KEF reference model 205/2.
Source: Manufacturer loan
KEF is one of the most distinguished names in audio, with roots extending back to its founding by acoustics engineer Raymond Cooke in 1961. KEF has long been associated with the 'rationalist' school of speaker design, where engineering and building on its knowledge base are key elements in producing its loudspeakers. To this end, KEF has frequently been on the cusp of new design technologies: laser inferometry, breakthroughs in driver design and materials, and more recently, in the use of finite element analysis to improve its designs. They also have had a long association with the BBC, developing drivers like the famous T27 tweeter, which were incorporated for many years in broadcast and studio monitors such as the famous LS3/5A. KEF also pioneered the use of separate enclosures tailored to the needs of individual drivers in order to optimize performance, which many other companies later adopted. KEF continues to be based in England, and makes its own drivers; cabinets are made to its specification in England.
Under analysis here is a speaker from KEF's venerated "Reference" series of speakers, which go back several generations; they also produce lower cost loudspeakers which use technology which has trickled-down from innovations developed for previous reference designs. The Model 205/2 represents a second-edition of the current Reference line--I rather wish they had used 205.2 rather than 205/2 to denote this, because the "slash" is a reserved character in Windows, and I had to use aliases in my filenames. (No big deal, but it explains why some of my charts used my moniker and not KEF's).
The major design improvement in the/2 series was devoted to refinement of KEF's unique "Uni-Q" driver, which incorporates a 6.5" midrange and tweeter mounted coincidentally (or coaxially)--the tweeter is positioned at the apex of the midrange cone, also known as the throat. KEF wanted to extend the Uni-Q's frequency response, obviating the need for a second super-tweeter, and also in reducing edge texture distortions, smoothing the amplitude response, and improving the already excellent radiation patterns. The coincidental mounting models a "point source", long a design paradigm promising superb dispersion and rock solid imaging, assuming it is properly optimized, and properly crossed over. KEF improved the tweeter's drive system, with a new 3-piece magnet motor system, and a braced (2-piece) titanium dome. The midrange driver was re-shaped to optimize dispersion and reduce throat resonances; even the surround is shaped to prevent interference, in a manner I have not seen in any "off the shelf" design from a major speaker supply house--this is one benefit of constructing one's own drivers, and is typical of the "spare no expense" philosophy exhibited in the Reference series--one which admittedly the purchaser pays dearly for.
The bass system was also improved, and uses 2 fiber-reinforced 8" woofers in the 205/2. KEF has a white paper dealing with the woofer design, and makes a good case for reducing distortion by the use of Faraday rings (also known as shorting rings) to keep the magnetic drive system linear, and reducing distortion by an order of magnitude (20 dB) over designs without the rings above and below the voice coil. I must note that they are not unique in this approach--other manufacturers do this, but not all. The woofers in my Orion also use shorting rings in this manner.
Of course, a vented-bass loudspeaker with forward-mounted drivers must have an inert cabinet to avoid the colorations of typical "monkey coffins", and the 205/2 has a beautifully built and finished cabinet. The sidewalls are curved, both for strength and to eliminate a major source of internal standing waves. The woofers further avoid the standing waves many tower speakers encounter by having independent chambers, loading, closed-cell foam insulation, and external vents for each woofer. This design is complicated and requires meticulous execution to avoid phase interactions that could create nulls in the amplitude response. Massive bracing keeps things very rigid, and the crossover boards are carefully mounted to avoid buzzing of the individual components. The crossover is a complex 4th order affair, and the pictures show very careful attention to component placement--I recall another British reference loudspeaker that had severe cross-coupling from electro-magnetic interaction between the bass coils when under power. All this results in a cabinet that is as solid and resonance free as any I have encountered before, and while the Orion's "boxless" design still seems preferable, it does impose placement restrictions that would be impractical in nearly any studio environment, and in many of the open floor plans popular in the USA. The 205/2 was designed to allow placement as close as 9" to the front wall-raising its placement flexibility and spousal acceptance factor over the Linkwitz design, which still invokes "why do they have to be so far from the wall?" inquiries from my spouse. C'est la vie.
Finally, I should mention that a luxury-class loudspeaker should have perfect fit and finish, and this was the case with the 205/2's. My pair was finished in satin sycamore, with no less than 3 pairs of Euro-spec (insulated, no dual bananas need apply) WBT connections, allowing tri-amping, or Heaven knows, tri-wiring. There is a wood presentation case provided that contains several cabinet footer solutions, a frequency sweep of each pair's performance standard (matched to within .5 dB of one another), and several little WBT screws, part of the "Uni-Balance" system KEF offers to allow some frequency balance tailoring based on room placement needs and user preference; these allow a -2dB cut with a knee at 300 Hz for close proximity wall locations, and a +.75/-1.5 dB adjustment above 2kHz for the mid/tweeter unit. I set the bass flat, but dialed-in -.75 dB for the upper range--the wide dispersion coincidental drivers resulted in a bit too much brightness in my lab's sparsely furnished environs. In my big Room 1, flat was optimum. KEF was wise to include this adjustment capability; measurements were done in flat setting position, by the way. Some have complained about the eccentricity of this adjustment setup, and while it is not intuitive, neither is it likely to be re-set often--it is more of a set it and forget it adjustment than a recording to recording tone control.
Of course, all this would just be pedantic were the end result not worth the trouble and expense. And overall, the KEF 205/2 emerged as a truly exceptional performer.
Listening was done using the 300 wpc Innersound (CODA) amplifier, which is at the top end of KEF's recommended power range. First impressions were of high transparency and clarity, and as I walked about listening to de-correlated pink noise to optimize placement, I was struck by the complete lack of coloration as I moved up and down, side to side, and all around. This hinted at the KEF's outstanding radiation pattern capability. Extended listening to all sorts of music did nothing to challenge my initial impressions; indeed, they were only strengthened as I threw the most challenging music in my collection at these "reference" loudspeakers. Human voice, male and female, was rendered with startling realism and transparency--baritones were never boomy or chesty, yet the full power of their voices made for realistic ensemble, chorale, and especially operatic reproduction. The same held true for the even more difficult sopranos--they came through without a hint of harshness, yet superb inner detail. Listening fatigue was non-existent, and I actually listened to Mozart's The Magic Flute in one sitting! My favorite string pieces revealed a speaker capable of removing veil after veil from recordings I've played many, many times. The 205/2's encouraged the listener to close one's eyes and get lost in the beauty or energy of the music being reproduced. And if, like one of my former colleagues who is a renowned audio critic, you like to play Toscanini and move about the room flailing your arms, the lack of tonal change the KEF's provided when one listened above axis meant freedom to move about at will; this of course also means they will sound great when your entertain standing guests--no midrange or treble suck-out was ever exhibited here.
The bass was a surprise, in that the published specification (-6 dB @ 35 Hz) did not look as promising in deep base extension as some other speakers I have analyzed of late, such as the Paradigm Reference Studio 100's or the NHT Classic 4's. Yet, I never felt they were bass shy on any recording, whether the source was classical bass viol, or strong pop bass lines or instruments like synthesizers. The close front wall positioning the KEF's allowed no doubt helped here, with typical room gain usually supporting bass below about 50 Hz in my room by 4-5 dB. Bass quality was as high as I have heard from a vented system, especially in mid and upper bass "quickness", by which of course I mean freedom from resonance/bass overhang. I still prefer the open enclosure, dipole bass of my Orions and the sealed bass performance of my subs, or my classic Allison 3's. But I was impressed by the 205/2's ability to play deep bass quite loudly--as well or better than the Paradigm or the NHT's or the Orions below 40 Hz. This indicates to me that the KEF design team made some excellent bass tuning choices, the drivers indeed are low distortion, and the system sensitivity good. Dr. Andrew Watson and his team have done a great job on this speaker.
Sound-staging and imaging were absolutely first-class. The 205/2's threw a wide and tall soundstage, and within it instrument placement and stability were better than any forward-mounted driver loudspeaker I have experienced. Superbly delineated and solid.
The crossovers seem very well executed. The mid to tweeter handoff is absolutely undetectable. About the only issue I had at all with the 205/2's was a slight sense of discontinuity between the woofers and the Uni-Q. Since the crossover between them is right in the range where room effects start to dominate, I might well have been hearing an amplitude anomaly, and not a defect in the handoff. On the other hand, note that the flagship of the KEF Reference line, the mighty $20,000 207/2, has a 10" driver covering the lower midrange exclusively from 120 to about 350 Hz, so maybe I was on to something. But also consider that the 207/2 uses 10" woofers, whose dispersion might well not integrate ideally with the Uni-Q where the 205/2's 8" woofers would have less of an issue. But this is not a serious fault, if a fault at all.
Overall, the performance of the 205/2's was absolutely top notch--day after day, I looked forward to challenging them and they never failed to deliver terrific sound.
As is my custom, I performed the measurements last. They were done with the Uni-Balance set to flattest position for bass and the Uni-Q.
The impedance (not shown) showed a saddle in the bass response typical of a vented design, centered around 32-34 Hz. This corresponds closely to the system tuning, based on the measured port output, normalized to the driver radiating areas. I consider this a slightly over-damped bass system, and it may explain the deep bass extension without any sense of boom. The impedance minimum was about 3.5 ohms, and over much of the bass and midrange, the impedance hovered between 5 and 8 ohms, rising to peak of about 15 ohms right at the 2.3kHz crossover to the Uni-Q. There were no spikes indicative of a cabinet resonance. System sensitivity is rated at 90 dB, but since it drops close to 5 ohms over the range I typically measure sensitivity, an amplifier would actually be working harder than at 8 ohms, so I would call the sensitivity moderately high at 88, which is excellent for a large tower speaker with reference pretensions. Although I used a powerful amplifier, this speaker can be driven by much less power, or effectively by multiple amplifiers, assuming they have equal gain or can be matched by level controls.
It is important to note something as you look at the measurements. I measured Figures 1 and 2 in the center of a large room, well away from any wall surface. So there is little to no bass re-enforcement, and the bass below 50 Hz tracks very closely to KEF's own measurements provided for each pair. Remember, room effects dominate below 300 Hz or so. Figure 3 uses a different scale, because I use pink noise to do the free-field RTA, and have to drive the speaker hard for sustained periods to do averaging, and I lower the scale to protect both the tweeter and my own well being.
That said, look at how smooth the response of the KEF 205/2 is in Figure 1. (The elevation at 63 Hz results from the near-field measurement, and the slight dip around 140 Hz is floor bounce). That is truly reference response, as good or better than I have achieved with other speakers measured under identical conditions. Even more amazing is the 45 degree off-axis response--it hardly differs at all, tracking very closely to on-axis, and any differences may just be the result of moving the mic, since I do not move the speaker. Only above 8 kHz or so do we start to see a divergence, and even that is just handful of dB. Outstanding.
Even more outstanding is the result shown in Figure 2. Note that I generally measure 15 degrees above and below axis, but the KEF's radiation pattern is so excellent in the vertical plane that no significant variation was found at 15 degrees! So I went to 30 degrees, and even then the variation was minor, and the consistent tracking, above and below axis, was better than any other loudspeaker I have measured. I did not try a higher off-axis measurement, as it would have put the mic too close to the ceiling! The performance of the KEF explains why there is no variation in timbre when standing or sitting, or even lying down--that Uni-Q really does deliver the goods in terms of modeling a point source. No wonder I heard such detail--it is greatly unaffected by reflected energy with varying amplitude response, as seen on first or second order crossover designs, or drivers widely spaced and poorly integrated.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
All this comes together in Figure 3, which is an RTA analysis of the averaged, temporally weighted response at the listening position, 12 feet from each speaker, both speakers operating. The 63 Hz peak now shows a dip--this is a room mode caused by the 8' ceiling. The floor bounce also remains. But from 800 Hz up, note how smooth and extended the response is right up to 12.5 kHz--the roll-off above that is typical room response--if it were flat, the speaker would sound unacceptably hard and bright. Interestingly, I did measure a shallow valley centered around 450 Hz, which may explain the discontinuity I mentioned in my listening comments earlier. It could well be a room effect, as KEF's measurement shows no anomaly at all; nor did my I meter sweeps.
All told, the KEF 205/2"s measurements are beyond reproach.
While this is a very expensive loudspeaker, it does provide outstanding measured and perceived performance, and lets music pass uncolored into the room as no other tower speaker in my experience. It surpasses the performance of my previous reference front-firing reference loudspeakers, the Waveform Mach 17, which retailed near $9,000 in its final days. While some will say that an $11,000 speaker does not represent "sensible sound", I disagree. First, one would have to demonstrate that the same performance and quality of materials are available for less elsewhere. Even then, the beauty of the speaker should be factored in, as well as the likelihood that it will provide reference-quality sound for many years, allowing the listener to amortize the cost over time, and perhaps in avoiding buying a number of lesser quality designs in the search for great quality sound reproduction.
The KEF 205/2 is an unqualified success, and a smashing good loudspeaker, worthy of the Reference name it carries. It delivers a musical experience of the highest quality, and does so in a manner that allows anyone in the room, sitting or standing, to revel in its rich, uncolored sound. It can be placed with great flexibility, and provides ample compensation for nearly any practical use. Whether a luxury loudspeaker "makes sense" is just as difficult as justifying an exotic automobile, a Breguet watch, or single-barrel Kentucky bourbon--but like all of life's luxury amenities, that's your call, not mine. If you value music and have the scratch, this one is very highly recommended. Otherwise, pray that the technology trickles down to one of KEF's many less expensive designs. As for me, there may be a pair of KEF's in my future.
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|Author:||Strauss, Glenn Oliver|
|Article Type:||Product/service evaluation|
|Date:||Apr 1, 2008|
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