PURPOSE: To prevent useless power consumption at the time of middle or small output while preventing the occurrence of the distortion of a signal at the time of large output.

CONSTITUTION: An amplifier circuit 5 which amplifies a control signal varying the amplitude and the like of a sound signal is provided. The amplifier signal of the amplifier circuit drives a control signal generating part 6, and outputs a voltage control signal corresponding to the amplifier signal. The voltage control signal is inputted to a voltage control circuit part 1 which varies power voltage to a sound output amplifier 4 provided for the last stage of a sound output equipment. The voltage control circuit part 1 varies and controls the power voltage of the sound output amplifier 4 in accordance with the voltage control signal. As the output signal of the sound output amplifier 4 varies in accordance with the control signal, the power voltage of the sound output amplifier 4 varies automatically to a necessary value in accordance with the output signal.