Patent GB-1332336-A (GB)
Title: Audio system including electrostatic loudspeaker
IPC: H04R19/02
Filed: 1970-12-08
Abstract: 1332336 Electrostatic loud-speakers DAYTON WRIGHT ASSOCIATES Ltd 8 Dec 1970 [8 Dec 1969] 58236/70 Heading H4J Various systems comprising an audio frequency voltage output source and a D.C. biased electrostatic loud-speaker are described. Loud-speaker.-One loud-speaker 10 (Fig. 1) comprises a diaphragm 14 made of materials such as "SARAN", "PLIOFILM" or "MYLAR" (Trade Marks) having one or more highly resistive conductive layers 20, 22 formed from a graphite or carbon/molybdenum suspension. The resistivity is preferably at least 100 megohms per unit square. The one or more layers 20, 22 are bounded by a portion of the diaphragm surface which is substantially electrically non-conductive. Electrodes 16, 18 of perforated metal, metal screen or wire cloth are rigid and are acoustically transparent. They have dished portions 24, the bottoms of which are planar. Four resistors 26 (see also Fig. 8) are connected to the conductive layers 20, 22 by a  foil strip (46) Fig. 6 (not shown) or a strip of  conductive paint. The diaphragm 14 is placed  between two pairs of end shells 30 having  recesses 32 to accommodate resistors 26. Nut and bolt fastenings 40, 36 provide electrical connection from the junction of two resistors 26 to a lug 38. Cups 42 in the end shells 30 support the ends of electrodes 16, 18 (see Fig. 3, not shown). Side braces 48 fit between the diaphragm 14 and electrodes 16, 18. The entire assembly is secured together by epoxy cement spread around the periphery of the electrodes 16, 18 where they contact the end shells 30 and side braces 48 and between the end shells 30 and side braces 48. A stiffener rib (50) Fig. 5B (not shown) preferably extends down the centre between the dished portions 24 of the electrode 16 and is cemented to the electrode 16 and the diaphragm 14. Electrical connection to electrode 16 is made by a bolt 54 having a lug (56) Fig. 2 (not shown). Electrical connection to electrode 18 is made through a bolt 60 which extends through insulators 64, 66 to a lug 62. The electrostatic loud-speaker may be enclosed in a gaseous medium such as sulphur hexafluoride. In one alternative arrangement (Fig. 11) there are two diaphragms joined together by an hermetic seal 504 the space between them being filled with such gaseous medium. Circuitry.-An audio transformer 70 (Fig. 8) has the centre tap of the secondary 76connected through a D.C. source 80 and high resistance 82 to the conductive layers 20, 22 of the diaphragm. The electrodes 16, 18 are connected across the transformer secondary 76. Negative feedback may be applied from the centre tapping of the transformer secondary 76 to the input of the final amplification stage (Fig. 12, not shown). Alternatively a D.C. source (Fig. 10) has an earthed centre tap. The negative and positive sides of the D.C. source are connected through high resistances 182 to the electrodes 223 which are conductive but have high resistivity. One terminal of the transformer secondary 76 is connected to an electrically conductive surface 225 of a diaphragm. The other terminal of the secondary is earthed. In a further alternative arrangement a D.C. source 280 (Fig. 11) also has an earthed centre tap and two movable diaphragms 321 mounted on opposite sides of a rigid electrode 324. The faces of the diaphragms 321 facing electrode 324 have high resistivity conductive layers. The positive and negative sides of the D.C. source 280 are connected through resistors 382 to the high resistivity layers. A transformerless system is also described having cross-connected negative feedback (114, 116) Fig. 13 (not shown) and a voltage divider is provided comprising resistors (118).