OCR-excerpt from HifiNews
Printed November 1991


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The Test of Time; Quad ESL63
 
Ten years after its introduction, the Quad ESL63 electrostatic loudspeaker remains a worldwide reference model
 
by Martin Colloms
 
The 'new' Quad electrostatic speaker has enjoyed strong sales since 1981, and what with minor improvements and the passage of time, it seemed appropriate to produce an entirely fresh assessment using a new sample pair. The listening tests were carried out as part of a major group speaker test project, and it was extremely interesting to put the ESI.63 up against more recently-introduced models.
 
As today's panel speakers go, the Quad now seems a relatively compact model, and in terms of full-range electrostatics it is still realisticallv priced.
 
Radiating almost equally from front and rear, it is very sensitive to wall reflections and needs to be spaced away from the rear wall by a minimum of several feet, as well as being angled in towards the listener.  Easier to drive than its predecessor, the ESL63 simply needs a good amplifier in the 25- 100W range, and should present no difficulties aside from the need for a mains supply (of minimal current drain) to each speaker.


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Sound quality
 
Despite the difficulty encountered with this model in serving a listening panel of six in a relativelv confined space, its innate quality won through, achieving commendable panel ratings of 68%. Take it from me (as a long-term if occasional Quad user), this score does not truly reflect the quality perceived at central or near-central listener situations, and in situations where the speaaker has more space to breathe and integrate. Its high quality was nevertheless confirmed in this unsighted auditioning, but it is worth noting that recent listening panels are showing a more critical attitude in regard to the bass. The Quad was found to be weaker here, in quality, quantity and power-handling. Never its strong point, the LF was described as 'compressive' and 'puffy' (in the context of the excellent mid-range), yet the speaker could still play bass tunes well. But the mid was a marvel, showing a broad even, accaccurate balance - yet also sounding fast, articulate, detailed, and with quite good transparency. The treble was not thought to be quite as clear - beautifully integrated with the mid, highly accurate, but marginally dulled in the upper ranges in terms of both clarity and dynamics.
 
In a good room, stereo images are wide and deep, with a fine representation of the recorded acoustic and a satisfying degree of transparency at the price. As signal sources have continued to improve, the Quads have shown an ability to keep up with progress, their quality being sufficient to exploit ongoing improvements. For moderate levels, both rock and classical music are served well. Classical quartet and piano (excellently reproduced) can be played at any sensible volume, but large-scale bass-heavy material needs some care to avoid overload.


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Design and technology

Though smaller than most others of the genre, this bi-polar open-panel design benefits from good siting and some stand elevation. Essentially a one-piece thin-film electrostatic, with a high-sensitivity constant-charge diaphragm, driven push-pull and hence low in distortion, the ESL63 has its directivity and frequency response controlled via multiple concentric ring drive. The radiating area naturally and correctly decreases with rising frequency, thus achieving high off-axis uniformity. A complex combination of a high-voltage transformer and compensated delay-lines provides the electrically contoured drive voltages. Proofed against overload, dust and humidity, the exterior is a neat piece of industrial design which has not dated.


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Lab report
 
Computed at the listening point, the loudspeaker offered an eqluivalent sensitivity of 85dB/W, which was a little below average. The present model is not too difficult an amplifier load (fig.3), and was rated at 6ohms nominal. A maximum sound level of 100dBA was possible from a pair in a typical listening room, but peak programme powers much over 100W are not recommended.
 
On-axis at 2 metres (necessary due to nearfield effects) the response is very well balanced (fig.1). The bass resonance tuning is evident, as are some mild interference effects in the high treble, but overall it worked pretty well. Pair matching was very fine. With 3rd- octave smoothing (fig.2), the even response trend is perfectly obvious, as in the fine integration of off-axis outputs up to 30degrees. Between 30degrees and 40degrees the response quickly becomes more directional.
 
At 86dB (fig.4) the distortion was very low above 200Hz, and still rated 'good' below that frequency. But at the 96dB sound level (fig.5) the LF range was nearing overload in the lower regions, which is poor for a model in this category - but still satisfactory in practice if used with care. Above 200Hz the 3rd-harmonic distortion content was commendably low under all test conditions. The room curve (fig.6) suggests a mildly lightweight sound, but smooth, well balanced, and with real bass extension - particularly if not driven too hard.


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MLSSA measurements
 
The main part of the review work was carried out some time ago, so my editor suggested that I should add some MLSSA impulse-derived data to the test in order to bring it into line with current magazine review practice. The Quad is an especially interesting example since its essentially non-resonant, electrostatically driven film diaphragm and non-cabinet construction should ensure very low stored energy. This part of its behaviour is associated with its natural 1ow-coloration sound.
 
Looking at the ETC results, the leading curve is for the unwindowed, unfiltered presentation. The initial decay is tidy and rapid but levels out at around -40dB, this continuing for an extended period at least to 8ms. Again, the Blackman Harris windowed result is exeellent for the first 0.6mS but after this, the speaker does not quieten as much as one would hope; compare with, for example, the B&W 804 dynamic design reviewed in September. The high-frequency structure is discontinuous and appears responsible for the significant slowing of the decay rate.
 
Turning to the waterfall views, the early decay is superb the first 0,6mS is a superb example to designers of exactly what can be achieved. The low frequency ripples are partly a function of measurement and display limitations but the 'hash' on the treble range is real enough. Above 6kHz, the Quad is less well behaved after the initial clearing phase.
 
Extending the dynamic window to 60dB, with the more frequency-conscious 0.2mS window risetime, the early clearing is still evident as is the good mid-range performance, but some decay 'clutter' is obvious in the treble. I feel this is associated with my subjective characterization of Quad treble as mildly masking the depth and transparency, with a hazy veiled effect. Quad treble does not quite reach the absolute clarity of a good ribbon, for example.
 
Conversely, the results do confirm the excellent transient accuracy of the first arrival sounds from the speaker. As in all things, the final performance is something of a compromise, but a distinctly different compromise to that found with dynamic or moving-coil systems.


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Conclusion
 
This classic performer has endured well. Fundamentally offering front-rank neutrality, the broad mid-range foundation was covered in an exceptional manner - of true monitor quality. The treble was not as clear and detailed as one might wish, while the bass performance was dated by comparison with more recent standards. To assess the degree of bass limitation a personal audition is strongly advised. The mid-range was a joy to hear and made up for an awful lot, while classical programme, if not played too loud on the bigger compositions, worked very well in practice. But loud dynamic rock is really out of the question.
 
I.ab measurements confirmed the innate neutrality and exceptional homogeneity, thanks to the unique design. Modern amplifiers drove it without fuss, and the new protection circuits were naturally less obtrusive and less potentially damaging. This speaker continues to stand up to the competition, offering one of the most accurate electrostatic mid-ranges ever evolved. A strong recommendation is assured, but with the proviso that the purchaser check that it is suited to his or her particular application and room environment.


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Quad ESL63 measurements