Abstract: A duplexer coupling a subscriber line of a PCM telephone system to a modulation channel and a demodulation channel comprises a plurality of transistorized amplifier stages with input connections from the demodulation channel and output connections to the modulation channel. The modulation channel is decoupled from the demodulation channel by being connected, directly or through one such amplifier stage, to a neutral point of a resistance network inserted between the demodulation channel and a transistor connecting the latter channel to the subscriber line; the neutral point may be the junction of two resistors or the output of an operational amplifier having inverting and noninverting inputs connected to cophasally energized points of the resistance network. Each of the two channels includes a filter and an associated amplifier which may be combined into an active filter of the Tchebycheff type. In the quiescent state of the system the power supply to the active filters may be cut off.