GB518442A - Electro-mechanical vibration translating systems - Google Patents

Abstract

518,442. Electro-acoustic apparatus; thermionic amplifiers. ELECTRICAL RESEARCH PRODUCTS Inc. Sept. 29, 1938, No. 28295. Convention date, Oct. 7, 1937. [Class 40 (iv)] [Also in Group XL] In an electro-mechanical vibration-translation system, e.g., for recording sound, having a magnetic circuit defining two spaced air gaps 14, 18 and a vibrating assembly comprising a resiliently-mounted member 20 supporting a driving coil 15 in one gap and a driven coil 19 in the other gap, the electrical energy generated by the vibration of the driven coil is fed back to the driving coil in phase opposition to the driving energy and the mass and stiffness of the vibrating assembly are so proportioned that it has a single resonant frequency in the range of frequency over which the system is intended to operate. The gaps are formed by an annular pole piece 13 and a centre pole piece 12 and by a spaced pole piece 16 and an extension 17 secured to the centre pole piece. A shield between the driving and driven coils is provided by rings 26, 27 of copper, and a fixed coil 29 located adjacent to coil 19 is connected in opposition to neutralize current induced in the coil 19 by the field of the driving coil. The coil-supporting member 20 is conical and is resiliently mounted on a diaphragm 24 and supported at its lower end by a V-shaped cantilever spring 23 secured to the pole piece 16. The diaphragm 24, which has a corrugation 25, and the conical member 20 are formed with perforations 28 to prevent the generation of additional resonances. The driving and driven coils may be mounted on a former having cylindrical portions joined by a frusto-conical portion, Fig. 5 (not shown). In a modification, Fig. 6, the upper portion of the former is cylindrical and tapers to a flat portion 70 carrying the driven coil 71 with its plane at right-angles to that of the coil 15. As shown in Fig. 3, the output from the driven coil 19 is fed over a loop 40, 41 and a phase-changing network 42 to the control grid of a valve 32, the output from the microphone 30 and amplifier 31 being fed to the suppressor grid 38 of the valve 32. In addition the amplifier employs negative feed back, and voltage from the output transformer 51 is fed through a phase changing network 45, 46 and a condenser 47 to the grid 44 of a valve 33. A condenser 48 and resistance 49 are also incorporated to minimize feed back at high frequencies. In a modified circuit arrangement, Fig. 8 (not shown), the feed back energy from the driven coil is supplied through a phase-shifting device to the control grid of a pentode valve, the driving energy from the microphone amplifier being applied to the suppressor grid of the valve. In a further modification, Fig. 9, the feed-back impulses from the driven coil 109 of the recorder are fed through a phase-shifting device 120 to a transformer 139 and to the grids 140, 141 of valves 131, 132 in push-pull. The signals to be recorded are fed from the microphone 101 to the driving coil 106 through amplifiers 133, 134 and the valves 131, 132.