Portable audio devices, such as for example MP3 players, radios, or external headphone amplifiers, are typically powered by a battery. The battery's voltage limits the maximum amplitude of the amplifier's output, and thereby the volume of an attached speaker or headphone. The amplifier's output voltage is inherently limited to stay below the supply voltage of the amplifier's end stage. Audio clipping, a distortion of the audio signal, occurs, if the amplifier's end stage becomes saturated.
Class G audio amplifier circuits are known to conserve power and avoid audio clipping by adjusting the voltage that is supplied to an amplifier's end stage. An exemplary class G headphone amplifier circuit is disclosed in U.S. patent application Ser. No. 12/255,537, which is hereby incorporated by reference. The known audio amplifier circuit comprises a voltage generator that generates pairs of differential output voltage at variable amplitudes which are supplied to a headphone amplifier. The voltage generator is controlled by an automatic signal level detector, which analyzes the amplifier's input signal.
Similarly, U.S. patent application Ser. No. 12/434,424 discloses amplifier circuits and methods of operation thereof. The '424 application discloses a variable voltage power supply, which may be designed to output a plurality of discrete output voltages. The variable voltage power supplies are controlled by the input signal to the amplifier. U.S. patent application Ser. No. 12/434,424 is hereby incorporated by reference.
U.S. Pat. No. 5,834,977 discloses an amplifying circuit comprising a step-up converter, which increases the supply voltage to an amplifying circuit from a battery voltage of 12 volts to an increased voltage of 18 volts based on a comparator which compares the output voltage Va of the amplifier with a fixed reference voltage of 10 volts.