Abstract: "Hands free" telephone sets in which there is connected in parallel with the terminals (A) and (B) of a first path (C.sub.1) comprising the coupling circuit of the telephone set in series with a controllable amplifier (11) of the receiving path (6), a second path (C.sub.2) comprising a controllable amplifier (12) which is controlled by a linear regulator (13) which keeps its output signal constant. The gains of the two amplifiers (11) and (12) respond to each other. A device is provided so as to ensure that the gain in the second path remains higher than the gain in the first path. A third controllable amplifier (5) is provided between the microphone (4) and the input terminal (A) which is used in common by the first and second paths; the said means (14) are adjusted in such a way that the electro-acoustic loop which is closed by the second path (C.sub.2) is permanently the source of non-annoying oscillations.

Abstract: In this subscriber line interface circuit which comprises a pair of amplifiers whose outputs are connected to the subscriber's line, the input terminals of these amplifiers receive by means of negative feedback voltages having opposite phases depending on the transversal current I produced by the line interface circuit and independent of the longitudinal currents I accidentally produced on the subscriber's line. The two load impedances, which are connected between the supply terminals of the amplifiers and the terminals of a d.c. voltage supply source are controlled by means of their control terminals by voltages which have the same characteristics as the voltages applied to the input terminals of the amplifiers but which in addition have been shifted with a certain shift voltage V.sub.z. Consequently, the amplifiers of the line interface circuit operate completely independent of the longitudinal currents.

Abstract: Apparatus for detecting the closure of a subscriber's telephone line in the presence of an alternating ringing voltage superimposed on a direct voltage comprises circuitry for producing a unidirectional current from the combined alternating and direct voltages. When the line is open no direct current will flow and hence the peak value of successive half cycles will be equal whereas when the loop is closed and direct current flows the peak values of successive peaks will be different. A capacitor (26) is charged by a voltage (U.sub.1), which is proportional to the combined voltage, through a transistor (24), provided that the voltage (U.sub.1) is greater than the voltage across the capacitor, and discharged through a resistor (27). The time constant of the discharge circuit is selected so that when the line is looped the smaller peak is lower than the voltage to which the capacitor is discharged and hence charging current flows into capacitor (26) on alternate peaks of the voltage (U.sub.1) only.