Abstract: To ensure that alternating current supplied to a load has an accurately predetermined voltage waveform each terminal of the load is connected to a d.c. source by way of a respective switching regulator receiving as its reference voltage an alternating voltage of the predetermined waveform and of appropriate phase. If the load is a reactive load each switching regulator is a push-pull regulator enabling the respective load terminal to be connected to either terminal of the d.c. supply. The switching regulators preferably operate at a duty cycle having a repetition rate which is a high multiple of the frequency of the alternating current to be supplied. The regulator operation may be additionally controlled to ensure that the alternating voltages appearing at the load terminals are balanced to earth.