This power supply for a high intensity gas discharge or fluorescent lamp exhibits near unity power factor with no third harmonic distortion, and facilitates highly efficient lamp operation with dimming capability. This is achieved by using an inverter to drive a resonant network that includes the lamp load. A feedback circuit adjusts the switching rate of the inverter to equal the resonant frequency of the network. Sinusoidal output voltage is achieved, together with voltage multiplication to a level sufficient to ignite the lamp and keep it lit during dimming. The inverter is driven by a switching regulator that is supplied with unfiltered, rectified ac power. A filter capacitor, connected across the regulator output, is sufficiently large so as to filter at the frequency of the rectified ac input. The regulator switching duty cycle is controlled in response to the average dc level across this filter capacitor. Lamp dimming is achieved either by adjusting the switching regulator dc output level or by varying the duty cycle of the inverter. The high power factor low distortion regulator concept may also be applied in other application where direct current power is derived from an ac line.