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Increase dynamic range ofthis versat i le unit .

MANY OF US have tapes in eitherthe reel to reel format or oncassettes which leave a lot to bedesired in terms of signal to noiseratio. It is not that we necessarilymade a bad job of, the recording inthe first place, but rather thelimitations of our equipment andtape were generally just a little bit

too much compared with what isavailable today. And because thesignal to noise ratio is so poor,many of these tapes (and quite afew records as well) tend to lie onthe shelf because of their audibleinadequacies. Apart from this it isby no means unknown for commer-cially pre-recorded tapes andrecords to be below an acceptablestandard.

Many people arbitrarily think thatthis problem is what the Dolbysystem is intended to resolve. But

this is not so. The Dolby systemhelps maintainthe original signal tonoise ratio when recording from onemedium to another but it has verylittle to offer when faced by existinginadequacies.

DYNAMIC RANGEAnother problem that plagues

many of us is the poor dynamicrange of our tape recorders or of thepre-recorded material that we buy.For example, the majority of cas-sette recorders are hard pressed to

offer even a 55 dB dynamic range.Many of them offer little more than40 dB. As if this were not badenough, few records have a dyna-mic range exceeding 50-55 dB andeven this is soon degraded to 40-45dB after a dozen or so playings in adusty environment.

THE SOLUTIONAudio volume expansion is thesimplest and most effective way ofincreasing the apparent signal tonoise ratio of a worn or noisy

recording. There is also no moreeffective way of preserving the fulldynamic range of a sound than byrecording with volume compres-sion, and replaying with equalvolume expansion. However, forthese applications, the compression

tape record ings or reduce record sur face noise wi th

and expansion must be done in aprecise and reproducible manner;which is by no means as simple as itfirst appears.

The Compressor-Expanderdescribed here is relatively inex-pensive to build, yet its perfor-mance is quite adequate for allpractical purposes. It is sufficientlyversatile to interface with mostexisting audio equipment, atnominal signal levels from about 25millivolts to 1 volt.

CONSTRUCTIONDue to the relative complexity of thecircuit a double-sided printed-circuitboard has been used to simplify the

construction, and we stronglyrecommend that this board be used.A single-sided board would bemuch larger and would require agreat number of wire links.

Begin construction by assem-bling the components to the boardin accordance with the component.overlay Fig. 2. Take particular carewith the orientation of componentsas marked on the overlay. Whensoldering component leads to thetop of the printed-circuit board use asoldering iron which has a small tip

and use a small gauge of solder (1mm recommended). Take care notto bridge solder between the ICpads. It is easy to miss solderingconnections on the component sideof the board and these should bedouble checked.

ELECTRONICS TODAY INTERNATIONAL-MAY 1976

Take care to insert the electroly-tics with the polarity as marked onthe overlay and even more care withthe orientation of the diodes. A

reversed diode can result in thedestruction of one or more of thedual transistors.

The resistors in the signal side ofthe circuit and those in the cur-rent-sink circuit should be 2% orbetter. Alternatively they may beselected from 5% values. In select-ing values an ordinary multimeter(operated at about the centre of therange) suffices. The resistors inquestion are all values between R3 7a n d R 6 5 .

For best results the two 12 volt

zeners should also be matched butin practice any slight discrepancymay be compensated by using thenormal stereo-balance control.

A value of 1 microfarad for C5allows compression or expansion tofollow the signal amplitude sorapidly that the ear is unlikely todetect the attack or release, which isvirtually complete in about 20milliseconds. However, with thisvalue, low frequency signal com-ponents (50 Hz or lower) will not beaveraged out in obtaining the gain

control voltage, and severe inter-modulation and 3rd harmonicdistortion will result. At the otherextreme, a value of 4.7 microfaradsfor C5 will prevent this distortionright down to the lower audiblelimit, but the attack and release time

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(about 100 milliseconds) is so longthat the effects can be audible,although not necessarily unplea-sant. A value of C5 equal to 4.7

microfarads will be found quiteacceptable by most people.

Potentiometer R is used tomatch the signal *revels of thecompressor-expander with those ofthe associated equipment. Poten-tiometer R s h o u l d b e a wire-wound typeyand for the front-panelcalibration to apply, it should have

an effective electrical rotation of280, and the midpoint of rotation. - - - - -should be set opposite the 1 .O indexline.

Capacitor C5 should be chosenin accordance with the particularcompromise that suits the user ofthe unit. Alternatively a switch maybe used to select different values.

The box used in our prototypemeasured 200 X 125 x 63 mmand, although a little cramped didadequately hold the unit. The next

POWER SUPPLYThe output of the transformer isrectified by a full-wave bridge toprovide 22 volts, as set by theZener diodes. The voltages obtainedfrom the MC1 468L regulator. arethe 15V required for correct,operation of the compressor-ex-

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pander. marked X is shorted to gro und.Monitor the left channel output wi th

the input of fset voltage o f t hecurrent sinks, a n d ensures that the

SETTING UP~~

a high sensitivity meter (or ampli- audio gain wi l l be control led

With the power supply connected fier) and adjust RV3 to the point correctly at the low end. Remove(check for correct polarity), apply a where the output JUST disappears. the input signal and t he s hort

strong (about 1 volt) audio signal to Repeat with the right channel and circuit.

both stereo inputs, while the point RV4. This procedure balances out RV1 i s se t b y the fo l l ow in g

-ELECTRONICS TODAY INTERNATIONAL-MAY 1976 3 1

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procedure:(1) Connect the compressor-ex-

pander to its associated equipment,and supply an input of moderatelevel (e.g. music of average loud-ness). RV1 should be fully clock-wise when viewed from the inputedge of the board.(2) Turn the compress-expandcontrol to full compression, andadjust RV1 to bring the output up toits original level (loudness).

(3) Turn the compress-expandcontrol towards the expansion end,and note any obvious change inoutput level.(4) If a decrease in level occurs, turnRV1 slightly anticlockwise; if anincrease occurs, turn RV1 slightlyclockwise.(5) Repeat steps (3) and (4) until thelevel remains reasonably constantover the whole range of compres-sion and expansion. Note that this

adjustment is subjective, and it doesnot need to be done with any greataccuracy.

If RV1 cannot be adjusted asdescribed, it means that the signallevel is outside the optimum rangeof. the compressor-expander. Some-what higher signal levels can beaccommodated by increasing thevalue of R1 and R2, whilst for lowersignal levels, R4 shou ld bedecreased. If correct adjustment of

3 2 ELECTRONICS TODAY INTERNATIONAL-MAY 1976

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RV1 is obtained well towards theanticlockwise end,improved signal-to-noiseresults if R34 and R36 areincreased to 18K, and the stereooutputs are each attenuated by a470 ohm/3.9K divider. However,this modification is not essential.

With no input signal appliedadjust RV2 such that the voltage atits wiper is zero volts. Now fit theknob such that the pointer lines upwith the 1 .O calibration. Now checkthat the potentiometer travelapproximately matches the scale. Ifnot reverse the two outside leads toit.

HOW TO USEThe use of a compressor-ex-

pander need not be confined tothose s i tuat ions where such adevice is really needed. Practicallyall tapes and many records becomemore listenable with a small amounto f expansion.. On the other hand,background music is far less

Interior of the unit.

obtrusive if the volume is com-pressed to some extend. The key to One final word of warning - this

listening pleasure lies in the hand- device is quite capable of outputting

ling of the compress-expand con- a signal of 10 volts. It would betrol. Dont move it far from the 1 .O wise to ensure that your amplifier is

position un less there is some capable of accepting this voltagedefinite reason. without damage.

HOW IT WORKS equation:The heart of an audio compressor- Eexpander is invariably a voltage

out = -4.15logE Incontrolled amplifier; that is, an The inverse of E is obtained

amplifier whose gain is set by means from amplifier lC2)/2 a n d b yof an applied voltage. This voltage connecting the compression-expan-itself must be derived from the sion control potentiometer as shown

amplitude of the audio input signal, between the input and output of thisaveraged over some preset period, stage, any voltage between E a n dand modified to give the required -0.3E can be obtained. ICS, lC6compression or expansion charac- and Tpt are combined as anteristics. In the circuit of Fig. 1, each antilogaiithmic or exponent ialportion of the circuit is identified amplifier which is the exact inverse

according to its function. These of the logarithmic amplifer, so thatportions, in turn, are grouped into the effect of all these operations onthree main sections; an AC to DC the input signal is to give to aconverter, a power function genera- positive DC output voltage, equal intor, and a stereo analogue multiplier.

The two channels of stereo inputmagnitude to the input voltageraised to the power k, where k can

are mixed in buffer amplifier IC1 / 1, have any value from -0.3 to 1.and the gain of this stage is set so In the analog multiplier sections,

that an output of about 1 volt is this voltage (E ) is converted togiven by a signal which corresponds current by am/;lifiers IC7 /2 andto moderate loudness. Amplifers IC8/2 thus sett ing the effectiveIC1/2, and IC2 / 1 are used to gain of the differential amplifiersobtain precision full-wave rectifica-tion of the mixed input, and the

TP3 and TP4. These are direct lycoupled into the output buffers IC7

resulting positive DC voltage is and IC8/ 1 so that the stereo signalsstored in capacitor C5. The choice of reaching the outputs have beenvalue for C5 is important, and it will amplified by a factor which dependsbe discussed in detail later on. on the average amplitude of the

Amplifiers IC3 and IC4 together signals, and the compression-ex-with the transistor pair TP/ 1 consti-tute a logarithmic amplifier. With the

pansion control setting. The actualvoltage gain can vary from 0.0004

components shown, the behaviour to 14, which represents a powerof this amplifier is described by the gain range of 97dB.

PARTS LIST - ET1443Rl,R2.. . . . . . . . . . . . . . . . . 15kR 3 . . . . . . . . . . . . . . . . . . . . . 5k6R 4 . . . . . . . . . . . . . . . . . . . . . 4 7 RR5, R6, R9, R13, R20, R2 1 . . . 2 2 kR 7 , R 8 , R10,R11,R 2 2 . . . . . . 10kR 1 2 . . . . . . . . . . . . . . . . . . . . 4k7R14, R30 . . . . . . . . . . . . . . . 1 OOkR15, R25 . . . . . . . . . . . . . . . 150kR16, R17, R24, R28, R29 . . . . 2k2R 1 8 , R19, R 2 6 , R 2 7 . . . . . . . . 1M5R 2 3 . . . . . . . . . . . . . . . . . . . 8 2 0 R

R31, R32 . . . . . . . . . . . . . . . 2 7 0 kR33, R34, R35, R36 . . . . . . . . l k 5R66, R67 . . . . . . . . . . . . . . . . 10RAll W, 5%

R37, R39, R44, R46 . . . . . . . . 1 2 kR38, R45 . . . . . . . . . . . . . . . . l k 5R40, R42, R47, R49 . . . . . . . . 2 7 kR41,R43,R48,R50. . . . . . 4 7 0 kR 5 1 , R 5 2 , R 5 5 , R 5 6 , R 5 9 , R 6 0 , R 6 3 ,R 6 4 . . . . . . . . . . . . . . . . . . . . 10kR53,R57,R61,R65.. . . . . . . 2 2 kR54,R62 . . . . . . . . . . . . . . . . 15kR 5 8 . . . . . . . . . . . . . . . . . . . . . l kAll W, 2% (may be selected from 5%resistors)RV1 . . . . . . . . . . . . . . . 2k t r i mmerRV2 . . . . . . . . . . .

RV3, RV4

5k w i r e w o u n d p o t

. . . . . . . . . . 1 Ok trimmer

Cl,C2,C5.. . . 4.7 F25V tantal umc 3 , c 4 . . . . . . . 1 OF 25V t an t al umC6, C7 . . . . . . . . . . . . . . . . .C8,C11

33OpF. . . . . . . . . . . . . . . . .

C9, C102 2 p F

. . . . . . . . . . . . . . . .C l 2

15OpF

. . . . . . . . . . . . . . . . . . . .c 1 3 , Cl4

3 3 p F. . . . . . .

C 1 5 , C l 60.47 Fpolyester

. . .C 1 7 , C l 8

47OF 2 5V electrolytic. . . . . . . 0.01 Fpolyester

C19,C20 . . . . . . 1 uF 25V tan ta lumC21, C 22, C 23 , C 24 1OF 35V tantal umTP1,TP2, TP3, TP4

LMl 14 Dual transistor or equivalentIC1, IC2, IC7, IC8 . . . . . . . . LM747

IC3 . . . . . . . . . . . . . .

I C 4 , IC5, itsLM308

. . . . . . . . . . . . LM301IC9 . . . . . . MC 1468L o r e q u i v a l e n t

Dl, D2,D 3 , D 4 . . . . . . . . . . 1 N914D 5 , D 6 , D7,D 8 . . . . . . . . 1 N4001ZDl,ZD2 . . . . . . . . . . . 12V, 4 0 0 m W203, Z D 4 . . . . . . . . . . . . . . 22v, 1wTl . . . .

PCB ETI 4 4 3

24OV/36V CT transformer

4 Phono sockets 4 6mm spacersc h a s s i s a n d c o v e r 2OOx63x125mmapprox

nuts, bolts and assorted hardware

ELECTRONICS TODAY INTERNATIONAL-MAY 1976