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Figure 1
Figure 1 - Amplifier Schematic

As can be seen, it is not a complex amp, and in fact is absolutely identical to that for P3A. The PCB has connections (not shown) provided for a SIM (Sound Impairment Monitor), and there is also a resistor to allow bridging (not recommended with a Class-A amplifier!).

For use into 4 ohms (including bridging into 8 ohm loads), do not exceed ±25V, and do not exceed the 1.5A quiescent current. The amplifier will operate as Class-A up to around 9W into 4 ohms, and will go into Class-AB mode beyond that.

D1 is a standard green LED, and is not optional, nor should it be used as a panel indicator! Don't use a high brightness LED, or change the colour. This is not for appearance (although the green LED looks pretty neat on the board), but for the voltage drop - different coloured LEDs have a slightly different voltage drop. The LED sets the current through the differential pair input stage.

VR1 is used to set the quiescent current, and normally this will be a maximum of 1.5A. The amp will work happily at lower current, but will not be Class-A. The Class-A driver (Q4) has a constant current load by virtue of the bootstrap circuit R9, R10 and C5. Stability is determined by C4, and the value of this cap should not be reduced. With fast output transistors such as those specified, power bandwidth exceeds 30kHz.

With the suggested and recommended 25V supplies, Q4 will normally not require a heatsink. The output drivers (Q5 and Q6) will benefit from a heatsink, although it does not need to be large.

Although I have shown MJL4281A and MJL4302A output transistors, these are very recent and may be hard to get for a time. The recommended alternatives are MJL21193 and MJL21194.

It is no longer possible to recommend any Toshiba devices, since they are the most commonly faked transistors of all. The 2SA1302 and 2SC3281 are now obsolete, and if you do find them, they are almost certainly counterfeit, since Toshiba has not made these devices since around 1999~2000.

Before applying power, make sure that VR1 is set to maximum resistance to get minimum quiescent current. This is very important, as if set to minimum resistance, the quiescent current will be very high indeed (more than enough to blow the output transistors!).

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