PURPOSE: To prevent shock sounds or harsh reverberation at the power-off time by controlling an electric charge discharging transistor (TR) and a bias cutting-off TR of an amplifier circuit output coupling capacitor to the turn-on state by a starting TR when a circuit power source is turned off.

CONSTITUTION: When the power source is turned off, the base potential of a starting TR Q6 falls quickly though the emitter potential falls slowly, and therefore, the starting TR Q6 is made operatable. since the base current is supplied to a bias cutting-off TR Q7 then, the TR Q7 is turned on. Then, the potential of a bootstrap terminal 10 falls quickly and a required bias is not given to a power TR driving circuit, and an output amplifier circuit 1 is immediately set to the non-operated state at the power-off time therefor to prevent reverberation. Since the electric charge of a capacitor 11 for bootstrap is discharged, the voltage of an output terminal 2 is so quickly reduced that shock sound is not generated.