GB513678A - Improvements in loud-speaking telephone circuit arrangements - Google Patents

Abstract

513,678. Anti-side-tone circuits. RADIO GRAMOPHONE DEVELOPMENT CO., Ltd., and DUFFELL, H. F. Jan. 18, 1938, No. 1675. [Class 40 (iv)] In a loud-speaking telephone system, the coupling between the microphone and the line comprises two parts, each consisting of a valve and output transformer, the loud-speaker being connected across the secondary of one transformer and the arrangement being such that the potential applied to the loud-speaker by this transformer due to microphone output balances the potential applied thereto by the other transformer and incidental couplings. The valves may isolate the microphone from incoming speech. As shown, the output of microphone 4, amplified at 5 if necessary, is applied to the grid circuits of valves 1, 2 having output transformers 11, 21 connected in series across the line, the loud-speaker 32 being connected across the secondary of 11. A variable resistance 7 may be used to effect balancing. Both valves may have their cathodes connected to ground directly over biasing circuits as shown for valve 2, but the bias circuit of valve 1 is preferably connected as shown by lead 8 to line 20 and thence to ground at 33, whereby the valve 1 affords amplification of incoming speech. In systems where a ground at 33 is inconvenient, wire 20 is grounded and the bias circuit of valve 1 is connected to lead 3.