Patent GB-392573-A (GB)
Title: Improvements in or relating to loud-speakers, pick-ups, sound recording and like devices
IPC: H04R9/12
Filed: 1931-11-14
Abstract: 392,573. Sound reproducing and recording devices. RONNEBECK, H. R., 36, Highfield Avenue, Golders Green, London. Nov. 14,1931, Nos. 31589/31 and 18877/32. [Class 40 (iv).] Relates to loud-speakers, gramophone pickups or recorders, and the like, of the kind comprising a short-circuited movable winding of one or more turns encircling a magnetic'core, and means for causing a polarizing flux to traverse the winding, whereby induced eddy currents may be caused to circulate in the winding. The main feature of the invention consists in forming the said winding as a flexible coil which is clamped at one or more points and comprises a relatively rigid portion, adapted as a result of flexure of other parts of the coil to cut the polarizing flux without substantial deformation and' with a translatory motion. A further feature consists in providing, in a device of the kind specified, a branched magnetic circuit 'so formed by a transformer core and a permanent magnet that the flux produced by the magnet subdivides and traverses the core in opposite directions, an air gap between the' magnet and the core, a short-circuited winding arranged in the gap and one or more fixed windings associated with the core; with this construction the net polarizing effect upon the core is about zero, and the variations of flux in the core occurring when the coil vibrates do not affect the total flux traversing the permanent magnet. Figs. 1 and 2 show a pick-up in which a movable " winding " consisting of an aluminium sleeve 6 of rectangular cross section encloses a portion of one limb L of a laminated core 1, the sleeve being carried by a needle-holding lever 5 pivotally mounted at 4 so that it passes through a gap G formed between a thickened portion of the limb L and a polarizing magnet 7 secured to the opposite limb of the core. The sleeve 6 functions as the primary winding of a transformer, having secondary fixed windings 2 surrounding the core, and alternating potentials are induced in these windings as the sleeve 6 is vibrated by the needle. A modification is described in which, instead of being carried between pivot points 4, the vibratory member is mounted on a knife-edge or roller bearing on the lower side of the sleeve 6, which side is considerably shorter than that lying in the air gap and is clamped within a rubber liner. Figs. 7 and 9 show the moving "coil" consisting of a flexible stamping having flanges 6b adapted to be clamped to the laminated core which passes through apertures 6c. A needle-socket 5a is secured to one side of the stamping, which, when the needle vibrates, is thus caused to flex as shown in Fig. 9, so that induced currents are set up therein. A pick-up employing such a flexible " coil " is shown in Fig. 10, 76a being the flexible member and 77 a horseshoe polarizing magnet providing the air gap. Rubber, fluid, or other damping devices (e.g. rubber blocks 14, Fig. 1) may be provided in any of the constructions described. Electrical damping means may also be associated with the fixed windings, to reduce or eliminate resonances. A further modification is shown in Fig. 13, the vibratory member being a closed loop 106 of strip aluminium. The stylus lever (not shown) is attached at 105 to the side lying in the magnetic air gap, the opposite side of the loop being coiled round the core 71. Electromagnets may be used instead of permanent magnets for providing the polarizing flux in any of the constructions, where convenient. The first Provisional Specification also describes other modifications as follows : A loud-speaker diaphragm is connected by a stirrup to a shortcircuited coil which surrounds the middle portion of a laminated core forming a diameter of a ring-shaped magnet. Two radial poles project from the magnet at right-angles to the said core, and partially enclose the coil. The " coil " may consist of an aluminium tube which may be almost completely subdivided by saw-cuts. A further construction employs an iron core built up of " window shaped " stampings having an air gap near the end of one leg, a horse-shoe magnet bridging the-leg containing the gap, and the movable coil being slipped over the leg so that one side of the coil lies in the gap. The second Provisional Specification also refers to the use of a short circuited coil divided into two parts which may or may not be coaxial and are connected by one or more conductors running in any desired direction.