GB509613A - Improvements in electric signal amplifiers incorporating voice-operated devices - Google Patents

Abstract

509,613. Two-way transmission. RYALL, L. E. Jan. 20, 1938, No. 1852. [Class 40 (iv)] In a loud-speaking system with voiceoperated switching devices, break-in is permitted, but acoustic reaction prevented, by arranging that the output of the loud-speaker is deficient in low frequencies and that the switching devices which condition the transmitting path are responsive to the preponderance of low-frequencies present in actual speech. The deficiency in low-frequencies of the loud-speaker output may be due to such deficiency in the distant microphone which is natural in the carbon type and may be deliberately effected in, say, a moving coil type in order also to reduce the effects of room reverberation. The transmitting and receiving amplifier may as regards speech as distinguished from control paths favour the high frequencies and the loud-speaker may itself be deficient in low frequencies. The control path of the transmitting amplifier favours low frequencies. In the arrangement shown, which is similar in some respects to arrangements described in Specifications 415,767, 416,372 and 430,567, a bridge comprising resistances 13, 14 and valves 3, 8 of the transmitting and receiving amplifiers respectively, is adjusted so that normally the points 15, 16 are at the same potential and a small current flowing from 16 to 17 controls rectifiers 18-20 associated with a winding of the input transformer 7 of the receiving amplifier 3 and rectifier 24, 25 associated with a winding of the speech output transformer 5 of the transmitting amplifier 8, whereby the output transformer is effectively shunted and the input transformer is operative. Speech currents from the microphone are rectified in the secondary circuit of a control output transformer 4 and reduce the negative bias of the receiving valve 8 whereby the balance of the bridge is upset and current flows from 17 to 16 to cause the conditions of the input and output transformers to be reversed if the lowfrequency output of valve 3 predominates, transformer 4 feeding the rectifier being tuned to a low frequency. Speech incoming from the line is rectified at a secondary winding of transformer 9, another winding of which supplies the loud-speaker 10 and the rectifier current increases the effect of the normal current between 16 and 17. The rectifier load on the amplifying valves, in series with speech path load in case of amplifier 3 but in parallel in case of amplifier 8, gives rise to harmonic distortion in the speech paths and to minimize this effect, amplifier 3 is arranged for series negative feed-back over resistance 26 and amplifier 8 is arranged for parallel feed-back by means of a further secondary winding 23 of output transformer 9, whereby the outputs of the amplifiers are substantially independent of the load impedance of the associated rectified control-current supply circuits.