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An energy-efficient consumer device audio power output stage with gain control provides improved battery life and reduced power dissipation without clipping the audio output signal. A control circuit controls a power supply that supplies the power supply rails to the power amplified output stage. Th

An energy-efficient consumer device audio power output stage with gain control provides improved battery life and reduced power dissipation without clipping the audio output signal. A control circuit controls a power supply that supplies the power supply rails to the power amplified output stage. The voltage of the power supply rails is controlled in conformity with an input audio signal level, which may be determined from a volume control setting of the device and/or from a signal level detector that determines the amplitude of the signal being amplified. The gain applied to the audio input signal is reduced for a predetermined time period when a higher output voltage of the power supply is selected, to avoid clipping the audio output signal.

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1. An audio amplifier circuit for providing an output signal to an audio transducer, the audio amplifier circuit comprising: a power amplifier having an audio input for receiving an audio input signal, an audio output for providing the output signal, and a pair of power supply rail connections;a pow

1. An audio amplifier circuit for providing an output signal to an audio transducer, the audio amplifier circuit comprising: a power amplifier having an audio input for receiving an audio input signal, an audio output for providing the output signal, and a pair of power supply rail connections;a power supply for providing a power supply voltage across the pair of power supply rail connections of the power amplifier, wherein the power supply has a control input for controlling the power supply voltage, whereby power consumption of the power amplifier is reduced when a magnitude of the audio input signal is reduced;a control circuit for providing a control signal to the control input of the power supply in conformity with an indication of the magnitude, so that the control signal causes the power supply to increase the power supply voltage when the indication of magnitude indicates that clipping may occur in the output signal; anda gain control circuit for decreasing the magnitude of the audio input signal in conformity with the indication of the magnitude for a predetermined period until the power supply has increase the power supply voltage, so that the clipping is prevented. 2. The audio amplifier circuit of claim 1, wherein the control signal is a digital signal that selects between two or more discrete values of the power supply voltage. 3. The audio amplifier circuit of claim 1, wherein the power supply has a unipolar power input having a ground terminal and an input voltage terminal, and wherein the power supply voltage is a bipolar output substantially symmetric about a voltage of the ground terminal. 4. The audio amplifier circuit of claim 1, wherein the power supply is a charge pump power supply comprising a plurality of capacitors and a switching circuit for switching the capacitors alternatively from a series to a parallel combination, and wherein in response to the select input, a pattern of switching of the switching circuit is altered, whereby a rational relationship between the power supply voltage in the first operating mode and the power supply voltage in second operating mode is provided. 5. The audio amplifier circuit of claim 4, wherein the charge pump power supply includes a switched-capacitor voltage divider that uses a series-coupling of capacitors in a first switching phase of the second operating mode to produce the rational fraction of the first voltage and couples at least one of the capacitors to at least one of the power supply rail connections in a second switching phase of the second operating mode to apply the rational fraction of the first voltage to the at least one power supply rail connection. 6. The audio amplifier circuit of claim 1, further comprising a first amplifier stage circuit having an output coupled to the audio input of the power amplifier, wherein the first amplifier stage has a pair of first stage power supply rail connections connected to a fixed voltage power supply, wherein a fixed voltage of the fixed voltage power supply does not change substantially with selection of the operating mode of the power supply. 7. The audio amplifier circuit of claim 1, further comprising a control circuit for selecting the operating mode in conformity with a volume setting of the audio amplifier circuit. 8. The audio amplifier circuit of claim 7, wherein the audio amplifier circuit further comprises a potentiometer coupled to the audio input for controlling a magnitude of the output signal, and wherein the control circuit selects the magnitude in conformity with a control level of the potentiometer. 9. The audio amplifier circuit of claim 8, wherein the potentiometer is a digitally controlled potentiometer, and wherein the control level is determined from a digital control value supplied to the potentiometer. 10. A method of improving efficiency of an amplifier for providing an output signal to an audio transducer, the method comprising; receiving an indication of a magnitude of a signal to be amplified by the amplifier;supplying a power supply voltage to the amplifier from the power supply circuit in conformity with a control signal;generating the control signal in conformity with the indication of magnitude, so that the power supply voltage is increased when the indication of magnitude indicates that clipping may occur in the output signal; anddecreasing the magnitude of the audio input signal in conformity with the indication of the magnitude until the power supply has increased the power supply voltage, so that the clipping is prevented. 11. The method of claim 10, wherein the control signal is a digital signal that selects between two or more discrete values of the power supply voltage. 12. The method of claim 10, wherein the power supply circuit has a unipolar power input having a ground terminal and an input voltage terminal, and wherein the power supply voltage is a bipolar output substantially symmetric about a voltage of the ground terminal. 13. The method of claim 10, wherein the power supply circuit is a charge pump power supply comprising a plurality of capacitors and a switching circuit for switching the capacitors alternatively from a series to a parallel combination, and wherein in response to the select input, a pattern of switching of the switching circuit is altered, whereby a rational relationship between the power supply voltage in the first operating mode and the power supply voltage in second operating mode is provided. 14. The method of claim 13, wherein the charge pump power supply includes a switched-capacitor voltage divider that uses a series-coupling of capacitors in a first switching phase of the second operating mode to produce the rational fraction of the first voltage and couples at least one of the capacitors to at least one of the power supply rail connections in a second switching phase of the second operating mode to apply the rational fraction of the first voltage to the at least one power supply rail connection. 15. The method of claim 10, further comprising a first amplifier stage circuit having an output coupled to the audio input of the power amplifier, wherein the first amplifier stage has a pair of first stage power supply rail connections connected to a fixed voltage power supply, wherein a fixed voltage of the fixed voltage power supply does not change substantially with selection of the operating mode of the power supply. 16. The method of claim 10, wherein the receiving receives a volume control level corresponding to a gain of a signal path of the signal and wherein the selecting is performed in conformity with a magnitude of the volume control level. 17. The method of claim 16, wherein the amplifier includes a potentiometer for controlling a magnitude of the output signal, and wherein the control circuit selects the magnitude in conformity with a control level of the potentiometer. 18. The method of claim 16, wherein the potentiometer is a digitally controlled potentiometer, and wherein the control level is determined from a digital control value supplied to the potentiometer. 19. The method of claim 10, wherein the receiving receives an indication of amplitude of the signal, and wherein the selecting is performed in conformity with the indication of magnitude. 20. An integrated circuit, comprising: a power amplifier having an audio input for receiving an audio input signal, an audio output for providing an output signal, and a pair of power supply rail connections;a power supply for providing a power supply voltage across the pair of power supply rail connections of the power amplifier, wherein the power supply has a control input for controlling the power supply voltage, whereby power consumption of the power amplifier is reduced when a magnitude of the audio input signal is reduced;a control circuit for providing a control signal to the control input of the power supply in conformity with an indication of the magnitude, so that the control signal causes the power supply to increase the power supply voltage when the indication of magnitude indicates that clipping may occur in the output signal; anda gain control circuit for decreasing a magnitude of the audio input signal in conformity with the indication of the magnitude until the power supply has increased the power supply voltage, so that the clipping is prevented.