GB687363A - Improvements in electrodynamic pick-up's - Google Patents

Abstract

687,363. Gramophone pick-ups. HANSEN, H. C. May 1,1950 [May 4,1949; May 5, 1949], No. 10707/50. Class 40 (iv). In a moving-coil pick-up the polepieces are embedded in a non-magnetic body, preferably of thermoplastic material, to present between them an elongated air gap in which the armature is arranged, the armature bearings serving to close the ends of the air gap and render it substantially dust-proof. As shown in Fig. 1, the pole-pieces 10, 12, which have wedge-shaped ends to produce a strong field in the air-gap 14, are sealed or moulded in the body 18, which is preferably of thermoplastic material, so as to leave a cylindrical gap between them. The gap has enlarged end portions to receive resilient bearings 34, 36 (Fig. 4) of rubber or the like which support the armature. The body 18 has plate-like extensions 38, 40 (Fig. 1) and a vertical rib-like member provided with a hole 44 to allow fixing of the body to a casing as by a screw. Front portions 48, 50 (Fig. 1) of the polepieces are left exposed as also are the corresponding rear portions. Moulding of body. The polepieces are inserted in a form in engagement with a core member corresponding to the cylindrical air gap and abutting against portions of the form to produce the exposed polepiece surfaces. The form is filled with thermoplastic material and after this has hardened the form is dismantled and the core member removed. Armature (Fig. 4). This comprises a rod-like member 56 having head portions 58, 60 of increased diameter with transverse slots 66, 68 therein. The coil 70 is wound in the slots to lie wholly with a cylindrical surface surrounding the head portions. The stylus arm 72 is fitted to the armature by inserting the bifurcated extension 62 through the holes 76 and deforming its ends. The ring members 34, 36 fit over the head portions 58, 60 and serve to mount the armature accurately in the air gap. Leads from coil. These extend axially from the top of the armature and are soldered to tags 94, 98 (Fig. 1) on the body. Magnetic system. After assembly three rodlike magnets which together form three sides of a rectangle are pushed in from the rear to abut against the rear exposed surfaces of the polepieces, and are then magnetized by bringing the poles of an energizing magnet into contact with the front exposed surfaces of the polepieces.