an apparatus for driving speakers includes user - configurable amplifier cells for driving selected speakers , a synchronous rectifier circuit , and a control system . each cell connects between power rails . the rectifier circuit provides current on the rails for consumption by the cells . the current depends on the configuration . the control system implements a model of the rectifier circuit and cells and uses it to control an audio input signal in response to information concerning electrical outputs of the amplifier cells . this indirectly limits current drawn from the synchronous rectifier circuit .