Quad ESL loudspeaker

High fidelity took a giant step forward in 1956 with Peter Walker's introduction of the Quad ESL. Walker's research efforts had been motivated by his firm belief in the superiority of the electrostatic dipole over the box loudspeaker, but actually to take the economic plunge and market such a speaker was surely an act of bravery. After all, those were the pre-stereo, pre-audiophile days of the mid '50s, and the public's tastes and expectations were relatively unsophisticated. The average front end was abominable by today's standards, so that making definitive assessments of loudspeaker quality was a difficult task at best.

At the time, the "hornless" direct-radiator loudspeaker reigned supreme. Rice and Kellogg's moving-coil loudspeaker, which had displaced the competition during the radio boom days of the '30s—primarily because of its ruggedness and low cost—had had 25 years to be perfected. A credulous public was besieged by a variety of loudspeakers whose usual claim to fame was some sort of "innovative" cabinet design housing one or more paper cones. There were infinite baffles, misguided baffles, and a bounty of boomy phase-inversion (bass-reflex) enclosures. And Edgar Villchur's air-suspension principle, in the shape of the AR1, was about to revolutionize the mass market.

Against this background, what chance for survival did this exotic new kid on the block have? It would seem that the forces of history were stacked against the newborn Quad, which appeared to be destined to play out its life cycle as a "small voice in the wilderness." With hindsight, however, it now appears obvious that the Quad ESLs were at the right place at the right time. The age of stereo and the resultant audio boom were just around the corner; indeed, during the '60s, the reputation of the Quads grew exponentially. Early audiophiles quickly became aware that the conventional box loudspeaker was a very weak link in the sound reproduction chain, and many of them discovered to their amazement the clean, transient-quick, transparent, and focused sound of the Quads.

The rest is history. For over a generation, the Quads were proclaimed a reference standard, being used by countless reviewers, as well as a multitude of audiophiles. Around 60,000 of the original Quads were produced, and most of them are still in active use. In 1984 production of the ESL was finally wound up in order that the company could concentrate their resources on the Quad ESL-63, introduced in 1981. However, as Fred Yando at Quad America (footnote 1) tells me that there are enough spare parts on hand to safely see Quad owners through to the 21st century, it seemed appropriate to assess the Quad classic's sonic worth in a modern setting. Is it still a viable speaker or simply an anachronistic relic? Do you have to be over 50 to enjoy them? Do mods improve the sound? These are some of the questions I will attempt to answer.

Prologue
It is commonly assumed that the electrostatic speaker is a fairly recent high-tech innovation. Nothing could be further from the truth. Its roots are firmly planted in 19th-century scientific invention. For example, in 1881 Professor Amos Dolbear of Tufts University patented a condenser-type telephone receiver and transmitter. I quote from Professor Dolbear's US patent (No. 240,578): "My receiver is based upon the discovery that one terminal of an open circuit will attract and be attracted by a neighboring body when the terminal is charged." The commercial success of radio during the 1920s generated much interest in the electrostatic speaker, although it seems that only two commercial ESLs were produced in any numbers: a German design, and the US Kyle condenser speaker which was incorporated into the Peerless radio of around 1930. ESL designs in those days were handicapped by the lack of suitable materials. Light plastic films and mylar were not available, and inventors such as Colin Kyle had to resort to india-rubber diaphragms, aluminum foils, and mosquito netting for insulation. Needless to say, there were reliability problems, and these early ESLs were quickly driven off the market by the advent of the rugged and more efficient dynamic loudspeaker.

The original Quad is a full-range electrostatic loudspeaker which uses the well-known push-pull grid geometry, but with an important new wrinkle: constant-Q or constant-charge operation. This means that a constant charge is maintained on the diaphragm, usually by using a high-resistivity coating. If this condition is not maintained, it can be shown that the electrostatic forces acting on the diaphragm will be nonlinear, ie, dependent on the location of the diaphragm between the grids. Constant-charge operation leads to effectively linear drive conditions.

887quad.old.jpg

A Quad ESL is made up of two bass panels and three centrally located "strips." The two outside or midrange strips operate in parallel and roll off above 2-3kHz, while the middle or treble strip reproduces all the frequencies above 600Hz. At serial number 16800, additional high-pass filtering, in the form of an RC network, was added to protect the treble unit from damage due to large low-frequency signals. The bass units are biased at 6kV, and the strips at 1.5kV nominal. Plastic dust covers are used to isolate the grids from dust accumulation and to preserve the life of the high-resistivity diaphragm coating. Expanded-metal grilles are used front and rear to protect the dust covers and drive-units and to prevent curious hands and fingers from getting zapped. Thick felt damping is provided behind the central treble unit, while "burlap" damping material is glued to the inside of the rear grille to control bass panel resonances.

The ESL Sound
It is difficult to conceal one's respect and admiration for the midrange quality of these "antiques" in stock condition. In a cognitive sense, you can readily dissect the Quad's sonic attributes: excellent resolution of low-level detail, transient quickness, and cohesive harmonic textures. But the instant emotional impression is one of naturalness. They don't scream, shout or wave their hands at you with a phony sonic signature. Instead, you're confronted with a clean and harmonically convincing sonic window that captures the heart of the musical experience. The perspective, however, is quite distant, even on closely miked recordings: you're invariably transported to the back of the hall. This is readily explainable on the basis of the Quad's frequency response, which features a broad valley from about 2.5 to 8kHz, or from the upper mids through the lower treble. Another side effect of this response anomaly is an occasional slight honky quality to female voice, no doubt caused by a slight suppression of the upper speech formants.

In matters of dynamic contrasts, say from very soft to loud, the Quads are outstanding. True, they are limited in ultimate SPL to about 95dB at a realistic listening position, but within this range they are capable of excellent dynamic bloom. The Quads are startling performers indeed, and the "startle index" is quite high through the middle octaves—I'd say about 8 on a scale of 10. What is the "startle index," you ask? Well, it's my slightly tongue-in-cheek attempt to quantify (footnote 2) the physical sensation of involuntary bodily movement associated with a sudden SPL change. (The startle index should not be confused with the wet-spot, or ring-around-the-sofa, syndrome, which is a measure of a speaker's ability to scare you.)

No, you don't have to listen to the Quads in diapers. There are no chest-crushing crescendos. There is no bass punch. There isn't even any deep bass, though with the right amplifier and the modifications described later, the midbass is tight and very well delineated. They're not monster speakers. King Kong is not likely to own a pair, though Godzilla might. After all, he appears to have audiophile instincts. Why else would he periodically destroy the city of Tokyo, if not to vent his frustration with the Japanese audio mass market?

The Quads possess a definite sweet spot, which is to say that the upper octaves are rather beamy—sort of like the way a flashlight concentrates light in the forward direction. The spatial extension of this sweet spot is defined primarily by the dispersion in the vertical plane, which is a mere 15° in the treble. Beamy treble is not a problem unique to the Quads, or to electrostatics in general, but is common to all speakers where the active dimensions of the treble driver are on the order of the radiated wavelengths or larger. For example, the length of the tweeter strips is around 24", which corresponds to the wavelength at 550Hz. At frequencies above 550Hz, therefore, the Quads become increasingly directional in the vertical plane. It becomes necessary then to contain the listening seat to the area of the sweet spot.

The speakers must first be toed in toward the listener and the height of the listening seat adjusted so that the ears are no higher than the top of the panels. The sweet spot is then defined (according to DO, at least!) by an area centered on a line bisecting the speakers, six to eight feet from the plane of the panels and one foot wide on either side of the centerline. The optimum distance from the speakers is somewhat room-dependent and should be selected on the basis of active experimentation in your room. Within the sweet spot, the sound of the Quads is quite cohesive and capable of excellent imaging. Outside of the sweet spot, their imaging ability and tonal balance deteriorate. The Quads are therefore a one-person speaker. That's fine with me; I prefer to do my private listening solo and reserve mingling with the crowds to live performances.

As you can see, I prefer to listen to the Quads rather close-up—almost using them as headphones—but a caveat is in order here: at distances less than about six feet, the speakers don't quite gel, so ultra-nearfield listening is to be avoided. The idea of listening in the nearfield, however, is to maximize the ratio of direct sound to room reverberation, and thereby reduce the sonic signature of the room. Intense, early room reflections are the most significant offenders, capable of defocusing the soundstage and, even more importantly, of modifying instrumental timbres.



Footnote 1: As of 2016, Quad products were distributed in the US by MoFi Distribution, 1811 W. Bryn Mawr Avenue, Chicago, IL 60660. Tel: (312) 738-5025. Web: www.mofidistribution.com. Their service center in Fairfax, Virginia will service all vintages and versions of the loudspeaker, including the orignal ESL.

Footnote 2: I've always been amazed at the facility Martin Colloms shows (at least in his writings for Hi-Fi News & Record Review in the UK) in nailing down sonic quality to a single percentage point (eg, 83% sonic score for a component under review). I can't do that. It is extremely difficult to quantify personal sensations, and is very likely to be of limited usefulness to someone else, considering the variability in individual response to physical stimuli.—Dick Olsher

ARTICLE CONTENTS

X