This circuit is an AGC system for audio-frequency signals. AGC systems usually consist of three parts: an amplifier, rectifier, and controlled impedance. In this circuit the functions of an amplifier and a rectifier are performed by a single op amp. This makes the system simple and cheap.The rectifier is made with the output push-pull cascade of the op amp and Rs, RL, and CB. The transistor Ql and D1 are used as a voltage-controlled resistance (Z).
The input signal is (Z + R^/Z times, diminished by the voltage divider and 1 + R2/Ri times, amplified by the op amp. C2 eliminates influence of dc bias voltage. R3 protects Ql and D1 from excessive current.