Abstract: A ringing signal supply, provided at a concentrator remote from a switching office, compares a fraction of its output with a low level reference ringing signal supplied from the switching office to product an error signal. The error signal, if it exceeds a positive or negative threshold, is gated during respective half-cycles of a rectangular alternating voltage produced by an inverter to control a bidirectional switch to connect the inverter output to the input of a low pass filter, whose output constitutes the output ringing signal. In this manner the instantaneous voltage of the output ringing signal is modified to reduce the error signal, whereby the output ringing signal has the same frequency and waveform as the reference ringing signal. The inverter can produce the alternating voltage commonly for a plurality of ringing signal supplies.