A loudspeaking telephone set incorporating an anti-Larsen device which upon detecting instability controls the insertion of an attenuator in either or both of the listening and transmission channels. Such device incorporates a detection circuit (6) for detecting a start of instability, consisting of two analogue multiplier circuits (11), (15), low-pass filters (18), (20), and two absolute-value convertors (19), (21), these elements being arranged in two parallel chains. One of them is phase-shifted approximately 90 DEG at the Larsen frequency, by a phase-shift circuit (16). At the output of the absolute-value convertors, the signals are added together by an adder (23) which delivers at its output an attenuation control signal after passing through a threshold comparator (24). |