The device comprises a first controlled attenuator (13) placed ahead of the amplifier (7) of the loudspeaker (8) and supervised by an oscillation detector (9) at the output of the microphone (1). It is supplemented with a second controlled attenuator (80) inserted at the output of the microphone (1) and controlled by a variable-threshold level detector (83) receiving on an input (831) a threshold-adjusting value delivered by a level-threshold generator (84) operating from the signal for exciting the loudspeaker (8), and on another input (83), the output signal from the microphone (1). This second controlled attenuator (80), brought into service when the signal from the microphone (1) passes below a variable threshold proportional to the level of the signal for exciting the loudspeaker (8), enhances the effectiveness of the anti-Larsen device and improves the subjective listening comfort of the telephone set. Its effect is improved by the presence of a third controlled attenuator (83) inserted ahead of the amplifier (7) of the loudspeaker (8) and controlled in opposition with it. IMAGE
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