Patent GB-545321-A (GB)
Title: Improvements in amplifying hearing aids
IPC: H04R25/04
Filed: 1940-09-07
Abstract: 545,321. Amplifying-sets for deaf persons. SONOTONE CORPORATION. Sept. 7, 1940, No. 13985. Convention date, Sept. 13, 1939. [Class 40 (iv)] [Also in Group XL] In a deaf-aid amplifier, a small coupling inductance or auto transformer having a core made of high-permeability material such as Allegleny metal or the material sold under the Registered Trade Mark "Mu-metal" and with a cross-section of a tenth of a square inch or less is used to enable sufficient amplification to be obtained with a single voltage-amplifying stage. The Specification illustrates by a series of curves the amplification by a pentode valve of various frequencies with different values of screen voltage and consequently of plate current and shows that variations of screen voltage can be used to control the tone. Another tone control may be provided by shunting the primary of the power-valve output-transformer by a condenser in series with a variable resistance. The bore-conductor receiver described in Specification 444,175 may be used with the amplifier. Set construction.-In the arrangement shown in Figs. 1 and 2, the whole of the apparatus is mounted on a plate 63 fitted into a shallow casing 23 having a cover plate 60. The latter is provided with holes opposite the elasticallymounted microphone and is recessed to prevent the user's clothing from rubbing against the edges of these holes and causing noises. A single plugcoupling is provided for associating the battery and the receiver with the unit, the leads from the latter being connected by plugs to the movable part of the coupling. Controls for the volume and tone-adjusting devices are provided by discs 70 projecting from the edges of the casing. The inside of the casing has a conductive covering and the leads are screened by sheets of conducting material connected to this covering which is also connected to the metal frame of the apparatus elements. The coupling- choke and transformer are separated by a highpermeably magnetic shield. In a modification, Figs. 15 ... 18 (not shown) the microphone is resiliently suspended towards the lower part of the casing and separates the coupling choke and the output transformer. Circuit arrangements.-Volume is controlled by means of a potentiometer 41, Fig. 5. Tone is controlled by a variable resistance 54 shunted, in series with a condenser, across the primary of the output transformer to the secondary of which the normal electromagnetic receiver 29 is connected. When a piezo-electric receiver 90 is used, however, it is connected across the primary which is shunted by a resistance, and the secondary is left open. In a modification, Fig. 14 (not shown), the tone is also modified in accordance with the current flow in the intervalve coupling element, which, in this case, is wound as an auto-transformer by varying the screen voltage of the first valve with the aid of a potentiometer. The saturating effect of current flow in the primary of the output transformer may be counteracted by passing a suitable current through the secondary. The Specification, as open to inspection under Sect. 91, comprises also an arrangement, Fig. 18 (Cancelled) (not shown), in which there are two resistance-coupling voltage amplifying stages and a tone-controlling arrangement similar to that provided in the other systems in the output circuit of the power stage. This subject-matter does not appear in the Specification as accepted.