagnitude, the vent area is too small (orany duct attached to it too long). Enlargethe vent, or shorten the duct, and test again.

If the lower frequency peak is ofgreater magnitude (more likely with the

design guidance given here), the ventarea is too large or any duct is notlong enough. Either reduce the ventarea, add a duct, or increase the lengthof any duct already fitted, and testagain.

Repeat the procedure until the twopeaks are of equal magnitude. Someexperts tune to a slightly higher fre-quency. This depresses the higher fre-quency peak and, it is claimed, resultsin a more uniform bass response. Theimpedance plot of the test bench speak-er, after tuning, is given in Fig.1.

It is preferable to install a duct, rather

than reduce vent area, in order to lowerresonant frequency. Hold ducts in placewith sticky tape during the setting upprocess.

If desired, a duct can be mountedexternally and adjusted until its length isalmost correct before fixing it behind thevent. Duct volume will then reduce cabinet

volume, so err on the long side whenadjusting its length in this way.

The speaker unit has an extended

bass response and, when driven bythe 8W amplifier described in PartOne (May ’02), sound levels aremore than sufficient for a domestic“hi-fi” installation.

Vent output makes a significant con-tribution at low frequencies (it will

extinguish a candle held close to the aper-ture), and there are no audible resonances.The speaker is most certainly not a “boombox” with honking, one-note bass.

The middle range is clear but there issome colouration at high power levels withmusic that has a heavy bass content.Performance at the higher audio frequen-cies depends very much on the tweeterused: the enclosure is certainly worthsomething better than the cheap unit fittedin the prototype.

When the crossover network isswitched to act as a “speech frequencybandpass filter”, signals overlaid by noiseare greatly clarified. Communicationsenthusiasts, or readers involved in sur-veillance, may find this circuit of interest.It certainly makes the unit more versatileas a bench speaker.

The Low Frequency Oscillator and

Resonance Detector units can, of course,be used to investigate any speaker system.The rating of resistor R1 in the ResonanceDetector is only sufficient for testing atcomfortable listening levels. If speakersare to be checked at high power, fit a 5Wcomponent and use silicon instead of ger-manium rectifier diodes.

Although the test equipment willrespond to very slight changes in venting,especially when the enclosure is small,only a refined ear could detect any audibledifference, even when quite large adjust-ments are made.


Fig.8. Block schematic diagram showing the interconnecting set-up for checkingspeaker resonances.

Completed circuit board for theResonance Detector.

10k INPUTATTENUATOR

LOUDSPEAKERUNDER TEST

OSCILLATOR POWERAMPLIFIER

RESONANCEDETECTOR

TESTMETER

COMMERCIAL guitar amplifiers, eventhose intended for practising, tendto be fairly expensive and have

many features such as gain and tone con-trols which are seldom used, while lackingmore useful ones such as an extra input fora microphone or another guitar. The bud-ding musician’s money could be betterspent on other accessories or even a betterguitar, especially as a simple practiceamplifier for use with headphones caneasily be built around a cheap integratedcircuit.

Even a more ambitious version for dri-ving a speaker providing an output of a fewwatts, which would be quite loud enoughto annoy the neighbours or for playing in asmall hall, only requires the addition of acheap power amplifier i.c and a few morecomponents.

Although the cost and number of compo-

nents required is small, audio power ampli-fier circuits do not lend themselves to asimple stripboard layout and the problemsassociated with designing and making a suit-able printed circuit board are likely to put offall but the most cost conscious or deter-mined constructors. The simple project to bedescribed here solves this problem and has

been designed for easy construction withvirtually no off-board wiring apart from themains transformer, speaker and an optionalheadphone socket.

Since the printed circuit board is readilyavailable, the circuit can be “knocked up”in a very short time and you should havesome change from £25. The finished cir-cuit can be mounted in the same cabinet asthe speaker (these can be salvaged from adefunct hi-fi unit) and even if a speaker hasto be purchased separately it should not setyou back very much.

The full circuit diagram of the Guitar

Practice Amp shown in Fig.1 is very con-ventional and consists of an inverting pre-amplifier stage, IC1, feeding a single chippower amplifier, IC2. The op.amp pream-plifier IC1 has a variable gain set by presetVR1 to enable this to be set to any requiredlevel (up to 100) and should, therefore, besuitable for even the most inefficient guitarpick-ups.

Many small commercial guitar ampsoften feature tone controls but these arereally superfluous as most electric guitarshave perfectly adequate tone controls fittedand so these have not been included in thisdesign.

The output of the preamplifier stage(IC1 pin 6) is fed via Volume control VR2to the power amplifier IC2, which is basedaround the popular TDA2030. This devicecan supply up to 24W of audio powerdepending on the supply voltage andspeaker impedance used, provided we arenot too bothered about the distortion whichin this application can almost be consid-ered to be an advantage.

With the lower supply voltage specified,a more reasonable output power would beabout 6W to 10W which should be morethan sufficient for our purpose. The poweroutput can be easily increased if requiredby reducing the speaker impedance orincreasing the supply voltage, and nochanges in the component values arerequired.

It should, however, be remembered thatthe maximum supply voltage for both i.c.sis 36V. The TDA2030 is a very well pro-tected device featuring both short circuitand over dissipation protection althoughfrom a reliability point of view it is cer-tainly not advisable to run the device ineither of these conditions.

Music generally tends to have many peakswhile the average power dissipated remainslow so that in practice, despite the use of therelatively small heatsink specified, the tem-perature of the device will remain well with-in its safe limit even with prolonged loudplaying. Also, as the circuit is permanentlyconnected to the speaker (except when inheadphone mode) the possibility of a shortcircuited output is much reduced.

An (optional) output socket SK3 is alsowired in circuit to enable headphones to beconnected in place of the speaker LS1.

84 Everyday Practical Electronics, February 2002

This is arranged so that inserting the head-phone jack plug automatically disconnectsthe speaker. It also switches in a resistor,R11, in series with the headphones to pre-vent overloading, see Fig.1 and Fig.3.

Both the resistor and the headphonesocket are mounted off the board and itwill be noticed that the headphones whichnormally have an impedance of 32 ohms(each) are connected in series.

The circuit is completed by a conven-

tional power supply consisting of mainstransformer T1, bridge rectifier REC1 andsmoothing capacitors C12, C13. It pro-vides a d.c. supply of +12V and –12V andalthough a single rail supply could havebeen used, the advantage here is that theusual large speaker coupling capacitor isnot required.

This may not seem to be such an advan-tage when it is realised that two capacitorsare now required in the power supply, butit does mean that the annoying “switch-onthump” normally associated with theseamplifiers (due to the speaker couplingcapacitor charging up) is eliminated. Therelatively low impedances in the circuitmean that hum and noise pick-up is low sothat an l.e.d. D4 Power On indicator hasbeen included to remind the user to switchoff!

Most of today’s top hits are songs and

playing chords on their own does notsound very good, it is far better if the“artist” can sing along while playing. Withan electric guitar a microphone is requiredto avoid having to shout rather than sing.

Nowadays headphones which include amicrophone are available from any com-puter store for around £5 and these areeminently suitable for this application.Many practice amplifiers however, haveonly one input and cannot easily accom-modate a microphone but this deficiencyhas been rectified in this design by addinga simple mixer.

The microphones incorporated in thesecheap headsets are usually “electret” types.The microphone element constitutes ineffect a very high impedance source and abuffer amplifier (consisting of a field effecttransistor or f.e.t.) is normally incorporat-ed within the microphone capsule asshown inset in Fig.1.

This requires a small supply voltage(between 1·5V and 5V) and a load resistorto operate and so the components associat-ed with the microphone input have beenadded to supply this. A nominal 5V supplyis derived from the main supply rail viaresistor R1 and Zener diode D1 while R2forms the load resistor for the f.e.t. insidethe microphone capsule.

Note that a stereo jack socket (SK2) isused for the microphone with the secondterminal supplying the +5V while the sig-nal is picked up from the centre pin (tip)and the outer earth (0V) connection in thenormal way. (The centre pin and the sec-ond terminal are connected inside themicrophone). This allows a differentmicrophone such as a dynamic type forexample, which does not need a supplyvoltage or resistor, to be connected and inthis case the 5V supply will simply be

Everyday Practical Electronics, February 2002 85

+

-

+

+

1 12 2345 5 6+

+

+

+

+

+

+

+

R1

22k

R2

4k7

R3

10k

R4

10k

R10 1k

R5

47k

R6

1k5

R7

4k7

R8

1k5

R9

10Ω

R11

120Ω

C1

47µ

C2

47µ

C3

47µ

C6

47µ

C8

47µ

C5

47µ

C4

100p

C10

470p

C11

100n

C7

100n

C9

100n

C13

2200

µ

C12

2200

µ

D1

BZ

Y88

4V7

ZE

NE

Rak

SK

1

SK

2

MIC

GU

ITA

R

IC1

TL0

81

2 3

47

6

VR

147

0k

VR

210

k LO

G

VO

LUM

E

D2

RE

D

a k

IC2

TD

A20

30

1 2

3

4

5

D3

1N40

01

D4

1N40

01

a

k

k

aH

EA

DP

HO

NE

S

SK

3

LS1

8Ω

RE

C1

2A

T1

9V 9V0V

0V230V

FS

110

0mA

(S-B

)S

1

ON

/OF

F

230V

AC

MA

INS

SU

PP

LYL EN

NC

NC

NC

TIP

TIP

NC

= N

O C

ON

NE

CT

ION

GA

IN

0V VE

+V

E

TIP

PL2

MIC

1

MIC

RO

PH

ON

E

1 2 CO

M(0

V)

Fig.1. Complete circuit diagram for the Guitar Practice Amp.

shorted to earth by the microphone’s monojack plug causing no damage to either themicrophone or amplifier.

The signal from the microphone is fed to

the input of the amplifier via another inputresistor R4, the value of which togetherwith the feedback control (resistor) VR1defines the gain of this channel. A 10 kil-ohms resistor was found suitable in theprototype but this may be changed ifrequired, a higher value resulting in a lowergain and vice-versa.

This stage (IC1) of the circuit forms anideal signal mixer since the inverting input(pin 2) of the amplifier is a “virtual earth”so called because the op.amp IC1 main-tains the voltage at its inverting input atzero volts. It does this by changing its out-put voltage when a change in the inputvoltage tries to upset this and as the feed-back preset VR1 has a higher value than

t h ei n p u tr e s i s t o rR4, the out-put voltage changeis higher resulting in avoltage gain.

Another way to visu-alise this is to realise that anop.amp always tries to main-tain both of its inputs at the same potentialwhich in this case is 0V. This means thatthe microphone channel will not be affect-ed by any changes in the volume or tonesettings of the guitar which is also con-nected to this point via its own resistor R1.

A general circuit of a “virtual earth’’

mixer is shown in Fig.2. and there is noth-ing to stop you connecting another guitaror other signal source such as a tape or CDplayer in the same way simply by addinganother input socket, connected to IC1’sinverting input by its own resistor asshown. The values of the resistors would,

of course, have to be chosen carefully toavoid over driving the amplifier. The out-put of a CD player for example would bemuch larger than that of a guitar so that itsresistor would need to have a higher value.

Alternatively, each channel could have aseparate volume control fitted as shown. Itwould also be a good idea to fit d.c. block-ing capacitors to prevent any d.c. on theoutput of the CD player or other deviceupsetting the bias conditions of theop.amp.

No separate provision for controlling thevolume of the microphone channel hasbeen made in this version as the relativevolume of the guitar can be controlled atthe instrument itself while VR2 controlsthe overall volume.

This is a mains operated circuit and

its construction should not be attemptedby those who are not suitably experi-enced or supervised.

The use of a printed circuit board(p.c.b.) makes the circuit

very easy to buildand, with only five con-nections to the board, itshould be possible to assemblethe Guitar Practice Amp without anymajor errors. The topside p.c.b. compo-nent layout, interwiring and full-sizecopper foil master pattern are shown inFig.3. This board is available from theEPE PCB Service, code 336.


ResistorsR1 22kR2, R7 4k7 (2 off)R3, R4 10k (2 off)R5 47kR6, R8 1k5 (2 off)R9 10R10 1kR11 120

All 0·25W 5% carbon film or better

PotentiometersVR1 470k carbon preset, lin.VR2 10k rotary carbon, log.

CapacitorsC1 to C3,

C5, C6, C8 47 radial elect. 50V(6 off)

C4 100p ceramicC7, C9,

C11 100n ceramic (3 off)C10 470p ceramicC12, C13 2200 axial elect. 25V

(2 off)

SemiconductorsD1 BZY88 4V7 Zener diodeD2, D3 1N4001 50V 1A rectifier

diode (2 off)D4 5mm red l.e.d.REC1 2A 100V in-line bridge

rectifier (see text)IC1 TL081 j.f.e.t. op.ampIC2 TDA2030 audio amplifier


Fig.2. Adding extra inputs to the “virtual earth’’ mixer circuit.

R1

R2

INPUT 1

INPUT 2

GAIN = Rf/R1

GAIN = Rf/R2

VIRTUALEARTH

OUTPUT

FURTHER INPUTSAS REQUIRED

RfDC BLOCKINGCAPACITOR+

+

+

0V

VR1

VR2

VR3 etc.


excluding speaker & case

MiscellaneousSK1 6·35mm (¼in.) moulded mono jack socket, with

2 switched break contactsSK2, SK3 3·5mm stereo jack socket, with 2 switched

break contacts (2 off)MIC1 sub-min. omni-directional electret microphone

insertS1 s.p.s.t. mains rated on/off toggle switchFS1 100mA 20mm slow-blow fuseT1 18VA 230V a.c. mains transformer, 9V-0V-9V

secondaries (see text)

Printed circuit board available from the EPE PCB Service, code336; 8-pin d.i.l. socket; panel mounted fuseholder; aluminiumheatsink, size 38mm x 58mm approx.; control knob; multistrandconnecting wire; mains cable; 8 speaker, type to choice; solderpins; solder etc.

Completed p.c.b. showingthe supply smoothingcapacitors, on/off indicator

l.e.d. and in-line rectifier.The mains trans-former, fuseholderand on/off switch are

mountedoff-board.

EPE Online


Assembly of the board should begin byinserting the terminal pins which will beused to connect the speaker and trans-former to the p.c.b. These usually require acertain amount of force to insert into theboard which could damage adjacent com-ponents if this were done at a later stage.

Once the solder pins have been fitted,the board may be completed by mountingresistors, diodes, capacitors etc. in ascend-ing order of height. Care should, of course,be taken to ensure that diodes and elec-trolytic capacitors are inserted the correctway around. Note also that a wire link(made from a discarded component lead)and a resistor (R10) are mounted underC12 and C13 so that these componentsmust obviously be fitted before the elec-trolytic capacitors are mounted on theboard. A second wire link is also requiredbetween C6 and VR2.

Although IC1 is not a CMOS device,and thus not particularly sensitive to static,it is worth fitting an i.c. socket to preventany possibility of overheating it during thesoldering operation this will also facilitateits easy removal should this be required.

The audio power amplifier IC2 is moredifficult to fit and before this is done it isbest to prepare the small heatsink accord-ing to Fig.4. In the prototype this was madefrom a piece of L-shaped aluminium extru-sion normally sold in DIY shops but shouldthis not be easily obtainable a suitable

piece of sheet aluminium bent to shape anddrilled as shown will do just as well.

IC2 should be mounted on the board butits leads should not be soldered for themoment. Once this has been done, theheatsink can also be mounted on the boardand secured to it using two nuts and bolts.When it is secure, IC2 should be bolted toit, via its metal tab, and it is here that the

Everyday Practical Electronics, February 2002 87

230V 0V

9V9V 0V

T1

EL

N

336

6.4IN (162.6mm)

1.5I

N (

38. 1

mm

)

R10

C1

R1VR1

C2

IC1C3

R9

R5

R2

R6

R3

R4

C7

D1

C10

C11

R7

REC1

IC2

R8

R11

C9

D2

D3

D4

C5

C4

C6

C8

C12 C13

VR2SK2

SK1

a k

a k

k ak a

+

+

++

+

+

+

+

12

34

5

2 5 4 1

3

TIP

1 4 5 2

3

SK3

TIP

LS1

LS1

HEATSINK

SOLDERTAG

FS1S1MIC

GUITAR

1

2TIP(1)

+V(2)

0V(3)

0V

PL2

SK2

MIC1

Fig.4 (top right). Heatsink dimen-sions and bending details.

Fig.5 (top, far right). Pinout detailsfor IC2, the TDA2030 audio amp.

Fig.3. Printed circuit board compo-nent layout, wiring and full-size cop-per foil master. The wiring to themicrophone insert jack plug PL2 isshown inset below.

OUTPUT(4)

V

+V(5)

INPUT( )(1)

+

INPUT( )(2)

TAB CONNECTEDTO

V(3)

10mm

12mm

12mm

22mm

5mm

20mm

38mm

38mm

Fig.4 Fig.5

advantage of delaying the soldering of thisdevice will be seen as this will allow a cer-tain amount of tolerance in the final posi-tioning of the device relative to theheatsink.

Once IC2 is secured to the heatsink, itsleads can be soldered and trimmed in thenormal way. Note that it may also be nec-essary to bend the leads slightly to enableit to fit the holes in the board, see Fig.5.

Most devices are supplied with the leadsalready pre-formed although it shouldbe noted that the TDA2030 isavailable with the leadsformed for bothv e r t i c a land hori-z o n t a lmounting.Both typesare identicalbut the verticaldevice is to bepreferred as quite alot of lead bending would be required to fitthe horizontal device.

A smear of silicone grease between theheatsink and IC2’s tab will help to conductheat away from the i.c. but this was notfound necessary on the prototype. What isimportant however is to ensure that there isa good electrical path between the tab andthe negative supply p.c.b. copper track. Forthis reason no mica washers or any otherinsulation should be fitted between the tabof IC2 and the heatsink.

The heatsink is used as a negative sup-ply connection to the chip and it must notbe earthed or connected to any other partof the circuit. The pinout details of theTDA2030 are shown in Fig.5 for refer-ence.

The only other component worthy ofindividual mention is the bridge rectifierwhere a 2A device is specified. A 1Adevice could also be used but this was notavailable in the author’s spares box. Theseare available in many variants and shapesand although any of these devices will do,the board has been designed for an in-linepackage and so this type should be pur-chased if possible to avoid a lot of leadbending.

After careful checking of the board to

ensure that there are no solder splashesbetween the tracks and that all the jointsare sound, the speaker and mains trans-former connections should be made to theboard. The transformer used in the proto-type had wire leads but if another type isused, then wires may need to be fitted.

Printed circuit board mounting typesshould be avoided as these usually lackmounting brackets and in this case thetransformer will need to be mounted on achassis or in the wooden cabinet containingthe speaker. The final arrangement willdepend to a large extent on circumstancesand is therefore left to the individual tosolve.

Care should be taken to ensure that atransformer with a centre tapped sec-ondary (or with two secondary windingswhich can be connected in series) is usedand although a voltage of 9V-0V-9V isspecified, a slightly higher output couldalso be used. It should be remembered thatthe output of a transformer is always

quoted as an r.m.s. value when deliveringits rated current.

After rectification and smoothing thefinal d.c. output will be nearer the peakvalue (approximately 1·4 times the r.m.s.value) and as amplifier circuits of thistype draw a relatively low currentwhen no signal is present,the final supply

voltage could be even higher depending onthe transformer used. The supply voltageshould, therefore, be measured to ensurethat it does not exceed the ratings of the i.c.s(i.e. plus and minus 18V). The centre tap ofthe secondary must be connected to the 0Vrail (corner terminal of the p.c.b.) while theother two leads may be connected to theother two terminals either way around.

The mains wiring should be carried out

carefully and all joints well insulated toensure that they cannot be touched inad-vertently when the unit is in operation. Amains On/Off switch and a fuse shouldalso be fitted in the live mains lead and themains cable securely clamped to the box orcabinet using a suitable strain reliefmounting bush.

The speaker will also need to beconnected to the output terminals usingsuitable lengths of wire. If a socket forheadphones is to be included, this shouldbe arranged to disconnect the speakerwhen the jack plug is inserted so that aswitched socket will be required (see Fig.1and Fig.6).

The finished p.c.b. is quite light and sono special mounting hardware is required.It should be adequately supported by thepotentiometer spindle and the input jacksockets but the final details of this are leftto the constructor and will depend to alarge extent on the cabinet in which thep.c.b. and speaker are mounted.

When fully assembled, check the wiring

again, especially around the headphonesocket and transformer primary and if all iswell, connect the unit to the mains andswitch on. The voltage across each of thetwo smoothing capacitors can be measuredand this should be about 12V d.c but nohigher than 17V.

A slight hum or hiss may be audible

from the speaker if the Volume controlVR2 is turned up fully. Turn down the vol-ume and connect a guitar which shouldnow be heard.

The only adjustment to be made is to setthe gain of the preamplifier stage (IC1)and this should be done with the volumeturned up to maximum on VR2 and theguitar. Starting with preset VR1 turnedfully clockwise the gain should beincreased until distortion is heard when astring is played. An oscilloscope is usefulhere but not necessary as it is the finalsound that is important and not the appar-ent purity of the output waveform.

If required, the headphones can beplugged in and, provided the wiring hasbeen done correctly, this should switch offthe speaker. With this “adjustment”complete, the stage act can be perfectedwithout interference from the rest of thehousehold. Take it away Eric . . .


Fig.6 (right).Headphone jack plugPL3 wiring. Theheadphone jacksocket (SK3) con-tacts break when theplug is inserted,disconnecting theloudspeaker LS1.

LEFT RIGHT

TIP(LEFT)

RING(RIGHT)

COMMON

HEADPHONES

COM R LPL3

Completed amplifier circuitboard showing the audio poweroutput i.c. bolted to its heatsink.

The loudspeaker/headphones are wired to twooutput solder pins hidden behind the volume control.

Copyright © 2000 Wimborne Publishing Ltd andMaxfield & Montrose Interactive Inc

EPE Online, May 2000 - www.epemag.com - 338

Intended primarily as ameans of processing microphoneinputs to computers, theSSM2166P integrated circuit (IC)– manufactured by AnalogDevices – has a wider range ofpossible applications. Publicaddress and surveillance systemsimmediately spring to mind, andthe device will be of particularinterest to radio enthusiasts,especially now that the popularPlessey 6270 IC mic/pre-amp,with voice gain, is no longeravailable.

This article describes how thenew IC can be used for a varietyof signal inputs, and additionalcircuitry is given for readers who

AMPLIFIERSThe input impedance of

buffer amplifier, A, is 180kilohms (180k) and its gain canbe set, by external feedbackresistors, between 0dB and20dB. There is a standing DCvoltage on the input, and ablocking capacitor must beused.

The input and outputimpedances of the controlledamplifier, D, are 1k, and 75ohms, respectively. A standingDC voltage necessitates the useof a blocking capacitor at theoutput.

Use one of the latest chips on the block to produce an audio pre-amp with AGCcompression, limiting, and noise reduction.

VERSATILE MIC/AUDIO PREAMPLIFIERby RAYMOND HAIGH

require a signal-strength meter.THE CHIP

The various amplifying andcontrol stages built into theSSM2166 chip are shown inFig.1.

Signal inputs are bufferedby opamp A, internallyconnected to a rectifier stage,B, which produces a DC voltagewhich varies in proportion tosignal strength.

After processing by thecontrol circuit, C, the DCvoltage is used to fix the largeand small signal gain of asecond opamp, D.

BUFFERAMPLIFIER

BUFFERAMPLIFIEROUTPUT

CONTROLLEDAMPLIFIER

+ +

6

13

12

4

7

8

35

A D

B C

VOLTAGECONTROLLEDAMP INPUTS

TRUE R.M.S.LEVELDETECTOR

CONTROLCIRCUITRY

BUFFERAMPINPUTS(AUDIO IN)

PROCESSEDOUTPUT

POWERDOWN(STAND-BY)

SET A.G.C.TIMECONSTANT

2 SETGAIN

111091

GROUND (0V) SET SQUELCHTHRESHOLD

SETCOMPRESSIONRATIO

SETLIMITINGTHRESHOLD

14

+5V LIMITING IS IMPOSED IN THIS REGION INORDER TO HOLD THE OUTPUT BELOW APRE-DETERMINED LEVEL

THRESHOLD OFLIMITING SETBY VR4

DOWNWARDEXPANSIONOR SQUELCHTHRESHOLDSET BY VR2

SIGNAL OUTPUT

SIGNAL INPUT

IN THIS REGION GAIN REDUCES ASSIGNAL STRENGTH INCREASES INORDER TO COMPRESS THE DYNAMICRANGE. COMPRESSION IS SET BY VR3

IN THIS REGION GAIN REDUCES ASSIGNAL STRENGTH REDUCES INORDER TO PREVENT HIGH-LEVELAMPLIFICATION OF NOISE UNDERNO-SIGNAL CONDITIONS

Fig.1. Internal block schematic for theSSM2166P microphone preamplifier, withvariable compression and noise gating.

Fig.2. Relationship between limiting, com-pression, and downward expansion or

“squelch”.



Provision is made forsetting the nominal gain of thecontrolled stage between 0dBand 20dB, but AGC action willincrease amplification, at thelowest signal levels, to as muchas 60dB. The output can bemuted.

Interestingly, the noisegenerated by the controlledstage is designed to be at aminimum when its gain is at amaximum, and this significantlyimproves the overall signal-to-noise ratio of the system.

RECTIFIERThe circuit of the rectifier, or

level-detector stage (B), hasbeen specially developed forthis application. It produces aDC control voltage, which isproportional to the log of thetrue RMS value of the inputsignal.

The speed at which thecontrol voltage responds tochanges in signal level, or the“attack time”, can be controlledby the user. Response to high-level changes is automatically

speeded up by the IC in order tominimize the duration of anyoverload.CONTROL CIRCUIT

The control circuit (C)enables the user to program theperformance of the IC in a verycomprehensive way, and theamount of signal compressioncan be set between zero and60dB.

Signal limiting can also beapplied to prevent theoccasional transient exceeding

the desired maximum output. Itcan be set at outputs rangingfrom 30mV to 1V. Above thisthreshold, the maximumcompression ratio of 15:1 isapplied.

The response of the systemto very low level inputs can bereduced in order to prevent theamplification of noise under no-signal conditions. The thresholdof this downward expansion (thelower the signal the less it isamplified), can be set at inputs ofbetween 250uV and 20mV.

Constructional Project

bc

e

VR522k

VR610k

VR710k

VR3100k

VR14k7

R61k

R310k

R11k

R210k

GAIN

OUTPUTSIGNALLEVEL

COMPRESSION

INPUTSIGNALLEVEL

C722µ

C41µ

C91µ

C64 7µ

C10470µ

C14 7µ

C54 7µ

IC1SSM2166P

VR21M

VR447k

R72M2

R915k

R41k

R51k

R8SEE TEXT(TABLE 2)

SQUELCH LIMIT

C8100n

C310n

C2100n

0V

0V

AUDIOINPUT 1

AUDIOOUTPUT

AUDIOINPUT 2

WIRELINK

TR1BC547

+

VR810k

ME150 A TO 1mASIGNALSTRENGTHMETER(SEE TEXT)

µ

R104k7

++8V TO

18VIC2LM78L05

INOUT

COM

AUDIO IN (1): ELECTRET MICROPHONES ANDINPUTS REQUIRING A D.C. BLOCKING CAPACITOR

AUDIO IN (2) DYNAMIC (MOVING COIL) MICROPHONES

5 3 9 11 14

4 8 10 1 2

6

7

12

13

LK1

SCREEN

SCREEN

+5V

Fig.3. Complete circuit diagram for the Versatile Mic/Audio Preamplifier.



Provision is made for thedevice to be placed in a “power-down” or stand-by mode, andthis feature will be of particularinterest when it is used insophisticated surveillancesystems. In this state, currentconsumption is reduced toaround 10mA and the input andoutput ports assume a highimpedance.

User programmable controlcircuitry, coupled with thecomplex rectifier or level-detector, contributessignificantly to the chip’sperformance. The relationshipbetween the noise reduction,compression and limitingfunctions is displayed in Fig.2.

RATINGSNo doubt with computer

circuit compatibility in mind, theSSM2166 is designed for a 5Vsupply. The absolute maximumsupply voltage is 10V. Currentconsumption is approximately10mA.

The maximum input to thebuffer is 1V, and the maximumoutput from the controlledamplifier is 14V RMS for 1 percent total harmonic distortion.Frequency response extendswell into the RF spectrum.

Static discharges candamage the IC, and the usualprecautions (discharging thebody) should be taken whenhandling and connecting it intocircuit.

The SSM2166P is embeddedin a 14-pin, dual-in-line package,and the suffix “P” refers to thestandard-size version. This is thetype most likely to be stocked bysuppliers. However, surface-mount types are alsomanufactured: these carry thesuffix “S”.

CIRCUIT DETAILSThe full circuit diagram for

the Versatile Mic/AudioPreamplifier, incorporating asignal strength meter, is given inFig.3. Provision for controlling somany functions results in aplethora of preset potentiometercontrols. However, they doenable the signal processing tobe tailored to individualrequirements, and theiradjustment is not critical ordifficult. A summary of theirvarious functions is set out inTable 1.

Preset VR1 permitsadjustment of the input signallevel to prevent overload and tooptimize the performance of the

circuit. Its value is appropriatefor moving coil and electretmicrophones, and for audiosignals derived from mosttransistor circuits. Keeping the


COMPONENTSResistors

R1, R4, R5, R6 1k (4 off)R2, R3 10k (2 off)R7 2M2R8 (see Table 2)R9 15kR10 4k7

See also theSHOP TALK Page!

All 0.25W 5% carbon film

CapacitorsC1, C5, C6 4u7 radial electrolytic, 10V (3 off)C2, C8 100n ceramic (2 off)C3 10n ceramicC4, C9 1u radial electrolytic 10V (2 off)C7 22u radial electrolyticC10 470u radial electrolytic

SemiconductorsTR1 BC547 (or similar, e.g. BC239, BC548) npn low-power transistorIC1 SSM2166 microphone preamp (Analog Devices)IC2 LM78L05ACZ +5V 100mA voltage regulator

MiscellaneousME1 50uA to 1mA FSD moving coil meter (see text)

Printed circuit board availablefrom the EPE Online Store, code7000260 (www.epemag.com);14-pin DIL socket; screened cable,solder pins, solder, multistrandconnecting wire, etc.

$27Approx. CostGuidance Only(Excluding meter)

PotentiometersVR1 4k7 enclosed carbon preset, horizontalVR2 1M enclosed carbon preset, horizontalVR3 100k enclosed carbon preset, horizontalVR4 47k enclosed carbon preset, horizontalVR5 22k enclosed carbon preset, horizontalVR6, VR7, VR8 10k enclosed carbon preset, horizontal (3 off)

Preset Value

VR1VR2

VR3VR4VR5

VR6VR7

VR8

4k71M

100k47k22k

10k10k

10k

FunctionSet input signal level: clockwise to increase.Set threshold of downward expansion (squelch):clockwise to lower.Set compression: clockwise to increase.Set threshold of signal limiting: clockwise to lower.Set gain of controlled amplifier: clockwise toincrease.Set output signal level: clockwise to increase.Set signal strength meter pointer at full scale(when strongest signal being processed):clockwise gives clockwise pointer movement.Set signal strength meter pointer at zero (underno-signal conditions): clockwise gives clockwisepointer movement.

Table 1: Preset Control Functions

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value below 5 kilohms increasesthe stability margin of IC1.

Power can be supplied to anelectret microphone’s integralFET (field-effect-transistor)buffer via resistor R1, and C1acts as a DC blocking capacitor.The input arrangements foralternative microphones andother signal sources arediscussed at greater lengthlater.

The input signal to IC1 isapplied to the non-inverting (+)input of the buffer amplifierstage (Pin 7 – see Fig.1) viablocking capacitor C2. This IChas an extended frequencyresponse and C3 introduces ameasure of roll-off above 20kHzor so, again in the interests ofstability.

BUFFER GAINThe gain of the buffer

amplifier is set at 6dB byresistors R2 and R3, and this islikely to be sufficient for mostpurposes. Gain can beincreased to a maximum of20dB by decreasing R3 to about12 kilohms. Adding this to thegain of the controlled amplifierresults in an overall systemgain, when signals are too smallto initiate compression, of 80dB.

This is a great deal ofamplification in a smallpackage, and particular caremust be taken with thescreening and routing of inputand output leads, and theconnections to a shared powersupply, if instability is to beavoided. Separate ground, or0V leads, from signal sourcecircuitry, the preamplifier, andthe power amplifier, should berun to a common point at thepower supply. The screeningbraid of signal cables should beconnected to ground at thepreamplifier end only.

Constructional ProjectIf desired, the gain of the

buffer can be set at unity bydeleting R3 and inserting a wirelink in place of resistor R2 (toconnect pin 5 and pin 6).Blocking capacitor C4 maintainsthe correct DC conditions.

CONTROLLEDAMPLIFIER

The output from the bufferstage (pin 5) is connected, viaDC blocking capacitor C5 to thenon-inverting (+) input (pin 3) ofthe controlled amplifier stage. Acapacitor of identical value, C6,at pin 4 connects the inverting(–) input to ground (0V). (Thisconnection makes any electricalnoise on the ground rail appearas a common mode signal tothe controlled amplifier and thedifferential input circuitry rejectsit).

The nominal gain of thecontrolled amplifier can be set,by preset VR5, between unityand 20dB. Resistor R6 ensuresthat the gain does not fall belowunity.

Switched muting can beachieved by grounding pin 2 viaa 330 ohm resistor (the switchshould be located at the groundor 0V rail end). Switch clickscan be suppressed byconnecting a 10nF capacitorbetween pin 2 and ground.

The IC can be put in stand-by mode by disconnecting pin12 from ground and connectingit, via a 100 kilohms resistor, tothe +5V rail. (Provision has notbeen made for muting orpowering-down on the PCB.)

The processed output istaken from pin 13 andconnected, via DC blockingcapacitor C9, to preset VR6.This enables the output signallevel to be adjusted to suit theinput sensitivity of the power

amplifier.

ATTACK TIMEThe response or “attack”

time of the AGC system can becontrolled by adjusting the valueof the rectifier reservoircapacitor C7. The ICmanufacturer suggests a valuewithin the range 22uF to 47uF,with smaller capacitors beingsuitable for music and the largerfor speech.

Too low a value will result in“pumping” effects, withbackground noise “rushing up”between bursts of speech. Thiswill become increasinglyapparent as the compressionratio is raised.

Conversely, too high avalue will excessively slow theresponse of the system tochanges in signal level. The22uF component specified forC7 has been found to work wellwith both speech and musicinputs.

The attack time is controlledmainly by the value of C7, butthe much longer “decay” time isdependant upon this capacitorand the internal control circuit.Fast attack and slow decay helpto reduce the pumping effect,which seems far lesspronounced with this IC thanwith simpler audio AGCsystems.

COMPRESSIONThe amount of compression

is determined by preset VR3,which connects pin 10 toground. There is nocompression with thepotentiometer set to zero. Whenits resistance is at maximum, a60dB change in input level(above the downward expansionor squelch threshold) changes



the output by less than 6dB.The onset of limiting is

controlled by preset VR4.Setting this potentiometer tomaximum resistance fixes it at30mV. With VR4 at minimumresistance, it is around 1V RMS.Above the threshold of limiting,a 15:1 compression ratio isimposed, irrespective of thesetting of compression controlVR3.

NOISE REDUCTIONPreset potentiometer VR2

sets the threshold below whichdownward expansion (gainreduces as the signals becomeweaker) is applied. Withmaximum resistance, downwardexpansion starts at signal levelsin the region of 250mV. Turnedto zero resistance, the thresholdis raised to around 20mV.

Gain rises to a maximumunder no-signal conditions withall conventional AGC systems,and the amplification of externaland internally generated noiseproduces a loud and tiresomehiss in the speaker or ‘phones.The IC’s noise reduction facility,which operates as a “squelch”control, is very effective inovercoming this. It can reduceoutput noise below the level ofaudibility when signal levels fallto zero.

With any squelch system, aneed to resolve very weaksignals overlaid by noisecompromises the usefulness ofthe feature. Radio enthusiastswith a particular interest in itcould mount VR2 as a panelcontrol so that the thresholdcould be adjusted to suitreception conditions.

POWER SUPPLYThe maximum safe supply

voltage is 10V, and it should be

noted that, under a light load, afresh 9V alkaline battery willusually deliver a higher voltagethan this.

However, in order to ensurethe correct operation of thedevice, and provide a highdegree of isolation from otherequipment sharing the samepower supply, a 5V 100mAvoltage regulator, IC2, isincluded in the circuit. Thisenables supplies with outputsranging from 8V to 18V (ormore, depending on IC2 rating)to be used.

Bypass capacitors C8 andC10 shunt the noise in theregulator output to ground. Notethat C8 is essential to thestability of IC1 and it must belocated as close as possible topin 14, even when the unit isbattery powered.

SIGNAL STRENGTHMETER

Some readers, especiallythose wishing to incorporate theunit into a radio receiver, maywelcome the provision of asignal strength meter. This isincluded in the circuit diagramof Fig.1 and consists oftransistor TR1, meter ME1 andassociated components.

The AGC control voltageappears on pin 8 of IC1. Itranges from 290mV under no-signal conditions toapproximately 720mV with highlevel inputs.

Transistor TR1, configuredas a DC amplifier, ensures thatIC1’s AGC line is only lightlyloaded, even when a 1mAmeter is used. It forms one armof a bridge circuit, the otherthree being its collector load,R9, and the potential dividerchain comprising preset VR8and resistor R10. The bridge isbalanced, and the meter set atzero under no-signal conditions,by preset potentiometer VR8.

When a signal is beingprocessed, the rising AGCvoltage on the base (b) of TR1increases its collector currentand, hence, the voltage dropacross resistor R9. Thisunbalances the bridge anddrives the meter pointer over.Preset VR7 adjusts thesensitivity of the meter so thatthe pointer can be set just shortof full-scale deflection (FSD)when registering a strong signal.

The circuit can be made toaccommodate meters with full-scale deflections ranging from50mA to 1mA by adjusting thevalue of resistor R8. Thisresistor controls the flow ofcurrent through the base-emitterjunction of transistor TR1, andvalues to suit a range of meterFSDs are given in Table 2. Biasresistor R7 provides a measureof negative feedback whichhelps to stabilize the operationof the circuit.

Almost any small-signal npntransistor should prove suitablefor TR1, and a 2N5827 or2N5828 could be used inaddition to the types listed in theComponents list. These deviceshave different case styles andthe base connections must bechecked.

CONSTRUCTIONAll the components, with the

exception of the meter ME1, are


Meter FSD R8

50uA100uA500uA1mA

1M470k100k47k

Table 2: Signal StrengthMeter ( Values of R8 for differ-ent meter sensitivities)



assembled on a small, single-sided, printed circuit board(PCB). The topside componentlayout, together with an(approximately) full-sizeunderside copper foil masterpattern, is shown in Fig.4. Thisboard is available from the EPEOnline Store (code 7000260) atwww.epemag.com

Commence construction inthe usual way by mounting thesmallest components firstworking up to the largest, but fitIC1, IC2, and TR1 last (seeearlier comments about thestatic sensitive nature of IC1). Aholder for IC1 will facilitatesubstitution checking. Solderpins, inserted at the lead-outpoints, will ease the task of off-board wiring.

SPOT-CHECKSWhen all the components

have been soldered in positionon the PCB, double-check theorientation of electrolyticcapacitors, the ICs, and thetransistor. Also, check the PCBfor bridged tracks and poorsolder joints.

Next, with IC1 “out ofcircuit”, connect a supplyvoltage of between 7V and 9Vand check that the output fromregulator IC2 is producing 5V. Afault in this device, or its wrongconnection, could result in thedestruction of IC1 when highervoltages are applied.

Once all is well, place IC1in its socket (checkingorientation), connect, viascreened cable, a signal sourceand a power amplifier. Adjustthe various presetpotentiometers until theprocessing meets yourrequirements. All presetfunctions are summarized inTable 1 for ease of reference.


BC547BC239BC548

LM78L05ACZ

e b c

C2

C8

+

+

+

+

+

+

+

C4

C6

C7

C9

C5

C10

C1

R8

R1

R10

R7

R3

R2

R4C

3

R5

R6

R9

VR6

VR1

VR5

VR4

VR3

VR2

VR8

VR7

AUDIOINPUT 1

AUDIOINPUT 2

INPUTSIGNALLEVEL

SIGNALSTRENGTHMETER

SQUELCHDOWNWARDEXPANSION

THRESHOLDOF LIMITING

COMPRESSION

OUTPUTSIGNALLEVEL

GAIN

INPUTGROUND(0V)

POWER SUPPLY

NEGATIVE + +8V TO 18V

INCOMOUT

IC2

TR1

e b c

INCOM

OUT

+

–

SET METERAT FULLSCALE

SET METERAT ZERO

SIGNAL OUT

LK1

UNDERSIDEVIEW

UNDERSIDEVIEW

Fig.4. Printed circuit board component layout, inter-wiring de-tails and (approximately) full-size underside copper foil master.

MICROPHONESThe unit works well with

dynamic (moving coil), electret,crystal, and ceramicmicrophones. Screened cable

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must, of course, be used toconnect any type of microphoneto the preamplifier.

Very high quality studiomicrophones can be insensitiveand require balanced feeders tominimize hum pick-up. The pre-amplifier described here isconfigured for unbalancedinputs, and is not likely to besuitable, as it stands, formicrophones of this kind.

A few words about thevarious types of signal inputmay prove helpful.

Dynamic Microphones aremanufactured with impedancesranging from 50 ohms to 600ohms. Output tends to begreatest with the higherimpedance units.

This type of microphoneshould be connected to Input 2(i.e., directly across presetVR1), and the wire link must beremoved to isolate resistor R1from the 5V rail.

Electret Microphones are amodern development of thecapacitor microphone (apermanently chargeddiaphragm, the electret,

eliminates the need for anexternal charging voltage). Theoutput from the actual unit islow and at a high impedance, sothese microphones have anintegral FET buffer. The drainload for the internal FET isprovided at the amplifier end ofthe cable (resistor R1 in Fig.3),to facilitate line powering.

Electret microphones mustbe connected to Input 1, and thewire link must be in place toconnect resistor R1 to thesupply rail. The 1 kilohm drainload (R1), fed from the 5Vsupply, should ensure theoptimum performance of mostmicrophones of this kind.

Crystal and CeramicMicrophones rely upon thepiezo-electric effect to producea signal voltage. The vibratingdiaphragm induces stresses in awafer of crystal, often Rochellesalt, or in a barium titanate

element in the case of ceramicunits.

These microphones shouldbe connected to Input 2. Theyhave a high impedance, andfeeding them into preset VR1will reduce their response to lowaudio frequencies. Lowfrequency roll-off is, however,desirable for communicationswork, and more is said aboutthis later.

The use of long connectingcables will attenuate the signalbut have little effect onfrequency response (cablecapacitance is modestcompared to the self-capacitance of thesemicrophones, which can be ashigh as 30nF).

If an extended frequencyresponse is required frommicrophones of this type, theuse of an external, line-powered, FET buffer, as builtinto electret microphones, is


g

d

s

VR12M2

C1

0V

+5V

R1

SCREENED CABLE

2N3819HIGH IMPEDANCEMICROPHONE

MICROPHONE CASE

R1, C1 AND VR1 ARE LOCATED ONTHE PREAMPLIFIER P.C.B.

1k10µ

Fig.5. The line-powered buffer stage built intoelectret microphones can also be used for ce-ramic and crystal types. (Most FETs will func-tion in this circuit with the source grounded,eliminating the need for the source resistor

and bypass capacitor.)

Layout of components on the completed circuit board. The“Signal Strength Meter” components, except the meter, have

been included on the board (bottom right).




recommended. This will alsoprevent signal losses when longcables are used.

A suitable circuit diagram isgiven in Fig.5 and the circuitcan be built inside themicrophone case. When thisarrangement is adopted, resistorR1 must, of course, beconnected to the supply rail,and the signal must be fed toInput 1.RADIO RECEIVERS

Audio derived AGC is oftenincorporated into directconversion radio receivers.Even simple superhets canbenefit from this form of control(sometimes a conventional RFderived system is not veryeffective when amateur single-side-band signals are beingprocessed).

Directconversionandregenerativereceivers willrequire asingletransistoraudioamplifier,after theproductdetector orregenerativedetector, inorder to

ensure sufficient signal voltagefor the SSM2166P. The outputfrom the detector stage in mostsuperhets will be more thanadequate.

Radio receivers should beconnected to Input 1, and thewire link removed. Theorientation of electrolyticcapacitor C1 will usually becorrect when the receiver has anegative ground or 0V rail.

However, some diodedetectors in superhets areconfigured to provide an outputwhich is negative going withrespect to ground (to suit thereceiver’s AGC circuit). Thepolarity of C1 will need to bereversed when equipment ofthis kind is connected.

SIGNALPROCESSING

The preamplifier’sfrequency response isreasonably flat from below100Hz to more than 20kHz.Speech clarity, especially undernoisy conditions, can beimproved by “rolling off”frequencies below 300Hz andabove 3000Hz, and active orpassive band-pass filters areoften used for this purpose.

A big improvement can,however, be made by modifyingsome of the coupling andbypass components in thepreamplifier. Constructorswishing to limit the frequencyresponse in this way shouldreduce the value of capacitorC9, to 47nF (a Mylar or ceramiccapacitor is then suitable). Thiswill attenuate the lowerfrequencies.

Wiring a 220nF ceramiccapacitor across preset VR1 willattenuate the higherfrequencies. Although extremelysimple, these measures arequite effective.

www.uspy.com

www.bardwells.co.uk

UNTIL recently, the author’s house-hold hi-fi system had a piece ofscreened cable hanging down the

back. This was left connected to the ampli-fier’s high-level (auxiliary) input. Whensome piece of experimental audio equip-ment needed to be tested, the cable couldbe retrieved and connected to the circuit. Itwas then possible to listen to the result.

This method was far from satisfactory,

so a small battery-operated “bench’’amplifier was designed for such purposes.As well as having an in-built loudspeaker,it has the facility for connecting personalstereo type headphones or an externalloudspeaker. Also, it will accept both high-level and low-level input devices.

Magnetic record player cartridges anddynamic microphones provide a low-leveloutput while the “line output’’ socket fittedto many pieces of consumer equipment(such as CD players and video recorders)provide a high level.

Many readers will, no doubt, wish toconstruct the amplifier for experimentalpurposes. However, it could have a varietyof other applications. Examples include asmall practice amplifier for electronicmusical instruments and as the basis for anintercom, or toys and games.

Being battery-operated it may be set upoutdoors and, with just a microphone(possibly with an extension lead) and apair of headphones connected, it could beused to listen to wildlife.

Readers should note that the amplifierhas been designed to be small and relative-ly inexpensive to construct. Although themaximum output power (one watt approx-imately) and sound quality are perfectlyadequate for the applications suggestedearlier, it is not suitable for critical appli-cations such as serious music listening.

The stated power rating of 1W is applic-

able when the amplifier is connected to an8-ohm load. If a 4-ohm loudspeaker wereto be used, the rating would be some 1·5W.In fact, the subjective difference betweenthe two is not great and battery life isreduced at the higher power. It is thereforerecommended that an 8-ohm loudspeakeris used. A 4-ohm unit had to be used in theprototype for availability reasons.

The low-level input has its own gaincontrol while the overall gain is set usingthe master volume control. This allowsjust about any input source to be connect-ed, including microphones, musical instru-ments and consumer audio equipment.

Although not ideal, the headphonesocket fitted to many pieces of electronicequipment provides a signal which willdrive the high-level amplifier input. Whendoing this, the volume control on theequipment would need to be adjusted toobtain the correct input level. When thiswas tried with a small TV, the sound wasbetter than from the TV itself.

Most of the time, the amplifier willprobably be used in conjunction with the

internal loudspeaker. However, bettersound quality is obtained when usingeither headphones or a good-quality exter-nal loudspeaker. Although the amplifier ismonophonic (that is, not stereo), whenused with headphones, the output isapplied equally to each one. This gives amore comfortable effect than with onlyone headphone operating.

The completed Handy-Amp is shown in

the photograph. For convenience, therotary controls and all sockets and switch-es are mounted on the front panel. Theseare a jack and phono-type socket for theLow-level and High-level inputs respec-tively, together with the input selectorswitch, low-level Gain and master Volumecontrols, light emitting diode (l.e.d.) indi-cator and on-off switch, headphone jacksocket, external loudspeaker sockets andoutput selector switch. On top, there is amatrix of holes to allow the sound to passout from the internal loudspeaker.

There are several possible batteryarrangements and the one chosen will bedetermined largely by the space availableinside the case. This, in turn, will dependto a great extent on the dimensions of theinternal loudspeaker. Whatever battery isused, it must have a nominal voltage of9V (say, six 1·5V cells connected inseries).

Cells should not have a capacity lessthan alkaline “AA’’ size. Note that a PP3type battery would be totally unsuitable.The prototype unit was powered usingtwo 4·5V alkaline “3LR12’’ batteriestaped together and connected in series.These have around twice the capacity ofalkaline “AA’’ cells.

The standby current requirement of thecircuit depends on the load. In the proto-type, it is 100mA. However, there will bepeaks of several hundred milliamps and,depending on how the amplifier is used(operating time, load and volume), a life ofsome 15 hours may be expected from apack of “AA’’ alkaline cells. This would besufficient for occasional use.

With headphones connected, the stand-by current requirement of the prototypeunit was only 50mA giving a longer bat-tery life. The l.e.d. indicator reminds theuser to switch off the unit after use.

Although battery operation is conve-nient and safe, for long periods of opera-tion the use of a plug-in power supply unitmight be more appropriate. More will besaid about this later.


Approx. CostGuidance Only $29.60 29.69

excluding batts. and case


ResistorsR1 680R2, R3 47k (2 off)R4 to R6 22k (3 off)R7 560


PotentiometersVR1 470k min. panel mounting, lin or log rotary carbonVR2 10k min. panel mounting, log rotary carbonVR3 47k min. preset, vertical carbon

CapacitorsC1, C3, C7 10 radial elect. 63V (3 off)C2 22 radial elect. 63VC4 47 radial elect. 63VC5 22 radial elect. 63VC6, C8 100n polyester, 5mm

pin spacing (2 off)C9, C10 220n polyester, 5mm

pin spacing (2 off)C11 1000 radial elect. 16V

SemiconductorsD1 red l.e.d., 3mmIC1 NE5534AN op.ampIC2 SSM2211 power amplifierIC3 7805 5V 1A voltage regulator

MiscellaneousSK1 6.35mm plastic body mono jack socketSK2 phono jack socket, single-hole fixing (see text)SK3 6·35mm stereo jack socket, plastic bodySK4, SK5 2mm socket or as required (see text) (2 off)S1, S2 s.p.d.t. toggle switch (2 off)S3 s.p.s.t. toggle switchLS1 small 8-ohm loudspeaker, 2W rating minimum

(see text)

Printed circuit board, available from the EPE Online storecode 273; aluminium case, 203mm x 127mm x 51mm; 8-pin d.i.l.socket (2 off); 3mm l.e.d. panel clip; control knob (2 off); alkalineAA-size cells (6 off – see text); holder and connector for cells (oras required).

µ

µµ

µµ

µ

µ

Ω

ΩΩ

Ω

Fig.1. Complete circuit diagram for the Handy-Amp.

The full circuit diagram for the Handy-Amp is shown in Fig.1.The design uses two main integrated circuits (i.c.s), IC1 and IC2,together with voltage regulator IC3.

Battery B1 provides a nominal 9V supply to the regulator whichthen gives a 5V supply for the main circuit. This will be main-tained until the battery voltage falls to some 7V, whereupon theregulated output will fail. Thus, as the battery ages, the supply willremain constant throughout its useful life.

Note that the l.e.d. on-off indicator, D1, is connected in serieswith current-limiting resistor R7 directly across the battery supply– that is, it is not subject to the effect of the regulator. It will beobvious when the batteries need to be replaced because the ampli-fier output will become weak and distorted and the l.e.d. willbecome dimmer.

Capacitors C9 and C10 promote stability of the regulator.Capacitor C11 charges up from the battery and can then maintain thesupply on the output current peaks when the amplifier is deliveringmaximum power. This helps to provide a distortion-free output.

If using a plug-in power supply unit, C11 will provide addition-al smoothing if a poorly-smoothed supply is used. This should notbe necessary with a good-quality unit but will be useful with inex-pensive ones.

When a low-level device such a microphone is connected, viasocket SK1, its output voltage is first boosted using a low-noisepre-amplifier, based on operational amplifier (op.amp), IC1. High-level (line) signals are input via socket SK2, thus bypassing IC1.


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The signal source is selected by switchS1 and, via volume control VR2, passedon to the power amplifier section centredon IC2.

Op.amp IC1 is configured as a voltageamplifier used in inverting mode. Pins 7and 4 are the positive and 0V supplyinputs respectively. Blocking capacitor C1allows the alternating current signal froma source connected to socket SK1 to passvia resistor R1 to the inverting input, atpin 2.

The input impedance is set by the valueof R1 and this will provide a good matchfor dynamic microphones. The op-ampnon-inverting input, pin 3, receives a d.c.voltage equal to one-half that of the sup-ply (nominally 2·5V) due to equal-valueresistors, R2 and R3, which form a poten-tial divider connected across the supply.

The pre-amp gain is set by the ratio of

feedback resistance (R4 plus VR1) to inputresistance, R1. With VR1 set to minimum,this provides a gain of about 32 and at max-imum, rather more than 700 (the fact thatthis is an inverting amplifier and the gainhas a negative sign is of no real conse-quence here and may be disregarded).

This range of gain will suit micro-phones and other low-level input devices.VR1 is the low-level gain control (labelledsimply “Gain’’ on the front panel). In use,this will be adjusted to take account of thesensitivity of individual input devices.

The output from IC1 appears at pin 6.

This passes via blocking capacitor C4 tothe “Low level’’ (Low) contact of two-way“Select Source’’ switch, S1. With this set asshown, any low-level signal passes to thecommon contact and hence through thetrack of potentiometer VR2 to the 0V line.

The sliding contact (wiper) of VR2draws off the required fraction of the signalvoltage and passes it, via capacitor C5 andresistor R5, to the input of the power ampli-fier (IC2 pin 4). If switch S1 is set to thealternative position (High), the output fromIC1 is disconnected from VR2 but now anysignal applied to the high-level input sock-et, SK2, is directed through VR2 instead.VR2 is the master volume control (labelled“Vol’’ on the front panel).

Power amplifier IC2 is an interesting

device and a block diagram showing itssimplified details is given in Fig.2.Basically, it consists of two operationalamplifiers, A and B. The output of op.ampA (pin 5) provides one of the outputs (Out1). However, it also feeds the invertinginput of op.amp B (via the 50k inputresistor) whose output (pin 8) becomesOut 2. The loudspeaker is connecteddirectly between Out 1 and Out 2.

Op.amp A is configured as an invertingamplifier. Thus, the signal appearing atOut 1 is an amplified and inverted versionof that at the input, pin 4. Referring backto Fig.1, its gain is set by the value ofexternal fixed resistor R6 and preset VR3connected between Out 1 and input pin 4.

The non-inverting input (pin 3) is con-nected externally to pin 2 which sets it at a

d.c. voltage equal to one-half that of thesupply. This is due to the potential dividerconsisting of two internal 50k resistorsconnected between supply positive (pin 6)and 0V (pin 7). Pins 2 and 3 are then con-nected to one end of the external bypasscapacitor, C6, with the other end connectedto the 0V line. This may be compared withthe biassing arrangement used for IC1.

Op.amp B is also configured as aninverting amplifier and because the inter-nal input and feedback resistors haveequal values (50k), the gain is set atminus one. Thus, any signal appearing atOut 2 is an inverted copy of that at Out 1.In this way, the input signal at pin 4 has anamplified but inverted copy of itself at pin

5 and a “straight’’ copy of itself at pin 8amplified by the same amount. This isknown as a bridge output configuration.Correct working depends on the twoop.amp sections being exactly balancedbut, of course, this is not easy to achieveprecisely.

In theory, when no input signal is pre-sent, Out 1 and Out 2 will be at the samevoltage. No current will then flow in aloudspeaker connected between them.When a signal is present, either Out 1 willdrive current through the loudspeakerwinding, which then sinks into Out 2, orOut 2 will drive current through the loud-speaker in the opposite direction and sinkinto Out 1. This will then reproduce thepositive and negative excursions of thea.c. waveform presented to the input.

In practice, there will be a small voltagedifference between the outputs in theabsence of an input signal. A small stand-ing current will then flow through theloudspeaker coil and the lower its imped-ance, the greater this current will be. Thisis added to the small current needed by thei.c. itself (for the working of op.amps Aand B, and for the current drain throughthe internal potential divider). The overallcurrent requirement is therefore somewhatload dependent.

The SSM2211 amplifier used as IC2

has a shutdown feature. Thus, if pin 1 ismade high, the i.c. is put into“sleep’’mode and requires very little cur-rent. However, this feature is not used hereand is disabled by connecting pin 1 to the0V line along with pin 7.

The gain of IC2 is calculated by theratio of feedback resistance (VR3 plus

Fig.2. Block diagram of the SSM2211power amplifier.

Fig.3. Handy-Amp component layout and full size copper foil track master pattern.

R6) to input resistance (R5) multiplied bytwo. This “multiplied by two’’ aspectcomes about because of the bridged outputconfiguration giving twice the voltageswing to the loudspeaker compared with asingle op.amp.

With VR3 set to minimum resistancethe gain is therefore two, and at maximumresistance is just over six. VR3 is a presetpotentiometer which allows adjustment forthe desired gain.

With switch S2 (“Select Output’’) in the

position shown in Fig.1, the internal loud-speaker is connected between Out 1 andOut 2. With the switch in the alternativeposition, the output is directed to both the“Phones’’ socket, SK3, and the externalloudspeaker sockets (SK4 and SK5).

It is thought unlikely that anyone wouldwish to connect an external speaker and apair of headphones to the amplifier at thesame time. However, even if they did, theload would not fall below the minimumimpedance providing an 8-ohm loud-speaker was used.

When using headphones, a greatlyreduced power is available to them com-pared with a loudspeaker. This is becausethe (usually) higher impedance allows lesscurrent to flow. The impedance of typicalpersonal stereo type headphones is about30 ohms for each unit.

In this design, the left and right units areconnected in parallel giving a combinedimpedance of some 15 ohms. However,because headphones provide acousticenergy direct to the ears, only a very smallamount of power is needed for them tosound with acceptable loudness.

A metal case should be used as an

enclosure for the Handy-Amp. A vinyl-effect aluminium box was used for theprototype unit because it gave a goodappearance. Do not use a plastic box sincethis will not provide any screening andhum pick-up might be a problem.

Construction is based on a single-sidedprinted circuit board (p.c.b.). The topsidecomponent layout and full size undersidecopper track foil master are shown inFig.3. This board is available from theEPE PCB Service, code 273.

Most of the components are mounted onthe p.c.b. although there are quite a fewoff-board parts which will be hard-wired

to one another and to various points on thep.c.b. later.

Begin by drilling the fixing hole in thep.c.b. then solder the sockets for IC1 andIC2 in position (but do not insert the i.c.sthemselves at this stage). Follow with allfixed resistors and capacitors. Note thatthe resistors are mounted vertically.

There are seven electrolytic capacitorsand it is important to solder all of thesewith the correct polarity. The negative (–)end is clearly marked on the body and thecorresponding lead is slightly shorter thanthe positive (+) one. Solder preset VR3 inplace but not panel potentiometers VR1and VR2 yet.

Fit the control knobs to VR1 and VR2.Measure how much of each spindle needsto be cut off then remove the knobs again.Hold the end of the spindle (not the poten-tiometer body or it could be damaged) in avice and cut off the required length using asmall hacksaw. Smooth the cut edgesusing a file and check that the knobs fitcorrectly.

Cut off the panel-location tags fitted tomost potentiometers. If these are left inplace, the bodies will not seat flat againstthe front panel when the p.c.b. is in posi-tion. The potentiometers should now besoldered to the p.c.b.

Identify the l.e.d. end leads. The cathode

(k) is usually shorter than the anode (a) lead.Also the body has a small “flat’’ to denotethe cathode end. Solder the leads to the“D1’’ pads on the p.c.b. observing the cor-rect polarity. Bend them through right-angles, as shown in the photograph, so thatthe body ends up in line with the centre ofthe potentiometer spindles and standing outto about the centre of the bushes.

Solder pieces of light-duty strandedconnecting wire to the following points onthe p.c.b.: “Low-Level Input’’, “S1’’ (Land C), “Out 1’’ and “Out 2’’. Using dif-ferent colours of wire will help to preventerrors when connecting them up.

Solder the red and black battery connec-tor wires to the “+9V’’ and “0V’’ pointsrespectively on the p.c.b. (or use pieces ofsimilarly-coloured stranded wire if sol-dered connections are needed to the batter-ies). Adjust the wiper of preset VR3 toapproximately mid-track position.

Solder regulator IC3 in position notingthat the back is towards the centre of thep.c.b. (the part that protrudes is towardsthe edge).

Note that the specified regulator has acurrent rating of 1A. Although the averagerequirement of the circuit is much smallerthan this, in use there are peaks of severalhundred milliamps and this regulator willcope well.

Due to the low average current, only asmall heat sink is needed. In the prototype,this consisted of a piece of sheet alumini-um size 50mm × 15mm bent through rightangles as shown in the photograph. It wasdrilled with a small hole and attachedsecurely to the back of IC3.

When choosing the internal loudspeakeralso take into account the size of the bat-tery pack to be used. The loudspeaker usedin the prototype was a 90mm × 50mmelliptical type as used in small radios andTV receivers. Make sure that the powerrating is sufficient because many smallloudspeakers are inadequate in thisrespect. Do not use one having a rating ofless than 2W.

Decide on a position for the p.c.b. andbattery pack by arranging them on the bot-tom of the box. Consider also the loud-speaker which will be mounted on the lidsection and the various off-board socketsand switches.

When the p.c.b. is finally attached, theremust be several millimetres of clearancebetween the copper tracks and the bottomof the box. This will avoid any possibilityof short-circuits. Take care that the heatsink does not touch wiring or other electri-cal connections.

Mark the positions of the holes for the

mounting bushes of potentiometers VR1and VR2, also for the l.e.d. mounting clip.Drill these through and, gently bending thel.e.d. leads out of the way for the moment,secure the p.c.b. to the case using thepotentiometer fixing nuts. Place washers(or spare fixing nuts) on the bushes on theinside of the case so that only a smallamount of each bush protrudes through itshole. Mark through the p.c.b. fixing holethen remove the board again.

Mark the positions of the switches, thelow-level input jack socket (mono 6·35mmtype), the phono socket, the headphones


Close-up detail of heatsink mountedon IC3.

output jack socket (stereo 6·35mm – seeImportant Note) and external loudspeakersockets. In the prototype, 2mm socketswere used for the loudspeaker, but the typeused will depend on personal require-ments. Drill the holes and mount the sock-ets, switches and l.e.d. clip.

The case itself is connected to 0V

(earth). It is not acceptable for the head-phone stereo output socket, SK3, to be ofa type where any of its contacts touch thecase. If they were to, a short-circuit wouldbe formed and this could damage IC2.This precludes using the ordinary metalsleeved type of 3·5mm jack socketbecause, when mounted in position, itsouter (sleeve) connection would makecontact with the metalwork.

There are various ways to avoid this.One method would be to use an insulatingsleeve and insulating washers on a stan-dard 3·5mm unit. However, the methodused in the prototype was to use a 6.35mmplastic body stereo jack socket. This hadall its connections isolated from the case.Headphones are then connected to it via a6·35mm to 3·5mm converter.

Mount the socket and check, using ameter, which tag is which and that none ofits tags make contact with the case.

The mono jack socket, SK1, used forthe low-level input, unlike the headphoneoutput socket must have its sleeve con-nected to 0V (earth). Since this socket willprobably have a plastic body, it will not bedone automatically and the sleeve connec-tion will need to be hard-wired to a soldertag attached to the case.

The phono socket (SK2 – high-levelinput) must also have its sleeve connectedto 0V. If using the specified single-hole fix-ing type, this will be done automatically.Note that this socket usually has a solder

tag on its bush and this may be used for theSK1 earth connection. If the phono socketis of a fully-insulated type, you will need tomake a connection between the sleeve tagand the case using a solder tag (which willalso be used for SK1).

Referring to Fig.4, carry out all the

internal wiring using light-duty strandedconnecting wire. By using differentcolours, you will avoid errors (rainbowribbon cable is ideal). Note that the twonon-sleeve (tip) tags of the headphonesocket are joined together so that bothheadphone units are connected in parallel.

Remember to leave all wires intercon-necting the various points on the p.c.b.with off-board components long enough toenable the p.c.b. to be removed withoutstraining them, should this ever becomenecessary. Also, the loudspeaker wires

should be sufficiently long to allow the lidof the box to be removed without strainingthem.

Place the loudspeaker in position andmark the fixing holes on the lid of the case.Take care to avoid the p.c.b. (especially theheat sink on IC3) and battery pack posi-tions.

Mark out the holes which are neededto allow the sound to pass through. Drillthese using a small (say, 1·5mm) drillthen increase the diameter to 5mmapproximately. Work carefully becausethe appearance of the finished projectwill be spoilt if the holes are drilledcarelessly.

Carefully clean away any metal parti-cles then attach the loudspeaker. Solderthe wires to its tags and apply some strainrelief so that they cannot pull free whenremoving the lid of the case. In the proto-type, this was done using a solder tag


Fig.4. Off-board component connection details.

having a long “tail’’. This was attached toone of the loudspeaker fixings.

The wires were protected using a shortpiece of sleeving and the tail of the soldertag was gripped gently around them. Takecare that the wires are not so tightly heldthat a short-circuit is produced.

Attach the p.c.b., making sure that it isparallel with the base of the box.Measure the clearance between the cop-per track side and the bottom of the case.Cut a plastic stand-off insulator to thesame length. Slide it into position andsecure the p.c.b. using a thin nylon nutand bolt.

Gently bending the leads as necessary,push the l.e.d. into its clip. Attach the con-trol knobs to the potentiometer spindles. Ifthe knobs have a white line or spot, thisshould be arranged to be verticallyupwards when the control is at its half-wayposition.

Fit self-adhesive plastic feet to the bot-

tom of the case to protect the work surface.Attach the battery pack using a smallbracket or adhesive fixing pads (stickyVelcro pads were used in the prototype).Do not connect the battery yet.

Immediately before unpacking and han-dling IC1 and IC2, touch a metal objectwhich is earthed (such as a water tap). Thiswill remove any static charge which mightexist on the body. This is a wise precautionbecause the i.c.s are static-sensitive andcould be damaged by such charge. Insertthem into their sockets with the correctorientation.

Place the lid of the case in position butdo not actually attach it. Make a finalcheck that nothing is obstructed and,especially, that the heat sink on IC3 iscompletely free of all wiring and internalcomponents.

Make sure switch S3 is off. Beforeconnecting the battery, make certain thepolarity is correct. The circuit will bedamaged if the polarity is incorrect.Make certain the positive battery connec-tion cannot make contact with the case orthe battery will be short-circuited. Thiscould result in damage to p.c.b. tracks.Switch on S3 and check that the l.e.d.indicator lights up.

Begin testing by using the amplifier

with a high-level input source, such as theline output of a CD player, cassette deck orthe audio output from a video recorder; ifthis is stereo, use only one channel.Connect it to the phono socket using apiece of mono screened lead with suitableconnectors at each end. Set switch S1 to“High’’ and S2 to “Internal Speaker’’.Turn both VR1 and VR2 fully anti-clock-wise.

Switch on the input device and slowlyincrease VR2. The sound should be heardclearly. Adjust preset potentiometer VR3so that the sound is undistorted when VR2is at maximum. You will find that the set-ting is not particularly critical. Almostmaximum resistance was correct for theprototype (that is, the sliding contactalmost fully clockwise when viewed fromthe right-hand edge of the p.c.b.).

Leave the system operating for aboutten minutes then switch off and check thatthe regulator heat sink is not excessively

hot. If it is uncomfortable to touch,increase its area. When satisfied on thispoint, attach the lid section.

Turn VR2 to minimum again. Switch S1to “Low’’ and connect a dynamic micro-phone to the low-level input jack. IncreaseVR2 to approximately one-third of its totalclockwise rotation then increase VR1slowly while speaking into the micro-phone. The sound should be clearly heard.

If the controls are turned up too far, orthe microphone is placed too close to theunit, acoustic feedback will become evi-dent. This usually manifests itself as a loudsquealing noise from the loudspeaker.

Acoustic feedback is a potential prob-lem with any loudspeaker/microphonesystem. It comes about because soundfrom the loudspeaker re-enters the micro-phone and builds up in a positive feedbackloop. To prevent it, turn down the controls,move the microphone away and/or point itin the opposite direction to the loudspeak-er. Acoustic feedback may be largely elim-inated by using headphones instead of aloudspeaker.

It is unlikely that the low-level gainwill need to be increased. If it is found tobe necessary, decrease the value of resis-tor R1 to 560 ohms or even 470 ohms.Note that excessive gain leads toinstability.

When connecting an external loud-speaker, always remember to switch offthe amplifier first. This will avoid anypossibility of loudspeaker connectionstouching the case and possibly damagingIC2.

If you wish to use a plug-in power sup-

ply instead of a battery, use a 9V d.c.type having a current rating of 800mAminimum. A fuse and polarity-protectiondiode need to be included (see later) ifdamage to the unit itself or to the circuitare to be avoided.

Attach a power-in type socket to therear of the box to suit the output plug onthe power supply unit. If its sleeve con-nection does not make contact with themetalwork automatically, you will needto hard-wire this to a solder tag attachedto the case.

Preferably, the power supply unitshould have a fixed polarity with the cen-tre (pin) on the output plug being the pos-itive and the sleeve the negative. If thepolarity can be reversed, make sure thatthe pin is made positive. If the polarity isincorrect, the circuit will be damaged. Thisis why a diode should be connected in thepositive feed wire. If the polarity is incor-rect, the diode will not conduct and noth-ing will happen. The fuse protects againstpossible short-circuits.

Referring to Fig.5, sleeve both endwires of a type 1N4001 diode. Solder theanode (non-striped end) to the centre (pin)connection of the socket. Attach a 20mmchassis fuse holder to the bottom of thebox in such a position that the cathode ofthe diode can reach one of its tags. Solderthis in position. A wire from the other fusetag should then be taken to the “+9V’’point on the p.c.b.

Insert a 20mm 1A quick-blow fuse inthe fuse holder. Make sure none of theconnections to the diode or fuse can touchthe case. Use insulation if necessary.

Fig.5. Using a fuse and diode as pro-tection devices when a power supplyadapator is used.


THIS communication system wasoriginally designed to help in theproduction of short commercial

videos. With it, the “director” is able tohold a two-way conversation with any oneof up to three camera operators. It is alsopossible to speak to all the operatorssimultaneously.

No doubt, such a system could findmany other uses, such as in amateur stagework, concerts and sports events etc.

In the prototype arrangement, the direc-

tor sits at a small desk console and theremote operators wear units clipped on totheir belts. Cables, which may be of anyreasonable length, link the remote stationsto the main unit.

The director (“Master”) and remote(“Slave”) operators wear headsets whichare plugged into their units. These headsetsconsist of a pair of headphones (or a singleheadphone) having a small boom micro-phone attached (see photograph).

For the target applications, headsets are

more convenient than loudspeakers. Theyprovide “hands free” operation and allowthe remote operators to move around freely(within the limits set by the interconnect-ing cables). Incoming speech cannot enterany microphone used to pick up the soundof the performance and cannot be heard bythe audience.

Headsets (while worn) are free fromacoustic feedback (the howling noisewhich is produced when the sound from aloudspeaker re-enters a microphone andbuilds up in a loop). The close proximity ofthe microphone to the speaker’s mouthprovides very clear communication evenwhen there is a lot of extraneous sound orwhen he or she only whispers.

Power is supplied using four AA sizealkaline cells housed inside each unit. Thecurrent requirement is 25mA approximate-ly (40mA for the master unit) and the spec-ified batteries should provide at least 50hours of operation. For safety reasons, thesystem MUST NOT be operated using amains-derived supply such as a plug-inadaptor.

The Master unit is built in a sloping

front instrument case (see photograph).The headset is plugged into a pair of sock-ets on the front and sockets on the rearpanel connect the cables leading to theslave units.

On the top, there is an on-off switch andassociated l.e.d. (light-emitting diode) “On”indicator. There is also a three-positionSlave Select rotary switch (S2) whichselects which slave (A, B or C) is to beplaced “on line”, a momentary-action push-button switch which provides the “Talk toAll” function and a Volume control.

Rotary switch S2 has three associatedl.e.d.s (Red, Yellow and Green) which con-firm the slave unit selected. These will befound useful when the unit is being usedunder dim conditions. Note that while the“talk to all” switch (S3) is being operated,only the remote station set by the S2 can beheard.

Each slave unit is built in a small plastic

box having a belt clip attached (see photo-graph). As well as sockets for the headset

and the cable leading to the master unit,there is an on-off switch, l.e.d. “on” indi-cator and volume control.

One particular feature of this circuit is thatthe operator’s voice is heard in his or herown headphones. This practice is used intelephony and helps the speaker to regulatehis or her voice level. It also allows the userto hear someone speaking direct without themuffling effect of the headphones. Theamount of voice feedback may be adjustedfor each station at the setting-up stage. Itmay even be reduced to zero if required.

The basic circuit for the Headset

Communicator is shown in Fig.1 and thisis the same for both Master and Slaveunits. Each unit may be considered as hav-ing one input and one output – the Listen(L) and Talk (T) lines respectively – plus acommon “Earth”.

By linking the talk line of one unit to thelisten line of another and the listen line ofthe first to the talk of the other and alsomaking the common earth connection,two-way communication would be estab-lished. Of course, additional switching is

734 Everyday Practical Electronics, October 2002

The Headset Communicator system units showing (left to right) the master unit,three slave units and a headphone with “boom” mic.

needed in the Master unit to select theslave unit to be communicated with. Thisaspect of operation is looked at later.

Six-volt battery B1, supplies currentthrough On/Off switch S1 and diode D2.The diode provides reverse-polarity pro-tection. Thus, if the supply were to be con-nected in the wrong sense, D1 would failto conduct and no current would flow, thuspreventing damage to semiconductordevices.

Note that a Schottky diode is specifiedfor D2. This introduces a smaller forwardvoltage drop than a conventional diode.

Capacitor C8 provides a reserve of ener-gy and allows peaks of power to be deliv-ered especially when the battery is nearingthe end of its useful life. Light-emittingdiode, D1 is the on indicator and operatesthrough current-limiting resistor R12.

The microphone section of the headset,MIC1, is connected to the circuit via socketSK1. This microphone is of the electret typeand so requires a power supply for its inter-nal preamplifier. This is derived from thenominal 6V supply through resistor R1.

The speech signal is applied, viacapacitor C1 and input resistor R2, to the

at the end of construction to provide asuitable gain for the particular micro-phone used. If tests prove the gain to betoo small, the value of resistor R2 couldbe decreased.

The output signal from IC1a flows, via

capacitors C3 and C9, to the Talk (T) pinof input/output socket SK3. In addition,some of this signal flows through presetpotentiometer VR2. The sliding contactselects a fraction of this and passes it, viacapacitor C4 and resistor R6, to theinverting input (pin 6) of IC1b. The non-inverting input (pin 5) biasing arrange-ments are the same as for IC1a, usingfixed resistors R7 and R8 in conjunctionwith capacitor C5.

A further signal arrives at IC1b invert-ing input from the Listen (L) pin of socketSK3 through capacitor C10 and resistorR9. This has been derived from the “talk”output of the remote unit.

Op.amp section IC1b may be regardedas a mixer for the local and distant signalsand since feedback resistor R10 is equal invalue to input resistors R6 and R9, the gain

Everyday Practical Electronics, October 2002 735

inverting input (pin 2) of operationalamplifier (op.amp) IC1a. This is one halfof a dual unit. The function of the othersection, IC1b will be looked at presently.

The non-inverting input of IC1 (pin 3)

is connected to a nominal 3V referencederived from the potential divider com-prising fixed resistors R3 and R4 work-ing in conjunction with capacitor C2.Since the op.amp is powered from singlesupply rails (+6V and 0V), this proce-dure allows for a “false zero” to be setallowing both the positive and negativehalf-cycles of the input waveform to beamplified.

Fixed resistor R5 and preset VR1 con-nected in series apply negative feedbackbetween IC1 output (pin 1) and the invert-ing input (pin 2). The value of the feed-back resistance divided by that of inputresistor R2, determines the gain.

With preset VR1 at minimum adjust-ment this will be unity and when atmaximum 23. In fact, these values arenegative but this has no practical conse-quence here. Preset VR1 will be adjusted

Rear panel shows the three XLR type sockets for connecting up the Slaveunits.

Completed Slave unit with belt clip attached tothe lid.

Fig.1. Circuit diagram for the Headset Communicator. This is the same for both the Master and each Slave unit.

is unity (actually –1). The level of the local(own voice) signal may be adjusted usingpreset VR2.

The output of IC1b (pin 7) is applied, via

capacitor C6, to the top end of the potentialdivider comprising fixed resistor R11 con-nected in series with panel-mounted poten-tiometer VR3. A fraction of the signal isobtained from the sliding contact andapplied to the input (pin 2) of power ampli-fier IC2.

This device has been designed to allow an8-ohm loudspeaker to be connected betweenits outputs (pin 5 and pin 8) to develop onewatt approximately. Here headphones areused and, since these have a higher imped-ance than a loudspeaker (30 ohms approxi-mately), the available power is reduced.

However, only a small amount of poweris needed to drive the headphones at fullvolume so this method works well. Theheadset volume may be adjusted usingVR3.

The specified power amplifier (typeTDA7052 – having no suffix) does notrequire a connection to pin 4. However,there are variants of this device having asuffix and which have a “d.c. volume con-trol”. If one of these must be used, then pin4 will be used to control its gain.

To match the characteristics of the spec-ified unit, it would be necessary to imposea voltage greater than 1·5V on pin 4 whichsets it to maximum. This could be doneusing a potential divider and more will besaid about this later.

How the Master console is connected to

the slaves is shown in Fig.2. The masterListen and Talk lines are directed to one ofsockets A, B or C using switch S2. Thisswitch is a 4-pole 3-position type.

The talk and listen lines are connectedvia switch S2a and S2b respectively whilethe l.e.d. corresponding to the chosen sock-et receives current via S2c and current-lim-iting resistor, R13. Pole d is not used.

The “All Talk” function (enabling theMaster to speak to all slave units simulta-neously) is provided by connecting themaster talk line to all three sockets. This is


ALL UNITS(Master and Slaves – as required)

ResistorsR1 10kR2, R5 1k (2 off)R3, R4, R6,

R7, R8, R9,R10, R11 47k (8 off)

R12 270Rx 56kRy 22k

(Rx and Ry not needed if IC2 is asspecified – see text)All 0·25W 5% carbon film.

PotentiometersVR1, VR2 22k sub-min. enclosed

preset, vertical (2 off)VR3 10k min. rotary carbon,

log.


(2 off)C2, C5 22 radial elect. 16V

(2 off)C3, C6,

C9, C10 10 radial elect. 16V(4 off)

C7 100n ceramicC8 220 radial elect. 16V

SemiconductorsD1 3mm red l.e.d.D2 1N5817 1A Schottky

rectifier diodeIC1 TL072 dual op.ampIC2 TDA7052 (no suffix)

power amplifier(see text)


MiscellaneousS1 s.p.s.t. rocker or toggle

switch SK1, SK2 3·5mm stereo jack socket

(or as required forheadsets used) – see text regarding head-phone socket (2 off)

B1 6V alkaline battery pack(4 x AA), with holderand connector clip

Printed circuit board available from theEPE PCB Service, code 369; headsethaving electret microphone and an ear-phone or earphones (impedance 30ohms approximately); 8-pin i.c. socket (2off); commercial XLR leads (or home-made leads) – total of 3 required; con-necting wire; small fixings; solder, etc.

ADDITIONS FOR MASTERR13 270 0·25W 5% carbon

filmS2 4-pole 3-way rotary

switchS3 d.p.s.t., momentary

action, push-to-makeswitch

D3 to D5 3mm l.e.d.s, one eachred, yellow, green

Sloping front instrument case with alu-minium top and plastic sides, size 170mmx 143mm x 55/31mm; XLR panel mount-ing socket (3 off); plastic feet; solder tag.

ADDITIONS FOR EACH SLAVEPlastic box size 114mm x 76mm x

38mm; panel mounting XLR plug; beltclips if required; 6V alkaline battery pack(4 x AA) with holder and connector clip.


excl headset, leads, case & batts

Fig.2. How the Master console unit isconnected to the three Slave units.

(Master +one Slave)

EPE Online


carried out using a double-pole momentaryaction switch S3.

In the prototype system, the intercon-

necting leads were of the commercial vari-ety fitted with a 3-pin XLR line plug onone end and a matching line socket on theother. These connectors are widely used inthe industry and are normally used for bal-anced audio applications. Before purchas-ing XLR leads, check that they are of thestandard pattern.

Some cheap cables intended for unbal-anced microphones, have only one innerconductor with the screening connected totwo of the pins. For this circuit, you needtwo available inner conductors plus thescreening. You could, of course, use home-made leads constructed using two-corescreened wire and stereo-type jack (orXLR) connectors.

Construction of the Headset Communi-

cator is based on four identical single-sidedprinted circuit boards (p.c.b.s). This, ofcourse, assumes that three slaves arerequired. These boards are available fromthe EPE PCB Service, code 369.

The p.c.b. topside component layout andfull-size underside copper foil master pat-tern are shown in Fig.3. Begin constructionof each p.c.b. by drilling the two fixingholes as indicated.

Next, solder the i.c. sockets in position,also the link wire connecting IC2 pin 2with Volume control VR3 sliding contact,all resistors (including the presets) and thecapacitors. Apart from C7, the capacitorsare all electrolytics so take care with theirorientation. Note that there are four holeswhich will have been left empty – see later.

Now solder pieces of stranded connect-ing wire to the talk (T), listen (L) and earth(E) points on the completed p.c.b. Connectsimilar pieces of wire to the MIC1 andVR3 positions. Use different colours toavoid errors later. Adjust presets VR1 andVR2 to approximately mid-track position.

It is advisable to check the operation

of each circuit board at this stagebecause it is then much easier to correct

minor problems. Solder the battery con-nectors to the +6V and 0V p.c.b. pads,taking care over the polarity (red wire for+6V).

Solder jack sockets (or the required typeto match the headset) to the MIC1 andPhones wires. Note that the sleeve of themicrophone plug must connect to right-hand MIC1 wire on the p.c.b. – that is, theone connected to the 0V line. In the proto-type unit, the microphone plug was a3·5mm stereo jack type but either “tip”connection could be used because theywere connected together internally.

The prototype headphones were alsowired to a 3·5mm stereo jack plug. In thiscase, each tip connection was responsiblefor one unit while the “sleeve” wascommon to both. This enables the head-phones to be used individually for stereoapplications.

Here, both tips need to be connectedtogether so that the units appear in paralleland provide mono operation. The commontips connect to one wire and the sleeve tothe other. This procedure may need to bemodified depending on the plugs fitted tothe headsets.

Referring to Fig.5,the Slave unit wiring,solder potentiometerVR3 tags to its wiresin the sense shown.Adjust it to approxi-mately mid-trackposition.

Insert the i.c.s intotheir sockets. Sincethese are CMOSdevices, they couldbe damaged by staticcharge which mayhave accumulated onthe body. To avoidpossible problems,touch somethingwhich is earthed(such as a metalwater tap), beforeunpacking them andhandling the pins. Donot throw away thepackaging because itwill be needed againlater.

Do not put the headset on initially incase of sudden loud clicks and other nois-es. Satisfy yourself on this point beforeputting it on.

Connect the battery and note that the Onl.e.d. operates. If acoustic feedback is evi-dent (which should not occur when theheadphones are worn) adjust Volume con-trol VR3.

Listen to the headphones and speak intothe microphone. If you can hear your voiceclearly, the circuit is working. If it is obvi-ous that the microphone gain is too small(quiet sound even with VR1/VR2/VR3 setto maximum) reduce the value of resistorR2 to 560 ohms (after switching off andremoving the i.c.s).

Repeat all this with the other circuitboards then, observing the anti-static pre-cautions mentioned earlier, remove the i.c.sfrom their sockets and replace them in theiranti-static packaging. De-solder the jacksockets, potentiometer and positive batteryconnector lead. Connect a piece of strand-ed wire to the +6V p.c.b. point instead.

Completed prototype circuit board.

Fig.3. Printed circuit board componentlayout and full-size copper foil master.


The sloping front aluminium instrument

case used for the prototype Master unitgives a professional appearance, seephotographs. There is an advantage inusing a box that is of part plastic construc-tion. This is because a case made entirelyof metal will need additional insulation onthe Phones output socket.

Find the best positions for the switches,panel potentiometer, l.e.d. indicators andsockets. The headset socket should belocated on a plastic part if possible.

Decide whether commercial XLR leadsare to be used or whether leads are going tobe made up so that the appropriate connec-tors may be chosen. In the prototype, XLRsockets were used in the master with amatching plug on each slave unit. Drillholes for all these parts.

Mark out and drill the holes for mount-ing the p.c.b., battery holder and anyremaining parts, including one for thesolder tag (in a metal part). Drill smallholes to correspond with the anti-rotationtabs on the rotary switch and

potentiometer. This prevents their bodiespossibly turning in service and breakingoff soldered connections.

Attach all internal components and,

referring to Fig.4, complete the interwiringto off-board components. Note how resis-tor R13 is connected. Apply some sleevingto the joints at the l.e.d. leads and any barewires to prevent short circuits. Using amultitester, check that the solder tag makesgood contact with the metal part of thecase. The wires connected to it should betwisted together and hooked through thehole before soldering.

Note that neither Phones socket connec-tion may make contact with 0V (earth) –that is, any metal part of the case. If, as inthe prototype unit, the socket is mountedon a plastic part, there will be no problem.

If the socket must be mounted on metal,the best approach would be to use a fully-insulated jack socket. Unfortunately, mosttypes make automatic connection of thesleeve to the case.

If necessary, you will need to make aninsulating sleeve (or a shouldered plasticbush) and use plastic washers to isolate itfrom the metalwork. Use a multitester tocheck that the sleeve does not make electri-cal contact with “earth” before proceeding.

Take care to wire up the Listen/Talkselector and the Talk to All switchescorrectly. The pole lettering and contact tagnumbering (see inset dia.) is as shown onmost switches of this type.

If using XLR connectors, pin 1 should beconnected to Earth (0V) along with the soldertag which connects to the metal body. In theprototype, pin 2 and pin 3 are used for theTalk and Listen connections respectively.

All the wires connected to these socketswill need strain relief. In the prototype, thiswas done by means of a cable tie passedthrough slots in the bottom of the case.This will help in preventing the wires frombreaking free in service.

The microphone input socket may be

mounted on a metal part because its sleeve


Fig.4. Interwiring from the Master circuitboard to off-board components.

must be connected to earth (0V). However,it will probably be mounted next to thephones socket for cosmetic reasons. If it ison plastic, you will need to hard wire itssleeve connection to the solder tag.

Note the sense of the wiring to theVolume control (VR3) potentiometer tags.This gives conventional operation – clock-wise rotation increasing the volume.

Note also that only one current-limitingresistor, R13, is needed for the slave indi-cator l.e.d.s. This is because only one l.e.d.can be illuminated at a time.

Choose plastic boxes of appropriate size

for the Slave units and fit the belt clips ifrequired. Check the layout of internal parts

and drill holes for them. Do not forget thesmall hole needed for the Volume controlpotentiometer anti-rotation tab.

Attach all slave parts and, referring toFig. 5, complete the internal wiring leavingplenty of slack in the wires. Note that cer-tain connections will be close together somake sure they do not touch and cause ashort-circuit. Use additional insulation asnecessary.

Check that the connections to the plugpins allow the interconnecting lead to makethe appropriate connections (Talk to distantListen and Listen to distant Talk). In theprototype, pin 2 was used for listen and pin3 for the talk. Connect pin 1 to the soldertag on the plug that connects to the metalbody. Take care over the sense of the

connections to thepotentiometer tags.

Attach the controlknobs to the spindlesof the switches andpotentiometers in allunits. Leave the lidsremoved from thecases for the momentto allow presets VR1to be adjusted.Observing the usualanti-static precau-tions, insert all thei.c.s into their sock-ets taking care overthe orientation.

Begin final

checking with all

the units switched off. Fit the batteriesthen plug in the interconnecting leads andheadsets, with integral microphone“booms”. Turn all the Volume controls tominimum and switch the units on.

The l.e.d. On indicators should operate.The headphones should be listened to withcaution in case the Volume controls havebeen wired in the wrong sense and a sud-den loud noise develops.

Test the operation between the Masterand each Slave unit. Preset VR1 should beadjusted in each unit so that the maximumvolume set by VR3 is not too great and thatthere are no signs of instability. Adjust pre-set VR2 in each unit for the preferreddegree of voice feedback. Check the “talkto all” function.

When satisfied, attach the lids of thecases and label the controls. You will knowwhen the batteries need to be replacedbecause the sound will become weak ordistorted and the l.e.d.s will glow lessbrightly.

In use, always start with the volumeturned down to minimum and switch on allunits before wearing the headsets. This willavoid any loud clicks.

If it is impossible to obtain the speci-

fied power amplifier (i.e. a TDA7052without a suffix letter) and you must useone having a “d.c. volume control”, itsgain will need to be configured to maxi-mum to match the characteristics of thespecified unit. This may be done by sol-dering resistors Rx and Ry in the unusedpositions on the p.c.b. Resistor Rx will bein the upper position which connects toIC2 pin 1 and Ry to the lower positionwhich connects to IC2 pin 4.

Resistor Ry may need a 1µF capacitorconnected in parallel with it. This could beplaced on the underside of the p.c.b. Notethat this set-up has not been tested andsome experimentation may be needed toobtain correct operation.


Fig.5.Interwiring detailsfor one Slave unit.

Packing the components into theSlave unit.

General layout of components on the Master unit metalfront panel.

LAST month we examined the basicprinciples which allow CMOS invert-ers to be used as oscillators, conclud-

ing with an example of a Colpitts oscillator.We conclude this two-part series by firstexamining a ccrystal oscillator circuit.

The high frequency crystals used to set

the clock frequency in computers canreplace L in the Colpitts circuit of Fig.10.The circuit is then sometimes called aPierce oscillator (Fig.11), although thisnomenclature is dubious.

Since a crystal blocks d.c., a resistance(R1) must be added to allow d.c. negativefeedback to set the working point. Thisresistance should be high enough not toimpair the oscillation.

Crystal manufacturers specify the valueof shunt capacitance needed to trim thefrequency to its nominal value. In the pi-net-work, the two capacitances are effectively inseries so each should be twice the quotedshunt capacitance. The frequency can be finetuned by adjusting one or both of them.

It is possible that oscillation may be tooviolent. A feedback control (VR1) mayalso be used as with the Colpitts oscillator.Crystal manufacturers may specify a safeoperating voltage and VR1 can be set to

ensure that it is not exceeded. Generallyspeaking, it is sufficient to set VR1 so thatreliable oscillation (in the face of fallingsupply voltage, etc.) is just feasible.

For crystals designed to generate fre-quencies below about 1MHz, or aboveabout 10MHz, special circuit arrangementsmay be needed. Consult the manufactur-er’s data sheet.

The need for transformers or twin

capacitors can be avoided by using a so-called two-terminal oscillator circuit. Thismeans that the frequency-determining LCcircuit can be connected by just two leads,those marked X in Fig.12.

With R1 = R2, A2 has a gain close toone, so it is just a voltage inverter. Then A1must provide the gain needed for oscilla-tion. The critical condition is that VR1should be just less than the effective resis-tance of the LC circuit at its resonantfrequency fo.

The effective resistance is called thedynamic resistance and is Q times thereactance of L or C at fo. For a usable coilthe Q “quality factor” is unlikely to be lessthan five, and may be several hundred.

Good sine waves are obtainable at theLC circuit when VR1 is considerably lessthan the critical value, but to get a purewaveform at A2 output, VR1 must be setso that the circuit just oscillates. It may besimpler to pick off a sine wave output atA1 and extract it via buffer A3. This has again of R4/R3. The circuit may be used upto about 1MHz.

If VR1 is calibrated it can be used toobtain a reasonably accurate indication of thedynamic resistance of the LC circuit. Simplyadjust VR1 to the maximum value for oscil-lation. Then VR1 is the dynamic resistance.From this the Q can be calculated:

Q = dynamic resistance / reactance of Lor C at fo

This circuit has overall d.c. positivefeedback. It would latch up if the d.c. gainof A1 exceeded one. Fortunately, the lowd.c. resistance of L keeps gain well belowone, so it is d.c. stable.

Resistors R1 and R2 set the gain of A2to unity (–1). Driving A2 directly wouldcause over-violent oscillation, The ratioR2/R1 could be increased to up the loopgain but this is not necessary with typicalLC values.

In A3, R3 and R4 set the gain and work-ing point and R3 also provides somebuffering. With VR1 set correctly there isno protection-diode conduction. Thisimplies a VR1 of slightly less than thedynamic resistance 2fLQ or Q/(2fC).However, VR1 can be less than optimumwithout seriously impairing the sine waveat the LC.

The reactive (RC) arms of a Wien bridge

(Fig.13) can be used to set the frequency ofa sine wave oscillator formed around anop.amp (Fig.14). In a Wien bridge, whenR1 = R2, C1 = C2 (the usual case) balance(zero output) is obtained when V2 = V3, inwhich case C then has a reactance equalto R.

This occurs when the input frequency finis 1/(2CR), usually called fo. Tuning isconveniently effected by using a two-gangpotentiometer for the two controlling resis-tors (R1 and R2) so that they are alwaysequal. In this way balance is maintained asthese resistors are adjusted.


Part Two

Fig.11. Pierce crystal oscillator. Herethe crystal replaces L in the Colpittscircuit.

π

Fig.12. Two-terminal LC oscillator. A2provides the required phase inversion.A3 can be added as a output buffer.

In oscillators, use is made of the factthat RC arms of the bridge form a frequen-cy-selective voltage divider whose outputis greatest at fo. At frequencies away fromfo, output falls. When this network is usedas a positive-feedback path in an amplifier(Fig.14) and the gain is just sufficient foroscillation, a sine wave at fo is generated.

Unfortunately, the Wien network is onlyvery weakly frequency-selective. It does apoor job of discriminating against harmon-ics produced by the amplifier overloading.The waveform is distorted.

A solution used in commercial Wienoscillators for audio work is to provide a dis-tortionless means of automatically restrict-ing gain to be just sufficient for oscillation.Very pure sine waves can then be obtained.A common method is to use a negative tem-perature coefficient (n.t.c.) thermistor for theR3 resistance. As oscillation builds up thesignal warms the thermistor whose resis-tance falls. This increases the negative feed-back to the inverting input terminal, damp-ing down the oscillation.

The standard circuit (Fig.14) does nottranslate into inverter-oscillator formbecause an inverter has only one input ter-minal. It can, however, be adapted to a 2-inverter circuit, as illustrated in Fig.15.

Inverters A1 and A2 are used in their“linear” mode and the parallel-RC armnow creates negative feedback to A1 whilethe series RC arm conveys positive feed-back from A2 to A1. The circuit oscillatesat fo when the gain of A2 (adjusted byVR2) slightly exceeds two. An extra preset

scales which are very cramped at the high-frequency end. Frequency sweeps(max./min.) of 10 are then a practical limit,though the circuit will oscillate over awider sweep.

The circuit can be used as a selectiveamplifier with input injected via a high-impedance buffer A3. In this case VR2 is asharpness control and for greatest selectiv-ity is set for “just not oscillating”. Thebuffer amplifier may also be used, ifrequired, to inject a frequency-locking sig-nal into the oscillating circuit.

An injected signal of a few mV can syn-chronise the oscillator. How long it stayssynchronised depends on the frequencystability of both the oscillator and the syncinput. Injecting a larger signal increasesthe locking range but at the risk of falselocks where one frequency bears somefractional relation to the other. (Often thewaveform then shows some periodic dis-tortion.)

Multi-band operation is possible byswitching-in different pairs of capacitorsC. For consistent performance each pairmust be very accurately matched.

An inverter with feedback from output

to input via a capacitor (as with A1 andA3 in Fig.17) has a gain which falls offas the frequency is raised. In a sine waveoscillator this reduces the harmonicswhich result from distortion. The abilityto yield good sine waves without specialamplitude control circuitry is especiallyuseful at very low frequencies, whereconventional control using thermistors isdifficult. (The resistance of the controldevice varies over the oscillation cycleand causes distortion.)

An inverter with capacitive feedbackproduces a phase shift. Two inverters, eachgiving a phase shift of 90° in the samedirection, give a total of 180°, which isphase inversion. When cascaded with asimple inverter and connected in a ring, theoverall feedback is positive at the 90° fre-quency. Here this is the frequency forwhich the reactance of C equals R.

An inverter with capacitive feedback isoften referred to as a Miller integrator, orjust an integrator. The frequency generatedby the type of circuit in Fig.17 is the sameas for a Wien network oscillator (fo =0·16/(RC)). With the values shown the


resistance, VR1, hasbeen added. Without itthe circuit wouldcease to oscillate as Ris reduced towardszero. The oscillationfrequency is:

fo = 1/(2C(R + VR1))

In fact, there is ahidden component inthe series arm: this isthe output resistanceof inverter A2 and itmust be compensatedfor by an increasedresistance in the paral-lel arm. If this is notdone, feedback variesas R is adjusted and itis impossible toobtain a good waveform over the tuningrange.

No device for automatic amplitude lim-

iting is shown in Fig.15. The job could bedone by substituting a thermistor for thefeedback resistance across A2 as in Fig.16.VR2 would then provide oscillation leveladjustment and should have a mid-valueequal to the working thermistor resistance.

Unfortunately, there are really no suit-able thermistors available to the averagehobbyist. The sub-miniature bead thermis-tors needed are very expensive. Cheaptypes are physically too bulky and do notheat up enough at the small signal levels inthe circuit.

Vout must drive enough current throughthe thermistor to reduce its resistance suf-ficiently to obtain low distortion. SinceCMOS inverters cannot deliver much cur-rent it is desirable to keep the thermistorresistance fairly high, say 10k. The a.c.voltage across it is unlikely to exceedabout 3V r.m.s. The power available towarm the thermistor is then 0·9mW. Forreliable operation over a range of ambienttemperature this amount of power mustcause a temperature rise of at least 20°C.

If very low distortion is not required, afairly good sine wave can be obtained fromthe circuit as shown in Fig.15 if set-upcarefully, as follows:

Set R to maximum. Set VR2 for “justoscillating”. Set R to minimum (zero).Without altering VR2, set VR1 for “justoscillating”. Repeat this procedure then, ifnecessary, make minor adjustments so asto obtain the best compromise perfor-mance over the tuning range.

The final result will depend on how wellthe two sections of the potentiometer arematched. Linear-law two-gang pots areusually better than log-law, but give tuning

Fig.13. Wien bridge.

π

Fig.14. Wien bridge oscillator using anoperational amplifier.

Fig.16. Using a thermistor in place ofRF in Fig.15.

π

Fig.15. Inverter gate version of Wien oscillator. The A3 sec-tion can be added to inject an external synchronising signal.

range is roughly 300Hz to 3300Hz. Therange can be switched by substituting otherpairs of capacitors, accurately matched.

When R is in megohms and C is inmicrofarads, the frequency is in Hertz(Hz). Because of the good discriminationagainst harmonics it is easier to achieve arespectable sine wave than with the Wienoscillator.

The circuit also has the useful propertyof yielding two equal output voltages (V1and V2) phased 90° apart (“in quadra-ture”). On the other hand setting up toachieve a good performance over the tun-ing band (by adjusting VR1 and VR2)involves using an oscilloscope and doing afair amount of fiddling.

Start with VR1 and VR2 set halfway.Trim VR1 to equalise V1 and V2. TrimVR2 for the best waveform. The tuningrange is somewhat affected by these set-tings. To achieve the best amplitude sta-bility one of the fixed resistances inseries with the tuning resistances mayneed to be trimmed (at the h.f. end of theband).

The three inverters of Fig.18a are con-

nected in a loop or ring. If the input to A1is positive then the output of A3 is nega-tive. Since this is fed back to A1, it oppos-es the positive input. The ring is a negativefeedback loop with total feedback and(accidents barred) it will be stable.Accidents do happen, though, as will beshown later.

Referring to Fig.18b, if we now inter-pose between successive stages networkswhich produce 60° phase shift to signals at

some frequency then, going round theloop, the three phase shifts add up to 180°.This is inversion.

The reactance is twice the resistance forseries C, shunt R, and the reverse for seriesR and shunt C.

The fed-back signal at A1 is now in stepwith the original signal. Feedback is there-fore positive and the circuit oscillates. Ifthe 180° phase shift occurs at only one fre-quency then that will be the frequency ofoscillation.

Two standard ways of achieving phase

shift are shown in Fig.18c to Fig.18d. Thefirst is passive – the required 60° shiftoccurs at the frequency at which theseries arm has twice the impedance of theshunt arm. At that frequency the attenua-tion factor is two (i.e. half the voltage islost). This is likely to be much less than

the gain of an inverter so the circuit oscil-lates strongly.

Unfortunately, the strong oscillationdrives the internal protection diodes intoconduction. The effect is to raise the fre-quency spectacularly but unpredictably. Itwould be possible to add swamping resis-tances but a better alternative is to use thecircuit in Fig.18d. Here the phase shiftingis done by incorporating the RC networkinto an integrator, the amplifier being oneof the inverters. The inverter input terminalis now a virtual earth point and the signallevel there is low enough to avoid the worsteffects of protection-diode conduction. In aring of three such integrators each pro-duces a lagging phase shift of 60°. Theoscillation frequency is theoretically

fo = 0·08/(CR)

As before, fo is in Hertz when CR is inmegohms times microfarads and so on.

If, in circuits using Fig.18c, the resis-

tances and capacitances are reduced to zerothe circuit reverts to that in Fig.18a. Itmight be expected to display a stubbornstability. Far from it! It oscillates, but at ahigh frequency.

The explanation is simple. We mayhave removed our Rs and Cs but the cir-cuit has its own built-in equivalents. R isnow the output resistance of each invert-er and C the input capacitance of thefollowing one.

In a particular case R might be 10kand C might be 10pF. These act like thosein Fig.18c. The 60° frequency is:

fo = 1/(RC) = 3MHz approximately.

Both the output resistance and the inputcapacitance of an inverter are affected bythe operating voltage. The output resis-tance is especially strongly affected.

In experimental tests using a CMOS4069 inverter, biased to operate in the lin-ear region of the input/output curve, theoutput resistance measured 16k whenVCC was 5V, falling to 5k when VCC was15V.

This means that the “zero component”ring of Fig.18a is in reality a voltage-con-trolled oscillator, with VCC as its controlvoltage. Oscillation may be possible atVCC down to 2V, where the frequency isquite low. At high VCC it may be tens ofmegahertz.

Note that there is a real risk, at high VCC,of the current drawn becoming excessiveand overheating the chip. Note also thatwhile standard CMOS i.c.s like the 4069


π

Fig. 17 Dual-integrator oscillator. Oscillation level is set by VR2. The two outputs V1and V2 are equalized by VR1 and are 90° apart in phase.

Fig.18. (a) Three-inverter ring. (b) With added phase-shift circuits. (c), (d)Alternative phase shift networks.

π

Fig.19. Dual-quadrature oscillator. Each twin RC network produces 90° shiftat fo.

are rated to work at up to 15V their modern“equivalents” like the 74HC04 have muchlower maximum VCC ratings.

It is possible to bring down the fre-quency while retaining voltage control.Add real capacitors for C while leavingR at zero.

A ring with three equal phase shifters

(Fig.18b) is a neat means of generating athree-phase signal. But suppose you needsome other number of phases. Any numberover two can be provided, with one precau-tion. The total number of inverters in thering must be odd. If it is even there is over-all d.c. positive feedback and the circuitlatches up.

If you need an even number of phasesyou have to add one plain inverter (with noassociated phase shift components) to keepthe d.c. feedback negative.

One potentially useful arrangement is tohave four shifts of 45° each. This enablesoutputs to be selected at multiples of 45°,notably 90°. The necessary fifth invertercan be used as a gain-adjustable stage to setthe oscillation level. The frequency is thatat which R and C have equal impedances,i.e. fo = 1/2CR.

The loop shift must be 180°. For a 3-sec-tion phase shift the average per section mustbe 60°, for four sections 45°, and so on.

It is also possible to generate outputsphased 90° apart with a 3-inverter ring(Fig.19). Here two pairs of double RC net-works each generate a 90° shift. The fre-quency is about 1/(2RC).

In theory, three or more RC (or CR) net-

works can be cascaded to give an overallphase shift of 180°. A single inverting ampli-fier can then maintain oscillation, see Fig.20.

These circuits are usually referred to as“phase shift oscillators” (though of coursephase shifting is involved in all the oscilla-tors we have just been discussing).

Phase shift oscillators may look neatbut they have two major disadvantageswhich stem from the fact that the secondRC section loads the first, the third loadsthe second and so on. This greatlyincreases the attenuation at fo. For anetwork with three cascaded RC or CR

sections, all withequal R and C, thegain needed to sus-tain oscillation isnearly 30. For a four-section network it isnearly 20. A singleinverter may not pro-vide enough gain.

The second snag isthat it is no longerpossible to pick offoutputs evenlyspaced-out in phase.Also, the voltagediminishes at eachsuccessive section.

A third problem isthat the gain is notreadily adjustable. If,however, one inverterprovides more thanenough gain a reduc-tion can be made byshunting off some ofthe current into a sec-ond inverter (Fig.21),which presents a loadof R1 and can be used as an output buffer.(This trick can be used with other oscilla-tors.)

For a three-section RC network fo =0·39/RC. For a four-section RC network fo= 0·19/RC.

Attenuation can be reduced by “taper-ing” the networks. Successive resistancesare multiplied by a factor N and successivecapacitances divided by N. As N is madevery large the 3-section attenuation factorfalls towards eight and the 4-sectiontowards four. Making N = 10 achievesmost of the improvement and even N = 2 isworthwhile.

The RC network discriminates againstharmonics and even if the input to a multi-section network is a square wave the outputis a fairly pure sine wave. However, itoccurs at a high-impedance point and canonly be used if picked off by a very highimpedance buffer. This adds its own quotaof distortion.

Phase shift oscillators are fascinating

circuits which over their long history

(they go well back into the valve era)have elicited from circuit analysts someformidable feats of mathematics. But ifyou need a low-distortion oscillator youwill be well advised to leave them aloneand stick to Wien or dual-integratorcircuits!

Whilst we have concentrated on the useof basic CMOS inverter gates, the princi-ples can equally well be applied throughthe use of dual-input inverting gates, suchas NAND and NOR.


Fig.20. Phase-shift oscillators. (a) Three-section RC. (b)Four-section RC.

≈

Fig.21. Gain-adjustment circuit. R1acts as a load on A1.

Tel: 01254 830761 Fax: 01254 830408Email: [email protected]: www.magtrix.co.uk

TM

!"#$%& '()*%#+,-../"0)1.).*23*2.+)1.).*2(45.%0

THE two previous Interface articles weredevoted to the use of the MSCOMM

ActiveX control to permit serial communi-cations with Visual BASIC programs. Theadvantage of this method is that it willwork with any 32-bit Windows operatingsystem, including Windows XP withoutthe need for any third-party add-ons.

The main drawbacks are that this con-trol is not included with anything lessthan the Visual BASIC ProfessionalEdition, and it is something less thanstraightforward in use.

MSCOMM and VBASoftware topics usually pro-

duce a certain amount of feed-back from readers, and thepieces on MSCOMM are cer-tainly no exceptions. A fewreaders pointed out that thiscontrol is included withMicrosoft Word and Excel aspart of VBA (Visual BASIC forApplications).

On checking two PCs thathad Microsoft Office installedbut had never been loadedwith Visual BASIC Profes-sional, one had MSCOMMand the other did not. VBA isnot only included withMicrosoft applications, it isalso provided with some soft-ware from Corel, Autodesk,etc. However, VBA is notalways installed when the“Typical” option is chosen dur-ing installation. It is sometimes necessaryto return to the installation disk in orderto add VBA.

The presence or absence of MSCOMMprobably depends on the exact softwareinstalled on the PC. The more upmarketthe software the greater the chances ofsuccess. It would certainly seem to be thecase that it is not included with all ver-sions of Microsoft Office.

It is not difficult to ascertain whetherMSCOMM is present on a PC. LaunchWindows Explorer and then use the searchfacility to scan the hard disk for a file calledMSCOMM.OCX. The MSCOMM ActiveXcontrol is not installed if this file is not pre-sent on the hard drive. If this file is present,it would probably be possible to use it withone of the free versions of Visual BASIC aswell as with VBA.

Same DifferenceVBA is not really intended for produc-

ing normal software, and its usual role isin the production of extra commands forapplications programs. However, “at apinch” it can be pressed into service as ameans of producing software for use withPC based projects.

The first task is to launch VBA fromwithin the host application, and it isnormally accessed via the Tools menu.

With Microsoft Word for example, it islaunched by selecting Macro from theTools menu and choosing Visual BASICEditor from the submenu.

No form is produced when VBA hasfinished loading, but a form can be addedby selecting User Form from the Insertmenu. You then have something likeFig.1, which is similar to the normalarrangement in Visual BASIC.

The next task is to go in search of theMSCOMM control, and the first step is tochoose Additional Controls from the Tools

menu. This brings up a window like theone of Fig.2, and it is then a matter ofscrolling through the list looking forMSCOMM. It will not be called MSCOMMin this list though, it is more likely to becalled “Microsoft Communication Controlversion 6.0” or something similar to this.

Having found the right entry in the list,tick its checkbox and then operate the OKbutton. A yellow telephone icon shouldthen appear in the Toolbox, and thisenables MSCOMM to be added to theform in the usual way.

VB or not VBAlthough VBA seems to be widely

regarded as identical to Visual BASIC,

there are differences. The fact that VBA isnot designed to produce standalone pro-grams enforces a few changes, but thereare differences in the code, such as theexact structure of conditional routines.

Programs written for Visual BASIC willusually require at least a small amount ofrewriting in order to make them workwith VBA. This point is demonstrated inthe first VBA listing (Listing 1), which isfor a simple program that reads singlebytes from a serial port and displays themon a label component.

In addition to MSComm and a form, itrequires two buttons and a label. The cap-tions for buttons one and two(CommandButton1 and CommandBut-ton2) are respectively changed to STARTand EXIT.

Listing 1

Private Sub UserForm_Click()End Sub

Private Sub CommandButton1_Click()MSComm1.PortOpen = FalseEndEnd Sub


INTERFFAACCEERobert Penfold

Adding MSCOMM Active-X control to your PC

Fig.1 (above). TheVisual BASIC forApplications (VBA)set up and readyto use.

Fig.2 (below). AddingMSCOMM, if it isavailable.

Private Sub CommandButton2_Click()MSComm1.RThreshold = 1MSComm1.InputLen = 1MSComm1.Settings = “9600,n,8,1”MSComm1.CommPort = 1MSComm1.InputMode =

comInputModeTextMSComm1.PortOpen = True

End Sub

Private Sub MSComm1_OnComm()If MSComm1.CommEvent = 2 ThenLabel1.Caption = Asc(MSComm1.Input)

End Sub

Operating the START button switcheson communication with the serial port,selects the required port, and sets therequired operating parameters. Thisworks in the same way as the code for theVisual BASIC version described in a pre-vious Interface article.

The routine used for MSComm1 readssingle characters from the port, convertseach one to its ASCII value, and thenwrites that value to Label1. In the originalprogram an If...Then...End If structurewas used to check that the rightOnComm event had occurred. If the rightevent had occurred (i.e. a new byte of

data had been received), the port wasread, the conversion was made, and datawas written to the label.

With VBA the If...Then...End If struc-ture is not quite the same, and the origi-nal routine just causes an error messagewhen used with VBA. In this case the rou-tine can be reduced to a single line ofcode, with no End If statement requiredat the end of the routine. In fact it must beomitted or an error message will beproduced.

The routine for the EXIT button simplycloses communications with the serialport and closes the program. The VBAversion of the program works as well asthe original Visual BASIC version, and itcan be seen working within VBA in Fig.3.

OutputThe second VBA listing is for a simple

serial transmission program. The form isequipped with START and EXIT buttons,as in the serial port reading program. Italso has a label, but this time it is used toshow the value generated by a scrollbar.

The latter is used to generate the valuesthat are transmitted, and its MAX settingshould be set at 255. It will then generateintegers from 0 to 255, or single bytes ofdata in other words.

Listing 2Private Sub CommandButton1_Click()MSComm1.PortOpen = FalseEndEnd Sub

Private Sub CommandButton2_Click()MSComm1.PortOpen = TrueEnd Sub

Private Sub Label1_Click()

End Sub

Private Sub MSComm1_OnComm()End Sub

Private Sub ScrollBar1_Change()MSComm1.Output =

Chr$(ScrollBar1.Value)Label1.Caption = ScrollBar1.ValueEnd Sub

Private Sub UserForm_Click()

End Sub

In this case the VBA program can bemuch the same as its Visual BASIC equiva-lent. It is the routine for the scrollbar thatactually transmits the data, and the newvalue is sent each time that a change occurs.

The Chr$ function is used to convert thevalue from the scrollbar into an equivalentASCII character which is then sent to theserial port for transmission. Theunprocessed value is displayed on the labelcomponent so that the user can see whatvalues are being sent. Again, the VBA pro-gram works as well as the Visual BASICversion, and it is shown running in Fig.4.

Lockout SituationPrograms are saved using the Save

Document option under the Edit menu.Once the document has saved, thisoption changes to Save XXXX whereXXXX is the program name that you

chose. Note that the main Word docu-ment can be empty, and there is no needto add any dummy text. To use the pro-gram on another occasion, load the rele-vant document and go to the VisualBASIC Editor again. This should containthe program.

There can be a problem when tying torun the program, with an error messageappearing. This points out that Macroshave been disabled and that the programcannot be run. Macros are disabled bydefault as a means of reducing the riskfrom macro viruses.

Selecting Macros from the Tools menufollowed by Security from the submenuenables the security setting to bechanged. A dialogue box appears and ithas radio buttons that offer three levels ofsecurity.

The lowest level enables macros to berun with “no questions asked”. You willbe asked whether or not you wish to runthe program if the middle setting is select-ed, and macros are blocked if the highestlevel is used.

If you are used to VBA and its versionof the BASIC dialect, VBA programs canbe a valid approach to producing soft-ware for your PC projects. Even if you donot have MSCOMM on your computersystem, VBA can still be used with thirdparty add-ons such as Inpout32.dll toaccess the serial and parallel ports.

One of the free versions of VisualBASIC probably represents a better

starting point for those starting “fromscratch”. Either way, it is possible to getinto visual programming at no cost.

Binary ModeA couple of readers have pointed out

methods of using MSCOMM in binarymode so that the string conversions canbe avoided. This is a subject that will beconsidered in detail when the problemhas been investigated fully.

Strangely, the Microsoft documenta-tion recommends that the text mode isused for all data transfers usingMSCOMM. A possible reason for this isthat some facilities of MSCOMM seemto disappear when the binary mode isused. The text and conversion method isa bit cumbersome, but it does have thesaving grace that it actually works quitewell.


Fig.3. The serial reader program operating within VBA.

Fig.4. The serial transmission program. Values set on the slidercontrol are transmitted from the serial port.

DO YOU have a collection of old vinylrecords? If so, you might wish totransfer them to CDs. By doing this,

you will preserve their value because youwill only need to play them once.

It may even be possible to enhance thesound by removing some of the back-ground noise and clicks which are foundon worn recordings. If you have a CDplayer in your car or own a portable unit,you will also be able to play your work “onthe move”.

To transfer a recording to CD, you needa computer with a Compact Disc writerinstalled. Many new machines, of course,already have one of these. If yours is not soequipped, you will find that fitting aCD “burner” module is inexpensive andstraightforward.

You do not even need a particularlymodern machine. A Pentium 133MHz PCmay suffice but a new up-to-date machinewill be much quicker (that is, produce aCD at the higher speeds allowed by thewriter). Before purchasing any hardware, itis important to check compatibility withthe supplier/manufacturer.

To record sound files on to the harddrive before transferring them to a CD willrequire quite a lot of spare capacity. If yourdrive is almost full, you will need to backup files in order to clear sufficient space.To record stereo tracks in 16-bit resolutionat 44·1kHz (CD quality) you will needsome 600MB for one hour of work andyou could run into trouble if you do nothave at least 800MB available.

It is not a good idea to link the record

deck to the computer sound card direct byplugging it into the microphone input. Somepeople have done this thinking, quite cor-rectly, that a magnetic cartridge provides alow-level output comparable with that of adynamic microphone. Although this maywork, the results will be very disappointing.This is because no equalisation has beenapplied to the signal. It will be found that the

copy recording is deficient in bass (low fre-quencies) but have excessive treble (highfrequency content). In other words, it willsound very “tinny”. More will be said aboutequalisation presently.

A better method would be to use anexisting hi-fi amplifier. The record deckwould be plugged into its “Phono” inputand a Line (high level) output obtained atthe back (the one used for tape recording).This would be connected to the line inputon the sound card using a piece of twin-screened wire fitted with the appropriateconnectors. The phono connection wouldprovide the necessary equalisation.

Unfortunately, many modern amplifiersmake no provision for playing “old fash-ioned” vinyl discs. You may therefore findthat it has no phono input. Even if you dohave a suitable amplifier, it may need along connecting lead to reach the comput-er station and this could result in hum pick-up and degraded performance.

The circuit described here is a small bat-

tery-operated stereo preamplifier whichprovides equalisation and boosts the output

of a magnetic cartridge to line-level. Thereare also Scratch and Rumble filter push-button switches. These may be used toreduce the effects of surface clicks andlow-frequency motor or turntable noiserespectively.

As well as being useful for making CDs,the preamplifier will be found handy byenthusiasts who simply wish to play theirvinyl records using a hi-fi amplifier thatdoes not have a phono input. Some readersmay even use it for tape or Mini Disc workor for making MP3 files to be sent over theInternet.

In operation, the circuit requires some40mA and the four AA size cells housedinternally will provide up to fifty hours ofservice. A front panel mounted l.e.d. indi-cator requires some 15mA so, if the usercan be trusted to switch the unit off afteruse, the l.e.d. may be omitted. This wouldgive a significant increase in battery life.For extended periods of use, a larger bat-tery could be placed externally.

This unit must not be powered using amains-derived low-voltage supply (suchas a plug-in adaptor).

Returning to the topic of equalisation,this must be applied if analogue recordingsare to be reproduced with any degree of

Everyday Practical Electronics, September 2002 665

fidelity. To understand why this is neces-sary, you need to know something aboutthe recording process.

Imagine the sound has three “bands”comprising the low, intermediate and highfrequency content. When the groove wascut in the master disc, the low frequencypart was reduced in level (volume) whilethe high frequencies were increased. Onlythe intermediate band was left unchanged.

Leaving the low frequencies as theywere in the original sound would haverequired more violent movements of thegroove cutter (that is, heavier modulation).This would have produced a wider grooveand a consequent reduction in availableplaying time. Also, the playing stylusmight have difficulty following such agroove and it may tend to jump out. Byreducing the level of the low-frequencysound, it is possible to obtain a uniformgroove width and a longer playing time.

Equalisation is the process by which thehigh and low frequency content from thecartridge are restored to their original stateand, in theory, should be an exact mirror ofthat used during recording. Note that byrestoring the high frequencies, the surfacenoise present during playback (which ismade up chiefly of high frequencies) isreduced. It thus provides a simple meansof noise reduction.

Unfortunately, different equalisationstandards have existed regarding the val-ues of the cut-off frequencies defining thelow, intermediate and high bands and alsothe degree of “cut” or “boost”. The samecircuit will therefore not provide perfectresults with all records.

However, most vinyl discs producedsince the 60s have followed the RIAA(Recording IndustryAssociation ofAmerica) standard.In practice, anequaliser designedfor this standardwill also providegood results whenapplied to record-ings using a differ-ent one (AmericanStandard Recordand British Micro-groove format). Itshould also be suit-able for 78s.

Practical equali-sation circuits canrange from the sim-ple (which provideonly a coarse cor-rection) to the verycomplex. This circuit lies somewhere nearthe middle of the range and provides goodresults without special adjustment.

The graph shown in Fig.1 illustrates theideal (theoretical) RIAA equalisation com-pared with that provided by this circuit.Note that this is for illustration only and isnot drawn to scale.

The section to the left-hand side labelled“A” provides a “roll-off” of frequenciesbelow some 10Hz. This reduces the “rum-ble” that is transmitted from the motor orturntable bearing to the cartridge through theturntable. This is much more pronounced

It is, therefore, the higher-frequency sig-nals which develop a greater voltage atIC1a pin 3. In other words, the low fre-quencies tend to be filtered out.

With the Rumble switch contactsclosed, the pair of capacitors C1 and C2give the same effect as a single unit havinga larger value. This decreases the overallimpedance and the circuit rolls off at alower frequency.

The output of IC1a at pin 1 is connectedto its inverting input (pin 2) through theparallel arrangement of resistor R5 andcapacitor C4. This works in conjunctionwith resistor R4 to set the gain.

The other end of R4 is connected to themid-point of a potential divider consistingof equal-value resistors R1 and R2. Thissets a d.c. voltage nominally equal to one-half that of the supply – that is, 3V. Thisprovides a “zero” reference so that the a.c.input signal will rise and fall with respectto it.

If the reference was a true 0V (the volt-age of the 0V supply line), the negativehalf-cycles of the wave would not beamplified. This is because the output volt-age cannot fall below 0V. As it is, the out-put signal will swing above and below the3V level.

Ignoring the effect of capacitor C4 forthe moment, the gain of this section isapproximately eight times. However, withC4 in place, the impedance of the feedbackloop will fall as the frequency rises. Thisreduces the gain at higher frequencies andprovides the “fall-off” characteristicshown by Fig.1 section “B”.

Section IC1b of the circuit is configuredas a unity-gain amplifier (buffer). The sig-nal from IC1a output, at pin 1, passesthrough resistor R7 (or R6 connected inparallel with it when Scratch switch con-tacts S2a are closed) to IC1b’s non-invert-ing input at pin 5.

High frequency signals now flow moreeasily through capacitor C5 (due to itsreduced impedance) and hence to a further“false zero” derived from the potentialdivider made up of resistors R8 and R9. Thevoltage appearing at IC1b pin 5 will there-fore be less than with higher frequencies.The higher frequencies therefore tend to befiltered out (shown by section “C” in Fig.1).

With Scratch switch S2a contacts

closed, resistors R6 and R7 are placed inparallel and provide near-RIAA high-fre-quency attenuation. With the switch con-tacts open, resistor R7 alone provides amore dramatic cut-off and provides the“scratch reduction” effect. These valuesmay be experimented with or a “tone con-trol” could be fitted to give a continuouslyvariable effect. More will be said aboutthis later.

The output from IC1b, pin 7, is nowequalised but still at a low level. The nextsection, centred around IC3a, is an ampli-fier used in inverting mode. This boosts thesignal by a large factor making it suitableto drive the line input of a sound card orexternal power amplifier.

Capacitor C7 allows the output signalfrom IC1b pin 7 to pass with little loss(due to its relatively low impedance at

666 Everyday Practical Electronics, September 2002

with a cheap unit and without such a “cut”would be accentuated due to the low-fre-quency boost made during equalisation.

Before proceeding to construct this cir-cuit, check that you have a good qualityrecord deck available. This must be fittedwith a magnetic cartridge (not a ceramicone). If you wish to transfer 78 r.p.m.records, make sure your turntable willoperate at this speed (many are designedfor 33/45 only) also that it is fitted with thecorrect type of stylus.

The full circuit diagram for the

Vinyl To CD Preamplifier is shown inFig.2. This is built around three identicaldual low-noise operational amplifiers(op.amps) – IC1a/IC1b, IC2a/IC2b andIC3a/IC3b.

Equalisation of left and right channels iscentred around IC1 and IC2 respectively,while IC3 is a “straight” amplifier whichboosts both channels to line level.

It is only necessary to describe theaction of one channel (the left-hand one)since the other is the same. Note that thecomponent numbering for the right-handchannel is prefixed with a “one hundred”.Thus, R2 (left) corresponds with “R102”(right). Components which are common toboth channels, the i.c.s, switches andinput/output sockets are numbered as ifthey belonged to the left channel.

The first section of the circuit is a non-inverting amplifier IC1a. The signalobtained from the input cartridge (left-hand channel) at SK1 is applied to the non-inverting input, pin 3, via capacitor C2 (orC1 and C2 in parallel if Rumble switchS1a contacts are closed).

This, in conjunction with fixed resistorR3, determine the anti-rumble characteris-tics of the circuit (the roll-off below 10Hzlabelled “A” in Fig. 1). Resistor R3 alsosets the input impedance making it suit-able for a standard magnetic cartridge.

Anti-rumble processing comes aboutbecause the impedance of capacitor C2rises as the frequency falls. High frequen-cy signals will then flow more easilythrough resistor R3 and hence throughcapacitor C3 (which has a relatively highvalue and therefore negligible impedanceat these frequencies) to 0V.

Fig.1. Equalisation graph (not to scale): a) roll-off; b) fall-offand c) high frequency filtering.



excl. batts. & case

ResistorsR1, R101,

R2, R102, R8, R108, R9, R109 2k2 (8 off)

R3, R103,R4, R104R10, R110,R11, R111 47k (8 off)

R5, R105, R6, R106 330k (4 off)

R7, R107 120k (2 off)R12, R112 15k (2 off)R13, R113 1M5 (2 off)R14 270

All resistors 0·6W 1% metal film.

PotentiometersVR1, VR101 1M min. enclosed carbon

preset, vert. (2 off).

CapacitorsC1, C101 470n polyester film (2 off)C2, C102 330n polyester film (2 off)C3, C103,

C6, C106 22µ min. radial elect. 16VC8, C108 (6 off)

C4, C104 10n polyester film (2 off)C5, C105 2n2 polyester film (2 off)C7, C107 1µ polyester film (2 off)C9, C109 10pF ceramic (2 off)C10, C110 10µ min. radial elect. 16V

(2 off)C11 220µ min. radial elect.

16V

SemiconductorsD1 3mm red l.e.d.IC1 to IC3 NE5532AN dual low-

noise op.amp (3 off)

MiscellaneousS1 to S3 d.p.d.t. interlocking push-

button switch – see text(3 off)

B1 6V battery pack (4 x AA alkaline cells)

SK1 to SK4 phono socket, singlehole, panel mounting (see text) (4 off)

Printed circuit board available from theEPE PCB Service, code 366; 8-pin d.i.l.i.c. socket (3 off); aluminium instrumentcase, size 150mm x 100mm x 75mm;battery holder and connector; 3mm l.e.d.clip; screened cable; multistrand con-necting wire; solder, etc.


Fig.1. Complete circuit diagram for the Vinyl To CD Preamplifier.

VINYL TO CD PREAMPLIFIER

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EPE Online


audio frequencies) through resistor R12and hence to IC3a inverting input at pin 2.Ignoring capacitor C9 for the moment,fixed resistor R13 connected in series withpreset potentiometer VR1 provides nega-tive feedback and, in conjunction withR12, sets the gain.

This will be some 170 times with VR1set to its maximum value and 100 times atminimum. Preset VR1 will be adjusted atthe end to provide a suitable output for theparticular cartridge being used.

The value of resistor R13 could beincreased to provide a greater gain if this isshown to be necessary at the testing stage.By adjusting preset VR1 in conjunctionwith its opposite number in the other chan-nel (VR101), the circuit will also be “bal-anced” to provide equal outputs for bothchannels.

Returning to capacitor C9 which appears

in IC3a feedback loop, its small value pro-vides an extremely high impedance ataudio frequencies. It therefore normallyhas negligible effect.

However, if radio-frequency signals hap-pen to be picked up by the circuit, theimpedance of C9 will be low. This willlower the impedance of the feedback loopand reduce the gain at these frequencies.This prevents instability.

The output signal finally passes fromIC3a pin 1, via capacitor C10, to LineOutput socket, SK3 (Left channel).

Construction of the Vinyl To CD

Preamplifier is based on a single-sidedprinted circuit board. This board is avail-able from the EPE PCB Service, code 366.The topside component layout and actualsize underside copper foil master patternare shown in Fig.3.

Commence construction by drilling thethree mounting holes as indicated. Solderthe spring-loaded, pushbutton switches inposition. If the specified type is notavailable, use toggle or slide units andhard-wire these to the appropriate pointson the p.c.b. at the end of construction.Next, solder in position the three i.c.sockets.

Follow with all resistors, preset poten-tiometers and capacitors – taking particularcare over the polarity of the electrolytics.Solder the battery connector to the +6Vand 0V points on the p.c.b., again, takingcare over their polarity. Adjust presets VR1and VR101 to approximately mid-trackposition to provide a medium degree ofgain for each channel.

Note that this circuit must be housed in

a METAL box to provide adequate screen-ing against hum pick-up.

Decide on a suitable layout for the inter-nal components. Measure the positions ofthe switches and l.e.d. on the p.c.b. Markthese on the front panel of the box at thehalf-height level and drill them through.Mark and drill the p.c.b. mounting holesalso those for the battery holder and theinput and output sockets.

Cut plastic stand-off insulators to thecorrect length so that, when the p.c.b. is inposition, the switch buttons will passthrough their holes with a little clearance.

Secure the p.c.b. and make sure the switch-es operate freely.

Attach the battery holder and the inputand output sockets. If these are of the spec-ified type, you will need to scrape away thepaint on the inside surface of the box toallow the outer (“sleeve”) connections tomake good metallic contact with the case.

Attach one of the solder tags suppliedwith the sockets under the fixing nut of oneof them. This will be used to “earth” the“0V” wire leading from the circuit board.

If you are using sockets of the fully-insu-lated type rather than the specified pattern,the sleeve connection of each must be con-nected to the case (0V) using a separatesolder tag.

Referring to Fig.4 and photographs,complete the internal wiring. Take care thatleft and right inputs and outputs maintaintheir identity during the wiring process(that is, they do not become interchanged).Set all switches to the “out” position, insertthe batteries and attach the lid of the case.


Fig.3. Printed circuit board component layout and full-size underside copper trackmaster pattern for the Vinyl To CD Preamplifier.

Unless the stylus on the record deck is

known to have given very little service,renew it. Styli cost very little comparedwith that of your record collection. Also, anew stylus will give better results. If youare going to transfer 78s you must have thecorrect stylus fitted – do not use one madefor 33s/45s.

It would be useful to have the turntablemanual available to help make optimumstylus pressure and anti-skid adjustments.Sometimes a slightly greater pressure thannormal will give better results. Althoughthis wears the record more quickly it maybe worthwhile since the record need onlybe played once.

For initial testing, connect the output ofthe preamplifier to the line input of a hi-fiamplifier using twin-screened cable fittedwith the appropriate connectors. Do notconnect it to the computer sound card atthis stage.

Connect the turntable to the preamplifierinput sockets. If possible, use a valuelessrecord to make initial tests. Turn theVolume control on the amplifier to mini-mum and switch on both units. Check thatthe front panel l.e.d. operates.

It may be found convenient to use head-phones to monitor the sound. Start playingthe record and gradually increase theamplifier’s volume control. The musicshould be clearly heard. Compare the vol-ume with that playing similar music from acommercial CD.

If the levels are not similar, adjust VR1and VR101 so that they are. If one channelis quieter than the other, adjust presets VR1or VR101 as appropriate to bring the weak-er channel to the level of the stronger one.This procedure ensures that the output isat line level and balanced between thechannels.

Check the effects of the Scratch andRumble switches. The rumble effect is verysubtle and may not be noticed. Note that, asdescribed, pressing the switches in pro-vides the anti-scratch and anti-rumbleeffects.

The frequency balance and anti-scratcheffects could be altered by changing thevalue of resistors R6/R7 and R106/R107.By increasing the appropriate resistor val-ues slightly, the high-frequency responsewill be “cut” and vice versa. Beware –small changes make a lot of difference!

An alternative method would be toreplace resistors R7/R107 with a dual-ganged, panel-mounted, potentiometer(stereo). This would allow for continuousvariation and switch S2 could then beignored.

When setting up the equipment to make

CDs, the turntable should not be placed onthe same surface as the computer (other-wise you could introduce hum due tovibration being transferred to the cartridgefrom the computer). Check that theturntable is “true” using a spirit level.

Connect the preamplifier output to theline input of the PC sound card using twin-screened wire. Check that Left and Rightchannels are connected correctly.

Before making a recording, clean thesurface of the disc using a proprietary anti-static cleaner. If it is very dirty, it will needspecial treatment to remove the debriswhich will have become deeply embeddedin the groove. You could try playing it onceor twice in an attempt to allow the stylusitself to remove the contamination.

Check the stylus after every playing for

any build-up of fluff and dirt. Leaving thiswill spoil the high-frequency response andalso tend to cause the stylus to jump out ofthe groove. Use a proprietary stylus clean-ing kit (a fine brush and cleaning fluid).Styluses are easily damaged so follow theinstructions and work carefully.

Refer to your CD recording software

instructions to make optimum sound levelsettings and make some tests using the oldrecord. For your final recordings, you willprobably be able to observe the file oscillo-scope-style. It is then possible to removethe heaviest clicks by highlighting anddeleting them.

However, this must be done with greatcare. Some CD recording software allowsfor sophisticated restoration work to beundertaken. Automatic click suppressioncan be a problem because many sections ofthe intended waveform are click-like.

One final point – do not use the scratch fil-ter unless the result sounds better. This isbecause it gives a markedly “dull” effect.


Layout of components inside the metal case.

Fig.4. Interwiring details from the printed circuit board to the rear panel mountedinput and output phono sockets.

PIC ALARMDear EPE,I’ve been building your PIC Controlled

Intruder Alarm (Apr ’02) – great application! Itseems, though, that you can only arm the alarmwhen the entry zone is set-up to be normally-open, is this so?

In your article you suggest feedback would bewelcome on the use of the RB4 interrupt for thepanic switch. I have linked pins of the S3 con-nector but can still trigger the panic event bygenerating mains noise, even pulling the plug outand switching to battery power sometimes gen-erates the event. I’m planning to add mains sup-pression etc.

Mark Jones,via email

Feedback is always welcome Mark, thanks.The entry zone restriction was not intentional,

but in practice I have never encountered a situa-tion where entering the main door zone couldrequire a choice between normally-open andnormally-closed contacts.

HOME SECURITYDear EPE,I am currently doing my final year project on

a home security system which involves a 4 × 3matrix keypad, PIR sensor, magnetic switch andglass break detector. I’m using a PIC16F84 andPICBasic to write the software. Can you pleasegive me some advice?

Brendon, Malaysia, by email

Sorry to disappoint you Brendon, but we can-not give specific advice for reader’s own designs,but you might find my PIC Controlled IntruderAlarm of April ’02 of interest. That uses amatrixed keypad.

8051 FREEWAREDear EPE,I know that most of your projects that use

microcontrollers are based around PIC devices,but I just want to let any of your readers who usethe 8051 microcontroller, or its many derivatives,know about a very good freeware open sourceANSI compliant optimising C compiler which Ihave been using for a few months, now calledSDCC. It’s available for download fromsdcc.sourceforge.net.

There are several discussion forums for itsusers also on the same site. It can also be target-ed at Z80, Gameboy Z80, AVR and PIC14xmicrocontrollers, and comes with a freeware8051 software simulator.

Keep up the good work on your magazine, Ihave been a reader since I was a schoolboyhobbyist.

Jez Smith, by email

Thanks Jez, undoubtedly we have some read-ers who are 8051 users as well as PIC addicts.And thanks too for your continued interest inEPE!

BASIC STAMPI have taught myself PICBasic and have a

great interest in microcontrollers. What I wouldlike to know is what industries use BasicControllers and is it hard to start a career usingand programming them? Any advice would begreatly appreciated.

Alex, via email

I suspect that in general industry does not usePICBasic types of program, preferring the moreuniversally used assembler codings in variousforms. Readers – what are your opinions?

SMOKE DETECTIONDear EPE,I am from Les Quennevais school in Jersey.

For my business GCSE project I am going tomake a photoelectric smoke detector, carbonmonoxide detector and heat detector for the deaf.I am wondering if you could send me some cir-cuit diagrams or tell me your suppliers as itwould largely help me in my project. Any infor-mation that you could give would be very helpful

Alan Morris,via email

Our Teach In 2002 series looked at smokedetection in the June ’02 issue, back issues canbe ordered via our Online site, or according tothe information published in each EPE issue. Wehave not done other smoke detectors in recentyears.

STYLOPIC OP.AMPDear EPEI am having problems finding the LM13600

transconductance op.amp for the StyloPIC ofJuly 2002, the RS 304-453 is now listed as “nolonger stocked”. Do you know what other devicecould be used as an alternative please?

Mike Mackellow,via email

You can use the LM13700 instead as a directreplacement – no mods needed.

RREEAADDOOUUTTJJoohhnn BBeecckkeerr aaddddrreesssseess ssoommeeooff tthhee ggeenneerraall ppooiinnttss rreeaaddeerrss

hhaavvee rraaiisseedd.. HHaavvee yyoouu aannyytthhiinnggiinntteerreessttiinngg ttoo ssaayy??

DDrroopp uuss aa lliinnee!!

WIN A DIGITALMULTIMETER

A 31/2 digit pocket-sized l.c.d. multime-ter which measures a.c. and d.c. volt-age, d.c. current and resistance. It canalso test diodes and bipolar transistors.

Every month we will give a DigitalMultimeter to the author of the best

Readout letter.

LETTER OF THE MONTH SHOCK HORROR TALE!

Dear EPE,I was re-reading some old EPE issues while

waiting for the latest to turn up here in NewZealand (I don’t suppose you could print EPEevery week, could you?), and something AlanWinstanley wrote in Circuit Surgery of Sept’00 made me laugh out loud. I hasten to say Ihave the greatest respect for Alan’s intellectwhich shines through everything he does, but Iwas reminded that there is sometimes a sec-ond, more amusing explanation for a set ofsymptoms.

A reader had queried Alan about “worrying”electric shocks from his dishwasher, and yethis RCD (residual current device) had nottripped the power off, and the RCD “checkedout OK”. Alan theorized a possible insulationfault but gave the excellent advice to get thedishwasher looked at by a professional.

The following story from my time as anelectronic repairman shows how a workingRCD might not trip even though the machine itis attached to is giving you electric shocks.

Some years ago I quickly attended a similar“fault” in an old, all-metal franking machine(stamps postage on envelopes) which had beenrelocated in an old office building and, while itwas running well, had been giving electric shocksto everybody since the relocation “even when itwas switched off”. I believe the NZ power distri-bution system is the same as UK, 230V a.c.,50Hz, multiple earthed neutral, so on the way tothe fault I was mentally going over things likeearth wire broken off in the old machine, wiringfaults and errors in the building, etc.

The ladies who used the machine were insome fear of their lives, and I had firmlyadvised them over the phone that this fear waswell-grounded (is that a pun?). When I arrivedonsite they were at first rather put out whenthey saw me dash in, wave my meter about themachine, glance around the room and burst outlaughing.

What I really did was to check the machinecompetently, and drew the conclusion that

when they walked over the nice new carpet intheir nice refurbished office in their prettyfeminine artificial “leather” shoes to themachine, all those thousands of volts of staticelectricity they had built up found a ready pathto ground through the well-earthed machine!And that was it.

After my careful and sympathetic explana-tion to the ladies about how to minimise sta-tic buildup, and how it wasn’t endangeringtheir lives anyway, they ruefully saw the rea-son for my amusement, but still didn’t wantto touch the machine. In the end I suggestedthey leave the office scissors (metal) near themachine, they could pick up the scissors and,holding them firmly, touch the machine withthe scissors first, thereby discharging them-selves with a mighty crack! of spark and notfeel a thing.

And that would be one way that EPE read-ers could get even severely-felt electric shocks“from” a machine and yet the machine’s fullyoperational RCD wouldn’t trip. Having saidthat, I very strongly advise people not toassume that electric shocks from machineryare just harmless “static”. Get it checked orplan your funeral, electricity is a good servantbut a bad master!

Stan Hood,Christchurch, New Zealand

Reminds me of a situation in my late schoolyears. While showering in the sports changingroom, I frequently felt tingles in my hand whenlightly coming into contact with the metalshower tap. For weeks the school authoritieswould not believe me when I said that the tin-gling was due to electricity being present onthe water piping.

Eventually the Electricity Board was calledin – yes indeed, there was an electrical prob-lem, affecting the adequate earthing of thatpart of the building. A lot of digging in theroad outside was required before the fault wasfound and cured! I would not be telling the talehad the current flow been more severe.

E-mail: [email protected]


STYLOPICDear EPE,Following on from your StyloPIC in July

’02, you might be interested in some info onthe original. There were three variations of thepocket model – standard, treble and bass. Thetreble and bass models being respectively anoctave higher or lower (mine is the standardmodel). Its big brother, the 350S, had manyextra features such as short or long envelope,staccato, two speed vibrato, wah-wah, andeight voices.

An innovative feature is a light sensor (l.d.r.)for hand control of vibrato or wah-wah. It alsohas two styluses (for playing “chopsticks”?). Anexternal amplifier was also available for eitherinstrument, with tone and tremolo controls. Onthe technical side, the circuit diagram for thepocket version is in the back of the instructionbook.

Tone generation is by a programmable uni-junction transistor so the waveform would bepulsed, however it is modified by what looks likea diode pump monostable so the mark-spaceratio would vary depending on the note frequen-cy (and presumably the harmonics generated).So the output waveform would be something likea square wave with slow rise and fall times.Vibrato is generated by a low frequency phaseshift oscillator to vary the programming voltageof the unijunction transistor.

I know John Becker likes to recycle his soft-ware so here is something to consider in a futureincarnation. It gives greater flexibility of the out-put waveform. And, of course, you can havemultiple waveform tables. This is only an exam-ple, other changes may be needed for it to workcorrectly.

OUTIT: call WAVFORMmovwf PORTAgoto MAIN

WAVFORM: andlw $7F ; Sinewave + 2nd harmonicmovwf PCL; 128 entries, amplitude

0 to 63

DT 00,00,00,00,00,01,02,04,06,08,11,13,16,19,22,26DT 29,32,35,39,42,44,47,49,52,54,55,57,58,59,59,59DT 60,60,60,60,60,59,59,59,58,59,59,57,57,57,57,57DT 58,58,59,59,60,60,61,61,62,62,63,63,63,63,63,63DT 62,61,60,59,58,56,55,53,51,49,47,45,43,41,39,37DT 35,33,32,31,30,29,29,28,28,29,29,30,31,32,33,34DT 35,36,38,39,40,41,42,43,43,43,43,42,42,41,39,38DT 36,34,32,29,27,24,21,19,16,13,11,08,06,04,03,01; (DT is “Define Table of retlw’s” in MPASM)

Peter Hemsley,via email

Thanks Peter. The technical stuff I did not findon the web. The table concept looks interesting. Idon’t know that I’ll ever upgrade StyloPIC – butwho knows?!

FLOW CHARTSDear EPE,PICs are not my strong point! However, I’ve

started to look at the code for your PICControlled Intruder Alarm (Apr ’02) with a viewto modifying it to suit my own purposes. Do youhave a flow chart that you could send me?

Trevor Brearley,via email

No, sorry Trevor, I don’t do flow charts for mysoftware – I keep concepts in my head and workto those!

Readers who do like to work with flowcharts will probably be interested in the FlowCode for PICmicro CD-ROM that’s availablevia our CD-ROM pages in this issue, and inTerry de Vaux Balbirnie’s review of it, also inthis issue.

BIOPIC LEADSDear EPE,I am building the BioPIC Heartbeat Monitor

(Jun ’02) and need to know the order code forBoots’ lead pack, together with the information

where to order from abroad. The TENS replace-ment electrode pads you specify are easy to findat almost any Boots shop, but the staff thereknow nothing about leads, nor how to order. I’vetried at several Boots shops on my last trip toLondon.

Cristian, via email

Mine came from Boots in Wimborne. I don’tknow the order code, they were being supplied asnormal stock items. If you can’t get any, use flex-ible wire with crocodile clips to clip onto thechest pads. They don’t need screening. You couldtry asking Boots HQ via email (www.google.comwill provide a web address).

SERIAL ADC PIC TRICKDear EPE,Readers might be interested in my PIC pro-

gram for use with the TLC548/9 8-bit serial ana-logue-to-digital converter. I use file registersCOUNT and TEMP as sort of “standard” regis-ters, COUNT for timing etc and TEMP as a sortof second W. It helps me get a mental view of myprogs.

In the program this routine comes from,COUNT has previously been reset throughDECFSZ, so I can get away with BSFCOUNT,3. I have run this at 6MHz withoutproblem, and it should go faster. The A-D valueis stored in file UNIT.

A2DIN: BCF PORTB,7 ; clear CS line tohold value tosend

BSF COUNT,3 ; set count to shift8 bits (makesure thatCOUNT clearedbefore this sec-tion or useMOVLW etc)

FETCH: RLF UNIT,F ; move bits oneplace left &store new valuein UNIT

BCF UNIT,0 ; set 0 valuebefore Portb,0bit test

BSF PORTB,6 ; set A2D clockpin high, releasebit for transfer

BTFSS PORTB,5 ; is bit 0 (DOUT)set ?

GOTO NEXT1 ; no, then leaveUNIT bit 0 valueas is

BSF UNIT,0 ; yes, set bit 0 ofUNIT

NEXT1: BCF PORTB,6 ; clear clock pinDECFSZ COUNT,F; is COUNT zero?GOTO FETCH ; no, get another

bit!BSF PORTB,7 ; yes, 8 bits

clocked out &held in UNIT,set CS line toget new value

RETURNGraham Card,

via email

Useful, Graham, thank you – I’ve put it in thePIC Tricks folder on our ftp site.

FREEZER ALARMDear EPE,I’ve been reading Humphrey Berridge’s

Freezer Alarm in the May ’02 issue, and I’mextremely impressed with the low componentcount for the functionality achieved, but I’d liketo make a suggestion:

The piezo sounder needs to be as loud as pos-sible, but it’s only being fed with 5V pk-pk frompin GP4 to ground. If you connect the sounderbetween GP4 and GP5, and feed GP5 with aninverted signal, you will get 10V p-p drive in abridge configuration – twice the voltage, at noextra cost!

The only changes required are to the sweep2and sweep3 routines:

sweep2 bsf output ; output highbcf output2 ; output2 low – addeddecfsz freq,fgoto sweep2movfw nfreqmovwf freq

sweep3 bcf output ; output lowbsf output2 ; output2 high – addeddecfsz freq,fgoto sweep3

Plus an extra define line:#define output2 gpio,5 ; inverted o/p to piezo

sounderNigel Goodwin, via email

Thanks Nigel!

LOTTERY PREDICTORDear EPE,I am studying GCSE Electronics. My father

has been purchasing EPE since 1994 and is stillenjoying each new edition. In the April ’95 issueI came across the National Lottery Predictorproject and am wondering if you could pleasesend me as much information on that topic aspossible to further my knowledge and passion.

Gopyr, via email

So sorry, but we cannot provide additionalmaterial for any published design. Regardingbuilding a circuit from 1995, we normally adviseagainst attempting to build a design that is overfive years old since parts could well havebecome obsolete during that time.

In this particular case, the p.c.b. is no longeravailable, nor will you be able to obtain the pro-grammed PIC as we are no longer in touch withthe authors, and they did not sell us the copyrightto their software (that was before we began toinsist that all project software must be madefreely available to readers).

EARTH RESISTIVITY LOGGERI am designing an “Earth Resistivity Logger”

for archeological use, inspired by Robert Beck’sEarth Resistivity Meter of Jan/Feb ’97. Mine isPIC controlled and will have its own non-volatilememory (data stays held even after switch off);possibly a graphics l.c.d. may show rough detailsof reading values as grey scale; serial interfacefor connection to PC for deeper analysis.

I am not an archeologist and am approachingthe design purely as an electronic problem to besolved – send an output signal, retrieve it from adistance and store the value. I am in communi-cation with a local archeological society, but Iwould be pleased to hear from any EPE readersinvolved in this field, with special regard to thefollowing: How many reading samples do you normally

take on a site in one main session? How many samples would you like the logger

to store before download to PC? Is powering it from a 12V car battery ade-

quate, or do I need ±18V as Robert had? What probing techniques do you use? I’m

assuming the twin-probe technique is best, asdescribed by Robert.

What maximum probe separation distance doyou use?

How deep do you insert the probes? Is a signal frequency of 137Hz as used by

Robert the best to use? In your experience, how likely is it that 50Hz

mains frequency is likely to occur on a sitebeing surveyed, and would thus need to be fil-tered out in some way?

Do you always plot the site squares in thesame regular order, or would you prefer tosample in random order, telling the logger thesquare number being sampled?Any answers would be appreciated, my email

is [email protected].


CONCERNS about finished projectsfailing to work are probably the

main reason for would-be constructorsfailing to “take the plunge”. It is not amajor concern for those with years ofproject building experience becausethey have the technical knowledge,equipment, and know-how to deal withpractically any problem. The opposite istrue for beginners who, on the face of it,have little chance of dealing with pro-jects that refuse to work.

Keep it SimpleIn reality the situation for beginners is

better than it might seem. Provided youstart with something reasonably simpleand follow the instructions carefullythere is a good chance of success. Pre-publication checking for both books andarticles containing electronic projectshas increased over the years, and thishas greatly reduced the chances ofbeing led astray by printing errors. Onthe rare occasions that an error doescreep in to an EPE article it is usuallyspotted quite early and corrected oneor two issues later.

In general, the complexity of modernprojects is greater, but your chances offailure if the instructions are followed “tothe letter” are much less than theywere. Like any creative skill, electronicproject construction would not be aworthwhile hobby if perfect results wereguaranteed every time with no skillsrequired. You have to be prepared toput in some effort and try to go aboutthings the right way.

It is worth repeating the importanceof choosing a project that is within yourcapabilities. It is tempting to divestraight in with a project that willimpress your friends, but the morecomplex the project the greater the riskthat you will make a mistake. In the pastit was not unusual to receive lettersfrom readers having problems withprojects that they clearly did not under-stand at all.

You do not need to know how aproject works in order to build it suc-cessfully, but you do need to have aproper understanding of what it is sup-posed to do and how it is used.Something like a household gadget is amore appropriate starting point than anadvanced piece of test equipmentwhere you need a degree in physics inorder to switch it on!

Fortunately, letters from readers whohave “bitten off more than they canchew” are relatively rare these days,but it is still a problem to take seriously.

Mains PointThe mains supply is potentially

lethal, as are projects that connect toit. Mains power projects are only suit-able for those with a reasonableamount of experience at project con-struction. Even if a project is very

simple, if it connects to the mains sup-ply it is certainly not suitable for abeginner.

Start with projects that are batterypowered. If you should make a seriousblunder it is possible that one or two ofthe components will be damaged, butyou should be perfectly safe. In mostcases all the components will survivethe experience as well.

The two main construction methodsused in modern projects are stripboardand custom printed circuit boards(p.c.b.s). While both types of board arepretty straightforward to use, customprinted circuit boards represent themore foolproof option. Stripboard is amulti-purpose circuit board that has aregular matrix of holes, and in mostprojects only a few percent of these areactually used.

As its name suggests, a customprinted circuit board is specificallydesigned for a particular circuit andnormally has just one hole per leadoutwire or pin. With a custom board thereis relatively little risk of making a mis-take in the first place, and any errorsthat should creep in are likely to bespotted almost immediately. With strip-board there are hundreds of unusedholes that are good at disguising mis-takes, and some very careful checkingis needed to detect them.

Bridging the GapHaving chosen a suitable project and

put it together with due diligence, whatdo you do if the finished unit fails towork? When a newly constructed pro-ject is clearly failing to work properly itis not a good idea to leave it switchedon.

Leaving a faulty project switched oncould result in damage to some of thecomponents, and the semiconductorsare particularly vulnerable. Alwaysswitch off faulty projects immediatelyand then recheck the component lay-out, wiring, etc.

The prudent project builder checksall this sort of thing very carefully dur-ing construction, and spotting errorsearly can save a lot of hassle latter. Inorder to properly check the unit youmay have to partially dismantle it inorder to get proper access to the circuitboard.

Years of practical experience sug-gest that the vast majority of problemsare due to “short-circuits” between cop-per tracks on the underside of the cir-cuit board. This is not exactly a newproblem, but the intricacies of modernboards make it even more problematicthan in the past.

Unless the board is coated with asolder resist that is designed to dis-courage solder bridges, it is likely thatseveral will be produced per circuitboard. Most of these bridges will bespotted while you are constructing the

board, and in most cases they are eas-ily removed using the bit of the solder-ing iron. If there is a lot of excess solderit is better to use a desoldering tool,and an inexpensive desoldering pumpis ideal for this application. It is advis-able to remove as much solder as pos-sible and then redo any joints that havebeen desoldered.

Hidden from ViewThe more difficult problem is minute

trails of solder that are often difficult orimpossible to see with the naked eye.The situation can be made more diffi-cult by the trails being hidden underexcess flux from the solder. This tendsto get liberally splattered across theunderside of circuit boards during con-struction. There are various productsthat can be used to thoroughly cleanthe flux from boards, but vigorousbrushing with a small brush such as anold toothbrush seems to do the job wellenough.

Good eyesight is not sufficient toguarantee that any solder bridges willbe spotted. Some form of magnifiernow has to be considered part of thestandard toolkit for electronic projectconstruction, and even a small magni-fying glass will greatly increase thechances of detection.

An 8x or 10x loupe (also sold aslupes) is better though. The inexpen-sive types sold as photographic acces-sories for viewing slides and negativesare perfectly adequate for the presentapplication.

Provided the board is thoroughlycleaned first, a careful visual checkusing a magnifier should reveal any sol-der bridges. As solder bridges occur sooften it is a good idea to clean and visu-ally inspect all completed circuit boardsprior to installing them in the case.

Hot SpotsDubious soldering is a common

cause of problems, particularlyamongst beginners. Soldering is likeany skill, and it is a case of “practicemakes perfect”. The more projects youbuild the more proficient you willbecome at completing soldered con-nections. There is insufficient spacehere for a “soldering tutorial”, but a

PRACTICALLY SPEAKINGRobert Penfold looks at the Techniques of Actually Doing It!


A “dry” joint.Solder failed toflow.

A good joint,nice and shiny.

Photos courtesy Alan Winstanley’s Basic Soldering Guide

good one is available at the EPE website. Some soldering irons and solder-ing kits are supplied with detailedinstructions, and it is well worthwhilestudying these.

Probably the most common cause ofso-called “dry” joints is the solderingiron being left unused for a few minutesbefore starting a new batch of connec-tions. If there is a substantial amount ofsolder left on the bit, any flux in it willburn away and it will probably start tooxidise. If you produce the next jointwithout cleaning the end of the bit first,the joint will contain a significant pro-portion of old solder, which may notflow over the joint properly.

The resultant joint might look plausi-ble and could seem to have goodmechanical strength as well. However,joints of this type usually provide onlyintermittent electrical contact or no con-tact at all, and are relatively weakmechanically.

Shining ExampleAlways make sure that the bit is tinned

with fresh solder prior to making joints.Practice soldering with some bits of wire,a few resistors, and a scrap of stripboardbefore you start building projects. Thiswill cost very little and will greatlyenhance your chances of success.

Checks with a continuity tester or thecontinuity function of a multimetershould locate dry joints, but thoroughlychecking even a small circuit board canbe quite time consuming. Largeamounts of excess flux are sometimesindicative of a bad joint, but this is of nohelp once the board has been cleaned.

Good joints normally have a char-acteristic mountain shape and thesurface of the solder is very shiny.“Dry” joints are often more sphericalin shape and the solder tends to havea relatively dull surface, possibly withsome crazing.

Clean BreakIf any joints look suspicious it is prob-

ably worthwhile desoldering them andthen re-soldering them. Before tryingagain it is a good idea to have a closelook at the two surfaces. These days itis unusual for dirt or corrosion on one ofthe surfaces to cause problems.Modern components are less vulnera-ble to corrosion on the leadout wiresand tags, and the flux in electrical sol-ders is very efficient at dealing withcontaminants.

However, there can still be occasion-al problems though, and if there is anysign of contamination it is a good ideato clean both surfaces before redoingthe joint. The best way to clean the sur-faces is to gently scrape them with thesmall blade or a penknife, a miniaturefile, or something of this type.

The driest joint of all is the one youforget to do! Missing joints are usuallyfairly obvious with custom printed cir-cuit boards, but can be difficult to seewith stripboard where there are numer-ous unused holes and no pads assuch. Firmly pulling on resistors, capac-itors, diodes, etc., will reveal any miss-ing joints, or ineffective joints that lookplausible.

Heat of the MomentApart from semiconductors, modern

components are reasonably tolerant ofheat. However, it is still possible thatdamage will occur if you take too longto complete joints. Heat damaged com-ponents usually show some obvioussigns of damage, such as a darkeningin colour or being slightly misshapen.Always replace any “off colour” ordeformed components, or any compo-nents that show significant signs ofphysical damage.

Integrated circuits (i.c.s) are mostlyfitted in holders, but transistors anddiodes are often connected directly tothe circuit board. Always take extracare when fitting these in place. Aspointed out previously, it is a matter of“practice makes perfect”, and you canavoid a lot of problems by learning tosolder quickly and neatly before dealingwith transistors and diodes.

Try and Try AgainHaving thoroughly checked both

sides of the board and made any nec-essary repairs it is time to reassemblethe project and test it again. Thoroughlycheck the hard wiring against the wiringdiagram, as it is relatively easy to makemistakes here. If the project still doesnot work, the most likely explanation isthat you have missed an error in thewiring or on the circuit board.

With this type of thing there is a ten-dency to blame others and not acceptthat you could have made a mistake. Inreality it is easy to make the odd mis-take here and there, and even “oldhands” make the occasional error.

Start by checking that every compo-nent on the circuit board is in the rightplace and has the correct value. Workthrough the components methodicallymaking sure that none of them areoverlooked. If you have managed tomiss out a component, this error shouldthen come to light. With stripboard con-struction make sure that any link wiresare present and correct.

Ideally you should get someone elseto check the unit against the construc-tion diagrams. A fresh pair of eyesmight spot something that you haveconsistently overlooked.

Wrong ConnectionThe components that must be fitted

the right way round are the most likelyto give problems. Layout diagrams and

the markings on components such asdiodes and electrolytic capacitors areusually quite explicit, so any errorsshould be easily spotted.

One exception is the type of diodethat has several bands rather than oneat the cathode (“k” or “+”) end of thecomponent. These have had somethingof a renaissance in recent times, so youmay well encounter them. The bandsindicate the type number using a varia-tion on the resistor colour code. A widerband at that end of the body (Fig.1)indicates the cathode (k) lead.

Light emitting diodes (l.e.d.s) canalso be problematic. If a project worksapart from a l.e.d. indicator, it is odds-on that the l.e.d. is simply connectedthe wrong way round.

A Pressing ConnectionBefore too long practically everyone

makes the classic mistake of forgettingto switch on the project or omitting thatall-important component – the battery.Battery connectors have always beennotoriously unreliable. Try pressing theconnector firmly onto the battery to seeif it makes the project burst into action.Slightly compressing the female con-nectors with pliers usually gets a looseclip to work reliably.

Battery holders for 1·5V cells arealso something less than totally reli-able. Ensure that the terminals of thebatteries and the holder are clean bygently removing any contamination withfine sandpaper.

Multi-checksA cheap multimeter is useful for

checking that the battery voltage isactually getting through to the circuitboard. It can also be used to check thatthe battery is in a usable state.

Even if you do not have much techni-cal knowledge, a multimeter can still beuseful for numerous basic checks. Forexample, it can be used for makingcontinuity checks on switches, whichmay not operate in quite the way youthink they do?

Have you confused the “on” and “off”settings? Often when a project seemsto be working irrationally it is just thatone of the switches does not functionas expected. The high and low rangesare transposed, or something of thistype.

A multimeter is also useful for check-ing cables for short-circuits or brokenleads, checking that that plugs andsockets connect together properly, etc.Even some of the cheaper digital typesnow have the ability to check resistors,transistors, diodes, and capacitors,which is clearly more than a little use-ful. A multimeter is a piece of equip-ment that no project builder should bewithout.

Because modern components arevery reliable you are unlikely to havea failure caused by a dud component.If you get everything connectedtogether properly your projects willwork, and it helps to keep this inmind. Of course, the projects willnever work if you do not pluck up thecourage to “take the plunge” andactually build them.


Fig.1. The wide band indicates thecathode (k) leadout of multi-banddiodes.

Circuitos de Audio

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