The circuit below shows a
4-transistor utility amplifier suitable for a variety of projects
including receivers, intercoms, microphones, telephone pick-up coils,
and general audio monitoring. The amplifier has a power isolation
circuit and bandwidth limiting to reduce oscillations and
“motorboating”. The values are not particularly critical and modest
deviations from the indicated values will not significantly degrade the
Three cell battery packs giving about 4.5 volts are recommended for
most transformerless audio amplifiers driving small 8 ohm speakers. The
battery life will be considerably longer than a 9 volt rectangular
battery and the cell resistance will remain lower over the life of the
battery resulting in less distortion and stability problems.
The amplifier may be modified to work with a 9 volt battery if
desired by moving the output transistors’ bias point. Lowering the 33k
resistor connected from the second transistor’s base to ground to about
10k will move the voltage on the output electrolytic capacitor to about
1/2 the supply voltage.
This bias change gives more signal swing before clipping occurs and
this change is not necessary if the volume is adequate. As before, the
two 4.7 ohm resistors may be replaced with a single 10 ohm resistor in
series with either emitter.