PURPOSE: To attain low loss and high efficiency of a class A output amplifier, by controlling power supply voltages variably up to positive and negative region in followup to the amplitude of input signals to be amplified in a power supply used for an audio reproducer.

CONSTITUTION: In a figure, 3 is a speaker and 5 is a control circuit, the anode of the 1st flywheel diode 17 is connected to a negative DC power supply 12 and the cathode of the 2nd flywheel diode 18 is connected to a positive DC power supply. Through the connection, the collector voltage of switching transistors (TRs) 13, 14 is swung to positive and negative polarity, and power supply voltages e1, e2 applied to a class A output amplifier 2, being output voltages of a variable power supply 4, are also swung to both the polarities. Thus, the output voltages e1, e2 of the power supply 4 are changed in follow-up to an output voltage e0 over the entire period of the output voltage e0 of the amplifier 2, an offset voltage E0 only is applied to an output TR of the amplifier 2, allowing to reduce power loss.