Elektor Electronics 2/2000

Be that as it may, the most obviousway of increasing power output is toraise the supply voltage of 12 V to ahigher level by means of a converter.Such a converter, however, is notexactly cheap and is, moreover, a noto-rious source of noise and interference.In view of the extensive electronic cir-cuits in modern motor vehicles, this lat-ter point is not to be underestimated.

Fortunately for the loudness-hun-gry, there is an alternative to a con-verter. Some years ago, Philips intro-duced a special integrated outputamplifier chip, the TDA1560Q, that isable to provide 30 watts into 8 Ω froma 12 V power supply (without the useof a converter). This output power is

44

Design by T. Giesberts

50+ watts from a 12 V battery

The integrated output amplifierdescribed in this article consistsof little more than one integratedcircuit. It is intended especially foruse in motor vehicles and other battery-operated applications. Although it appears

simple and hardly worth looking at, the amplifiercan produce an appreciable audio power output.

power output, their design is far fromsimple.

A simple calculation shows that aconventional amplifier operating from12 V (or maximum 14.4 V when thebattery is fully charged) cannot pro-vide a power output of much morethan 6 W. Use of a bridge arrangementmay push this up to some 20 watts, butthat is just about the maximum possi-ble.

Nowadays, many motorists, andmore particularly the younger ones,want considerably more power than6–20 watts. Loudness is their god, butthey unfortunately forget that this is anunforgiving god, leading them to earlydeafness.

Amplifiers come in all sorts. Most ofthe amplifier designs published in thismagazine are intended for domestic orstudio use. As such, they are usuallypowered by a 60–150 V supply, whichmay, moreover, be further divided intotwo balanced voltages. This makesthese amplifiers unsuitable for use inmotor vehicles.

Amplifiers that can operate from a12 V supply are quite different units,designed as they are for low-voltagesupplies. If, moreover, they arerequired to provide a substantial

AUDIO & HI-FI

designed for in-car use

one-IC audio power amplifier

obtained by operating the amplifier inClass H (see box elsewhere in this arti-cle).

An amplifier based on the Philipsdevice was described in the February1995 issue of this magazine.

Philips designers have furtherimproved a number of properties ofthe IC, among which the power out-put. According to the Philips datasheet, the improved device, theTDA1562Q, can deliver 70 watts into4 ohms, but that is at the cost of the dis-tortion, which at 10 per cent is rathertoo high, even for in a car.

The prototype of the designdescribed in this article provides54 watts into 4 ohms at 1 per cent dis-tortion. Since the number of requisiteexternal components is smaller than inthe case of the earlier device, theprinted-circuit board is even morecompact than that for the February1995 amplifier.

T H E T D A 1 5 6 2 T QThe internal circuitry of the new deviceis very similar to that of its predecessor.Since, however, not everybody can beassumed to be au fait with the February1995 article, the circuitry is describedanew.

The block schematic of the device isshown in Figure 1. Particular attentionis drawn to the boxes marked ‘lift sup-ply’, which are necessary in a Class Hsystem (see box).

The IC evaluates the input signaland estimates the consequent drive ofthe output transistors. When thesetransistors tend to go into saturation,the supply voltage is raised briefly byswitching the capacitors connected topins 3 and 5, and 13 and 15, in serieswith the supply voltage.

Apart from the input amplifier and

Class H output amplifier, the devicecontains several protection circuits.One of these protects the IC against anexcessive output current and againstshort-circuits.

The temperature protection circuitworks in two steps. When the firstthreshold temperature is exceeded, thedevice switches from Class H to Class Boperation. There is then no question ofincreased supply voltage. When thesecond threshold temperature is

exceeded, the drive to the output tran-sistors is reduced.

There is also a protection circuitagainst overvoltage and one againsttoo low load impedances. When theload impedance drops below a prede-termined critical level, operation isswitched from Class H to Class B. Aload impedance smaller than 0.5 Ω isseen as a short-circuit, which causesthe device to be switched off alto-gether.

45Elektor Electronics 2/2000

1

Figure 1.Block diagram of the innards of the TDA1562Q. Anumber of protection networks make the amplifier virtu-ally foolproof.

Brief technical dataPropertiesHigh power output through Class-H operationLow power dissipation during reproduction of music signalsProof against short-circuitsProtection against excessive temperaturesStandby switchNo power-on or power-off clicksVisible error indication

Measurement results (at Ub=14.4 V)Supply voltage 8–18 VSensitivity 760 mV r.m.s.Input impedance 70 kΩPower output 54 W r.m.s. into 4 Ω (f=1 kHz; THD+N=1%)Harmonic distortion (THD+N) at 1 W into 4 Ω: 0.046% (1 kHz)

0.29% (20 kHz)at 35 W into 4 Ω: 0.12% (1 kHz)

0.7% (20 kHz)Signal-to-noise ratio (with 1 W into 4 Ω) 88 dBAPower bandwidth 7.5 Hz – 185 kHz (at 25 W into 4 Ω)Quiescent current about 135 mA (‘on’)

C I R C U I TD E S C R I P T I O NThe circuit diagram inFigure 2 emphasizes how few externalcomponents are needed to construct acomplete output amplifier (in fact,fewer than half the number in the Feb-ruary 1995 amplifier). For instance, thenew device does not need compensa-tion networks to enhance the stability.Also, because of the absence of switch-on phenomena, there is no need for aswitch-on delay network.

There is, of course, still a need forsupply line decoupling capacitors.

Capacitors C5 andC6 are required forClass -H operat ion,about which more inthe box.

The value of inputcapacitors C1 and C2 isrelatively low, thanks to

the high input impedance of the IC.Switched RC network R4-C4 at the

‘mode select’ input (pin 4) serves toswitch the IC to ‘mute’ or ‘standby’.When the supply voltage is switchedon, the IC is first switched automati-cally to the ‘mute’ mode and to ‘on’only after a short delay. The time con-stant R4-C4 is a few tenths of a secondand this delay between the two statesis sufficient to obviate disturbing (andannoying) switch-on phenomena.

Switch S1 enablesthe amplifier to beswitched to ‘standby’when the use of theamplifier is not needed

for a period of time. When that timehas elapsed, the amplifier is quicklyreverted to normal operation. The cur-rent drain in the standby mode is vir-tually negligible at only 200 µA.

Resistor R3 prevents a short-circuitcurrent ensuing when S1 is beingclosed at the instant C4 is being dis-charged.

E R R O R I N D I C A T I O NThe diagnostic output (pin 8) of theTDA1562Q is one of the few facets ofthe device that is completely new. Asshown in the diagram, it can now beused to drive an visible error indicator,D1, directly. During normal operationthe diode should be out. Its lightingmay be caused by one of four possiblecauses.1. The amplifier is being overdriven.

The internal circuit responsible forthe indication is the ‘Dynamic Dis-tortion Detector’ (see Figure 1). Inpractice, this will be the case whenthe distortion rises above 1.6 percent at 1 kHz. The diode is, there-fore, a kind of clipping indicator.

2. There is a short-circuit between theoutputs or between one of the out-puts and the supply line. In the firstcase, the outputs are disabled,whereupon the protection networkascertains at short intervals of timewhether the short-circuit has beenremoved. The DIAG output is thendisabled for 30 µs at 20 ms intervals.In the case of a short-circuitbetween one of the outputs to thesupply line, the DIAG outputremains active.

3. The internal sensor measures a tem-perature of 145 °C, whereupon the

46 Elektor Electronics 2/2000

TDA1562

PG

ND

2

PG

ND

1

MODE

IC1

SG

NDVREF

OUT+

OUT–

STA

TDIAG

+IN

–IN

C2+C2–

C1+C1–

VP

1

VP

2

1217

16

11

1315

14

1053

4

8 9

6

1

2

7

C54700µ

25V

C6

4700µ25V

C1

470n

C2

470n

C7

100n

C3

10µ63V

C8

4700µ25VC4

10µ63V

R1

1M

R4

10

0k

R2

4k

7

D1

S1

standby

LS+

LS–

000004 - 11

12V

R3

1k

2

Figure 2. The circuit ofthe amplifier is con-spicuous by its sim-plicity. Diode D1 is anerror indicator.

000004-1(C) ELEKTOR

C1

C2

C3

C4

C5

C6

C7

C8

D1

H1

H2H3

H4

IC1

R1

R2

R3

R4

LS+

LS

-

0+

S1

Tstandby

00

00

04

-1 000004-1(C) ELEKTOR

3

Figure 3. The compactprinted-circuit boardfor the amplifier isavailable through theReaders’ services(towards the end ofthis issue).

COMPONENTS LIST

Resistors:R1 = 1MΩR2 = 4kΩ7R3 = 1kΩR4 = 100kΩ

Capacitors:C1,C2 = 470nFC3,C4 = 10µF 63V radialC5,C6,C8 = 4700µF 25V radial

(18mm max. dia., raster 7.5 mm)C7 = 100nF, raster 5 mm

Semiconductors:D1 = high-efficiency-LEDIC1 = TDA1562Q (Philips)

Miscellaneous:S1 = single-pole on/off switchFour spade connectors, PCB mountHeatsink for IC1 (Rth<2.5 K/W)PCB, order code 000004-1 (see

Readers Services pages)

relevant protection circuit isenabled.

4. The IC is in the power-up state.When it is switched to the ‘on’ state,the diode goes out. If, however, theoutput impedance is not to specifi-cation during power-up, the dioderemains lit.

A C O M P A C T B O A R DThe amplifier is best built on the single-sided printed-circuit board shown inFigure 3 (available through our Read-ers’ services – see towards end of thisissue). As mentioned earlier, the boardis small for an output amplifier. In fact,most of its surface is taken up by thefour car-type (spade) connectors viawhich the power supply and the loud-speakers are connected to the amplifier.

The small size of the board also cre-ates a few difficulties. If, for instance,the electrolytic capacitors are mountedfirst, the fitting of the IC becomespretty difficult. It is, therefore, advis-able to start with fitting the IC onto theheat sink (using plenty of heat con-ducting paste).

Also, ensure that after the board hasbeen placed in a suitable enclosure, itssolder (track) side remains readilyaccessible. After the board and heatsink have been secured in the case, sol-der the pins of the IC to the board.

Figure 4 shows a photograph of thecompleted prototype as built up in ourdesign lab.

Make sure that when the board isfitted in the case, the loudspeaker ter-minals are not (and cannot be) short-circuited to earth. Although the IC isprotected against much abuse and mis-use, it is always better not to temptprovidence.

In most cases, two amplifier mod-ules will be fitted in one enclosure: onefor the left-hand channel and the otherfor the right-hand channel.

Constructors who find 2×54 wattsinsufficient may consider driving thefront and rear speakers in the vehicleby separate amplifiers. This would takethe total number of modules requiredto eight, and these deliver some400 watts of audio power into the vehi-cle. A case of minding the windows!

[000004-1]

Text (Dutch original): S. van Rooij

47Elektor Electronics 2/2000

4Figure 4.There cannotbe many 50 W outputamplifiers that are thissmall.

Class-H operationA Class H amplifier is somewhat akin to a Class G amplifier, which is a power ampli-fier in which two Class B amplifiers with different supply voltages are combined. Small-amplitude signals are boosted by the one with the lower supply voltage, resulting inmuch higher average efficiency for speech and music. When signals exceed the low-voltage supply amplitude, the amplifier that operates from the higher supply voltage takesover, while the first one is switched off.

In a Class H amplifier the supply voltage is varied by an efficient Class S amplifieror the arrangement in the present design so that it remains just above the minimumvalue required to prevent saturation. This configuration also achieves a much higheraverage efficiency for speech and music signals. (A Class S amplifier is a pulse-width-modulated audio amplifier in which the active elements are switched by a control fre-quency several times higher than the signal frequency being amplified. This type ofamplifier has an efficiency of some 90 per cent).

In the present amplifier, Class H operation is achieved by the use of two switchedcapacitors, C1 and C2. These are charged to the battery voltage and switched intoseries with the 12 V supply line during brief drive peaks.

In the diagram, transistors T1–T4 are part of the ‘normal’ amplifier, while T5–T8 andC1 and C2 are added for Class H operation. At small-signal amplitudes, T7 and T8are on and C1 and C2 are charged to the battery voltage via diodes D1 and D2. Whenthe signal amplitude tends to drive T1 or T3 into saturation, the internal sensor(lift/recharge control) detects this and causes T7 and T8 to be cut off and T5 and T6to be switched on. When that happens C1 and C2 are in series with the supply line;diodes D1 and D2 prevent their being discharged into the battery.

When the input signal amplitude drops, the capacitors are linked to earth again byT7 and T8, whereupon their charge is topped up.

C1 C2

R1

D2D1

T1

T2

T6

T8

T5

T7 T3

T4

LIFT/RECHARGECONTROL

LIFT/RECHARGECONTROL

E1

950024 - 12

V3

V1 V2

V4

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