GB628136A - Improvements in variable tone control and other variable shaping circuit arrangements for thermionic valve circuits - Google Patents

Abstract

628,136. Valve amplifying circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., and ROOYEN, L. J. VAN. Nov. 7, 1946, No. 33165. [Class 40 (vi)] A tone control or other shaping circuit comprises a series network of two outer impedances with a signal-controlled valve connected between them, a potentiometer being connected across part of the network including the. valve with its adjustable tap connected via one or more reactive circuits to a point on one of the outer impedances, the output being taken from points on the network on opposite sides of the valve which are asymmetrically disposed with respect to the mid-point of the network. The figure shows a tone control circuit having outer impedances R3, R4 and the potentiometers R5, R5<1> having values large compared with the internal impedance of the valve. At the midpoint setting of the potentiometers, assuming resistors R3 and R4 to be of equal value, the capacitor C and inductor L have no effect upon the frequency response law of the circuit. As the tapping of potentiometer R5 moves downwards the gain of the stage is increased and as it moves upwards the gain is reduced at the higher frequencies. The potentiometer R5<1> enables the response at the lower frequencies to be controlled in a similar manner. The variable tap of the potentiometer may be connected to earth by a switch via any one of a variety of networks, Fig. 1 (not shown). The resistors R3, R4 should preferably be non- reactive and the tone control potentiometer may be ganged with a gain control so that the effect on the human ear of a flat frequency response over the audio-frequency range may be simulated over a wide range of levels.