Patent GB-919157-A (GB)
Title: Piezoelectric transducer
IPC: H04R17/08
Filed: 1959-02-16
Abstract: 919,157. Stereophonic gramophone pick-ups. ELECTRO-VOICE Inc. Feb. 16, 1959 [March 3, 1958; June 12, 1958], No. 5325/59. Class 40 (4). [Also in Group XL (b)] A stereophonic pick-up cartridge has a polarized piezo-electric rod with two pairs of diametrally opposed electrodes each extending longitudinally parallel to the axis of the rod, the rod being mounted in a casing by electrically conductive means at one end and carrying a stylus at the other end. In the cartridge shown in Figs. 1 to 4 the piezo-electric rod 46 is tubular with its inner surface coated with silver to form an earthed electrode 48 and has strip electrodes 52, 54 and 56, 58. A tubular casing 18 is closed at one end where it is transversely apertured on each side of diametral fins 72, 74, to admit electrically conductive rubber clamping blocks 68, 70, which embrace one end of the rod 46. The rod is also supported at its centre by a rubber ring 82 and carries a rigid arm 76 with a stylus 78. One of each pair of the electrodes 52, 54 and 56, 58 extend into the region of the rubber clamp between blocks 68 and 70, one electrode being in contact with one block, see Fig. 2. The casing 18 is a push fit, with a locating keyway 24, in a terminal housing 10 through the closed end 16 of which terminals 88 and 90 project so that when the casing 18 is inserted the terminals engage the conducting rubber blocks where they project out of the casing 18. A terminal 100 passing through the end 16 is cranked over to provide a contact for a conductive rubber plug 92 forced into the bore of piezo-electric rod 46 to connect up the earth electrode 48. By surrounding the rod 46 with a ring of resistive rubber 86 the electrode pairs are interconnected with resistance and the circuit is as shown in Fig. 7. By using separate resistors the electrodes may be interconnected as shown in Fig. 6. The mounting ring 82 may be made of resistive rubber to avoid the use of a separate ring 86. To prevent torsion of the piezo-electric rod the stylus arm 76 may incline away from the axis of the rod so that the stylus itself lies on the axis. Despite this, two styli may be mounted oppositely on the arm for grooves of different width. The arm is preferably made of polyvinyl chloride but may be made of rubber in which latter case the larger mass required enables the electrode pairs to be directly connected by U- shaped clips 122, 124 separated by an insulator 126 as seen in Fig. 10. The rubber arm 76A carries a metal mount 120 for oppositely directed styli. As an alternative to mounting the piezoelectric rod between rubber clamping blocks, transverse metal spring clips may be used and a wire spring may replace the rubber plug 92 to make connection with the earth electrode 48, see Figs. 8 and 9, not shown. As shown in Fig. 12 each of the four electrodes 222, 224 and 226, 228, may be in contact with one of the metallic mounting blades 230, 232, 234, 236, which pass transversely into the casing 210 and engage with four terminals 240 in a containing terminal housing. By suitable orientation of the electrodes relative to the stylus each pair of electrodes may be associated with one recording channel of the two channel hill and dale or 45 degree out recordings. Circuit connections between the pairs of electrodes with and without an earthed electrode are described with reference to Figs. 20 to 24, not shown. The piezoelectric rods may be of square, circular or cruciform cross-section, and may or may not have earth electrodes. At its simplest the rod may be square with an electrode on each face and be polarised as shown in Fig. 14. Earth electrodes may be provided either in a simple single bore or by a series of bores as shown in Fig. 16, or by a cruciform bore as shown in Fig. 17; the bores being coated with silver, graphite or with material known under the Registered Trade Mark " Aquadag." The rod may be built up of elements separated by a cruciform conductor as shown in Fig. 15, the cruciform conductor comprising thin metal sheet or being coatings on each element. As shown in Fig. 28 the rod may have a cruciform cross-section with earth electrodes in the angles and the electrode pairs being on the outer faces. It is said that this shape may be extruded and fired and the entire body may be coated with silver by evaporation or by pasting and firing. When coated the body is tumbled in powdered aluminium oxide until the conductive layer is abraded from the ends and edges to form the separate electrodes. To avoid total abrasion of the outer electrodes these may be made concave as shown. Alternatively the electrodes may be painted on. The piezo-electric materials preferred are barium titanate, strontium titanate with a ceramic binder and lead zirconium titanate. Specifications 787,115 and 841,657 are referred to.