Quad ESL-63 loudspeaker Larry Greenhill

Larry Greenhill reviewed the Quad ESL-63 US Monitor in February 1989 (Vol.12 No.2):

Electrostatic loudspeakers (ESLs) have always held a fascination for audiophiles. The Jantszen, Beveridge, KLH, Acoustat, Stax ESL-F81s, and Servo-Static models in the late 1960s and '70s, as well as the MartinLogans and Sound Labs of today, all promise faster transient response, low distortion, and a higher order of "transparency." Why? The driver, instead of being a cone with a mass of several ounces, is an extremely thin sheet weighing mere grams, often made of Mylar, and suspended between two charged plates. The musical signal, applied to the plates, causes the diaphragm to move. Many of the designs operate full-range, without complex crossovers and their attendant problems.

Like all exotic systems, there is a price to pay. Electrostatics, particularly the early models, are planar systems and have beaming problems. The bipolar dispersion pattern of the sound makes them very sensitive to placement. Although the low-mass membrane can move very quickly, it can not move far; deep-bass response is limited unless huge panels are used. Electrostatics require power supplies and transformers, and the impedance and phase angle of the interface can vary tremendously depending on frequency. Many solid-state amplifiers have had problems driving such complex loads. Charging the plates requires expensive, high-voltage power supplies, often mounted in the speaker's base.

Unreliability plagued many of the early models, arcing of the diaphragms giving a wonderful blue glow in the dark but giving the owner a sinking feeling—an expensive repair was in the offing. This unreliability added insult to injury, as the price of these systems can be quite high. Although we are now jaded with the thought that a full-range, "all-out" speaker system can cost in excess of $10,000, the early members of the electrostatic club in the late '60s clearly were buying then the most expensive speaker systems, at costs of several thousand dollars. All this was justified, for the dedicated hobbyist (and eventually neurotic, worried owner!) who could tolerate the expense, breakdowns, and cumbersome speaker enclosures, for these speakers offered low-distortion, non-fatiguing sound with superb imaging and detail.

Why review the Quad ESL-63 again, now presenting itself in the United States as the US Monitor? Because the Quad, now in its third version, is the longest-surviving consumer-grade electrostatic speaker on the market, if one counts the first version made in 1957. Only 11 full-range electrostatic systems are listed among the 1376 loudspeakers in Audio's 1988 Annual Equipment Directory, and these are manufactured by only four out of 257 speaker companies in the audio industry. Sound Lab makes three full-range ESLs and four ESL subwoofers; Acoustat offers six models; MartinLogan has only one "pure" ESL system; and there is the Quad US Monitor. (In addition, Stax still offers the ESL-F81, 'F83, and the ESTA-4U.) Stereophile's "Recommended Components" for April 1988 (Vol.11 No.4) lists only the Sound Lab A-3 and Quad ESL-63 in Class B. For many, the US Monitor will be a serious contender for the "best" ESL: accurate, superb imaging, no crossovers, with great sonic coherency, practical size, and high reliability.

The Early Quad
The first Quad electrostatic, which remained in production for 25 years, had all the electrostat's virtues and vices. As with all of Peter Walker's products, some new principle was applied—the first Quad employed the "constant-charge" technique, which insures an even distribution of charge across the entire diaphragm. The speaker was a curved rectangular panel, with the longer sides horizontal, a look that was copied by Jon Dahlquist for his DQ-10. These relatively small panels imaged beautifully and, for me, gave the ultimate in midrange accuracy, speed, transparency, and imaging. On the other hand, it could not play loud, had very limited bass response, and less than optimal dispersion patterns for stereo imaging. It would arc instantly (blue flame and hole in the Mylar) if you were rash enough to overload it even for a second using an amplifier that put out an instantaneous voltage exceeding 27V.

Finding the right amplifier was another part of the electrostatic owner's lifestyle. The amplifier had to be right sonically, of course, but also had to have exactly the correct voltage peak or it would literally "consume" the loudspeaker. The Acoustical Manufacturing company made a small solid-state amplifier, the 303, which was safe to use with their electrostat. In the early 1970s, John Curl and Marc Levinson designed another amp for the Quad panel, the ML-2. This product fit the exotic, hyper-expensive world of electrostats to a T. Sporting huge cooling fins, the ML-2 was a 65lb, monophonic, full-duty cycle, class-A amplifier than ran as hot as a space heater, putting 25W into the speaker and 150W of heat into the room. It cost then about $4000/pair (current special-order versions of the ML-2 are still available today from Madrigal at $9600/pair!).

The sonics of this speaker-amplifier combination were highly touted, and have since been regarded as one of the few "classic" pairs of audio components. This original Quad, for its extreme midrange transparency, did not offer as much at either frequency extreme, and required total dedication on the part of the owner. Some high-end dealers supposedly even taught their customers to repair the diaphragms themselves, using Mylar and a hair dryer!

The Quad ESL-63: the first 7 years
Peter Walker began to redesign the Quad in the early 1960s (the "63" in the ESL-63's name supposedly designates the year of the design). The new version was released at the CES of 1981, and seemed smaller because the long side of the speaker's rectangular frame was now vertical. Many exciting and clever technical inventions were incorporated into the '63 (detailed in an excellent article by Reg Williamson in Speaker Builder, Vol.3 No.1, pp.10-18, February 1982).

The first involved a new protection circuit, offering a technically sophisticated triac clamping circuit to prevent arcing. The circuit operated by limiting the input, and when that failed, by short-circuiting the input with a "crowbar" technique (the amplifier needed to have adequate protection against the speaker!). This crowbar circuit was actuated by an RF "sniffer" that was set to sense the high-frequency noise that accompanies the ionization of air that occurs when the speaker arcs.

The second innovation was the speaker's unique radiating element, which used driver plates that employed a printed circuit board of annular rings, like the ripples formed when a stone is dropped into a lake. These rings were fed by delay lines (employing some 11 miles of wire!) which allowed the flat diaphragm to radiate the sound first at the center and last at the periphery, as if it were a radiating sphere—the ideal shape for approximating sound emanating from a point source with an apparent location 12" behind the panels. The single element in the new Quad also meant the elimination of a venetian-blind, treble-beaming effect found in speakers with multiple panels. This design meant near-perfect phase coherency, as shown by Quad's show-stopper demos in which two squarewaves, out of phase with each other, are fed to two Quad speakers. A microphone placed between the speakers shows that the two signals cancel out completely, suggesting very low distortion in the speakers.

Many of this magazine's major reviewers have made excellent and critical statements about the '63's strengths and weaknesses. Bill Sommerwerck opened with a very technical description following the speaker's first CES showing in 1981, praising "FRED" (Peter Walker's technical name for the ESL-63, which stands for Full Range Electrostatic Driver) for its natural-sounding, pristinely focused, unstrained ability to capture the acoustical space in a recording.

JGH had a mixed opinion, praising the '63 for its imaging, but faulting it in other areas. He found the sound to be "warm, withdrawn, and overly rich . . . [with a] persistent dryness and slight top-end tizz" (Vol.6 No.4). The new Quads quickly shut down during orchestral climaxes, which led him to withhold his recommendation, "regardless of the sonic merits it possesses." The loudspeaker "simply did not have the power-handling capability" for program material then becoming available on CD (Vol.6 No.4).