An audio amplifier includes a plurality of power stages, a driving circuit, and a power stage control circuit. The driving circuit is arranged to drive the power stages. The power stage control circuit includes a feedback circuit and a control circuit. The feedback circuit is coupled to the power stages, and is arranged to generate a feedback signal according to at least one detection input, wherein the at least one detection input includes at least one of a power, a voltage signal corresponding to a switching time of the power stages, and a voltage signal corresponding to a switching frequency of the power stages. The control circuit is coupled between the feedback circuit and the power stages, and is arranged to generate a control signal according to the feedback signal, wherein the control signal is arranged to dynamically control a number of turned-on power stages in the power stages.