This is a compressor circuit that can be used for dynamic mic, condenser mic, or electric guitar pickup. This circuit doesn’t produce very good sound output, but it has very stable amplitude/volume. This compressor circuit design is unique since it uses base current control for manipulating the gain. Here is the schematic diagram of the circuit:
The first and second transistor is an ordinary pre-amplifier, but the first transistor’s bias is controlled by the output signal to get a negative feedback. The third transistor is employed to control the dc voltage level at the junction of 1M resistor and 10uF cap that provide the first transistor bias current. The control mechanism is also unique, where a full dc level at the capacitor is discharged at each half cycle of output when it exceed the base forward bias. The gain control method by decreasing the base bias current of first transistor near its cut-off point produce a distortion, which might be unacceptable by some people especially for music/singing, but can be acceptable for public address. For electric guitar application, this compressor can be a good option since the distortion characteristic gives a unique effect, and the overshoot and undershoot of the amplitude stabilization gives something similar to “wow” effect from old recording.