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Figure 1
Figure 1 - Amplifier Schematic

As can be seen, it is not a complex amp, but the performance is excellent. Connections are provided for a SIM (Sound Impairment Monitor), and there is also a resistor (R17) added to allow bridging. This resistor connects to the output of the other amplifier (the master). When used in this way, the input should be grounded - do not omit the capacitor, or DC offset will be too high. When used in bridge mode (also called BTL - Bridge Tied Load), the SIM should be taken from the master amplifier only.

* Components marked thus are optional - if you do not want to use the SIM or bridging, these may be omitted completely.

For use into 4 ohms (including bridging into 8 ohm loads), do not exceed ±35V (from a 25-0-25V transformer). Most applications will be satisfied with the lower voltage, and the reliability of the amp is assured with almost any load. In bridge mode, this amp will happily produce 200W into 8 ohms, and will do so reliably even for continuous high power levels. Never attempt to operate the amp in bridge mode into 4 ohms, as this represents an equivalent load to each amp of 2 ohms. The amp was not designed to handle this, and will fail. ±42V is the absolute maximum voltage, and should only be used where 4 ohm loads will never be applied.

D1 is a green LED, and should be a standard type. 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.

VR1 is used to set the quiescent current, and normally this will be about 50-100mA. The amp will work happily at lower current, but the distortion starts to be noticeable (on a distortion meter monitored by an oscilloscope) at less than around 20mA (the recommended minimum quiescent current). 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 35V supplies, Q4 and the output drivers (Q5 and Q6) will normally not require a heatsink. With 4 ohm loads, you may find that a heatsink for Q5 and Q6 is needed, but my experience is that these transistors should not get hot under most operating conditions.

If using the amp at ±42V, a small heatsink should be used for Q4, as the dissipation will be quite a bit higher and the device will get very warm.

Although I have shown MJL4281A and MJL4302A output transistors, these have been available for over 6 years and are still hard to get. The recommended alternatives are MJL21193 and MJL21194.

Note: 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 (almost certainly enough to blow the output transistors!).

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