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New QUAD Electrostatic Speaker.

Wireless World, August, 1981.
    
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The Quad ESL-63 Electrostatic Loudspeaker with the grille cloth removed. The concentric annular electrodes which 'spread' the sound pressure pattern across the diaphragm can be clearly seen.

For many years, whenever one read a review of a new loudspeaker, the 'standard' speaker used in a/b comparisons was always the Quad ESL. Now Quad have announced the ESL-63 (named because development began in 1963) known to its engineers as FRED (full range electrostatic doublet).

Peter Walker postulated that if a very light diaphragm could be made to reproduce the air particle motion found at an imaginary plane some distance from, and normal to the direction of propagation from a theoretical ideal source, the result to the listener would be the same as if he were hearing that ideal source. The Quad ESL-63 achieves this by means of a very light electrically polarised diaphragm suspended between two sets of rigid and acoustically transparent (they have hole in them) concentric annular electrodes to which the signal is fed through sequential delay lines. The sound pressure pattern produced is a replica of that from an ideal source placed some 30 cm behind the plane of the diaphragm. The motion of the diaphragm is roughly analogous to the wave motion which results when a stone is dropped into a still pool.

This configuration, says Quad, gives the designer complete control over the directivity of the loudspeaker. As a dipole with a figure-of-eight dispersion pattern, there is no radiation in the plane of the diaphragm and the ratio of direct to reflected sound is much higher than from an omni-directional source so there is a great improvement in the localisation of the stereo image.

Visually the ESL-63 is a great improvement over the old ESL and does not look like a room heater. It has a height of 92.5 cm and a width of 66 cm. The depth of 27 cm includes the base containing all the electronics. It requires an AC mains supply.

The nominal resistance is 8 Ω and this is almost purely resistive. It has a sensitivity of 1.5 μbars/V referred to 1M, which is 86dB/2.83VRMS. The maximum input is 10 VRMS continuous, 40 V for undistorted maximum peak output with a maximum permitted peak input of 55 V. The maximum output is 2N/m2 at 2 m on axis. The bandwidth with reference to -6 dB limits is 35 Hz to over 20 kHz.

It is expected that the ESL-63 will be sold at &Pound;1,000 a pair.

The ESL-63 advert from June 1981.

The ESL-63 advert from August 1981.

Image courtesy SDS Audio.

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