MartinLogan CLS loudspeaker:
Martin Colloms part 2
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