The transistors form a differential pair with an active current-source tail. This configuration, known technically as a variable transconductance multiplier, has an output proportional to the product of the two input signals. Multiplication occurs due to the dependence of the transistor transconductance on the emitter current bias. Tremolo (amplitude modulation of an audio frequency by a subaudio oscillator--normally 5-15 Hz) applications require feeding the low frequency oscillator signal into the optional input shown. The gain control pot may be set for optimum depth.