Company Address: 225 East Palmer St., Tallahassee, Fla. 32301.
Few speakers really introduce a new approach to speaker technology; most simply refine existing drivers and concepts, the vast majority of which date back to the '20s and '30s. The Eminent Technology LFT-III is different. While it builds on the work of other planar and dipole speaker technologies, it marks a significant departure from any other existing planar or ribbon design. It is the first full-range, push-pull dynamic planar speaker and is intended to combine the best sonic virtues of electrostatic, ribbon, and previous planar designs.
At $3,250 a pair, the LFT-III can scarcely be treated as just an interesting experiment. Fortunately, its performance is up to audiophile standards. It has a superb midrange and good performance at both frequency extremes.
Further, the speaker has gone through a long enough teething period to have been modified to the point where it is compatible with any good, high-current power amplifier.
Bruce Thigpen of Eminent Technology, the LFT-III's principal designer, traces the speaker's origin back to his first exposure to the original Quad electrostatic speakers and his conclusion that a box speaker could never sound as good as a planar. This led Thigpen to study speaker theory, and he went on to build his own electrostatic systems. In the process, he became interested in trying to build a push-pull planar speaker that could drive the entire surface area of the diaphragm uniformly and that could do so with the speed and detail of an electrostatic or ribbon design.
Thigpen also sought to create a speaker that would not present the drive-voltage and transformer problems of an electrostatic, one which would be more inherently linear than a ribbon. He also sought to create a magnet structure that would be fully linear, unlike some small planar tweeter and midrange designs that use opposing magnets with like poles and which tend to create a nonlinear distortion that can exaggerate the brightness and detail of the sound.
The end result is the LFT-III, a planar doublet system using a single-diaphragm transducer with a specified response from 35 Hz to 40 kHz. It is fully push-pull and has a linear magnetic field over the entire area of its diaphragm. The area of the diaphragm is 580 square inches, the foil thickness is 0.00033 inch, mylar thickness is 0.0005 inch, laminate adhesive thickness is 0.00015 inch, the gap between conductors is 0.03 inch, and peak-to peak diaphragm displacement is 0.2 inch. The conductors that make up the equivalent of a voice-coil in the LFT-III are etched onto each diaphragm. They are flat, with very thin traces and narrow gaps between each trace. Crossovers segment the diaphragm so that a 2-inch line source at the outer edge of each speaker reproduces the full frequency range, with good high-frequency and upper-midrange dispersion. The remainder of the diaphragm plays only low frequencies.
Unlike other planar magnetics, the magnets are on both sides of the LFT-III's diaphragm. This is the key to giving it its true push-pull operation and to allowing the use of a very low-mass diaphragm. It also means that the LFTIII has to be heavy and solidly built: It uses twice the weight of magnets as a single-ended planar speaker, and its magnets cost about eight times as much as those in a comparable full range cone system. There has to be an extremely strong structure because the magnets are aligned in repelling fields. The LFT-III uses a very heavy welded metal frame to achieve rigidity and proper magnetic "focus"; as a side benefit, the structure is far less resonant than that of most planar speakers. At the same time, the LFT-III does use diagonal flux lines across the diaphragm. This improves linearity, but it also leads to low efficiency.
The overall dimensions of the LFT-III are 27 inches W x 54 inches H x 1 inch D (12 inches at the base), and each unit weighs 95 pounds. The finished system is relatively good looking.
Eminent Technology provides more technical data on the performance of the LFT-III than is possible to summarize in a brief review. The specified frequency response is 35 Hz to 20 kHz, ±4 dB, and phase accuracy is said to be ± 20° from 100 Hz to 31 kHz.
High-frequency level is switchable; the settings are flat,-3 dB, and-6 dB. The speaker's rated sensitivity is 82 dB at 1 watt/meter, maximum SPL is 106 dB at 1 meter, and the lowest diaphragm resonance is 38 Hz. Minimum power requirements are 100 watts of high-current amplification into 4 ohms. The speaker's nominal impedance is 4 ohms, although a 16-ohm version, for use with output transformerless amplifiers, is available on special order.
As in most pioneering efforts, Eminent Technology changed many of the speaker's design details in the process of moving toward full production. The first version was a two-way design with a 16-ohm impedance. Its efficiency was only 77 dB with a 2.83-V drive, which was too low for most amplifiers.
The second version's impedance was 4 ohms, and it had a three-way crossover. The efficiency rose to 81 dB. This was still low, but workable, although most tube amplifiers could not drive the speaker. It also lost extension in the highs and definition in the bass, compared with the 16-ohm version.
These developments led to the third version, which has been in full production for the last year. It uses a multiple layer diaphragm in the bass panels; tension is adjustable in each layer in order to improve bass definition. According to Bruce Thigpen, the loss of bass definition in the 4-ohm version resulted from a loss of diaphragm control by the amplifier because the amp got less voltage feedback from the speaker. This caused the low-frequency resonances of the panels to become more audible and resulted in less definition. Multiple-layer diaphragms in the low-frequency section now allow the panel resonances to be spread out over the entire bass range, improving definition.
The magnet structure of the midrange and tweeter sections was also modified to increase magnetic field strength at the diaphragm. This provides more speed and transparency.
These changes required a redesign of the crossover and allowed an efficiency improvement to 82 dB. This efficiency is still relatively low but makes the LFT-III somewhat more competitive.
The end result of this mix of technical innovation and evolution is a speaker whose sound equals the best ribbon and electrostatic designs. At the same time, the Eminent Technology LFT-III has a sound character notably different from most competing high-end speakers.
One of the LFT-III's greatest strengths is an exceptionally flat response and natural timbre from the lower through the upper midrange, with exceptionally low coloration. While the LFT-III certainly has a different timbre from the Quad ESL-63, and is slightly more forward and detailed (with more extension and power) in the lower midrange, it is one of the few speakers to rival the Quad in overall midrange accuracy and coherence. If proper attention is paid to room placement-away from the rear and side walls, and with the stands firmly spiked to the floor-then the LFT-III is an exceptionally flat and neutral speaker in the band where this is most important for creating the illusion of an actual live performance.
It doesn't make much sense to talk about the treble and midrange coloration of this speaker. When properly set up, the LFT-III has very little audible irregularity from about 150 Hz up. The apparent speed of the upper midrange and treble are also quite good, superior to that of most dynamic planar speakers and electrostatics I've heard.
At the same time, some of the latest dome tweeters, EMITs, and ribbon speakers offer more apparent detail and air in the upper octaves. They appear to have more life and dynamic excitement and to give music a more immediate and involving character.
Accordingly, some trade-offs have been made. On the one hand, the LFTIII is very neutral, and now is much faster and more transparent than the earlier versions. On the other hand, it may seem just slightly recessed and flat in comparison with some of the leading competition. The designer claims that the LFT-III's sound reflects a lack of coloration present in the competition. Because of this, and because both reviewers and audiophiles disagree as to what kind of performance creates the most credible illusion of live music, any prospective buyer should audition the LFT-III carefully before making a decision to buy.
The LFT-III's bass response also involves trade-offs. If the speaker is carefully positioned according to the instructions in the owner's manual, and then is fine-tuned to find the point (away from the rear wall) where it gives the most natural bass sound, it produces relatively extended bass response-with a good signal, down to about 35 Hz. The bass is tight and controlled rather than warm and full.
Once again, because there is so much debate about what type of performance is right, the issue will be whether the LFT-III's performance is the kind you like. All I can say is that with really powerful high-current amplifiers (from Classé Audio, Mark Levinson, Krell, et al.), this speaker can deliver very powerful and convincing bass and good detailing of the organ, bass viol, and percussion. It may not be the speaker for bass freaks who are heavily into power and floor vibration, but it can be very musically convincing. Audiophiles who care about bass information and detail may appreciate that the LFT-III, in my opinion, outperforms virtually every planar of similar size and any type of speaker that cuts off at 35 Hz.
The LFT-III provides very good overall resolution of transient response and high-frequency detail. If you are familiar with some of the better electrostatic speakers in these areas, you will probably recognize a similar type of performance in the LFT-III. Its latest version now arguably seems more detailed and lifelike than the Quad ESL-63, which is high praise indeed. The lower mass of the diaphragm may contribute to the fact that it outperforms most cone speakers and dynamic planers in these areas. Yet the LFT-III is not as exciting in these respects as some ribbon speakers, and it does not seem to etch or emphasize detail and transient information with the special excitement of a speaker like the Infinity IRS Beta.
Once again, it presents the kind of trade-offs inherent in the choice of any high-end speaker.
With a good amplifier, the LFT-III plays loud by any standard, and its dynamics are very good. It is sufficiently lacking in frequency coloration and bass overhang to resolve sudden musical shifts from low-to high-level passages about as well as virtually any speaker around. Its dynamics do, however, still lack the apparent speed and power necessary to rank as excellent in comparison to some large high-end cone speakers, Apogee ribbon speakers, and some Infinity models.
The LFT-III provides a soundstage that is wide and coherent, with excellent placement of instruments and voice. When the tweeter switch is set for the most natural balance in a given room, there is no artificial detail or tendency to etch the imaging in place.
Depth is very good. If given careful placement, the LFT-III can sound very natural over a reasonably large listening area-say, a three-person couch at a reasonable listening distance. Yet, this speaker is limited in vertical dispersion and should be listened to while sitting down. To provide the best high frequency performance and imaging, the LFT-III should be tilted slightly back by carefully adjusting its feet. As for the apparent listening position, this system tends to have a middle-of-the hall character regardless of the setting of the treble switch. The imaging, timbre, and dynamic response tend to make the listener feel he is sitting slightly back in the hall, even when the music is closely miked and has a forward character. These aspects of the sound may please classical music fans more than jazz or rock fans.
The LFT-III offers reasonable flexibility in setup and room placement, although you do need at least 3 feet of clearance from the rear and side walls to get the best response. Further, the bass is likely to sound more natural and extended when the speaker is placed along the short wall of a rectangular room. This is very well explained in the owner's manual, which offers excellent advice about room setup.
Like all dipole speakers, getting the bass right requires very careful experimentation in terms of distance from the rear wall. In some rooms, a change of only several inches can make the difference between irregularities in the bass and smooth response. As mentioned, the spiked feet also need adjustment to angle the speaker so the top tilts slightly away from the listening position. In addition, there should be as little furniture as possible around the speaker and between the speaker and the listening position.
The treble energy is variable over a reasonable range, and the three level settings allow you to tailor the speaker to a given room and system. I would suggest experimenting with string music to determine the setting that best suits your conditions; I found that the -3 dB setting worked best in my room.
As for bass, the owner's manual tends to imply that re-tensioning the diaphragm is optional. It is not. You will need to do this to get adequate bass response. You will also need a true high-current amplifier that delivers at least 100 watts; consult your dealer as to which amplifiers are compatible.
The LFT-III should not be used with receivers or amps designed to meet test specifications rather than the far more demanding conditions imposed by today's high-end speakers. For best results, I would suggest keeping the volume high enough to approach natural musical sound levels. There is nothing to warm the bass or lower mid range at lower volumes, and the Fletcher-Munson effect means that the severe bass roll-off in your hearing will make any speaker seem to be lacking in bass.
The LFT-III has jacks for bi-wiring. It can also be rewired internally for use with an electronic crossover, but I can see no reason for such devices. This speaker will perform best in a straightforward system that has one amplifier per channel and emphasizes midrange purity and bass power.
The Eminent Technology LFT-III has evolved into a speaker having excellent performance capabilities but involving some sonic trade-offs. Its strongest suits are its lack of coloration in the midrange and its ability to provide remarkable detail and natural sound in the region most important to enjoying music. This outstanding performance shows up particularly well in terms of voice, but it is equally impressive on chamber music, small orchestral music, and jazz.
Performance at the frequency extremes is less outstanding but still very good. The LFT-III is better than any full range electrostatic in the treble, except for early Quads with their separate "tweeters." It scarcely has "sub woofer" bass, but it produces far more in this region than more expensive planar and electrostatic speakers. In short, the LFT-III is a speaker for the audiophile who wants outstanding midrange quality and musicality rather than spectacular sound. Like most high-end speakers, it needs a good, high-current amplifier and careful placement. With such care, however, it can provide an exceptional degree of transparency, transient detail, and musical accuracy and realism. For many music lovers, the Eminent Technology LFT-III may well be an ideal reference speaker.
-Anthony H. Cordesman
[adapted from Audio magazine/Feb. 1989 ]
Also see:
Eminent Technology Tonearm 2 and Talisman Virtuoso B Cartridge (Feb. 1987)
Apogee Acoustics Scintilla Loudspeaker (Dec. 1985)
Apogee Acoustics Slant 6 Ribbon Loudspeaker (Apr. 1997)
Quad Component System -- One-Brand Systems (Dec. 1983)
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