Quad ESL loudspeaker Page 2

It is well known that the direction of a broad-band soundsource is largely determined by interaural cues associated with the earlier-arriving direct sound to the neglect of later-arriving reflections. This is precisely the precedence effect. (Note that the precedence effect does not preclude the perception of two separate but simultaneous auditory events from a pair of loudspeakers if the spectral content of the left and right signals is greatly different.) Experimental evidence supports the notion that the time course of the precedence effect spans two "windows." Following the abrupt onset of a sound, the sensitivity of the ear/brain system to interaural intensity and time differences is greatly degraded over the following 0.5 to 10ms. This means that during this time, the listener cannot distinguish even intense early reflections from the direct sound. Early reflections arriving in this 10ms window fuse with the direct sound and the sum is spectrally dissected by the ear/brain in order to form an impression of timbre or tonal color.

Sizeable reflections arriving during the second window, from about 10 to 50ms, are largely recognized as such by the ear/brain and do not figure prominently in timbre perception. However, because they are still below the threshold where they are recognized as discrete echos, these later reflections are not perceived as separate auditory events; instead, they are localized with the direct sound and used in determining the size or diffuseness of the spatial impression. After about 50ms, the spatial persistence of the auditory system fades and the cycle repeats itself.

With this introduction out of the way, it should be easy to understand the sonic degradation introduced by nearby room surfaces during playback. For example, strong lateral room reflections will fuse with the direct sound and cause the perceived image to shift toward the source of the reflection. This may result in the extension of the soundstage width well beyond the edge of the speakers, but the rub is that the subsequent size of the instruments will appear broader or more diffuse. Timbre accuracy suffers when the room generates early reflections (delayed less than 10ms) from walls, floor, ceiling, or even a coffee table in front of the listening seat.

To prove to yourself that the ear/brain actually works this way, you might want to try this rather interesting and elegant experiment. Record a short passage of a familiar solo instrument on a two-track open-reel tape deck. Then play the passage forward and in reverse and compare timbres. Even though on a gross time scale of several seconds the spectral content of the passage in reverse is identical to that in the forward direction, the reverse presentation sounds strange and unrecognizable! That's because during the "reverse" playback, the ear's 10ms window integrates primarily over the decay part of the musical information. One can conclude from this that correct arrival times for the various frequency components are crucial for the interpretation of timbre, and that extraneous room reflections can influence timbre perception.

Positioning the Quads in the listening room should follow the rules of thumb applicable to any dipole radiator, and they should be positioned several feet away from the back wall and side walls. In addition to acoustical treatment of the back wall, the side walls should be treated—at least when the Quads are fairly close to them. Generally, you wouldn't think that a figure-eight radiator is likely to create much in the way of lateral wall reflections, but because of the toeing-in requirements, the back-wave can interact strongly with the side walls.

Choice of amplifiers is also important. The Quads work extremely well with the old Quad II tube amps, simply because these amps were designed around the speakers. Any candidate amp must meet strict stability requirements into a significantly reactive load (30–15 ohms in the range 40Hz to 8kHz, falling above 8kHz) and possess a voltage-limited output of no greater than 33 volts (peak), this limit representing the threshold of physical pain for the Quads. More than 33 volts and you will punch holes in the diaphragm. The Radford STA-25, Marantz 8B and, of course, the Jadis JA-30, are all good choices. The nature of the load tends to favor tubed amps, but small class-A solid-state designs like the Krell KSA-50 should also mate well with the Quads.

Quad Mods: from the Simple to the Sublime
As with any other piece of classic gear, the Quads have not escaped audiophile intervention in an effort to improve an already good product to further sonic heights. And considering the amount of time the product has been around, I believe that everything possible has been tried (footnote 3).

The simplest possible mods consist of discarding the cosmetic accouterments—namely, the back and front grilles. Removing the rear grille, together with the burlap padding, trades off bass control against enhanced midrange transparency. (Although with the Koval mod described later, bass control is not a problem and the reduced veiling and midrange congestion are worthwhile benefits.) Removing just the burlap padding and retaining the grille does not work as well. The gain in midrange transparency is not as great, and a standing-wave resonance develops in the response at about 4kHz, where the wavelength corresponds to the spacing between the back grille and the treble panels. Do not remove the thick felt padding behind the central treble unit, as it provides needed acoustic damping and controls what would otherwise be serious ringing in the mids.

Removing the front grille provides an even more dramatic gain in transparency and focus, which must be heard to be believed. It appears that the front grille acts as a diffraction grating or diffuser for the sound, veiling musical textures and reducing clarity. Without the front grille, clarity top-to-bottom is now competitive with any modern design—the MartinLogan CLS, for example. There are practical problems, however, inherent in removing the grilles—the least of which is the resultant ugly cosmetics. The dust covers are now exposed and vulnerable to rupture. High-voltage terminals are accessible to prying hands and curious pets. If there are small children or pets around your listening room, I'd think twice about these modifications. In my case, the Quads in my new listening room are quite isolated from such intruders, except from an occasional visit by one of JGH's cats.

The next modification is a lot trickier to implement. It is not one that I've personally tried, but I have it on good authority that it is sonically worthwhile. It involves replacing the dust covers with thinner mylar sheets. Such plastic sheets are readily available at hardware stores, and are commonly sold as paint drop-cloth. The reported improvement is in the same direction as that afforded by the removal of the grilles. Of course, no dust covers are better than even thin ones, so you may be tempted to dispense with them entirely. Don't! You will drastically shorten the life of the diaphragm by doing so.

Finally, the Koval mod (footnote 4). Amazingly, this mod was first advertised for public sale around 1979, and is still available in 1987 from John Koval. You'll need a pair of his mysterious-looking modules (one per channel), for which he asks a modest $160/pair, post paid, and there is a money-back guarantee in case you're not satisfied. The modules are completely potted-in, with only five wires and six connecting pins protruding from the body. They are to be installed in the bottom of the audio transformers.

The installation is not overly complex, but is time-consuming (figure on a few hours), and requires care and a lot of soldering. Electrically, the mod involves, among other things, allowing the two midrange strips to run fullrange, rather than rolling off above 2–3kHz. The woofer panels are also rebalanced to complement the new and much simpler radiation scheme, which eliminates the overlap and interference between the mid and treble strips. Another benefit is the elimination of capacitors from the signal path—no caps are better than good caps.

As you can see, the Koval mod borders on a redesign of the stock unit. Is all of this effort justified? The answer is a resounding yes! Sonically, the most significant improvement is in the area of tonal balance. The Koval mod eliminates the speaker's broad midrange recession. My measurements show the modified Quads to possess flat response on-axis (within 2dB) from 700Hz clear out to 17kHz. Below 700Hz, the room response is excellent but a bit more jittery (4dB glitches) down to about 55Hz. The change in balance is quite obvious and results in a more lively and spacious character through the mids and treble. It takes some getting used to, especially if you've tailored the rest of your system around the tonal balance of the original Quads. Initially you might think the sound a bit bright or the treble a bit tipped up, but in the final analysis, the Koval mod advances the Quads much closer to reference caliber. The focus of instruments within the soundstage is also tighter, improving imaging specificity. The bass (with both grilles removed), too, is better defined, with now what strikes me as an almost ideal Q. It is now much easier to follow bass lines and to identify various bass registers (footnote 5).

Of course, the Koval mod is not an overall panacea. The dynamic range and deep-bass limitations of the original Quads are still apparent. But the improvements are so significant that I think the Koval mod may be the best-kept secret in audio. It represents great value for money, and, together with the other modifications outlined above, pumps new life into the original Quads.

I have been using my modified Quads for quite a while now, not only as a reference tool, but simply for the sheer enjoyment of it all. It will be a cold day in hell before I retire these babies!

Footnote 3: Art Dudley described renovating and modifying a pair of original Quad ESLs in his July 2007 "Listening."—John Atkinson

Footnote 4: Linear Acoustic Labs, 11521 Cielo Place, Santa Ana, CA 92705 (1987).

Footnote 5: Bass transparency also improves if the Quads are used on rigid 15-inch stands, as recommended by Christopher Breunig in Vol.10 No.1.—John Atkinson