Quad ESL-57

A pair of charged plates and a diaphragm lighter than the air it moves. This is the speaker that made "boxless" a selling point.

Audio-Express (April 2002, p.64) noted that British amplifier designs such as the Williamson and its descendants had a lasting influence on American audiophiles. The Quad ESL-57 emerged in that context as a favorite among enthusiasts for years to come.

Overview

Two hundred times. That's the figure Quad's own literature leans on when describing the moving element inside the ESL — a diaphragm so light compared to the cone of a conventional moving-coil driver that the comparison stops being a spec and starts being a philosophy. The Quad electrostatic loudspeaker was the world's first commercial full-range electrostatic, and according to the company's own documentation it remains the standard by which all others are judged. Whether you accept the marketing framing or not, the numbers behind it are unusual: the panel is entirely free of cabinet resonances and colouration, and Quad's spec sheet puts maximum output at 93dB on axis in free space across 50Hz–10kHz, rising to 100dB between 70Hz and 7kHz.

The manufacturer quotes a bandwidth of 45Hz-18kHz, with attenuation asymptotic to 18dB per octave at the extremes. Independent accounts of the actual behavior tend to be a little more conservative. The real strength is the midrange, and the response typically extends from around 70Hz to perhaps 17kHz according to period commentary. Dispersion is approximately 70 degrees horizontal and 15 degrees vertical. That tells you almost everything about setup. This is a line-source-ish panel with a narrow vertical window, and getting your ears into that window is most of the battle.

Impedance runs 30-15 ohms across 40Hz-8kHz, falling above 8kHz. That's a difficult load for a great many amplifiers, and it's why Quad's own documentation is blunt about partnering: the speaker must be used only with a Quad amplifier or one specifically designed for this type of loudspeaker. The panel is driven by an audio transformer and a separate EHT supply unit. The EHT rail sits at a voltage high enough that the service notes tell technicians to disconnect AC power and wait two hours for it to discharge before going anywhere near the internals.

The physical package is modest for something with this reputation. Net weight is 18kg, and the panel measures 880mm wide by 790mm high by 270mm deep. The front grille is expanded aluminum, available in bronze or black, and the speaker is enclosed within those grilles with polished wood end frames and feet. Quad rates it for rooms up to 140 cubic metres. The AC voltage options were 100-120 or 200-250 volts at 50-60Hz, and the mains arrangement changed over the production run. The three-pin AC input connector was revised at serial number 44000, with the earlier PP1403A plug and PS18621 socket giving way to the PP579A and PSP587A. The two types are not interchangeable.

Service documentation treats the speaker as five discrete components: two bass units, one treble unit, an audio transformer unit, and an EHT supply unit. When something goes wrong, the guidance is to identify which of the five has failed and replace or repair that component rather than chasing faults across the whole panel. A fault in the audio transformer unit is noted as very unlikely, which pushes most intermittent breakdowns toward the speaker units or the EHT supply. There's also a modification history worth knowing if you're buying used. At serial number 16800 (March 1966), additional filtering was added to protect the treble unit from high-level low-frequency signals, and earlier speakers can be modified with a tagboard for use with the Quad 303 or comparable amplifiers. The supply indicator neon changed at serial number 45590, and again the old and new types are not interchangeable.

That low-frequency protection exists for a reason rooted in the design's physics. In the 1950s, amplifiers could reproduce low frequencies cleanly but turntables produced significant rumble. Unfiltered subsonic content could send the diaphragm into large excursions, enough to bring it too close to the charged plates and cause arcing. Quad limited the low-frequency extension of its own preamplifier to protect the speaker, and the company's matching II power amplifier was rated at just 15W. With sensitivity around 86dB, the Electrostatics could give good results from roughly 12W, though they were criticized at the time for being less sensitive than conventional designs. Valve-era power was scarce, and 20W was adventurous territory.

Quad's own modification guidance for stacking is unusually forthcoming for a manufacturer. The side pieces come off and are replaced by a single taller frame holding two speakers one above the other. The lower one is inverted so the connections sit close together, with the mains supply commoned and the speaker feeds paralleled from the amplifier. Room placement becomes a fresh experiment because low-frequency cancellation effects shift to different points than with a single pair. Removing the front grilles improves matters further, with the obvious warning that the dangerously high voltages are then laid bare. Experimenters have also removed the hessian-type absorbent material behind the bass units, with mixed results. The material is there to absorb rear radiation, which matters when the speakers sit close to a wall.

The dipole configuration gets listed first among the speaker's supposed weaknesses. The rear radiation is part of the presentation, and it demands effort rather than tolerance. The treble diaphragm is often called too large to perform well, and the bass diaphragms too small and not rigidly mounted enough to couple efficiently with room air at low frequencies. The Electrostatic was designed when replay bandwidth was far more limited than it is now, so judging it against modern expectations of extension misses what it was built to do.

One early review noted the family cat's fur standing on end the first time it walked past while the speaker was playing. That may have been the electrostatic charge. It may also have been the speaker recreating sound images with an uncanny accuracy.