PURPOSE: To prevent the generation of an eary rasping noise due to a BBD securely, by driving a muting circuit through a muting switch when a microphone is not in use and cutting off the output signal of the BBD.

CONSTITUTION: When a microphone switch 4 is turned on, an interlocking muting switch 11 is also turned on to ground the base of the muting transistor (TR)13 of the muting circuit 12, which is then not in operation. Therefore, the output signal of the BBD6 is applied, as it is, to the 1st mixing circuit 7 through a variable resistance 16. When the micropnone switch 16 is turned off, the interlocking muting switch 11 is also turned off, so a prescribed bias is applied to the base of the muting TR13, which turns on. Consequently, a noise output of the BBD6 is grounded through the variable resistance 16 and the collector-emitter path of the TR13 and not applied to the 1st mixing circuit 7, so the noise originating from to the BBD6 is outputted from a speaker 10.