Quad Electrostatic Loudspeaker

Acoustical Manufacturing Company had launched a ribbon-hybrid, horn-loaded "Corner Ribbon Loudspeaker" in 1949,[2] but this was not commercially successful.

The electrostatics were designed by Theo Williamson, and the company's founder, Peter Walker, from a principle elaborated by Edward W. Kellogg for General Electric that was granted a patent in 1934.

There are two perforated plates with a potential difference of up to 10,000 volts, between which a very thin film, coated with a mildly electrically conductive substance, is sandwiched.

[14] The electrostatic design does away with two major weaknesses of boxed speakers: enclosure colourations and signal degradation due to the crossover.

The crossover in the ESL consists merely of resistors which work with the inherent capacitance of the speaker elements to obtain a first-order response.

The driver, being only 0.00137-inch thick and weighing only 3 mg,[15] is thus capable of stopping and starting seemingly instantaneously with no damping necessary to the drive units.

Although a slight deviation did not seem to make a difference, it is in practice not possible for more than one person to enjoy a reasonably balanced sound stage with the ESL.

[5][16] Contrary to the flat appearance of the loudspeaker, all three panels are slightly curved along the vertical axis to improve quality.

A crossover network, set at 500 Hz, remains necessary to adjust for back-to-front interference causing loss in output at low frequencies.

[17] In fact, the instructions book states that any properly-regulated amplifier delivering no more than 33 V peak into any load is unlikely to cause damage to the speaker.

Late in the speaker's life, many owners found that the highly-stable 15-watt Naim NAIT launched in 1983 worked well with the ESL.

[17] The design's lack of structural integrity is seen as another significant weakness of the Quad ESL-63 since its inception, and lived on in its various successors.

[6][7][25] Other shortcomings include very limited sound pressure levels,[Note 3] meaning that it won't play loud, and its bass response was lacking.

Another "crowbar action" protection circuit could detect the high-frequency noise that accompanies the ionisation of air when the speaker arcs, and shuts down the power when that occurs.

As the rising Pound Sterling made the product much more expensive for US customers, it was decided to improve the specification of the ESL-63 and target the $4,000 price point with a "USA Monitor" version.

[1] Home Theater Review said it was "the most cherished hi-fi product ever", and that Walker had been elevated to god-like status in the audio world.

[31] Commenting on the ESL-63, J. Gordon Holt of Stereophile considered it was a major failure of the speaker for it to shut down during orchestral climaxes.

He said that "regardless of the sonic merits it possesses [the speaker] simply did not have the power-handling capability" for more recent program material on CD.

[14] A properly configured stereo pair would generate a very small 'sweet spot' where the sound stage snaps into focus.

This led to criticisms of the speaker's directionality which in practice was rarely a problem and had the benefit of reducing the effects of room resonances.

[10][32] Sam Tellig wrote in Stereophile in June 1989 of the neutral sound emanating from the ESL-63: "There is no discontinuity from bass to midrange to treble—it's all so natural.

[10] Its superior sonic characteristics were offset by its moderate power handling, and the need for a relatively large space in which to "breathe".