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stereo pair is considered. In the far field these can be expected to coalesce to some degree. Other Considerations 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 in the diaphragm, tensioning, 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 could be inappropriate. Conversely it is sufficiently robust to allow vacuuming! The Apogee has a physical weakness in that its diaphragms are relatively susceptible to damage; even a light finger touch will mar the fragile diaphragm surface and there are no grilles. Magneplanars The well-established Magneplanar MGIIIa, based on a proprietary planar stretched film drive unit in which a wire-conductor array is bonded to the diaphragm, is probably the most durable of the three designs under test. The exposed bass and mid diaphragms are at the rear, behind the magnet array; they are also sufficiently resilient to resist minor impacts. The true ribbon tweeter is also well guarded. The fact that the MGIIIa is a three-way design has given its designer more freedom to balance its frequency, response; it is arguably more successful in this particular respect than the two other models. The ribbon is pure aluminium, just 0.5inch wide with very good horizontal directivity and an extended high frequency reponse. Other advantages of the MGIIIa include a truly extended, wide-directivity high range, plus a respectable 86dB/W sensitivity and well-damped film diaphragms, achieved via a lossy coating and the use of a special adhesive for the wire conductor bonding. Acoustically speaking, the bass and mid sections are not as transparent and unobstructed as the Logan or the mid-treble section of the Duetta. Where the Apogee diaphragm are trapezoidal to help control the natural drum modes the rectangular examples of the Magneplanar have a set of judiciously placed modal clamps to help damp the more noticeable modes, which otherwise colour the lower mid range. Sound Quality: Apogee Duetta II
I shall leave sound quality rather more the more effusively indulgent characteristics of to my American colleagues and present (I hope!) a academic and objective view. This speaker has improved considerably since my first acquaintance with the original samples some 15 months ago, which I declined to review. In my system, problems were experienced with tonal imbalance, midrange glare, and a lack of precision in the high treble, though other critics seem to have enjoyed greater success with its sound. With the Mk.II, I found a greatly improved uniformity of frequency response, and a high-treble problem was avoided. The speaker was easy and relaxing to live with, and had great virtues. However, there was also what I feel to be a considerable flaw: the Duetta was demonstrably and unarguably rich in balance, and sounded dim when set against my established references, all of which have been directly compared with live sound sources under controlled conditions. If the Duetta is the only speaker you use, or perhaps the only speaker used that day, and you only play your brightest-sounding records, then you might become accustomed to its tonal imbalance. In my 80m³ room, the bass was almost overpowering in level, while the balance sounded as if down-tilted with increasing frequency all the way from 50Hz to 10kHz. (Imagine a Quad 34 or 44 tilt control rotated fully to slope downwards.) I felt a strong urge to beg, steal, or borrow the nearest Cello Palette and attempt some moderation of this speaker's slow, ponderous character. (with hindsight that rich balance was engineered partly to account for low frequency losses in the larger open plan US living spaces) In addition, while the problem I had perceived above 12kHz from the Mk.I had gone, so, it appeared, had the extreme treble itself. The treble sounded filtered, with little air or sparkle in the final audio octave. The end result was still very sweet, but lacked a sufficiently “open” quality. Perhaps surprisingly, however, the Duetta sounded best with the treble switch up (i.e., dimmer). With the switch set down, the treble was lifted slightly at the expense of a mild return of upper-mid “glare,” something I can do without. I also indulged in some strictly illegal fiddling: I tried placing 1.5-ohm resistors (25 watt wirewound) in series with the woofer panels, which also helped the tonal balance somewhat. The Duettas are amazingly insensitive; for example, Audio Research M100s (on their 4- ohm taps) sounded wonderful at low levels, but failed to stir the Duetta into life at high levels. Substitution of Krell KMA-100s (lovely amplifiers these, despite their being solidstate) essentially maintained the standard of reproduction but added some 6dB of level, bringing the Duettas to a satisfactory volume level. A quick lab check showed that the KMA-100 could churn out 200 watts into 8 ohms and 360W into 4, yet to obtain the full dynamic range I believe the Duetta capable of, I felt the need for a KMA-200 or the equivalent. In-room, this would establish maximum sound levels of typically 105dBA from a stereo pair. For reference, a KEF R107 fed a standard 100W (8 ohms) per channel will deliver 107dBA. An MGIIIa fed its power limit of 100W (8 ohms) would deliver a satisfactory 101dBA.
Page 1 and 2: _____________________ THE FLAT RESP
Page 3 and 4: in a true ribbon form. The magnet s
Page 5: lossy foam-plastic mounting is used
Page 9 and 10: This CLS was better balanced than b
Page 11 and 12: For the record, the Quad ESL-63, wh
Page 13 and 14: Duetta can be rated well enough . F
Page 15 and 16: 50Hz, and, in my view, showed exces
Page 17 and 18: FIG.10 The CLS offered a below-aver
Page 19: * Provisions have been made for the

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