Patent GB-416436-A (GB)
Title: Improvements in apparatus for reproducing and/or recording sounds
IPC: H04R27/02
Filed: 1933-03-28
Abstract: 416,436. Valve amplifying circuits. POLIAKOFF, J., and POLIAKOFF, A., 14, Sutcliffe Close, Hampstead Garden Suburb, London. March 28, 1933, Nos. 9323 and 22744. [Class 40 (v).] An amplifier for reproducing sound derived from wireless reception or gramophone records is provided with a loud-speaker for the use of persons with normal hearing and with headphones for the use of deaf persons ; and a switching arrangement is provided by means of which the loud-speaker is switched from the output circuit to an input circuit so that it picks up local sounds and enables them to be heard by the deaf persons. Alternatively, the loud-speaker may be disconnected, and a separate microphone switched into the input circuit. An ordinary wireless receiver may be adapted as a deaf aid instrument by the addition of a unit which is connected to the set through the medium of a valve adapter. The three-valve amplifier V1, V2, V3 of Fig. 1 has its input grid connected alternatively to receive broadcast programmes by closingswitch GS, or local programmes by closing switch GP. The output circuit includes head-phones H with volume control R6 for use by the deaf, and a loud-speaker T3 for normal listening, the latter being connected through switches 1, 5. For use as a deaf aid, the switch GP is closed, contacts 1, 5 are opened, and contacts 2, 3, 4 are closed. The speaker T3 is thus disconnected from the output valve V3, and is connected through contacts 2, 4 across a potentiometer R3, R4, the part R4 being in the grid circuit of the valve V1. The contact 3 connects a resistance R2 in the plate circuit of the valve V3 equivalent to the disconnected loud-speaker T3 ; this resistance may be omitted, and the disconnection of the speaker may be accompanied by a switching of the transformer ratio of the phone transformer T2. A gramophone pick-up may be connected to terminals PU across the resistance R4. The resistance R3 is shunted by a condenser C4 to accentuate higher frequencies, and tone may be further controlled by an intervalve transformer T1 constructed as in Specification 381,540, [Group XXXV]. In a modification, the circuit is modified so that a separate microphone instead of the loud-speaker is used for picking-up local sounds (Fig. 5, not shown). In a further modification (Fig. 2, not shown), the loud-speaker, when switched over to act as a receiver of local sounds, is used to modulate the local oscillator of a superheterodyne receiver, the switch serving to adjust its frequency to agree with the intermediate frequency. Fig. 3 shows the circuits of an adapter for converting an ordinary receiver for deaf use. The loud-speaker T3 and the plate lead of the output valve are disconnected from the valve plate B and connected to a terminal E on an adapter. When switch 11 is closed, the set operates as before conversion, except that phones H are now in circuit. Opening switch 11 and closing switch 12 disconnects the speaker T3 from the output circuit and connects it through a condenser C3 and a high-pass filter to the grid circuit of an earlier valve. Preferably this connection is made at the gramophone pick-up terminals, if such are provided. In another modification, Fig. 4, the adapter has three switch contacts, one 15 for disconnecting the speaker T3, another 14 for closing the circuit of a microphone M through a transformer primary T4, and the third 13 for connecting the secondary of transformer T4 to pick-up terminals PU.