Abstract: A novel and improved automatic gain control (AGC) circuit which is ideally suited for use with sound systems which are operated in a very large environment, such as a supermarket or auditorium. The preferred embodiment of the circuit comprises first and second input channels and a single output channel. The first input channel is a feedback signal provided by a microphone which is placed near one of the speakers of the sound system. The second input channel receives a music signal which is being played by the sound system. The output of the circuit is a music signal which has a constant ratio relative to the noise detected in the environment. Constant power ratio operation of the system is achieved through the use of logarithmic processing of the signals in each of the respective input channels. Control circuitry is provided to maintain the gain level of the system at a constant level during periods of time when no music is present on the second input channel.