Motorola Hi-Fi power amplifier



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

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what does c2 at the mc1741 output do


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.


This amplifier is easy to build even point to point it will work very well if tou use the right parts and clean soldering.
also building with some different parts is posible, but sometimes you must make a correction if you cann’t get the right parts. this amplifier is powerfull enough to use at home and the heavy ampli. you can use like a powerampli.
this amplifier works the best with some more capacitors of 4700 uF in the power supply.
A nice one to build and to listen to.


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