PURPOSE: To prevent noise from being produced at switch changeover and to avoid signal leakage by connecting output terminals of a 1st amplifier circuit and a 2nd amplifier circuit in common and providing a muting switch operating only either of the 1st amplifier circuit and the 2nd amplifier circuit.

CONSTITUTION: When an operation power supply is supplied to an amplifier circuit A through a switch S2, the amplifier circuit A is operated and other amplifier circuit B is not active, a voltage applied to an input terminal 1 is amplified and the result appears at an output terminal 2. When the operating power supply is supplied to the amplifier circuit B conversely, the amplifier circuit B is active and the amplifier circuit A is inactive and a ground voltage (=0) applying to a noninverting input terminal of the amplifier circuit B is outputted to the output terminal 2. Transistors(TRs) Q11-Q17 form the amplifier circuit A and TRs 21-27 constitute the amplifier circuit B. The switch S2 is thrown as shown in figure in this circuit, a TR Q18 is turned off, the TRs Q13, Q17 are turned off and the amplifier circuit A stops its operation.