MartinLogan CLS loudspeaker

John Atkinson & Various, September, 1986

The quest for a full-range electrostatic loudspeaker has occupied many engineers' minds for many years. The problems are manifold: large physical size (which can lead to room placement problems and poor dispersion), the difficulty of achieving high sound pressure levels, the need for a potentially sound-degrading step-up transformer, and the unsuitability for production-line manufacture. Even so, the potential rewards are so great that one can understand why loudspeaker designers keep on attempting the apparently impossible. Epoch-making models do appear at infrequent intervals, keeping the flame burning since the appearance of the original Quad in 1955: Acoustat, Sound Lab, and Beveridge in the US, Stax in Japan, Audiostatic in Holland, Quad, of course, in England, and now MartinLogan.

M-L's first model, the elegant Monolith, coupled an electrostatic treble and mid panel to a conventional dynamic woofer. I first heard this speaker in the Threshold room at a Summer CES some years back; the clarity was stunning. The CLS—the acronym stands for Curvilinear Line Source; I think that Clear LoudSpeaker would be just as appropriate—is the Kansas company's first full-range electrostatic, and again an elegant design, standing 5' tall. The most visually striking aspect of the CLS is its apparent simplicity: the Mylar diaphragm, just 2µm thick and with a total mass less than that of one cubic inch of air, is suspended between the two metal-plate stators, coated with black insulating paint, and punched with a matrix of holes to allow the sound to "escape." The diaphragm operates in push-pull, so should operate with low distortion.

And that's it, apart from a real-wood frame surrounding the diaphragm assembly and the box of electronics on the rear. No protective screen to keep the high voltages away from prying fingers; no dust cover that could introduce another set of HF resonances; nothing to get between the loudspeaker's moving element and the listener's ear. The unique aspect of the design, however, is the fact that, whereas other full-range electrostatics consist of flat panels, the CLS, like the Monolith, has a gently curved diaphragm. Exactly how this is done is M-L's proprietary process—though I know for a fact that Dorothy brought the secret back to Kansas from Oz—but the one-piece diaphragm is broken up into a number of different-sized, discrete horizontal elements by insulating damping strips, with long vertical strips running the full height of the speaker either side of the central array to preferentially handle low frequencies.

The reason for this techno-trickery is to avoid the beaming that accompanies a sound-source reproducing frequencies with wavelengths on the order of, or less than, the size of the source. Acoustat and Audiostatic attempt to overcome this problem by making the diaphragm narrow; Harold Beveridge used an array of waveguides to create a narrow apparent source in front of a flat diaphragm. By working out how to make a plastic-film diaphragm hold a curved profile, MartinLogan creates a virtual line-source behind the speaker which, because of its reduced width compared with a large diaphragm, will have excellent horizontal dispersion. (The Quad ESL-63 achieves a similar result from an almost flat diaphragm by driving discrete annular sections with different time-delayed signals, the crucial difference being that Quad's Peter Walker tries to achieve a virtual point-source behind the speaker.)

Setting up the CLSes presented no problems. The electronics module bolts to the rear of the panel with Allen-head bolts (a suitable tool is supplied) and acts as a support. The only fiddly bit is connecting the panel's internal Monster Cable wiring to the module via a high-voltage printed-circuit-board edge-connector. Screw-feet at the module's rear can be adjusted to tilt the speakers to the desired angle; being English, I naturally attached Chris Brooks Cones—Tiptoes with a British accent—to these feet. I also placed these cones under the front panel to ensure a solid "mechanical earth" for the diaphragm. M-L says that electric equilibrium will be 95% reached one minute after plugging in the speakers, and 100% after a half-hour of charge. I left them on for a day and night before listening to them seriously. It was no sweat, as I only played 20Hz, then pink noise, to run-in the diaphragms. (CD is great for chores like this.) On pink noise, the sensitivity appeared to be almost identical to the Celestion SL600 at around 83-84dB/W. 88dB/W was originally claimed, but M-L now says that you can knock 2-3dB off that figure due to modifications in production. At least I'm in the right ballpark.

While the speakers are running in, I'll tell you about the system. The front end consists of either my familiar Linn LP12/Ittok/Koetsu Red record player, or the California Audio Labs Tempest CD player that I reviewed in Vol.9 No.6. Preamplifier was an updated Audio Research SP-10—Yes, Bill Johnson had waved his magic wand over it—and amplification was first a Robertson Forty Ten, then a Krell KSA-50 Mk.II, and finally one of JGH's Audio Research D-250 Servos. My room is quite lively, particularly in the midbass, but ASC Tube Traps in the corners behind the speakers went some way to taming the acoustic.

So how was the sound?

Down the yellow brick road...
The running-in is essential, as the CLS sounds almost unbearably bright straight from the box. After being kicked around for 24 hours, however, the speakers were broken in. What record did I put on first? Having no Judy Garland records, it would have to be the Sheffield Lab James Newton Howard, this merry band of musicians being in real life members of Toto.

Well, I'll tell you first what impressed me about the CLS. Stereo imagery was spectacularly precise. Not only were instruments and voices hung in space between and behind the speakers—which quite disappear—with both a natural perspective and unexaggerated size, but the way in which that perspective and the instrumental balance continually shift as the recording engineers play with the mixing desk was ruthlessly laid bare. Do you want to be a record critic? Put on the Deutsche Grammophon CD of West Side Story—the CLSes let you hear how a team of highly-trained, experienced professionals can turn Bernstein's greatest score into alphabet soup! Hear the perspective of Nigel Kennedy's fiddle change in the second movement of his Elgar Violin Concerto recording. Do you want to be a recording engineer? Put on the Kings Singers' Flanders & Swann LP and learn how to distinguish EMI's Abbey Road studio from AIR London by the tonal quality of their echo-plates. And hear how different instruments in a typical rock mix have different colorations due to the mikes used.

Without a doubt, the CLS is the most revealingly transparent loudspeaker I have used. The Wilson Audio Beethoven violin sonata, for example, was presented spatially the best I have heard, the soloist being obviously around four feet in front of the piano, and the piano image naturally sized.

Tonally, the midrange is neutral, voices being rendered without undue coloration, but the lower midrange is slightly depressed. The exact amount of upper midrange energy seems to depend very much on listening height: my chair puts my ears about two-thirds of the way up the panels; lower than that and the midrange depresses, leaving the low treble a little exposed. The bass was a little lightweight, there being a lack of upper bass (though there is a slight bump in the response that makes male voices a bit chesty). The midbass was fine, but there was then a hole before the relatively sparse low bass came in. (In-room, spatially-averaged measurements using pink noise showed that the speakers rolled off rapidly below 60Hz.)

The low frequencies are also a little "slow": on the "Nature Boy" track on the Norwegian Radka Toneef album, which features a Scandinavian ice maiden, grand piano, and air-conditioning—all immaculately recorded—the piano appeared almost to anticipate the beat, the bass being definitely a little sluggish compared with the treble, which is quite "fast."

Above the midrange and bass, we now start to get to an area of tonal performance where I was less happy. It both sounds and measures as if there is a broad plateau between 1kHz and 4kHz, which adds to the feeling of transparency but gives the speaker a merciless quality. (I also felt that there was something weird going on at very high frequencies, around 14kHz.) The sound is detailed, yes; transparent, certainly; but there is a feeling of "glare" which, no matter how effective at revealing recorded detail, detracts from musical enjoyment. It is almost as if you are reading a book with fine print by the light of a 500W bulb. You are made aware of every detail, including the intricacies of the typeface and the texture of the paper, but you become a little fatigued. You become so aware of the "how" that you lose interest in the "why."

The longer I used the speakers, the less this character was apparent, and changing to the D-250 considerably improved things in this respect. Listening to the CLSes from a greater distance than the 6-9' I used will also help. But there was still some residual forwardness that gives the sound an overall "cold" balance. Pianos have too much "snarl," rosin noise on strings is accentuated, and recorded sibilance, as featured heavily on my favorite Clannad album on RCA, can often become intolerable.

Another aspect of the CLS sound bothered me more in the long term: a lack of dynamic range in the lower mids and upper bass. We are at 7000' in Santa Fe, so I was prepared to accept some compromise on ultimate loudness. The diaphragms have to move further to generate high SPLs in the thin New Mexican air, and run out of excursion sooner than at sea level. Nevertheless, I found that measured peak levels in the high 90s would cause random ticking noises from the diaphragms, particularly with recordings having a predominance of lower midrange energy (footnote 1).

The Cyprien Katsaris recording of the Liszt "Eroica" transcription has all its energy centered between 200Hz and 800Hz, and levels above 95dB (flat, average) caused the CLSes to misbehave. The dynamic range limitation seemed less of a problem when I switched to the Mk.II Krell KSA-50 from the Robertson 4010, but it was disappointing to not achieve the admittedly high levels at which I occasionally like to listen.

Could the CLS's load impedance explain what I was hearing, electrostatics in general having a bad name for being pigs to drive? Rated by M-L as a nominal 6-ohm speaker, the CLS actually varies considerably, reaching a maximum of 40 ohms at 1kHz but dipping below 5 ohms both in the mid and low bass, and above 4kHz. The impedance minimum is 2 ohms at 16kHz. This might go some way to explaining the subjective problems in the bass and high treble, particularly if the nature of the impedance at these minima is highly capacitive. I would have thought, however, that all the amplifiers I used would have no problem supplying the necessary current.

After a couple of weeks of living with the speaker, I had no option but to call M-L's Gayle Sanders to discuss what I was hearing from the CLSes. He nodded his head sagely when I mentioned the bass quality: "The adhesive we use can soften in transit, leading to too much midbass and a corresponding dip in the upper bass." He counseled running a heat gun up and down the edges of the panels, which should both even out the upper-bass response and increase the speaker's capacity for producing high sound levels. We then got on to the subject of the residual brightness that was bothering me. Again he nodded his head, and mentioned that the latest version of the electronics module had a modification to ameliorate this character. Apparently the original version I had could introduce amplifier instability at ultrasonic frequencies, due to the capacitive nature of the load up there in the stratosphere. A new set of modules would leave Kansas via Federal Express as soon as the Munchkins had finished applying the shrink-wrap.

The new bits arrived—serial numbers were 1870/1871 compared with the original modules' 1562/1563—and I duly set the speakers back up, having first played my hair-drier up and down the edges of the panels.

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Footnote 1: As a reference (for the thin New Mexico air), the Quad ESL-63s "crowbarred"—or arced, when the protection circuitry wasn't working effectively—predictably on 96-97dB peaks, depending on the frequencies being reproduced (the lower the worse). The CLSes did 2-3dB better, again with problems showing up sooner at low frequencies.—Larry Archibald

Well, I have to say that although the balance was still cold, the glare had gone. Driven by the D-250, the superb stereo imagery and the totally transparent midrange allowed me to hear further into recorded balances than, with one exception (footnote 2), I have ever heard before. The SL600 that has been my reference for three years now is excellent in this area, but sounds veiled beside the CLS. The CLS's upper bass had also improved somewhat, but the relatively limited dynamic range was still a feature, orchestral music producing "cracks" with average levels over 96dB (equivalent to around 103dB peak). I can only assume that I had been unable to apply enough heat.

Somewhere, over the rainbow?
This is a hard conclusion to write. The CLS has major strengths, but also, for me, a tonal balance that favors the treble overmuch, and a limited low-frequency dynamic range. To put this into perspective, I am talking about sound levels that are probably higher than many would like, and the CLS does play louder than the Quad ESL-63. However, I think a $2500 loudspeaker should play loud; certainly it should deliver levels approaching the real for some kinds of music. The bass was better with the KSA-50 than with the 4010, but I don't feel the CLS to be a loudspeaker for solid-state amplifiers. They were at their best with the D-250, and I would think that Quicksilvers, the C-J MV50, and similar tube designs would also bring out their best.

I am tempted to say that the CLS is an ideal speaker for small-scale chamber music—the JS Bach flute sonatas CD on Harmonia Mundi sounded magic—except that when I played the old Rostropovich Brahms cello sonatas CD on DG, which has a predominance of energy in the upper bass/lower midrange, the speakers again ran out of steam if played at musically satisfying levels.

Let us assume that this is only a limitation for my pair of CLSes, which might, of course, be atypical at Santa Fe's altitude. How then would I rate them? Magically transparent; neutral through the midband; capable of throwing a superbly delineated soundstage; the CLSes still proved unsatisfying on musical terms. With speakers like the Celestion SL600s, you can invite friends round, listen to a large selection of different music, and, though the characteristics of the recordings are always clear, they do not get in the way of the musical communication. Resultant conversations amongst listeners involve matters of performance and interpretation, or even just involve people sitting, tapping their feet. With the CLSes, those conversations take on a hi-fi flavor. Is the VTA correct? I think your turntable has a trace of wow. That's the sound of 6DJ8s aging. Listen to the faders on the mixing desk move. Was that an edit? Crossed hypercardioids for sure! Surely the engineer has placed the soloist too far forward in the mix.

Amongst all these dramatically revealed trees, one loses sight of the musical forest.

This, of course, may well be a matter of my taste being out of tune with that of the MartinLogan design team, and perhaps with yours. If you feel that the CLS's positive attributes that I have described outweigh my critical feelings, try to audition the speakers with your own set-up for a weekend before committing yourself to their purchase. If you still love the things they do right on Monday morning, and can ignore the shortcomings, they will be the right speaker for you.

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Footnote 2: The exception was Harry Pearson's awesome IRS system, which I experienced before the Sea Cliff fire. This system, however, does rather more than the CLS in the other areas of reproduction.—John Atkinson

Martin Colloms wrote about the CLS in January 1987 (Vol.10 No.1):

Panel speakers are back in fashion. After years of determined effort on the part of a few established manufacturers, the number of these designs has seen a marked increase, and many new exotic models have appeared. Stereophile has recently reviewed two leading examples of the genre, the $2780/pair Apogee Duetta and the $2490/pair MartinLogan CLS, (both in Vol.9 No.7). To judge by the tone of letters arriving at the magazine's offices, the reviews generated heated controversy. John Atkinson asked me, therefore, to conduct an in-depth examination of these two models, to give a fuller picture of what these speakers are capable of.

I hope to present rather more data than contained in a conventional review. To begin with, there will be new supporting evidence on the sound of the CLS generated in alternative rooms. Furthermore, the review samples were of recent vintage, with the Logan equipped with the latest electronics. The CLS frequency balance had also been changed for the UK market. This involved the substantiation of a higher resistor value in the treble section of the equalizing/matching network to provide a richer, and in my view, more accurately balanced sound. (Those comparing my remarks with JA's in Vol.9 No.7 should bear in mind that JA's samples had not been so modified.)

Design Compromises
The MartinLogan CLS fullrange electrostatic is an open-panel speaker and therefore must suffer the low-frequency rolloff limitation of a finite baffle. It copes with this by means of a designed resonance at 50Hz. The electrostatic principle is attractive in many respects, not least because it allows the use of an extremely light diaphragm—so light, in fact, that its vibratory contact with the air molecules may be used as a palliative, controlling, damping mechanism. Light diaphragms can move quickly, promising good high-frequency response, while their low inertia assures a low level of stored energy, promising an accurate transient response.

In principle, the electrostatic driver is very efficient in its conversion of moving electric charge into acoustic power. Problems arise, however, in interfacing the necessarily large electrostatic element to the room, as well as to the power amplifier. The science of electrostatics is the science of high voltages, with the polarizing field established by as much as 7kV applied to the inner shielded diaphragm. Ideally, the fixed-mesh electrodes on either side of the diaphragm need to be driven in push-pull at upward of 500V, and require isolation from the user. (In the case of the Quad, the electrodes are protected behind grounded mesh screens.) A thick, tough, black plastic insulating coating is applied to the CLS electrodes; these simultaneously form the external grilles of the speaker.

High-ratio step-up transformers need to be used to couple the amplifier to the system, while the input characteristic is not one which allows for optimum power transfer. An electrostatic offers an input characteristic which is predominantly capacitive, hardly an ideal load. By the time these considerations have been taken into account, as well as the need to improve the poor dispersion of a single large diaphragm, the much-vaunted efficiency has been whittled away. Nevertheless, most electrostatics have a basic sensitivity of around 85dB/W (8 ohms), which is rather better than that achieved by present large ribbon systems.

The CLS diaphragm is unusual, for although it is only 0.004" thick, and consequently very light (the whole system is reckoned to weigh the same as one cubic inch or 16.4ml of air), it is formed from a pretty rigid plastic and is almost self-supporting. No mechanical damping is used in the diaphragm; its stretched "skin" and "plate" vibration modes are partially controlled in the first instance at the boundaries, where a lossy foam-plastic mounting is used, and by the acoustic impedance of the air load imposed on the diaphragm. For comparison, the Quad ESL-63 uses an 0.00137" diaphragm, with 0.001" plastic film for the dust-cover membranes. The CLS uses a density of Mylar similar to Quad, giving a total mass of around 0.02gm. (The ESL-63 is even more extraordinary, its diaphragm weighing just 0.003gm, or 3 milligrams, not too far removed from the tip mass of a pickup cartridge!) Another interesting comparison is with a typical wide-band moving-coil dome tweeter, where the moving mass is normally around 200mg. Quads enjoy higher levels of air-damping than MartinLogans and, in addition, use an interlayer of the sheerest gauze to provide necessary resistive mechanical contact damping to the diaphragm surface.

From its appearance it is obvious that the CLS diaphragm is broken up into discrete radiating areas by damping strips. It is possible to acoustically excite these separate "cells" by blowing on them. As when air is blown over the necks of differently sized bottles, each one has its own distinct "sound." By this means, dominant resonances are moderated and dispersed by dimensioning each rectangular cell differently (rectangular, in any case, to reduce the resonance-mode symmetry).

The vertical side compartments of the CLS diaphragm are of fairly high Q, and are tuned to 50Hz. Above resonance, this level falls at a natural rate of 6dB/octave, partly compensating for the naturally rising 6dB/octave response of the panel as a whole. Beyond this frequency range, the double-section, frequency-compensated, step-up transformer applies a further reduction in mid/treble level. Finally, the distribution of the cells and the arc of the curvilinear diaphragm complete the frequency balancing.

A full-range planar diaphragm like the CLS becomes increasingly directional as the wavelength of sound approaches the size of the panel. In the Magneplanars and Apogees, this is solved by allocating the subsequent frequency range to narrower and narrower line-source elements. For the Quad, the diameter is subdivided no less than eight times, providing a controlled reduction of element size with frequency, and thus maintaining a good radiation angle. MartinLogan deals with the problem in a different way: the electrostatic panel is formed into a near-parabolic contour with a prime radiation approaching a vertical half-cylinder. With this broader-angled distribution, some of the natural rise in frequency response is also offset.

If the diaphragms were sufficiently well damped, and dimensionally stable to operate without the subdividing cells, this geometry would approach the ideal. In practice, the cells' boundaries terminate the high frequencies in a nonuniform manner, resulting in an array of treble radiators rather than a uniform whole. Such an array is likely to result in an off-axis polar response with a complex fine structure at high frequencies. This will generate random amplitude and phasing irregularities when a stereo pair is considered.

Electrostatics generally have a finite sound-level limit, due to considerations of peak voltage and electrical flashover, or saturation limiting in the cores of the step-up transformer. Other problems include aging—changes in the diaphragm tension—and dust build-up due to electrostatic attraction. The Quad ESL-63 is dust-proofed, but the Logan has no such protection, and I would suspect that its use in a dusty environment would be inappropriate.

Sound Quality
An earlier CLS had sounded quite impressive—lively, brilliantly clear, and full of dynamics as well as considerable musical information. Ultimately, however, it had proved a trifle wearing in that its tonal balance was uptilted—too bright—the converse of the Apogee Duetta. The current review sample sported the latest "electronics" comprising a factory-specified treble resistor selection, which balanced it closer to tonal neutrality.

This CLS was better balanced than before, yet much of its impressively exciting "liveliness" was still apparent. My listening notes contain a very good first impression which placed it in the true high end. The broad midrange was undoubtedly very fine, and sounded quite remarkable on plucked instruments, such as harp and acoustic guitar. As with the best panel speakers, the absence of the usual wooden-box colorations came as a welcome relief. The mid was highly informative and immediate, with that now much-sought-after "direct-coupled" character.

The speaker could play pretty loud with quite modest amplifiers, and showed a surprisingly healthy bass and bass-power handling. If I had stopped listening here, the CLS would have sailed through the subjective testing; however, as the listening period was extended, I became increasingly aware of certain adverse effects which ultimately moderated my good opinions of this model.

For example, I found the stereo imaging restless and aurally uncomfortable. To put it bluntly, the upper range, beyond 5kHz say, was "phasey" (footnote 1). By this I mean that small head-position movements or changes resulted in disconcerting shifts in apparent image position. Higher-frequency instruments were often presented in a forward, "over-wide" manner, which tended to detract from the impression of depth. Unless one sat perfectly still, head virtually clamped, with the two speakers perfectly and symmetrically aligned, the stereo focus was consistently imperfect in the upper registers. In addition, the upper treble possessed a distinct, though subdued, "edge" or "fizz" above 10kHz. Finally, the bass, at first impressive, began to resolve itself into a one-note emphasis or "hangover" located in the mid-bass, with little extension apparent below this point.

Small musical forces, such as a trio, replayed exquisitely, but with larger forces the CLS showed a tendency to lose this level of clarity. In addition, it could be made to clip on moderately high levels of solo piano. (JA mentioned this in his review; MartinLogan reckons it due to core saturation in the drive transformer at the power-handling limit).

Careful comparisons on master-quality sources suggested that the treble range suffered from some fine structure unevenness, while in terms of tonal balance it was felt to be somewhat midrange forward and showed some mild sourness on violin tone. Its overall performance brought to mind the inherent character of the Decca cartridge: "direct-coupled" immediate sound, odd bass, and suspect treble, all allied to a marvelously "live" midrange.

Conclusion
I find the CLS has considerable merit, particularly for smaller orchestral forces, offering an essentially uncolored, "fast" sound. Reservations remain, however, concerning the nonuniform impedance load, the peak midrange power-handling for its price and size class, the treble phasiness and related fine structure irregularities, and, finally, the almost one-note 50Hz bass.

The CLS is an interesting speaker of remarkable appearance. Only a careful audition will properly inform a prospective customer, especially since broad areas of its frequency range sound quite as transparent as it looks.

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Footnote 1: JGH also noted this quality when he heard the CLSes in my room. He refers to this as the "venetian blind" effect.—John Atkinson

Jack English wrote about the CLS IIA in December 1991 (Vol.14 No.12):

The CLS IIA is the most fascinating product in the MartinLogan lineup and, strangely, the most often overlooked. This fact is particularly intriguing given the CLS's stunning aesthetic appeal. Introduced in 1985 to widespread fanfare within the audiophile community, the original CLS was rightly praised for its literal and sonic transparency. It received rave reviews and became the most sought-after speaker for many an audiophile. Oddly, it has subsequently suffered from its early blockbuster success.

MartinLogan used the CLS technology to develop the Sequel, now in version II form. In addition to a curved, transparent electrostatic panel like the CLS's, the Sequel included a dynamic woofer and cost a grand less! The market responded enthusiastically, to say the least. M-L president Gayle Sanders responded with a revision to the even more potent Monolith, now in version III form, which also included dynamic woofers to fill out the much-needed bottom. The audiophile with some financial constraints had the Sequel, and those with greater financial resources had the Monolith. For those with no financial constraints, M-L offered the megabuck Statement. In 1991, still another MartinLogan speaker hit the market: the Quest, with the now familiar hybrid design of an electrostatic top and dynamic bottom, and priced to go head to head with the CLS. But where did this leave the CLS?

CLS I vs CLS II
Before moving on to the performance of the CLS IIA, it is appropriate to review the "report card" of the earlier CLSes. My impressions of the original CLS were mixed. The CLS I had early problems with reliability, something clearly a thing of the past, in my opinion. It did some remarkable things which were detailed in excellent reviews by John Atkinson (Vol.9 No.7), Martin Colloms (Vol.10 No.1), and John Nork in issue 45 of The Abso!ute Sound.

The CLS Is were often criticized for having a glare in the upper midrange/lower treble region that became most apparent during loud passages. There were also its difficult load characteristic, inadequate deep bass, some criticisms of a one-note bass character, limitations in both dynamics and ultimate loudness, phasey/beamy trebles, and an overall lightweight character. On the plus side of the ledger, the CLS Is were: visually stunning; state-of-the-art in overall neutrality and transparency, setting new standards in the resolution of inner detail while simultaneously eliminating much of the sonic grundge we had all grown so accustomed to hearing; devoid of box colorations (they had no boxes); incredibly fast; seamlessly coherent; had good dispersion; were reasonably efficient; and had a marvelous midrange immediacy.

In spite of this dazzling array of strengths, the CLS I really did suffer from significant limitations in deep-bass extension, midbass punch, and dynamics, as well as suffering from some upper-midrange glare. While I admired the CLS I, I did not think it would have wide acceptance due to these sonic limitations. In fact, Nork concluded that the original CLS speakers were destined to become rather controversial. They did.

MartinLogan claimed three significant improvements for the CLS II revision, which was released in January 1989: better bass extension and definition, better high-frequency extension and naturalness, and improved efficiency and power-handling capability. John Nork, Arthur Pfeffer, and John Cooledge discussed the relative strengths and weaknesses of the MartinLogan CLS I and II in issue 68 of TAS. Specifically, they talked about the limitations of the I and such later improvements as better bass extension and articulation, minimization of glare, and, although it isn't mentioned, significantly improved reliability. However, they felt the II was harder to drive and suffered from a loss of immediacy.

These comparisons were relevant for CLS I owners but moot for everyone else. In essence, the Is and IIs were different speakers. If you absolutely loved the original CLS, you may have felt too much had been sacrificed in taming the beast. I didn't love the Is, and felt the IIs were a significant improvement, making the speaker more appealing to a wider number of audiophiles.

The fundamental issue is sonic performance, and here the CLS IIs shone brightly. Bass performance was dramatically improved, as claimed by MartinLogan. The IIs had improved definition, better extension, and a reduction of the one-note character endemic to the Is. Soundstaging and imaging remained outstanding. Dispersion was good for a panel speaker, but was improved by raising the speaker off the floor. Treble extension had also been improved as claimed. The upper-midrange glare of the earlier model had been admirably tempered although not entirely eliminated. Speed, coherency, and resolution of detail continued to define the state of the art. On the whole, the MartinLogan CLS II was a truly great if still perplexing speaker.

...and now, the CLS IIA
Upgrading a pair of CLS IIs to IIA standard costs $150. If you are marginally competent technically, you can install this upgrade yourself. If not, your dealer can do it for you. Of course, the electronics can be shipped back to M-L for upgrading. The IIA upgrade consists of snipping out one component (for each unit) and soldering in a 9" piece of wire. The IIA has a bit more midrange energy and detail than the II. This change moves the sound of the CLS back toward more of the midrange immediacy of the original CLS I.

Though there is a $2520 "upgrade" available for the I, the CLS IIA is not an updated I. The only thing that isn't changed in the upgrade from a I is the wooden frame—everything else is replaced! The diaphragms, electronics, and transformers are all new. In short, the CLS IIA is an entirely different speaker and should be treated as such.

Considering it an upgraded version creates problems. For example, the CLS I could be effectively driven by a high-quality, low-powered tube amp. The impedance curve ran from 4 ohms at 20Hz through 32 ohms at 1kHz to 2.2/2.3 ohms at 18 to 20kHz. The impedance curve on the CLS IIA has less swing or variance over its range—it's nominally rated as a 4 ohm speaker—but it dips to a low of less than 1 ohm at 20kHz! With a low-powered tube amp this will result in a noticeably attenuated top end. Even higher-powered tube amps like the ARC Classic 120 will tend to attenuate the extreme upper frequencies.

Wrong amplifier matches have led to complaints often inappropriately directed at the speakers. If you hear a pair of CLS IIAs and there is no treble, they are probably being driven by the wrong amplifier. If a pair of CLS IIAs are inserted into a system that had been fine with CLS Is, it's not certain that the performance will be as good using the same electronics or cables. For example, both Pfeffer and Cooledge found the VTL 225 to be a poor match with the IIs, although the amplifier more than satisfied the stated power requirements and had been able to mate well with the CLS Is.

For the CLS IIAs to perform optimally, therefore, great care must be exercised in the selection of a matching amplifier. While MartinLogan addresses this question in terms of rated amplifier power, that is inadequate. Where low-powered tube amplifiers may have been ideal choices for the CLS I, solid-state amplifiers from companies like Krell and Classé may actually be a better match in a number of situations with the IIAs. Your dealer and MartinLogan should be able to provide more meaningful guidance on this issue.

Setup, etc.
Let's take a bit of time to review some mundane issues often overlooked by reviewers and yet of paramount importance to prospective purchasers. Given the track record of any new panel speaker in general and electrostatics in particular, reliability is often suspect. With more than seven years of actual production under their belt, MartinLogan's designs have cleared this hurdle with aplomb. Their products are reported as indeed being reliable and trouble-free, so much so that they now offer a three-year warranty at no charge. The purchaser simply completes and returns the certificate of registration within 30 days of purchase.

The packaging and shipping of the product is equally trouble-free. My CLS IIAs arrived in two cartons, one each for the speaker panels and the electronics. One electronics module is connected and fastened to each speaker panel. To complete the setup, the enclosed feet are affixed, speaker cables connected, and power cords plugged in. The final step is to properly position the speakers. M-L suggests locating them at least 2' from any wall and very slightly angled toward the listening position. That done, you're ready to go!

Not quite, of course. Remember, the CLS IIAs are electrostatic speakers and require a significant amount of time for the panels to properly charge, as well as a significantly longer period of time to break in. The more important factors here are the established reliability of the product, the integrity of the packaging, and the ease of unpacking and installing the speakers. All in all, a very straightforward and professional job with the consumer in mind.

Another important factor for the consumer is the care with which the CLS User's Manual has been put together. For the impatient there is a section entitled "Installation in Brief" to help you get set up and playing in the shortest amount of time. This section is the very first portion of the manual, not buried in an appendix or at the end, as is so often the case. For those with more patience, the manual includes a concise historical chapter giving proper credit for the electrostatic design to people like Rice & Kellogg (Bell Labs), Arthur Janszen (KLH), and Peter Walker (Quad). This is followed by an easy-to-read chapter on the electrostatic concept.

Only after providing all of this information on the evolution of the electrostatic design does the manual begin to discuss MartinLogan's unique contributions: the curvilinear line source (CLS) panel geometry (the CLS is still M-L's only full-range electrostatic speaker), the vapor-deposited film diaphragm, and transducer integrity. Appropriate cautions are provided on the speakers' use and maintenance. Seven pages of text are then devoted to room acoustics and proper speaker placement. There is even a section on recommended recordings. This is an excellent, instructive manual.

Although not immediately obvious, the CLSes are not identical. Each set is actually a mirror-imaged pair. The left- and right-hand vertical sections of each speaker are of different widths. The manual suggests setting them up with the wider outside section toward the side walls of the room, but goes on to say that this is not mandatory. Based upon my listening, I would say the suggested setup is indeed mandatory. With the larger vertical sections toward the inside, residual panel resonances were more obvious, the resultant performance significantly less realistic. I heard more of the speaker with the larger sections toward the inside. With these sections toward the side walls, the sounds of the panels themselves were less obvious, the speakers' performance significantly improved.

Sonics
For this review, I used the ideally suited Audio Research Classic 150 monoblocks (reviewed in Vol.14 No.11). These amps are in the upper recommended power-rating band and have no difficulty with the speaker's low-impedance load. (They offer output taps available for loads as low as 1 ohm.) Other equipment included the Versa Dynamics Model 1.0 LP player; Benz Micro MC-3 phono cartridge; CAT SL-1 Mk.II preamp; Magnan Type Vi, XLO, and ARC interconnects; Micromega CD-f1, Esoteric P-2, and Theta Data CD transports; Theta DS Pro Generation II processor; Kimber KCAG and XLO digital interconnects; and Cardas, MIT, TARA, and ARC speaker cables.

The remaining claims will be the crux of this review. Have the folks at MartinLogan simultaneously improved both bottom and top without sacrificing anything in between? Although not claimed by MartinLogan, is the CLS IIA more dynamic, with better bass and less glare? We shall see...

The CLS IIAs can be unpacked, set up, charged, and playing within minutes of their arrival. While the resultant sound is far from optimal, it nonetheless clearly indicates this speaker's strengths. I began my serious auditioning with Constance Demby's Sacred Space Music (Hearts of Space HS 11010-2). The CLS IIAs were a great match for her primary instrument, the hammered dulcimer, due mainly to their ability to handle both the attack and decay sides of the transients. There is nothing slow or sluggish in the sound. The IIAs maintain the model I's reputation for speed, being unsurpassed in this regard. The very lightweight diaphragms likely play a major role in the speaker's remarkable transient speed.

Equally obvious was the overall openness of the presentation, a wide and deep soundstage set back between and behind the speakers, with excellent retrieval of ambient information. There were certainly no box-like colorations anywhere. These are the impressions I would expect anyone to form in a quick A/B comparison or audition—the CLS IIAs are fast and open; no sounds are slowed by the drivers or trapped in nonexistent boxes. In many respects, they reminded me of my first audition of the Dahlquist DQ-10s. And no matter how many times you've seen the transparent panels of the MartinLogans, they remain visually stunning. Their physical characteristics always reminded me of one of their sonic strengths—that oft-debated phenomenon of transparency.

Of course, Constance Demby's sparse music provided few sonic challenges. More of the IIAs' sonic performance was evident using the ethereal-sounding Julee Cruise's Floating into the Night (Warner Bros. 25859). The trebles were indeed open and extended. Treble extension was directly attributable to the proper choice of amplification. I was somewhat surprised with the treble performance, given the gradually diminishing frequency-response curves included in the owner's manual. In particular, the percussive strikes and lingering decays of cymbals were very natural. The IIAs were clearly superior to the Is with respect to treble extension, as claimed.

The saxophone was satisfyingly rich and warm, Cruise's vocals sweet and delicate. Most captivating was the integrity of her voice. Every note, every overtone was cut from the same cloth. This is a key strength of the IIAs: top-to-bottom coherency. Of course, with all of the overdubbing, Cruise's voice does often get larger than life. Since that's what's on the recording, that's what the IIAs delivered. On the downside, the deep bass simply wasn't there, and the midbass was somewhat light. The overall impression of bass performance was somewhat akin to waves washing up on the beach. Depending upon frequency, sometimes the bass was there, sometimes it wasn't.

Detail resolution, again a strength of the original model, was superb. In fact, detail resolution with the IIA was superior to that of the II. Every instrument, every effect, and even every gain setting was abundantly obvious. In the song "I Remember," there is an interweaving series of complex sounds which were all clearly unraveled. Descriptions like "fast," "clear," and "coherent" only begin to describe some of the IIA's strengths. Like its predecessor, the IIA displays state-of-the-art resolution of inner detail.

The IIAs sounded great with such great recordings as John Handy's Excursions in Blue (Quartet Q-1005CD, distributed by Bainbridge). Most of this recording consists of naturally recorded jazz with bass, piano, drums, and sax. Here again, the strengths of the IIA dominated. Cymbals were fast and clear, although there could have been a touch more sparkle in the uppermost trebles. Brushes were clear. The piano got a bit sharp, but not hard, during loud passages, especially in the upper registers. This was a significant improvement over the glare found in the original Is, although it hasn't been exorcised entirely. Once again, the sax was rich and full, with adequate bits of bite as appropriate. There were abundant cues of real people at work: fingers hitting keys, depressing pads, plucking strings. The stunning detail resolution of the IIAs was captivating.

However, if I stood up, the tonal balance shifted significantly. This was a problem identified by JA that has not been corrected in the IIA. The CLS IIs do some wonderful things in the area of soundstaging, but they do suffer with respect to vertical dispersion. If you stand up, you lose a great deal of the top end. If you only listen to your music while seated, this really doesn't matter. If, on the other hand, you don't, this can be a significant irritant. The solution is simple. Raise the speakers 18" inches or slightly more off the floor. MartinLogan now offers a dedicated stand to do exactly this. When the speakers are raised off the floor, you can listen from either a standing or seated position.

I tested this assumption originally with a couple of cinderblocks. While seated, two other changes become obvious. First, the overall soundstage was raised higher off the floor; I looked up at, instead of down into, the stage. Second, the overall bass performance was improved, becoming slightly cleaner and tighter. As there was no deleterious effect from this change, I strongly recommend using some form of stands with the CLS IIAs. In addition to those from M-L, stands are available from Arcici, Underground Sound, and SimplyPhysics. For most real-world listening situations, the IIAs will perform better if raised off the floor.

But that bass...same old story. It was inconsistent in level at different frequencies, and overall lacked adequate power and body. The more I listened to the IIAs, the better I understood the decisions to include a woofer with the Sequel, Quest, and Monolith. The music simply lacked a sturdy foundation. The bottom, at times adequate, was all too often weak or missing. On balance, the speakers had superb "hi-fi" strengths but were tonally lightweight, with significant limitations in the bottom and a residual trace of glare in the upper midrange.

Over time, the speakers seemed to relax. This is a euphemistic description of what happened as the panels broke in. The bass, while never really becoming strong, definitely improved, adding much-needed body to the overall tonal presentation. This, in turn, altered my early impression of a lightweight sound to some extent. With respect to the Is, the IIA's bass was significantly improved in that it went deeper and was more clearly defined. While the IIAs had better bass than the Is, there was still not enough for most types of music. MartinLogan has fulfilled still another of its claims, but the bass performance of the IIAs, while improved, still falls short of what is needed in a full-range transducer.

The sensational strengths of the IIAs were abundantly obvious on certain types of music. An excellent example is the new-age beauty of Gabriel Lee's Seasons (Narada Lotus 61002). Using only an acoustic guitar, Lee paints a sonic picture of spring and summer with bright, light hues and voicings. The delicacy of each note and melodic line was pristine. The seamless nature of the tonal palette was retained intact. Lee's fingering was portrayed clearly and meaningfully. The transient speed was captivating. The IIAs just weren't there, leaving only the music in all its beauty.

Conversely, with other types of music, the IIA's weaknesses inhibited the performance. These shortcomings were all too obvious with recordings such as Art Zoyd's masterful score for Murnau's 1921 silent film classic Nosferatu (Atonal ACD 3008). For starters, this music is intended to be played loudly if it is to resemble the live experience. No matter how hard the IIAs are pushed, they simply can't achieve realistic volume levels. In addition, the music's terror and tension depend heavily on throbbing, powerful dynamic contrasts and a thunderous bass foundation. The IIAs lack the bass extension and requisite harmonic fullness to communicate these dark tonalities and effects.

The CLS IIA is indeed a better speaker than the original CLS I. For the most part, MartinLogan has effectively accomplished the improvements they claimed. The IIAs have better deep-bass extension, more realistic treble energy, a reduction of the one-note midbass character, a minimization of upper midrange/lower treble glare, and improved reliability. The amplifier/speaker interactions are different, though not necessarily better. The correct choice of amplification remains a critical variable in the IIA's performance.

More than any other product I have reviewed, the CLS IIA's overall performance is critically dependent upon the source material. With music that matches well with their strengths, the IIAs are stunning. With music that highlights their weaknesses, the IIAs are simply unacceptable. Since the major problems concern various aspects of bass performance, MartinLogan's decision to mate dynamic subwoofers with each of their other speakers makes very good sense. The CLS IIA desperately needs similar bass augmentation to be regarded as a full-range speaker.

Conclusions
MartinLogan has done its homework. The IIAs, in addition to the improved reliability and warranty, come well packaged for safety; are easy to set up once it is remembered that they are a mirror-imaged pair; have a first-rate manual; and are supported by an extensive dealer network.

The MartinLogan IIA is indeed a markedly improved speaker over the original CLS I. It now has a proven track record of reliability supported by a three-year warranty. Deep-bass extension has been improved; treble performance is more extended, although this depends on a correctly matched amplifier; the level of resonances has been somewhat tamed, virtually eliminating the one-note bass of the model I; and the upper midrange/lower treble glare has been diminished. While the IIA is more sensitive than the I, it is also a more difficult load, with an impedance curve that dips down below 1 ohm. Relative to the CLS II, the IIA has a slightly greater sense of immediacy and an even better resolution of detail through the upper midrange.

The IIA, like models I and II before it, has a state-of-the-art ability to unravel inner detail; a remarkable ability to handle transients; seamless top-to-bottom coherency; reasonably good dispersion (although they should be used on stands); and visual and sonic transparency which set them apart from the pack.

On the downside, the overall character of the IIA remains lightweight; deep bass is still attenuated; trebles are beamy; and both dynamics and ultimate loudness levels are restricted. When pushed to compensate for the lack of deep bass and restricted dynamics, the IIA can still sound somewhat hard and glary.

Taken alone, the MartinLogan CLS IIA is a perplexing speaker. In addition to its drop-dead looks, its bevy of strengths are at the leading edge of home music reproduction. On the other hand, it also has a set of weaknesses that are very difficult to accept over the long term. MartinLogan has dealt with the shortcomings by mating the CLS technology with dynamic woofers in all of their other speakers. Taken alone, the IIA is tragically flawed, yet enticing. If you listen to your music at moderate levels, are careful about selecting an amplifier, and tend to listen to smaller-scale works as opposed to orchestral or heavy metal, the IIA just might seduce you. If you're looking for a full-range, dynamically capable speaker, the IIA won't fill the bill.

It would have been easy to end my review with the last paragraph. It may appear to finish the story, but it doesn't really. The IIA simply has too many outstanding strengths to let it be. I will attempt to solve the problems of the IIA (without impinging on its impressive strengths) in a series of "Follow-Ups." Stay tuned.

Jack English wrote a Followup on the CLS IIA in February 1992 (Vol.15 No.2):

Acoustic Sciences Corporation Studio Traps had been selected specifically to deal with in-room problems identified with the use of MartinLogan CLS IIAs. Specifically, the CLS IIAs had the following problems that could potentially be minimized with the ASC STs: an emphasis of midbass energy, often giving the speakers one-note bass; a lack of sufficient deep bass, causing the speakers to sound lightweight overall; and a beamy/phasey character in the trebles which caused the speakers to have different tonal characters depending upon the listening position.

When the IIAs were located closer to the rear wall, the midbass resonances were worse but the deep bass extension was slightly improved. Placing the panels closer to the rear wall proved a mixed blessing. The STs worked splendidly with this setup, doing nothing to diminish the newfound deep bass extension but significantly minimizing midbass boom. With the STs, overall bass performance became more coherent. There was a reduction of the elevated midbass without losing any of the newfound deep bass extension. The net effect was a significantly more realistic bottom-end performance. Music once again had a semblance of bass foundation. The STs, coupled with the repositioning of the IIAs, led to a significantly improved tonal balance. In this application, the STs made improvements with virtually all recordings, including the Stones' mono hits.

With the IIAs well out in the listening room, the midbass was more boomy, the deep bass greatly attenuated. Listening to a recording such as the soundtrack from The Commitments (MCA MCAD 10286) was simply not satisfying. The performance lacked the foundation essential for this type of music. Moving the IIAs back toward the rear wall and adding the STs improved things significantly. The bottom became more believable, more realistic. With the firmer foundation, the music became more solid and natural. I was able to concentrate more on the vocals of Andrew Strong and Maria Doyle covering those classic soul tracks. My frustrations with the lightweight character of the IIAs, while not ameliorated, were at least somewhat pacified.

With the IIA panels placed closer to the rear wall and having the STs placed in the rear focal point of the back wave, the overall performance in the bottom was better but still far from complete. For example, the playful first movement of Shostakovich's Symphony 15, with Bernard Haitink and the London Philharmonic (London 417 581-2), was listenable and enjoyable, but sounded as if it had been written by someone else. The tympani whacks sounded like toy drums! I could hear something, but it certainly wasn't what was intended, or what was actually played.

However, tonal balance wasn't the only thing changed in this new arrangement. With the back wave damped, the overall soundstaging of the M-L speakers was altered. Without the STs, there were many more problems with front wave/back wave cancellations. This has often been described as a "venetian blind" effect. With the STs in place, much of the back wave was absorbed and diffused. The net result was less cancellation of the front wave.

Without the STs, the overall performance of the Shostakovich was more distant and spacious. In fact, the absorption of the back wave significantly diminished the overall spaciousness of everything played through the IIAs. This loss may be unacceptable for many listeners, as it robs the speakers of one of their unique attributes. With the STs in place, the performers were closer, the sound more immediate. The soundstage had less depth, but more precise placement. Spotlighted soloists were more obviously taken out of their proper soundstage locations. In fact, this was more accurate but less satisfying (to me). I preferred the more distant and spacious presentation of the soundstage without the Studio Traps.

A similar result occurred with the surreal sonic landscapes of Robert Rich and Steve Roach's Strata (Hearts of Space HS 11019-2). With the STs in place, the presentation was closer and more immediate. There was less depth and space. On the upside, the tonality was warmer and richer. Without the STs, the sound was more distant and spacious but placement was less precise. This hauntingly beautiful new age music was more "spacey" without the Studio Traps.

With music lacking either a naturally expansive soundstage (such as the Shostakovich) or a euphonically desired one (Strata), there was not as much of a difference with or without the STs in terms of soundstaging. A good illustration of this was heard on Paul McCartney's Unplugged (Capitol CDP 7 96413 2). The performers were spread across the stage with little natural depth. Since most of the instrumentation was acoustic, the performers were closely miked. They should sound like they are "in the room" as opposed to being located well behind the speakers. This is exactly what was heard, with or without the STs.

The MartinLogan CLS IIAs specifically, and other dipoles in general, present unique problems in a typical home listening room. The IIAs' problems include a midbass emphasis, limitations of deep bass extension, and various front wave/back wave cancellations. The Studio Traps worked very well to temper these problems. Use of the STs allowed the IIAs to be placed closer to the rear wall, which led to an improvement in deep bass extension. With the STs located at the rear focal point of the IIAs with the absorptive side facing the dipole, midbass bloom was meaningfully tamed. However, these improvements did come at a cost. In this case, the openness and spacious presentation of the IIAs was diminished. The soundstage had less depth and was located closer to the listening position. However, the placement of performers became more precise, the overall sound more immediate.

Jack English wrote about the CLS IIZ in June 1994 (Vol.17 No.6):

The $3500/pair CLS is the orphan in the MartinLogan range. Everyone's attention has been drawn to this company's remarkable hybrids: the modestly priced Aerius (reviewed Vol.16 Nos.6 & 10); the very impressive Quest Z (Vol.16 No.10); the popular Sequel (Vol.11 No.12, Vol.12 Nos.8, 9, & 12, Vol.14 No.2); the large, single-box Monolith (Vol.8 No.3, Vol.9 No.3); and the newest, the multi-piece Comment (WCES report, Vol.17 No.4). Each of these speakers marries an electrostatic panel, for the mids and highs, with one or more dynamic drivers for the bass. The CLS, in all its incarnations, has remained the only full-range M-L electrostatic speaker.

The evolution of the design has resembled the swings of a pendulum. The original CLS worked well with lower-powered tube amps that could handle very high impedance loads. That was good (moderate power demands). In response to criticism, M-L redesigned the speaker and gave us the CLS II, whose impedance load didn't stretch so far into double-digit ohms (good), but dipped so low that it was below a single digit (bad).

In fixing one problem, another popped up: The II required greater amplifier power (bad). One of the original's captivating strengths was its sense of immediacy (good). But in tackling the load, the II somehow lost a great deal of immediacy (bad). The speaker was taken back to the drawing board, and eventually the IIA was born. It recaptured some of the I's immediacy (good), but its impedance load dropped so low that it drove many amplifiers nuts (bad). Relative to the original—see JA's review in Vol.9 No.7 and my review in Vol.14 No.12—the IIA had significantly improved deep-bass extension, better midbass articulation, a more extended treble, and a minimized upper-midrange glare.

MartinLogan then released the fourth version of the CLS: the IIZ, which featured a new crossover board and power supply, and a reconfiguration of the transformers. While the IIZ purportedly maintains the immediacy of the I and IIA, it more than doubles the lowest impedance load, effectively ameliorating the major complaints people had with the II and IIA.

While that may seem like quite an improvement, remember that the IIA dropped down to 0.6 ohms. The improvement means that the IIZ drops only to 1.5 ohms. Relative to earlier versions of the CLS, this is indeed big news (good); relative to most other speakers, this is still a difficult load for many amplifiers (bad). The upgrade, which actually consisted of entirely new electronics, costs $695 (bad), but the price of the IIZ remains the same as the IIA (good).

The net improvements of the Z-mod (in terms of both sonic improvements and relaxed amplification requirements) have been obvious in the many consistently positive reviews MartinLogan has received for the updated/new Aerius and Quest hybrids. While the IIZ continues to suffer from deep-bass limitations, it has taken yet another step forward in terms of both sonics and practicality. I found the Z version, as advertised, easier to drive with a broader array of amplifiers. These ranged from the low-powered, inexpensive AMC CVT-3030 to the monstrous, wondrous Conrad-Johnson Premier Eight. While the speakers perform better with better amps, the IIZ can now be used in more systems.

Of course, all would have been for naught had the sonic performance taken a step backward. Fear not. As usual, the overall performance was remarkably coherent, with every nuance cut from the same cloth. The visually transparent panels continued to be remarkably coherent sonically, allowing the IIZs to retain the magic immediacy that has captivated so many CLS owners.

The inherent shortcoming in its electrostatic technology remains the limited reproduction of deep bass. MartinLogan's CLS IIZ is still agonizingly limited in this respect, but most people who see/hear the speaker are blown away by its combination of visual splendor and sonic capabilities. Non-audiophiles are consistently astounded that such a product can even exist. If you're out to impress your friends, it's hard to think of a better product to do so with.—Jack English

Sidebar 1: 1987 Measurements

Looking at the amplifier load factor, the impedance curve (fig.1) is typically "electrostatic." Low at low frequencies, and with no visible signs of the 50Hz diaphragm resonance, it climbs steeply to a harmless 33 ohms by 1kHz. Above this level, the load is almost purely capacitive, falling with frequency at 6dB/octave, reaching 2.5 ohms at 10kHz, and finally bottoming out at a value of 1.6 ohms at 20kHz. Wide-band synthesizers played at high level will not do the amplifier-speaker combination much good, while this load non-uniformity will cause more treble balance variations than usual with different cables and different amplifiers. In fact, one or two power amps may not be too stable with this load under heavy transient drive.

Fig.1 MartinLogan CLS, electrical impedance (solid) and phase (logarithmic vertical div.).

The CLS frame assembly seemed to show a minor mechanical resonance which was checked out acoustically (fig.2) and clearly placed at 160Hz. This resonance could be picked out on nearly all response curves taken.

Fig.2 MartinLogan CLS, frame resonance.

The forward response set is shown in fig.3. Taking the solid axial line first, the speaker output was high in the midrange, partly due to proximity. Above 1kHz, the output was even and well-extended, suggesting a basically good tonal balance. No loss was seen by 20kHz. The low frequencies also deserve some comment. As predicted by Gayle Sanders of MartinLogan, the main resonance was precisely at 50Hz, and, in my view, showed excessive amplitude. The bands above 50Hz were deficient, while a steep rolloff held below 50Hz, the output already -8dB by 40Hz, and -18dB by 30Hz.

Fig.3 MartinLogan CLS, 1/3-octave response on listening axis at 1m (solid), off-axis response in the vertical (small-dashed) and horizontal (long-dashed and dot-dashed) planes.

Given the smoothing advantage provided by 1/3-octave analysis, the off-axis responses of the CLS were remarkably good. It held a well-extended response over a wide range of angles, ±15 degrees vertical and up to 45 degrees horizontally. This is shown by the commendable closeness of the off-axis response group.

To explore the subjective comment of "phasiness," a narrow-band analysis was performed in the 500Hz-20.5kHz range for the axial response, and for a small 5 degrees lateral angle change (fig.4). The results show that the CLS does have a peak at 15-16kHz (heard on audition), and that the two responses, although subjectively alike, do not correlate well. Inter-response differences of 3-5dB are frequent at high frequencies which, of necessity, implies quite rapid phase shifts. These effects are typical of a multi-sourced, multi-lobed treble radiating system—the multiple cell configuration.

Fig.4 MartinLogan CLS, narrow-band analysis of on-axis response (top) and 15 degrees off-axis to the side (bottom).

The computer-averaged room response (fig.5) provides the final stage of measurement analysis. Here the 50Hz resonance can be clearly seen, while the energy loss above 60Hz is also apparent. The broad midrange is well balanced, while the upper treble is a little too well-extended at the highest extremity, suggesting an aural "edge" or corner. This is associated with the 16kHz axial prominence.

Fig.5 MartinLogan CLS, 1/3-octave, spatially averaged response in MC listening room.

Without claiming sonic superiority for the Quad ESL-63, the response for this model under identical conditions (fig.6) makes for an interesting comparison. Quad has attained a commendably uniform bass to 30Hz, with a broad sweep of uniform output held over the entire range, culminating in a naturally smooth room-energy rolloff in the highest frequencies. Yes, it can be done!

Fig.6 Quad ESL-63, 1/3-octave, spatially averaged response in MC listening room.

The CLS offered a below-average sensitivity of 84dB/W, with a power handling of 100W per channel. Maximum room sound-levels of typically 100dBA will be possible, though the speaker generally seemed to sound a mite louder than that.—Martin Colloms

Sidebar 2: Specifications

Description: Full-range, single-driver, electrostatic loudspeaker. Frequency response: 45Hz-22kHz ±2dB. Dispersion: 30 degrees horizontal, 4' vertical line source. Sensitivity: 86dB/1W/1m. Power Handling: 200W, 60V peak maximum. Recommended amplifier power: 80-200Wpc. Nominal impedance: 6 ohms (1986), 4 ohms (1987-1994). Minimum impedance: 1 ohm (1986), 1.6 ohms (1987-1992).
Dimensions: 28" W x 57.5" H x 14.5" D. Weight: 67 lbs each.
Price: $2490/pair (1986); $3500/pair light or dark oak, $3700/pair black oak or walnut, $3750/pair mahogany, $4030/pair teak (1994). Approximate number of dealers: 75.
Manufacturer: MartinLogan Ltd., 2001 Delaware Street, Lawrence, KS 66046. Tel: (785)-749-0133. Fax: (785)-749-5320. Web: www.martinlogan.com.