Patent GB-1094070-A (GB)
Title: Improvements in moving-magnet pick-ups
IPC: H04R1/16
Filed: 1964-12-23
Abstract: 1,094,070. Pick-ups. ELECTROACUSTIC G.m.b.H. Dec. 23, 1964 [Dec. 23, 1963], No. 52332/64. Heading H4J. A moving magnet pick-up (Fig. 2) has at least one magnetic circuit which includes polepieces 7a (7b) and a removable stylus subassembly comprising a replaceable oscillatable magnet 3 and a stylus arm 2 secured thereto, each polepiece 7a (7b) being provided with an end portion 8 in magnetic connection but not formed with the remainder of the polepiece 7a (7b). The polepiece end portions 8 are such that the distance between them varies along the length of the magnet 3 lying between them so that a cavity is created in which there is supported a flexible bearing 10 secured to the magnet 3. The connection may be by flexible, spring or adhesive rings 13 or the bearing 10 may be cemented or vulcanized to the magnet 3. When the stylus 1 is deflected, magnetic flux variations occur and voltages are induced in coils and are then transmitted to stereophonically related amplifiers (not shown). The bearing 10 is a plug fit in a cavity (9), Fig. 3 (not shown). Damping of the magnet 3 is effected by cushioning material 19 in a casing 17 and also by the bearing 10. The casing 17 is of non-magnetic material, and has a flange 20 which is secured to the bearing 10 and engages in grooves in the polepiece end portions 8. Assembly.-The stylus holder sub-assembly 1, 2, 3 and bearing 10 are placed in a transparent insert 14 from the right-hand side and are located as shown in Fig. 2 to abut against the end portions 8. The insert 14 is then placed in a housing 15 so that the front surfaces of the polepieces 7a, 7b abut against the bearing 10. A guide nose 24a on the insert 14 fixes the insert 14 relative to the housing 15. The end portions 8 and the polepieces 7a, 7b assist in registration by having surfaces in contact and such contact provides a magnetic connection. The casing 17 is flexibly connected to the polepieces 7a, 7b and its cylindrical wall bulges outwardly so that it is pressed inwardly by the pieces 7a, 7b. Polepieces 7a, 7b may be cast integrally with the housing 15 and with coils. A grip member 26 including a shield 27 for the stylus arm 2 is integral with or connected to the insert 14. In a monaural pick-up one magnetic circuit is omitted so that only lateral or vertical recordings are played back. For normal stereophonic recordings the magnetic circuits in the pick-up are rotated through 45 degrees while the orientation of the stylus 1 is left unchanged (Fig. 3, not shown). The portions 8 may be cast in the inset 14, which may be of aluminium. The bearing 10 may be narrower than its cavity so that an air gap exists on each side of the bearing 10. In another embodiment (Fig. 4) a cylinder 29 (see also Fig. 5, not shown), is open at one end 37 to admit the bearing 10 during assembly. A base 31 with openings 32 guides the polepieces 7a. The cylindrical wall of the cylinder 29 (see Fig. 5) is provided with radial openings for the insertion of polepiece end portions 30. A tubular part 33 is slipped over the cylinder 29. A nose (38) (see Fig. 5) on the cylinder 29 engages a groove (not shown) on the tubular part 33 to prevent relative rotation. The end portions 30 are cranked and engage the polepieces 7a by an integral arm 34. The bearing 10 is large and is covered by an outer flange 20 and a cover 39. The flange 20 and cover 39 are constructed with a bulge and are pressed flat during assembly. Alternatively (Fig. 6, not shown), the cylindrical edge (40) of the bearing 10 is slitted to provide a flexible tongue (41). Rubber packing (not shown) between the assembly 29 and the end portions 30 may assist in registration. Alternative embodiments.-Wall parts (43) Fig. 7 (not shown), of the end portions (8) and the polepieces (11) may be shaped to leave a wedge-shaped space (44) between the end portions (8) and polepieces (11) and the flexible bearing (10). Alternatively, the side walls of the bearing (10) may be bevelled. The bearing is then only supported at its periphery, thereby resulting in greater flexibility and a small restoring force for the bearing. The touching surfaces of the polepieces (11) Fig. 8 (not shown), and the end portions (8) may be flush with a side edge of the bearing (10). Alternatively, the touching surfaces of the polepieces (11) and end portions (8) may be at an angle of 45 degrees, Fig. 9 (not shown). A notch (50) in the end portion (8) receives the bearing (10).