Because of input of a digital signal, PWM-input, separately excited, class-D amplifier systems are less prone to be affected by noise than in the conventional case that an analog signal is input. Since a ramp wave that is synchronized with a PWM signal is used as a comparison clock, no beats occur between a PWM clock and a reference clock, making is possible to provide a class-D amplifier system which exhibits a large S/N ratio. Furthermore, the fact that an external clock can be varied provides another advantage that no beat noise occurs even in a set that is disposed close to a radio receiver.