In an audio amplifier having a D-class power amplifier, a noise upon muting is suppressed. A sampling rate converter circuit for sampling rate converting a digital audio signal into a digital audio signal, and a .DELTA..SIGMA. modulation circuit for re-quantizing the digital audio signal into a bit-reduced digital audio signal are provided. Further, a PWM modulation circuit for converting the digital audio signal into a PWM signal, and a D-class power amplifier to which the PWM signal are supplied. Still further, a dither signal forming circuit for superimposing a dither signal SDI on the digital audio signal, and a forming circuit for forming a muting signal SDET are provided. Upon muting, an input side of the sampling rate converter circuit is stopped by the muting signal SDET.