Abstract: A subscriber's line loop including a terminal station and symmetrically arranged in the loop relative to the station two constant current sources. Each current sources has a power unit defining an output connected to the terminal station, a feedback output, and a control input, a differential amplifier having a non-inverting input connected to a source of reference voltage. In order to suppress interference signals induced into the power unit, a bandpass filter is connected between the output of the differential amplifier and an adder connected to the inverting input of the amplifier. The other input of the adder is connected to the feedback output of the power unit. In a modification, a band rejection filter is interconnected between the control input of the power unit and the output of the differential amplifier.