KEF Reference 105/3 Floor standing speakers
KEF, a British company, has been in the forefront of applying the most modern loudspeaker technology. In the Reference Series Model 105/3, KEF has taken advantage of the latest in very powerful neodymium-iron-boron magnets to shrink a 1-inch dome tweeter and actually place it in the neck of a midrange cone loudspeaker. The 105/3 is the first of KEF's Reference Series high-end line to use the new "coincident source," coaxial-style, mid-high frequency driver, which they have dubbed the "Uni-Q." Previously, only a lower cost line of KEF loudspeakers utilized this technology.
The 105/3 is a floor-standing, four-way system that uses the Uni-Q 6-1/2-inch coaxial driver to reproduce all the frequencies above 350 Hz. The lower frequencies, up to 150 Hz, are generated by two 8-inch cone woofers in KEF's "coupled cavity" bass-loading system. This is a form of single-tuned, bandpass vented-box enclosure design that KEF has used in several previous systems, including the Model 107. Each woofer operates in its own sealed enclosure and is coupled to a third port-tuned, central enclosure which radiates bass energy into the room through its port. The two woofers are joined through the centers of their magnet structures by a heavy metal rod, which effectively cancels the opposing reactive motions of their frames. This helps minimize sonic coloration caused by unwanted box vibration triggered by driver reactive forces.
The remaining frequency range, between the bass section and the Uni-Q coaxial driver, is reproduced by two additional 6-1/2-inch cone drivers symmetrically, placed above and below the Uni-Q driver. This configuration provides a symmetrical, forward-facing, vertical coverage pattern that has no lobing error (i.e., its vertical directional pattern stays solidly aimed in the forward direction at all frequencies through crossover). KEF has chosen a relatively low crossover frequency of 350 Hz between the lower mids and Uni-Q coaxial driver, which effectively minimizes any additional lobing and beaming due to the relatively wide spacing of the midrange drivers. The two lower midrange drivers have the effective area of a much larger driver but the width of a small driver. This allows a narrow front panel that improves lateral imaging and reduces baffle-reflection and diffraction problems.
KEF makes extensive use of conjugate load matching to smooth the system's impedance, thus making it a much better load for amplifiers and their associated connecting cables. A system with a purely resistive impedance, independent of frequency, is a very easy load to drive. It is also very tolerant of high values of cable resistance that typically cause frequency response variations in speaker systems whose impedance varies with frequency. For a system with purely resistive impedance, the cable resistance causes only a reduction in power available to the loudspeaker, not any frequency response variations.
The Uni-Q driver is superficially much like a typical coaxial loudspeaker, but the tweeter is actually mounted at the apex of the midrange driver's cone, inside the midrange voice-coil instead of in front of the larger driver's cone. The latter technique offsets the acoustic centers of the individual drive units. The Uni-Q was designed to align the acoustic centers of the drive units (the points where the waves appear to originate) so that their individual directional patterns will match very closely through the crossover region. This would effectively provide a two-way driver that would behave as a single coherent source of sound, providing uniform coverage over a wide but controlled region.
This concept was originally implemented in the distant past with horn-loaded high-frequency drivers, where the high-frequency sound actually passed through the center of the low-frequency cone driver and was radiated by a horn mounted in the center of the bass device. The old Altec Lansing 604 coaxial was just such a system, and utilized a 15-inch woofer and multicellular horn tweeter. These coaxes are still living in the professional market, where systems such as the UREI 813 control room monitor (with updated horn) are still used quite extensively. The U.K.'s Tannoy has several good horn-loaded coaxial systems in its current line of speakers.
KEF's Uni-Q assembly extends the concept to a completely direct-radiator design utilizing a well-behaved dome tweeter with a very small, high-energy magnet, mounted in the center of a 6-1/2-inch midrange driver. The cone of the midrange driver actually acts as a waveguide for the high frequencies, and thus potentially matches the high- and low-frequency coverage patterns of the individual drivers. The 105/3 is designed to have wide but controlled coverage that improves its imaging capabilities by minimizing wall, floor, and ceiling reflections.
This model also follows KEF's established system of computer-matching all speaker and crossover components to minimize unit-to-unit variations. KEF tests all incoming components and numbers them according to their measured values; KEF schematics then either specify particular part values or show tables for the component combinations to meet the company's very tight specifications.
The 105/3 can be used with the KUBE 200 external active equalizer, which extends the response of the system down to 20 Hz and also compensates for response variations caused by speaker placement and listening room acoustics. The equalizer has two "Contour" controls, "HF" and "LF," which provide limited-range adjustment of high- and low-frequency level. The equalizer also contains a front-panel EQ bypass switch and complete tape monitor switching capabilities. In addition, the rear panel contains equalized and nonequalized outputs (one set with level control) that allow versatile interconnection and level adjustment in a biamplified system and also provide convenient capabilities for multi-room or audio/video systems.
Twin sets of gold-plated terminals on each speaker's rear panel, spaced 3/4 inch (19 mm) apart, allow bi-wiring or biamping. Removable, gold-plated connecting straps are installed at the factory for normal single-cable drive. The terminal can handle large cables up to about 1/4 inch (6.3 mm) in diameter. Unlike some high-end systems I have reviewed, the KEF has easily accessible terminals so that I could get my fingers on their knurled knobs to tighten them securely.
Separate p.c. boards hold the high- and low-frequency sections of the crossover, but both are mounted in the woofer enclosure. The crossover is composed of 51 separate components, divided up into 15 resistors, 13 inductors, and 23 capacitors. All component quality is first-rate.
The enclosure is very substantial and well strengthened by the construction of the internal woofer-mounting compartments and additional bracing. The two midrange drivers and the Uni-Q high-frequency drive unit are mounted on an independent, specially profiled module, precision-machined from solid medium-density fiberboard, 3 inches thick. This mid/high-frequency module holds a vertically symmetrical array of three sealed sub-enclosures, made of die-cast aluminum, behind the drivers. The module, with drivers attached, is mounted to the front of the enclosure by four respectable, 5/16-inch socket-head bolts, 3 inches long. The module actually protrudes about 3 inches beyond the front of the woofer enclosure, making the system look front-heavy with its grille off. The smoothly contoured port outlet for the central resonant-air chamber is mounted just below the module.
The woofer enclosure is finished on all five sides in rosewood, walnut, or black ash veneer. The sculptured grille assembly covers the top seven-eighths of the front of the enclosure and is secured to the cabinet with four powerful disk magnets. (And they are powerful! You had better keep your fingertips from between the grille and cabinet when replacing the grille assembly, or you'll get pinched when the grille snaps into place!) The grille itself is contoured to fit snugly over the mid/high module and provides no added sharp edges to diffract the high-frequency sound.
Use and Listening Tests
I listened to the KEF 105/3s in my listening room, which is furnished in normal living-room style with a carpeted floor and is approximately 15-1/2 x 27 x 8 feet. Driving equipment included Rotel RCD-855 and Onkyo Grand Integra DX-G10 CD players, a Krell KSP-7B preamp, a Krell KSA-200B solid-state power amplifier, and Straight Wire Maestro interconnects and speaker cables. I did about half of the listening before the measurements were made.
All of the listening was done with the 105/3s placed in my customary evaluation position, about 6 feet away from the short rear wall, and separated by 8 feet. The speakers were about 4 feet from the side walls. The first round of listening was done with the systems toed in and aimed directly at my head. Later listening was done with them cross-fired at a 10° angle. The measurements indicated that this cross-firing would significantly smooth out the system's top-octave response. I listened sitting on the sofa about 10 feet away, which placed my ears about 36 inches above the floor.
The systems were hooked up in a normal single-cable configuration, not bi-wired. At nearly 100 pounds apiece, the systems are quite heavy, but one person can move them by tipping them slightly and walking them on two of the enclosure's feet. If the grille is off, a strong person can lift the systems by placing one arm under the exposed mid/ high module and the other in the rear.
The KUBE 200 equalizer was used in the tape loop of the Krell preamplifier. A lot of my listening was done with the equalizer switched out. When it was switched in, I left the "HF" control in the flat position, though I did experiment with the "LF" adjustment. Switched in, the equalizer added a needed low-frequency heft or weight to most program material. However, if the program had any significant high-level material below 50 Hz, and was played loud, the equalizer's boost would often make the low frequencies sound mushy and distorted because of speaker overload.
When I first sat down to listen to the 105/3s, I placed a favorite orchestral CD in the player, turned the volume up to my usual level, hit the play button, and was promptly blown away by the volume. The KEFs are fully 6 to 10 dB more sensitive than the typical systems I listen to; the volume levels generated by typical speakers with your usual 200-watt amplifier can be generated by these KEFs with an amplifier of only 20 to 50 watts.
I was initially very impressed by the neutral character, precise imaging, and extremely even coverage of the KEF 105/3s. These impressions have not diminished after longer listening. The high peak output capability of the systems much improves their impact and realism in reproducing transient sounds that require high peak sound levels, such as drum rim shots, explosive sound effects (a la Ein Strauss-fest, Telarc CD-80098), and other program material with high peak levels. When played at realistic levels, the high sensitivity and power handling of the 105/3s directly translate into the ability to reproduce peak levels that closely match those of live sound, thus improving realism. Some digitally recorded jazz has crest factors approaching 30 dB, which makes it very hard to reproduce at realistic average levels without peak clipping. (Track 5 of the Flim and the BB's album TriCycle, DMP CD-443, has a measured crest factor of 29.9 dB!) If two speaker systems are compared with the same power amplifier and at the same realistic average sound level, the system with the higher sensitivity will reproduce higher peak levels. This is directly due to increased power amplifier headroom when driving the more sensitive system (assuming that the amplifier, when driving the low-sensitivity system, is being driven hard enough to cause significant clipping).
I was very impressed with the 105/3s' handling of the sharp, intense transient sounds and very distinct lateral imaging of the computer-synthesized plucked string sounds on track 14, "Silicon Valley Breakdown," on The Digital Domain CD (Elektra 9 60303-2). It is a great demo disc for imaging, dynamic range, and sound effects. The Huey helicopter sequence on track 16 could be played at very realistic levels. Due to high-level low frequencies on this track, it sounded much more realistic with the EQ switched out; when the equalizer was switched in, headroom suffered greatly. When played at highest level before audible distortion, the 105/3s produced sound levels of 92 dBA (105 dBC) with the EQ in and 99 dBA (109 dBC) when it was switched out. As long as we are talking sound effects, the 105/3s did an incredibly hair-raising rendition of the USAF F-16 fighter jet flyovers on track 3 of the Sonic Booms CD (Bainbridge BCD6276): 110 dBA, or 112 dBC, without the EQ. Great fun! Now on to more serious listening...
On the Winds of War and Peace CD of Lowell Graham conducting the National Symphonic Winds (Wilson Audiophile WCD-8823), the systems rendered the massed horns very cleanly and sounded excellent on the bass underpinning and snare drums on track 6. On Pat Coil's Steps (Sheffield Lab CD-31), the vocals, string bass, and particularly the strings on track 7 were reproduced effortlessly without any harsh or strident effects. Quite thrilling were the dynamics of John Arpin's playing of Louis Gottschalk's preragtime piano compositions on Cakewalk (ProArte Digital CDD 515).
The 105/3s passed the pink-noise stand-up/sit-down/walk-around test with excellent results, the best I've heard to date. With pink noise, I could reliably hear the on-axis/slightly-off-axis high-frequency roughness indicated in the measurements. On program material it was much more difficult to reliably hear the differences. The 10° toe-in configuration noted earlier improved the top-octave high-frequency smoothness at my normal listening position and also somewhat improved image stability for listening positions not on the center line. Not many systems can profit from toe-in, because their off-axis response is insufficiently even and they lack enough directivity to make it worthwhile.
On direct comparison at matched, moderately loud levels, I found that the frequency balances of the KEFs and my reference B&W 801 Matrix Series 2 systems were very similar. Generally, I had no preference for one system over the other. The B & Ws were very slightly brighter on material with significant high-frequency content. However, after getting over the initial thrill of the KEFs' high sensitivity and high peak output capacity, I have reservations about their low-frequency performance and low-frequency maximum output capabilities.
Although the KEF systems are very efficient and can handle large amounts of power at frequencies at and above the upper bass range, their efficiency and power-handling capacity (and hence maximum output capability) below 40 Hz cannot keep up with their high-frequency performance. The maximum peak acoustic output of the 105/3s in the bottom octave from 20 to 40 Hz is some 5 to 15 dB less than my reference 801s. (Note, however, that above 60 Hz, the maximum output of the 105/3s exceeds that of the 801s by about the same margin.) On the 20-, 25-, and 31,5-Hz third-octave, band-limited pink-noise cuts on the Pro Audio CD (Bruel & Kjaer CD-4090), the maximum clean output of the B & Ws would "walk all over" the output from the 105/3s. At 20 and 25 Hz, the 105/3s could not be turned up loud enough for the fundamental to be heard without excessive distortion. On sine wave, the comparison was only slightly better. These tests were done without the equalizer switched in. The 105/3s would profit very much from the use of a high-output subwoofer.
I am quite pleased with the performance of the 105/3s, particularly with their high sensitivity, generally smooth response, very high midband maximum peak output capabilities, and amazingly uniform horizontal and vertical coverage. Improvements can be made, however, in smoothing the system's on-axis high-frequency response and increasing its very low-frequency maximum output capabilities.
