Patent GB-602768-A (GB)
Title: Improvements in or relating to volume expansion systems for audio and like amplifiers
IPC: H03G7/04
Filed: 1945-10-23
Abstract: 602,768. Valve amplifying circuits. RUDKIN, E. P., and STANDARD TELEPHONES & CABLES, Ltd. Oct. 23, 1945, No. 27858. [Class 40 (vi)] In a contrast expansion amplifier, positive and negative feedback paths are provided, controlled automatically from the input to vary the gain of the amplifier. The control may be effective on either or both feedback paths and when pentode output valves are used, a further negative feedback associated with them may be increased with the input to reduce the distortion of the system at large amplitudes. The feedback paths are controlled by voltages derived from the input over an auxiliary amplifier and rectifiers, and the gain of the auxiliary amplifier may be controlled from a similar system involving a further amplifier and rectifier. The output of a R.C. coupled main amplifier, Fig. 3 (not shown), is applied over potentiometers 403, 431, Fig. 4, to the inner control grids of hexodes V1, V2, the output of hexode V1 being applied to the grid circuit of the preceding valve of the main amplifier to give negative feedback. The outer control grid is maintained at a low potential, in the absence of a signal input to the amplifier so that the negative feedback is fully effective. The input is fed to an auxiliary amplifier, the last valve V3 of which feeds a diode D2 which derives a negative controlling voltage, applied to the outer control grid of the hexode V1 to decrease the gain and hence the amount of negative feedback, the battery B2 giving a variable amount of delay. In a similar manner, the gain of hexode V2 is varied in the opposite sense by a controlling voltage from diode D1, the output A being taken from the cathode circuit to give positive feedback increasing in effect with amplitude of input signal. Inductors L1, L2 are included to minimize phaseshift effects. In a modification, Fig. 6, positive and negative feedback paths are provided for the main amplifying valves 600, 601 by resistor networks 604, 605, 607 and 611, 610, the resistors 605, 611 being shunted by controlling valves V5, V4, the bias of which is varied by diodes D1, D2 fed by the auxiliary amplifier 620. In a modification, Fig. 7 (not shown), two windings, in opposite senses on an output transformer, feed two controlled valves, the outputs being connected to a preceding amplifier stage. In a further modification, Fig. 8, the output from a valve 101 in the main amplifier feeds hexodes 102, 103, the anode circuits of which are connected to the cathode and grid circuits of valve 100 to give positive and negative feedback respectively. The gain of the hexodes is stabilised by feedback resistors 104, 105 and filters 150, 160 are included in the feedback path to give a rising bass response at low signal levels and rising treble response at high input. In a further modification, Fig. 9 (not shown), one of the series resistors in each of the feedback paths is replaced by a valve controlled by bias derived from the signal. In a further modification, Fig. 12, the amplifier valve 1500 feeds a bridge 1020, 1030, 1050 and 1060, the latter arm being shunted by the impedance of valve 1090. Negative and positive feedback paths are constituted by connections from the junctions of resistors 1020, 1030 and 1050, 1060 to the cathode-grid of an amplifier, the variation of impedance of valve 1090 causing the net feedback to become more positive with increasing signal. In a further modification, Fig. 14 (not shown), the output of the amplifier is applied to two cathode followers using tetrodes with the outer grid controlled by bias developed by rectifying the output of the auxiliary amplifier. The cathodes feed transformers connected to the grid or cathode circuit of an earlier valve to give positive and negative feedback. Specification 518,128 is referred to.