Patent GB-302190-A (GB)
Title: Improvements in sound-reproducing instruments
IPC: G11B3/31
Filed: 1927-07-11
Abstract: 302,190. Columbia Graphophone Co., Ltd., and Mitchell, F. A. July 11, 1927. Electric sound-boxes ; sound-boxes, mounting ; tool-arms.-Relates to a gramophone pick-up of the kind comprising a U or like shaped permanent or electro field magnet having four pole extensions and an armature mounted on a pivot situated within the space bounded by the pole extensions, and consists in an arrangement in which the axis of the armature pivot lies approximately in the plane of the field magnet and the, pole extensions overlap one another at their extremities. In the construction shown in Figs. 1 and 2 a magnet M, mounted within a casing C is provided with pole pieces N, S, each of which is formed with a pair of extensions N<1>, N<2> and S<1>, S<2>, the end of the extensions overlapping one another to form magnetic air gaps in which the ends of the armature A is located. The armature, which is formed in one with the stylus holder H, is pivotally mounted on the pole pieces by means of flat spring members P<1>, P<2>, the planes of which are at right angles to or otherwise intersect one another. The armature is surrounded by a coil D which is wound on a bobbin D<1> cut away on two opposite sides, preferably as shown in Fig. 5. The armature may be damped by means such as described in Specification 269,978 and comprising rubber &c. pads R, R<\> one or both of which may be adjustable. The stylus K may be held in the holder H without the usual screw grip as described in Specification 302,216. If necessary, a small spring H may be provided for preventing the stylus falling out when not reproducing, or the stylus may be retained by the magnetic, attraction of the pole-pieces N<1>, S'. The inner end H<3> of the needle hole may be inclined to guide the stylus into its correct position when placed on a record and mav also contain a rubber &c. damping pad. Fig. 8 shows the preferred means for connecting the reproducing device with the associated amplifier, the coil D being connected by leads L, L<1> to an auto-transformer E which in. turn is connected to the input transformer F of the amplifier through the regulating contact arm E<1>. Shunting or blocking condensers G, G<1> may be provided as shown so that it is possible to shunt the high-frequencies by means of the condenser G or to block the low frequencies by means of the condenser G<1>. Figs. 9and 10 show a modified form of pick-up in which the upper end A<3> of the armature is arranged close to but not between the upper pole extensions N<2>, S<2> and the combined armature and stylus holder is mounted upon ball bearings as described in Specification 298,685. One of the leads L from the coil D is attached to the metal case C by a screw C<1> whilst the other lead L<3> passes out of the case as shown. The case C is eccentrically pivoted, as shown in Fig. 12, in the forked member J which is mounted on the arm O pivoted at O<1> to the cabinet U. The lead L<3> is connected through the arm O to the lead L whilst the case C is connected to the lead L', the fork J being secured to, but insulated from, the arm O by the clamping screw O<3>, insulating washer J<2> and rubber tube O<2>. Thus, when the reproducing unit is in operation on a record disc W as shown in full lines in Fig. 12, the electrical impulses are conveyed along the two metallic paths C, J, L<1> and L<3>, O, L to the amplifier, but when the unit is thrown back into the rest position C<2> it touches the screw head O<3> and thereby shortcircuits the leads L, L'. Instead of mounting the unit on the arm O it may be supported on two or more link members or arms as described in Specification 300,704.