Abstract: A power transformer connected to an amplifier, said transformer being adapted to operate at maximum power output at a relatively high frequency, in the order of 20 KHz. There is a control circuit which produces control pulses at a frequency of 20 KHz. The voltage level at the output of the transformer is compared to the amplitude of the audio signal to be amplified to produce a control signal related to the difference between the two. This control signal acts through a modulator to pass portions of each control pulse, with the duration of the pulse portions that are passed being generally proportional to the magnitude of the control signal. These pulse portions in turn open and close a switch connected to the primary winding of the transformer to control the duration of current pulses which flow through the transformer at a 20 KHz frequency.