This Motorola Hi-Fi power amplifier circuit are very simple, low cost, and Hi-Fi quality power amplifier. You can build this circuit in 5 options power output, as shown in the table (from 20W, 25W, 35W, 50W, 80W RMS).
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.
