Patent GB-865012-A (GB)
Title: Telephone sub-sets
Inventor: LEMAN HUGH SPENCE
IPC: H04M9/08
Filed: 1957-12-20
Abstract: 865,012. Substation circuits. STANDARD TELEPHONES & CABLES Ltd. Dec. 19, 1958 [Dec. 20, 1957], No. 39641/57. Class 40 (4). [Also in Group XXXV] In a telephone subset comprising a hybrid coil for interconnecting the line, the receiver, the transmitter and a balancing network, unwanted out-of-balance signals received by the receiver are amplified and fed back into the balancing network, with such phase as to reduce the unwanted signal. As shown, Fig. 7, the hybrid transformer 8, 9, 15 has its left-hand winding 8 connected to line, its right-hand winding 9 connected to balancing resistor N, its centre tap connected to the input b of a loud-speaker amplifier, and its secondary winding 15 fed by the output of transistor amplifier T2 which is in turn fed by amplified microphone signals received over lead f. Unwanted out-of-balance signals appearing at the centre tap are amplified by transistor T1 and fed back into the coil 9 in antiphase in order to reduce their strength. Since this feed-back is also operative to produce an undesirable reduction on the centre-tap currents due to signals incoming from line, arrangements are provided to keep the gain of the transistor T1 low, by virtue of the emitter resistance R 3 , until the subscriber speaks, whereupon transistor T3 is biased to conduction over lead c, and short-circuits some or all of R3, thereby raising the gain of T1. A similar variation of gain of the amplifier T2 is effected by changing the conductivity of transistor T4 by potential applied over lead d. This ensures constancy of the loss around the acoustic feedback loop as an insurance against howling. Power supplies, Fig. 10.-These are obtained over the line from the exchange via a smoothing inductance L which feeds a rectifier bridge 26 whose output is independent of the line polarity. Zener diode Z provides a constant reference potential for the emitter of transistor T12 which compares this potential with that derived from a potentiometer R25, R26 across the output of the circuit and which is fed to the base of this transistor. Any variation in output voltage thus varies the collector current of T12 which is effective via transistor T11 to vary the emittercollector current of T10 in such a way as to minimize the change in output voltage. A small biasing voltage is provided over lead e and is developed over resistance R5 for use in the circuit of Fig. 11. Condensers C14, C16 ensure that the source has a low A.C. impedance. Microphone and loud-speaker amplifiers. Figs. 8 and 9 (not shown).-These are conventional transistor amplifiers incorporating equalization networks. Switching voltages for directional control, Fig. 11. The microphone amplifier output received over leads g, h feed the emitter follower T5, one of whose outputs feeds the hybrid secondary winding over lead f, as described above, and whose other output is rectified at 32 and amplified by T6 to charge condenser C17. The potential thereon causes T7, T8 to conduct, the latter cutting off T9, so that its emitter and consequently leads c, d become more negative to cut off T3, T4 in Fig. 7, with results as described above. A simple circuit of this type, for use where improved side-tone balance is desired, and using a carbon microphone, is also described, Fig. 12 (not shown).