PURPOSE: To keep the output constant irrespective of the number of microphones being used by providing a voltage control amplifier(VCA) for comparing the sum total level mixed at every channel and each microphone input, and controlling automatically the sound volume of each channel by its detecting signal.

CONSTITUTION: Pre-amplifiers 1a-1h consisting of an electronic balance circuit by a semiconductor are provided at every channel 1-8, and amplify and output a sound signal from a microphone. Limiter amplifiers 2a-2h receive outputs from the pre-amplifiers 1a-1h, and prevent a distortion at the time of excessive input from the microphone. VCAs 5a-5h control the sound volume at every microphone input level, and output it to two circuits of a voice line and a reference line. A mixed input comparing/detecting circuit 7 compares the sum total level obtained by mixing an output of the reference line, and the maximum value of each microphone input, and outputs its detecting signal. Individual input comparing/detecting circuits 4a-4h compare the input level of each microphone, and an all channel mixed level, and output its detected signal as a sound volume control signal to the VCAs 5a-5h.