An audio amplifier system comprising: a variable gain audio processor configured to receive a digital audio signal and output a processed digital audio signal, a digital to analog converter coupled to the audio processor, and configured to receive the processed digital audio signal, a variable gain amplifier having an input coupled to the output of the digital to analog converter and operably connected to a power supply having at least two power supply values, and a controller coupled to the variable gain audio processor and the variable gain amplifier and configured to switch the audio amplifier system between a first operating mode having a first power supply voltage value and a second operating mode having a second higher power supply voltage value; wherein the controller is operable in the first operating mode to set the audio amplifier system gain to a predetermined gain value, and in the second operating mode to maintain the amplifier system gain at the predetermined gain value by increasing the gain of the variable gain amplifier and decreasing the gain of the variable gain audio processor with respect to the first operating mode; wherein the variable gain audio processor further comprises a sigma-delta modulator (446,600), having an input connected to the digital audio input, the sigma-delta modulator comprising a gain corrector (608, 610) coupled to the controller, the gain corrector being selectable to increase the gain of the digital audio signal by a gain factor in the first operating mode and to apply a unity gain to the digital audio signal in the second operating mode; the gain corrector being arranged between a loop filter and a quantizer and a further gain corrector (614;616) arranged in the feedback path of the sigma-delta modulator and coupled to the controller, wherein the further gain corrector is selectable to attenuate the processed digital audio signal by the gain factor in the first operating mode and to apply a unity gain to the processed digital audio signal in the second operating mode; and wherein the controller further comprises a detector coupled to the output of sigma-delta modulator and wherein the controller is further configured to vary the gain of the variable gain audio processor and the variable gain amplifier in response to a predetermined number of consecutive zeros being detected in the processed digital signal by the detector. a variable gain audio processor configured to receive a digital audio signal and output a processed digital audio signal, a digital to analog converter coupled to the audio processor, and configured to receive the processed digital audio signal, a variable gain amplifier having an input coupled to the output of the digital to analog converter and operably connected to a power supply having at least two power supply values, and a controller coupled to the variable gain audio processor and the variable gain amplifier and configured to switch the audio amplifier system between a first operating mode having a first power supply voltage value and a second operating mode having a second higher power supply voltage value; wherein the controller is operable in the first operating mode to set the audio amplifier system gain to a predetermined gain value, and in the second operating mode to maintain the amplifier system gain at the predetermined gain value by increasing the gain of the variable gain amplifier and decreasing the gain of the variable gain audio processor with respect to the first operating mode; wherein the variable gain audio processor further comprises a sigma-delta modulator (446,600), having an input connected to the digital audio input, the sigma-delta modulator comprising a gain corrector (608, 610) coupled to the controller, the gain corrector being selectable to increase the gain of the digital audio signal by a gain factor in the first operating mode and to apply a unity gain to the digital audio signal in the second operating mode; the gain corrector being arranged between a loop filter and a quantizer and a further gain corrector (614;616) arranged in the feedback path of the sigma-delta modulator and coupled to the controller, wherein the further gain corrector is selectable to attenuate the processed digital audio signal by the gain factor in the first operating mode and to apply a unity gain to the processed digital audio signal in the second operating mode; and wherein the controller further comprises a detector coupled to the output of sigma-delta modulator and wherein the controller is further configured to vary the gain of the variable gain audio processor and the variable gain amplifier in response to a predetermined number of consecutive zeros being detected in the processed digital signal by the detector.