Q-Watt - Simple Audio Power Amplifier (110656) | Elektor Magazine

Before connecting the amplifier directly to the power supply, the quiescent current of the output stage needs to be set. As a precaution, we recommend connecting two 47Ω/5 W power resistors in series with the PCB power supply connection (positive and negative). If something goes wrong (a short circuit occurs somewhere), the amplifier itself will not be damaged, but in the worst case, the two power resistors will burn out. It is better to use a regulated power supply, but who can use a dual power supply that can be set to around +/- 56 V. Before turning on the power, turn P1 fully counterclockwise. The current consumption in the positive power cord should be approximately 30 mA (with activated output relay). The screw terminal K7 should be connected to the power transformer. Connect an ammeter in series (used with a power resistor), and then slowly turn P1 clockwise until the current increases by 30 mA, and the reading is 60 mA. This low setting is more than sufficient. If the heat sink temperature increases, the quiescent current will also increase slightly, but it should be kept below 90 mA. At high power output, the temperature of the junction of the two power transistors T4 and T5 will rise much more than the temperature of the heat sink, so the VBE multiplier T1 will not be able to fully compensate for this. The quiescent current will instantly rise to several hundred mA, but will drop when the power is reduced and the radiator cools again. In this case, the amplifier has good additional functions. You can say that the A-level setting of the output stage increases with the output power provided.

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