PURPOSE: To correct the frequency characteristic and distortion and to reduce noise by providing a switching circuit to switch the negative feedback quantity at muting to the negative feedback circuit provided between the output side and the input side of a low frequency amplifier circuit so as to control the negative feedback quantity properly.

CONSTITUTION: In the depression of a transmission pushbutton switch, a normally closed switch 17 of a switching circuit 12 is opened interlocking therewith and a transistor(TR) 13 is turned off. Thus, the collector and emitter of the TR 13 show a high resistance and sufficient negative feedback from the 2nd low frequency amplifier circuit 4 to the 1st low frequency amplifier circuit 3 is applied. For example, even with a gain decrease of nearly 50dB, noise invasion is suppressed. Moreover, if negative feedback quantity is too large, the possibility of abnormal oscillation might be caused due to phase rotation, but since a capacitor 18 is inserted, the frequency causing ease of oscillation is tapped to prevent abnormal oscillation.