to the second embodiment of the invention in this embodiment a voltage source control unit 21 is provided instead of the source voltage control unit 11 in the first embodiment the voltage source control unit 21 includes a headroom calculating unit 212 and a voltage control signal creating unit 213 the other configuration is the same as in the first embodiment and thus the configuration and operation thereof will not be described hereinafter the configuration and operation in this embodiment will be described on the basis of the features of this embodiment the amplifier 1 according to the second embodiment changes the value of a source voltage set and apply the source voltage lowering the source voltage when the input audio signal is relatively low and raising the source voltage when the input audio signal is relatively high which is supplied to the amplifier 14 from the voltage variable power source 12 to correspond to the variation in level signal level of an envelope of an input audio signal input to the amplifier 1 when the u201c headroom u201d to be described below is not considered the amplitude of an audio signal output from the amplifier 1 may become greater than the voltage value supplied from the voltage variable power source 12 and the audio signal output from the amplifier 1 would be distorted possibly to what extent the source voltage output from the voltage variable power source 12 can follow the voltage control signal input to the voltage variable power source 12 is determined depending on the capabilities such as the slew rate of the voltage variable power source.