An ultrasonic power supply for driving piezoelectric transducers in an ultrasonic cleaning apparatus comprises a full-wave rectifier for providing unfiltered direct current to an inverter circuit of the clamped mode resonant converter type, which provides alternating current output at the series resonant frequency condition of the transducers. The inverter circuit switching cycle is controlled responsive to means for causing a set average power level to be maintained, means for causing the resonant frequency to be maintained and including means for causing the resonant frequency to be swept symmetrically by an adjustable sweep frequency. Also, power modulation means are provided for optionally providing to the transducers power bursts of a set amplitude, but at the set average power level. As a result of the unfiltered direct current from the full-wave rectifier, the power supply will operate substantially at unity power factor.