Abstract: A gain control circuit is connected between an output terminal of a transistor detector circuit for audio-frequency or voice signal and an audio amplifier circuit. The gain control circuit comprises a control transistor having an output electrode at which an output potential is produced which is lowered correspondingly as the potential at the output electrode of the detection transistor is decreased, and is utilized for controlling the gain of the audio amplifier circuit. The gain control circuit further comprises time constant circuit, for controlling the control time and the restoring time of the potentials at the output electrodes of the above two transistors and a switching circuit.

Abstract: An automatic level controller comprising a switching transistor connected to the output stage of an audio amplifying circuit to be turned off when the output signal of the audio amplifying circuit attains a predetermined level, a control transistor connected to the input stage of the audio amplifying circuit for bypassing to the ground a portion of the audio signal applied to the audio amplifying circuit thereby controlling the audio level, a diode adapted to be turned on in response to the turn-off of the switching transistor for supplying current from a power source to the audio level control transistor as an audio level control signal, and a time constant circut for setting the attack period and release period of control by the audio level control signal.