Abstract: The invention refers to a circuit for telephone systems for the transmission of alternating current signals such as voice signals from a subscriber telephone system to a central office, wherein a signal transformer is provided with a by-pass for the direct current running in both wires to the central office. According to the invention a current path is provided by an amplifier inserted between the two wires to the central office. The amplifier is controlled by the dc current from the central office and alternating current provided by the transformer. The resistance of the amplifier changes according to control signals in the form of current supplied. The application of an amplifier as a current path between the two wires of the central office enables maintenance of direct and alternating current in the central office and simultaneous transmission and amplification of voice signals. The transformer need only supply a small part of the analog transmission signal energy.