PURPOSE: To prevent generation of a transient sound by performing turning-on and off at an AC zero level time of sound signals.

CONSTITUTION: Detector 3 outputs pulses shown in (F) by the AC zero level of sound signals (A); and when the high level of control signals (B) is given from terminal 8, the output of AND gate 5 becomes as shown in (G); and then, since the output of R-S flip flop 4 generates rise pulse (H) synchronized with the AC zero level of sound signals, transistor 1 becomes conductive, and sound signals are broken at an AC zero level point as shown in (I). In case of sound signals are outputted again, sound signals are similarly outputted from the AC zero level point by inverter 7, AND gate 6 and flip flop 4.