Patent GB-847138-A (GB)
Title: Improvements in or relating to pick-ups for sound recordings having a groove for mechanically oscillating a stylus
Inventor: COOPER PATRICK BERNARD
IPC: H04R11/12
Filed: 1957-11-05
Abstract: 847,138. Pick-ups. DECCA RECORD CO. Ltd. Feb. 4, 1959 [Nov. 5, 1957], No. 34508/57. Class 40 (4). In a variable reluctance pick-up for reproducing from a single groove two sets of signals recorded as oscillations in two orthogonal directions lying in a plane perpendicular to the direction of the groove, the magnetic circuit has at least three separate polepieces providing a corresponding number of parallel paths between which paths the flux through the armature may be divided, the polepieces being so disposed with respect to the armature that the division of the flux between the paths differs for the two different directions of oscillation. Three-pole arrangement.-In the pick-up of Fig. 1 a tubular armature 10 of mild steel, which extends parallel to the surface of a record, is resiliently mounted at 11 so as to permit freedom of movement in all directions perpendicular to the groove of the record, and carries at its free end a jewel stylus 12. Damping of the armature may be effected by the rubber block 18. The three polepieces 13, 14, 15 are connected to a common magnetic yoke 16 leading to one pole of a permanent magnet 17, the other end of which is adjacent the armature 10. The armature 10 is normally equidistant from the ends of all three polepieces and lies on the straight line between the polepieces 13 and 15. For reproducing from a record having lateral cut and hill-and-dale cut signals the arrangement of coils shown in Fig. 2 is employed, coils 20 and 22 in series reproducing the former, and coil 21 the latter. For the more common 45/45 recordings the coil arrangement of Fig. 4 may be employed, coils 20 and 21 in series reproducing the RH (right-hand) signal, and coils 21 and 22 the LH signal. Although the magnetic flux can saturate the armature 10 without introducing any distortion, thus facilitating the construction of a light-weight armature assembly, it must not be allowed to saturate the polepieces. Four-pole arrangements.-Although these are not so simple to construct as the three-pole arrangement they are preferred since the latter may give rise to second harmonic distortion. In such an arrangement (Fig. 5) the poles of a first pair of polepieces 30, 31 are arranged one on each side of the armature at a level slightly below that of the armature, and those of a second pair 32, 33 slightly above, the armature normally being equidistant from all four pole-, pieces. For use with a record having lateral cut and hill-and-dale cut signals, two sets of coils 34 to 37 and 38 to 41 are provided on the four polepieces as shown in Fig. 12, the coils of each set being wound in series in such senses that the required signals are additive, and that in the set 34 to 37 the unwanted (vertical) signals are cancelled out. An arrangement of coils for reproducing from 45/45 records is shown in Fig. 11, only one coil per polepiece being required. This arrangement will also give an output from lateral cut records across the terminals RH and LH. Another embodiment is described with reference to Figs. 6 to 9 (not shown) in which the polepieces 30 to 33 do not extend in directions normal to the groove of the record, thus giving a more compact construction which provides less magnetic leakage between the pole faces.