GB715401A - Improvements in and relating to gain control circuit means for line and radio telephony - Google Patents

Abstract

715,401. Automatic volume control. PLESSEY CO., Ltd. March 4, 1953 [Feb. 1, 1952], No. 2763/52. Class 40 (4) [Also in Group XL (c)] In a volume compressor arranged to mute in the absence of signals so as to suppress background noise, there is produced a negative D.C. potential proportional to the output, which is applied to a compressor valve as grid bias via two independent paths in parallel, which respectively effect muting and volume compression. Signals are applied to the compressor valve V1 via a transformer T1 and are taken from a subsequent output valve V1 via transformer T2. A rectifier M3 is connected by way of a potentiometer R12, 13 and blocking condenser C3 to the anode of valve V2, and produces across condenser C2 a negative potential proportional to signal output. In the absence of signals the valve V1 is given a positive bias via resistances R8, 10, 9, 2, so that grid current renders the input impedance low relative to that of series resistance R1: rectifiers M1, 2 are biased to be respectively conductive and non-conductive. As the output level and the negative D.C. potential across condenser C2 increases, the potential of the point "B" moves negatively until grid current in the valve V1 ceases, giving full gain. The rectifier M2 becomes conductive and clamps the potential of point "B". Upon sufficient further increase of negative potential across condenser C2 the rectifier M1 is rendered non-conductive: the potential is then communicated to the grid of valve V1 via resistance R6, 3 so as to cause volume compression. The point at which muting occurs may be adjusted by means of the potentiometer R8 and the compression point by resistance R5.