OCR-excerpt from HifiNews March1992
The Quad Gets a Bass Boost by Martin Colloms
The Gradient SW63 sub-woofer for the Quad Electrostatic lifts performance even higher.

Designed and made in Finland, this add-on woofer system is expressly intended for the Quad ESL63 loudspeaker. Last reviewed in HFN/RR Nov 1991, the '63 has been with us for some years now and shows every sign of repeating the long and illustrious production life of its predecessor, which ran nearly 20 years. To recap, the '63 is a relatively compact, open panel electrostatic loudspeaker design. A unique feature is the effective use of concentric elements which provide an unusually good off-axis directional performance for a bi-polar panel speaker. It has an unusually low diaphragm mass for an electrostatic, as its excellent transient performance testifies. When appropriately located away from the rear walls and (ideally) placed on some kind of custom frame or stand, the sound of the '63 is characterised by natural, low coloration levels, and a tonal balance close to monitor standards. Within its moderate dynamic range it has very low distortion and respectable bass to 45Hz. It performs very well for the vast majority of users, is loud enough for all reasonable purposes, and there is all inherent greatness in this speaker which suggests something might lift it further into the highest audiophile class.

An examination of its limitations suggests that the '63 should be relieved of the acoustic workload below 150Hz, the region of greatest excursion and nonlinearity. Up to 10dB greater dynamic range is then possible from the design, with consequent improvements in definition and clarity in the mid and treble ranges. The problem is in finding a bass system of sufficient caliber to justify the effort. Certainly there have been a number of such products, usually large sub-woofer designs with an omnidirectional radiation pattern. The Quad is bidirectional and on instinctive and theoretical grounds one always felt that a bi-directional bass system would be preferable. One example was the System 600 woofer designed by Graharn Bank for the Celestion SL600 miniature. Trials with the Quad were most encouraging, though there were obvious physical and visual mismatches. Now the Finnish company Gradient has developed a custom bi-polar woofer fully matched to the 63. As with most high quality sub-woofers, a second, preferably matched, power amplifier is needed. Priced at £1500 in the UK, the SW63 is approved by Quad as a compatible and complementary product, despite its independent distribution by KJ Leisuresound of London, W1. At a total cost of £3500 (excluding power amplifiers) this product is assessed here as a complete new speaker system and not simply all add-on accessory. The competition includes models such as the Marlin Logan Quest and CLS, the new Magneplanar MG3, the B&W 801 and Apogee Duetta Signature; all of these speakers relatively large models.

Design and technology

Inside the speaker box is a pair of 260mm pressed-steel-chassis bass units, pulp coned and fitted with low hysteresis half-roll surrounds of foam polyurethane. Placed side by side on the open baffle, one faces forward and the other back, providing some balancing of linearity.

The Gradient employs two low-frequency drivers in each cabinet, arranged as an open-baffle linear array to give dipole operation. 6dB per octave boost needs to be applied throughout the low frequency range to offset the falling open baffle response, and the bass units used need a linear, high excursion capability; ± 12mm is quoted. Before overload is reached, the degree of boost must be limited and this sets the degree of bass extension available. In this sense the SW63 is not a true sub-woofer in that it does not go that final 10Hz down to 20Hz. However, it does extend the power response of the Quad by almost an octave, whilst simultaneously increasing the dynamic range and linearity of the main system. This latter aspect might well be as important as bass extension.

The SW63 is rated at 250W  8 ohms and has a basic sensitivitv of 86dBW at the 16ohm setting - there is a 4-ohm setting where more gain is required (eg a lower power, higher current amplifier). A small self-powered active crossover is provided. A control offers some mild 0.5 and 1.0dB attenuating correction for the ESL63 upper mid range, while another switch sets the low frequency range to mono, useful in cases of excess low frequency rumble on analogue disc. Finally, there is a sensibly ranged level control for the bass to aid room matching and to compensate for programme deficiencies.

With the crossover point set at an ideal 110Hz, second order, the final bass roll-off is third order at 28Hz. However, there is a sting in this engineering tail [tale?]. For the equaliser/controller does not even come with a transformer. Instead, a small line cord device is added into the package by the dealer. A single 15V chip regulator is followed by a TL071 to divide the rail into two. By top audiophile standards this construction is mid-fi - for example the input reservoir is just 100uF while straightforward active filter circuitry, is used based on TL072CP integrated circuits. The design is well engineered but does not reflect high end practice, so it is perhaps just as well that all the sound quality checks were passed before the box was opened! I am happy to accept the fine subjective results as they come, and feel encouraged that perhaps a higher Standard still could well be available with a greater investment in equaliser engineering.

Sound quality

Amplification was supplied firstly by a Krell KSA 150 partnered by an ARC Classic 120. But it became apparent that the virtue of matched amplification outweighed other considerations, and the best results were obtained with a double set of Meridian 605 mono-blocks. Digital source components included Theta Data Pro Basic and the Accuphase DP-70V. No equivocation is required, quite simply, the ESL63-SW63 system was a first-rate high-end contender. It managed to capitalize on the Quad virtues, making it sound even better in terms of claritv and detail. The Gradient crossover did not appear to colour the reproduction significantly and an overall gain occurred, At low frequencies the system was exceptionally good. The mildly fat bass of the Quad was here replaced by a more correct, drier, crisper and cleaner output, one with audiophile slam, extension and dynamic range. Thinking about that final 10Hz, I have to say that the speed and tunefulness of this speaker is such that I would not willingly trade its performance for more bass extension. It was as dynamically exciting on rock bass guitar as it was on pizzicato bass viol. A big soundstage was developed at the 60cm height for the ESL63, and very good stage width and perspective were heard. It was something of a revelation to be able to play the Quads loud without fear of rattles or overload. The whole sound was open, stable and exceptionally free from strain, proving easy to listen to for long periods and consistently rewarding in the best musical sense. I found that it handled all types of programme well, especially with the rhythmic and dynamic delivery of the 605 power amps behind it. At the limit, dipole bass cannot be expected to provide the gut-wrenching power of a fully room-coupled box woofer, but on the other hand the SW63 could give a more open, more even and more articulate low frequency coverage than the vast majority of extended bass systems. Here I  rate quality higher than quantity. With a programme power rating of 200W, a sensitivity of 85dB and, aided by the Quad's directional nature, serious in-room maximum levels of 110dBA are possible.


The Gradient woofer has not transformed the ESL63 - rather it has released it from restrictions determined by a need to make the standard speaker a full-range device. With the SW63 the electrostatic blossoms into a true high end speaker of such integrity and ability, that no sensible criticisms are worth voicing. This is a most graceful performer which again shows the fundamental quality of Peter Walkers marvellous creation. It comes strongly recommended.

I can think of no better upgrade path for a '63 owner with ambitions, while its an all-in-one purchase the high review rating equally applies.

Test results

Basic tests on the equalizer verified the electrical characteristics and showed that it met good standards for distortion and the like, sufficient not to compromise the chain. The first graph shows three curves, with that for the ESL signal illustrating the second order roll-off set at 110Hz. By 40Hz the drive to the Quad had been reduced by 22dB, making its workload much lighter. In the solid line the woofer signal is seen, the roll-off complicated by the necessary boost occurring below crossover point. The boost stopped at around 35Hz and then this signal was rolled out at 18dB/octave to prevent overload on large very low frequency excitation. The lower dashed line is the actual acoustic output of the woofer measured in the nearfield, and the overall join looks fine. The second graph shows the room averaged response. Good extension was present to 32Hz while the benefit of the directional, dipole bass was seen in the remarkably smooth low frequency register developed in this room. More space behind the Quads would have improved the lower mid response. It proved less sensitive to room variations.



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