PA

High-power amplifier module_

PARTS LIST

Amplifier module

1 large finned heatsink, as for the lefthand side of the Playmaster Series 200 amplifier

1    PCB, codę 85ma9, 153 x 202mm

4    3AGfuse elips

2    3AG 7A fuses

1 6.8jiH choke (see text)

1 12V DPDT relay, type FRL264, 10A 250VAC contacts, 160fi coil

Semiconductors

3    BC547 NPN transisitors 1 BC557 PNP transistor

1    BC327 PNP transistor

3    BC556 PNP transistors

2    BF469 NPN transistors

1    BF470 PNP transistor

2    2SJ49 N-type Mosfets 2 2SK134 P-type Mosfets

5    1N4002 Silicon power diodes 5 1N914 Silicon signal diodes

2 9V 400mW zener diodes

Capacitors

1    470**F 25VW electrolytic

2    100/iF 100VW electrolytic

1    100/iF 16VW electrolytic

2    47/zF 50VW bipolar electrolytic

1    47/xF 16VW electrolytic

2    22/xF 100VW electrolytic 1 1 nF metallised polyester

4    0.22/zF metallised polyester

1 0.15/iF 250VAC dual dielectric (Philips MKT-P)

1 0.1 nF metallised polyester 1 .015/zF metallised polyester 1 330pF ceramic or polystyrene 1 18pF ceramic

Resistors (0.25W, 5% unless stated)

1 X 270kfi, 2 X 56kfi, 4 X 22kf2,

3    X 22kfl/0.5W, 1 X 1 2kQ/1 W, 2

X 4.7kfi, 2 X 2.2kfi, 1 X 1kfi, 1 X 6800,    4    X    2200,    1    X

1200/1 W,    5    X    1000,    1    X

6.80/1 W, 1    5000 miniaturę

trimpot

Power supply

1 300VA transformer, Ferguson PF4363, 94V centre-tapped at 3A, 30V centre-tapped at 500mA

1    6A 400V bridge rectifier

2    8000/tF 75VW electrolytic capacitors

2 10k0/1 W resistors

These may include large ripple signals (hum) or, morę importantly, harmonics of the input signal, which would otherwise lead to higher harmonie distortion.

The balanced output signals from the collectors of Q1 and Q2 are coupled to the second differential amplifier stage, consisting of Q4 and Q5, together with current mirror Q6.

Q6 operates in an interesting way. Notę that the base-emitter voltage drop of Q6 is roughly equal to the voltage drop across associated diodę D3. This means that, by the emitter-follower action of Q6, the voltage drop across its 1000 base resistor will be echoed (or “mirrored”) in the 1000 emitter resistor.

So if Q4 turns on harder and causes an inerease in the current through diodę D3, an equal current inerease will tend to occur through Q6. But at the same time as Q4 and Q6 are turning on harder, Q5 will be turning off by the same amount, due to the differential action of the Q4/Q5 pair. This means that Q6 tends to reinforce voltage changes at the collector of Q5 and thus inereases the overall gain of the stage.

The 12kQ/lW resistor in series with the collector of Q4 serves no purpose other than to reduce its power dissipation to the same no-signal level as for Q5, ie, about half a watt. The 12kfi resistor is bypassed with a .015/xF capacitor to avoid degrading the effect of the current mirror at high frequencies.

Q4, Q5 and Q6 have a collector voltage rating of 250V and are intended specifically for class-B driver stages in television receivers. That makes them ideał for this application where their voltage rating, good beta linearity and excellent gain-bandwidth product are put to good use in obtaining a stage with very Iow distortion.

The two differential stages just described provide all the voltage gain of the amplifier. In fact, the Mosfet output stage throws away some of that gain sińce the modę of operation, source follower (similar to emitter follower), has a gain of less than unity. Current gain is the forte of the output stage.

Zener diodes D5 and D7, in series with D4 and D6, set the maximum signal which can be delivered to the Mosfet gates, with respect to their source electrodes. Any signal voltage in excess of ± 10V will be clipped. Thus the diodes form an effective overdrive Circuit, preventing excessive power dissipation in the event of a short-circuit across the output.

220Q resistors are connected in series with each Mosfet gate. These function as “stoppers”, helping prevent spurious RF oscillation.

No source degeneration resistors or any other measures to ensure current-sharing between the parallel-connected Mosfets are used. They have not proved to be necessary. In practice, if one Mosfet tends to become hotter than another, due to taking morę current, its transconductance (measured in amps/volt) is reduced slightly and it is throttled back to take a morę even share of the current.

The 500ft trimpot connected between the collectors of Q5 and Q6 sets the quiescent bias voltage applied to the complementary Mosfet gates. This sets the quiescent (no-signal) current through the Mosfets and is used to obtain a Iow value of cross-over distortion.

Single pole lag frequency compens-ation is applied by the 18pF capacitor connected between the base and collector of Q5. This rolls off the amplifier’s open loop voltage gain so that it reaches unity at a high frequency where the phase rotation through the whole amplifier is less than 360°. If this condition was not met, the amplifier would oscillate with feedback applied.

Overall negative feedback is applied from the output via the 22kfi resistor to the base of Q2. The feedback is 100% for DC, ensuring that the amplifier has a very Iow DC offset at the output.

AC voltage gain is set by the voltage division ratio of the 22kQ and lkfi resistors connected to the base of Q2. These set the gain to 23. The bass rolloff is set by the 47/xF capacitor in series with the 1 kfi shunt feedback resistor.

A particular refinement of the design is the RLC network in the output Circuit. This is based on a paper in the September 1975 issue of Proceedings of the IREE and is included to make the amplifier unconditionally stable; ie, stable no matter what output load is connected.

Loudspeaker protection

The protection circuitry is also substantially the same as in the Playmaster Series 200 except that only one relay is used. It works as follows: When power is first applied, the 21V supply to Q15 is available almost instantly but Q15 is unable to turn on because the base of Q14 is at ground potential and so it, too, is off.

Q14 cannot turn on because its 56kfl base resistor (ignore Q11, Q12 and Q14 for the moment, they are not functional yet) is supplied from the 100/zF capacitor which is initially discharged. After switch-on, this capacitor is slowly charged up which allows Q14 to turn on. This then tums on Q15 and energises the relay. Thus the turn-on delay is a few seconds. This is useful in preventing any turn-on thumps being fed to the loudspeaker.

When the amplifier is switched off, the 21V raił drops to zero almost immediately, de-energising the relay and

36

ELECTRONICS Australia, October, 1985