Q

May 21, 2024

Simple audio power amplifier A kit with the PCB and all the needed components (except

Before connecting the amplifier directly to the power supply the quiescent current of the output stage needs to be set. We advise as a precaution to put two 47 Ω/5 W power resistors in series with the PCB power supply connections (in plus and minus). Should something be wrong (a short circuit somewhere) chance is the amplifier itself will not be damaged but in worst case the two power resistors will burn out. The use of a regulated power supply would be better but who has access to a dual power supply that can be set to +/- 56 V or so. Before switching on the power supply turn P1 fully counter clockwise. Current consumption in the positive power line should be about 30 mA (with activated output relay). Screw terminal K7 should be connected to the transformer of the power supply. Put an ampere meter in series (with the power resistor) and turn P1 slowly clockwise until the current increases 30 mA, resulting in a reading of 60 mA. This low setting is more than enough. If the temperature of the heat sink rises the quiescent current will also rise slightly but should stay below 90 mA. At high power output the temperature of the junctions of the two power transistors T4 and T5 will rise a lot more than the temperature of the heat sink and thus VBE multiplier T1 will not compensate this fully. Momentarily the quiescent current will rise to several 100 mA, but will fall when power is reduced and the heat sink cools down again. In this the amplifier has a nice extra. You could say the class A setting of the output stage rises with the supplied output power.