Motorola Hi-Fi power amplifier

Jacint Chapo     
Project non tested



This is a very simple, low cost, Hi-Fi quality power amplifier. You can build it 5 ways, like it’s shown in the table (from 20 W to 80 W RMS).


  • The first thing that you must do, is to measure the end transistors (T3 and T4) amplifying coefficient, the hfe or β. If their disagreement is bigger than 30 %, the amplifier would not give a clear sound. I used MJ3001 and MJ2501 transistors, and this disagreement was around 5%.
  • 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, to some uA-s, or in a lucky situation to zero. I was able to decrease it to 10 uA.



I am very contented with this amplifier. It gives a very good sound quality. Have fun with it!

Jacint Chapo – 6/5/2005

3 Response on “Motorola Hi-Fi power amplifier

    1. i am trying to found out the purpose of c 2 in output of the op amp output pin 6 in the Motorola hi-fi power amplifier circuit.

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