GB1318665A - Electromagnetic pickup cartridge - Google Patents

Abstract

1318665 Gramophone pickups J F GRADO 16 June 1971 [29 June 1970] 28160/71 Heading H4J A stereophonic gramophone pickup has a moving armature 46 (Fig. 1) which in its neutral position is sensibly out of the direct flux path of the profile of the air gaps between a magnetic plate 32 and four pole faces such as pole faces 16e, 18a while being in the fringe fields. The armature 46 moves in response to stylus motion to vary the reluctance of the two air gaps. A diametrically opposed pair of pole pieces 14, 16 have their axes in a plane at right angles to the plane of the axes of pole pieces 18, 20. The pole pieces 14, 16, 18, 20 have pole faces 14a, 16a, 18a, 20a (see Fig. 4) respectively. Coils 22, 24 on the pole pieces 14, 16 are in voltage adding and hum cancelling relation to each other, as are coils 26, 28 on the pole pieces 18, 20. The magnetic circuit includes a rear magnetic plate 30 serving as a pole piece support, a front magnetic plate 32 and an interconnecting permanent magnet 34. Four air gaps are thereby provided. A moulded plastics piece 40 includes a base 40a, a post 40b and an extension 40c (Fig. 3). A conical tip 40d carries a flexible pin 42 acting as a fulcrum for a stylus arm 44 (e.g. of plastics material or aluminium) which projects through an aperture 32b in plate 32. The armature 46 is frusto-conical and moves about a forward pivot P (Fig. 6) in response to stylus movement to vary the reluctance of one or two of the four air gaps. The armature 46 is adhered to a conical part 44a of the stylus arm 44 and is centred by an elastomeric member 48. The construction provides magnetic damping on the armature 46. In a second embodiment the pivot P is rearwardly of the armature, (146) Fig. 7 (not shown), and is surrounded by elastomeric material (148). The armature (146) is a right circular cylinder of magnetic material which, in its neutral position, is sensibly within the profile of the four air gaps. In a third embodiment (Figs. 8, 9, not shown) the single armature is replaced by a substantially cylindrical plastics carrier (246a) carrying four bar armatures (246a, 246b, 246c, 246d) at the requisite 45 degree orientation so that each is in proximity to an air gap, but in its neutral position is sensibly out of the direct flux path. The four bar armature construction is used where very low cross-talk is required.