The first thing you should do, is to measure the end transistors (T3 and T4) amplifying coefficient, the hfe or â. If bigger than 30 %, the amplifier would not give a clean sound. You can used MJ3001 and MJ2501 transistors, and this amplifying coefficient was around 5%.
Just before the first “turning on” you must short circuit the inputs of the amp, and put a mA-meter on the output, than turn the amplifier on, and tune the R13 pot, to decrease the DC current on the output, nearly 0, or in a lucky situation to zero. In this example, it was able to decrease it to 10 uA.
This Motorola Hi-Fi power amplifier 20W to 80W RMS circuit is very satisfying because of exellent sound quality.
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