The circuit above 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
performance.
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.
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.
4 Transistor Amplifier for Small Speakers Circuit Diagram
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.
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.
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