Patent GB-1319652-A (GB)
Title: Electrostatic loudspeakers and systems meploying same
IPC: H03F3/183
Filed: 1970-06-12
Abstract: 1319652 Electrostatic loudspeakers and circuits therefor H N BEVERIDGE 12 June 1970 [17 June 1969] 39549/72 Divided out of 1319651 Heading H4J  An electrostatic loudspeaker has a diaphragm 12 (Fig. 5) conductive on at least one face, and a conductive sheet 14c having, on the side facing the diaphragm 12, a dielectric portion 14a substantially thicker (e.g. 3 or more times thicker) than an air gap Ag which separates the dielectric portion 14a and the diaphragm 12 in a neutral position. The relative permittivity of the dielectric portion 14a is greater than 30 at audible frequencies. The diaphragm 12 is of polyethylene terephthalate carrying on each side a vacuum deposited aluminium coating. Two conductive sheets 14c are of an epoxy resin containing a dispersion of silver coated particles. The dielectric portions 14a are moulded from epoxy resin loaded with an equal weight of barium titanate and with carbon to approximately 5% of the weight of the epoxy resin to produce a dielectric of which the relative permittivity is between 30 and 40 and whose volume resistivity is in the range 10<SP>8</SP> to 10<SP>11</SP> ohm. cm. The portions 14a are sufficiently conductive to conduct off rapidly any charge if the diaphragm 12 should touch. The dielectric portions 14a have integrally moulded slots (Fig. 2, not shown) which diverge outwardly to permit passage of sound from the diaphragm 12. Edge members 14b which determine the size of the air gaps Ag are made by placing a removable spacer 40 on a glass plate 38 (Fig. 4) to support the inner surface of a dielectric portion 14a. The edge members 14b are of epoxy resin and are formed by jig members 42, 44. Local circuit.-A pentode T 3  (Fig. 7) acts as a constant current source connected to receive and amplify the input audio signal 52. Its output is applied between two series connected pentodes T 1 , T 2  functioning essentially as class B amplifiers, their linearity being enhanced by a negative feedback resistor 66. The anode of pentode T 1  is connected to the cathode of pentode T 2  by a resistor 68 shunted by a Zener diode. The output is taken to the electrostatic loudspeakers at point B. Further linearity improvement can be achieved by feedback from point B to the input 52. If desired, a second identical amplifier may have its output connected to the fixed electrodes, a 180 degree phase shift being provided between the two amplifiers. The circuit of Fig. 7 may be part of a system (Fig. 8) which includes feedback from the diaphragm to the pentode 72 (T 3 ). The loudspeaker diaphragm is in this instance connected to point B. A 260 KHz signal is applied by a generator 84 to the diaphragm and this signal induces corresponding signals at loudspeaker electrodes 86, 88. The induced signals on the electrodes 86, 88 are detected and summed at a displacement detector 87 whose output is applied to a differentiator 89. A feedback amplifier 90 applies the signal to the constant current generator 72. The signal fed back is significant only from about 20 Hz to 200 Hz and its phase in this range opposes the drive voltage.