GB655037A - Improvements in or relating to electrical mixer circuits - Google Patents

Abstract

655,037. Telephone transmission systems. STANDARD TELEPHONES & CABLES, Ltd. Sept. 16, 1947, No. 25280. [Class 40 (iv)] In a mixer circuit, Fig. 2, each of a number of generating devices 20-22, such as microphones or gramophone pick-ups, is connected to a common output channel 29 through a variable attenuator 23-25 and a compensating network 26-28. Each attenuator is of the ladder network type 19, as shown in Fig. 1 where the generating device 8 is of impedance L and the following apparatus 10 is of impedance 2L, with a balancing impedance 9 also of value 2L, the attenuator being controlled by a slider 12 moving on a series resistance 13. Across terminals 5, 6 is connected the associated compensating network 31, Fig. 4, of the minimum loss matching pad type, offering an impedance 2L to the ladder network 19 which in turn presents an impedance represented by a resistance 30, Fig. 4, to the compensating network 31, a resistance 34 representing the impedance presented by the rest of the apparatus. Formulµ are given for the various impedances required, which are so arranged that the adjustment of any one attenuator from one limit to the other has only a limited effect upon the impedance presented to the common channel and upon the output of any other generating device not associated with the said adjusted attenuator. Limiting values for such effects are given.