In some audio applications it is sometimes necessary to limit the amount of power used by an audio amplifier due to power constraints. In a particular application, an audio amplifier that drives the speakers for a computer sound system has a limited amount of power available to the amplifier. In this system, an audio power amplifier is powered directly from a personal computer's USB (Universal Serial Bus) port. This is desirable to reduce the audio speakers wiring to a single connection to the speakers from the USB port. The USB interface connection includes power as well as a data bus. Power limitations of the USB port are defined by an accepted bus interface standard. The power is typically limited to ½ amp at 5 volts.
Overdriving the audio without limiting the current drawn on the USB port could damage the PC's USB drive circuits or cause them to current limit and shut down. Some prior art circuits would cut the current to the audio amplifier if the current draw rose to high. This would case an undesirable pause or stop in the audio output.