How to build Motorola Hi-Fi power amplifier

Description

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).

Some comments:

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.

Circuit diagram

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

author: Jacint Chapo, jacintch@gmail.com
circuit from http://www.electronics-lab.com/