KEF R107 loudspeaker Martin Colloms 1987
Dick Olsher first reviewed the R107 in Stereophile in Vol.9 No.7, and readers should read that review for a complete description. Briefly, bass is handled by two 10" pulp cone woofers mounted in small infinite-baffle enclosures within the R107's main box. The chassis of these drivers are coupled by a bracing bar to reduce the mutual vibration contribution to the enclosure. Their acoustic outputs are summed in a central chamber with the filtered "bandpass" energy emerging from the large port/grille on the enclosure top surface, just under the head assembly. The air mass in the port is, in effect, the actual bass driver.
The head assembly itself is a molded and sculpted casing with a low diffraction shape. It is double-walled, and heavily loaded with damping mastic; it is thus highly nonresonant, and, with appropriate drive-units, allows for a very smooth frequency response. These units comprise a 5" polypropylene-cone unit based on the classic B110, and the T33 doped-fabric dome tweeter, the latter a recessed design with a short horn/phase plate. The R107 is essentially driver time-delay compensated, allowing the 24dB/octave crossover to operate with an in-phase electrical connection. The system is well styled and finished, requires only a small floor "footprint," and the main listening axis is at a good height.
The sophisticated crossover is fully compensated in terms of impedance amplitude and phase, the entire system presenting a constant 4-ohm resistive load to the amplifier. Electrical connection is via knurled, gold-plated 4mm binding posts on the rear of the bass enclosure. These are deliberately spaced too far apart for double-plugs to be used.
The KUBE electronics box connects either in the preamplifier tape loop or between pre- and power amplifiers, and includes a remote-cord power supply. The circuitry is based on TL0 series IC op-amps, with rather a large number required to implement all the equalization functions. Some measure of mono bass-blending is applied electrically at very low frequencies to reduce the audibility of out-of-phase rumble noise on recordings, such as background traffic, air conditioning, pressing rumble, and the like. These often become all too obvious when an 18Hz limit woofer with a healthy output is in operation. At present, the KUBE comes with permanently connected audio cables, terminated in gold-plated phono plugs, but production from early 1987 onward will be fitted with RCA sockets, allowing the user to optimize cable choice for his or her system.
The KEF features a very high standard of construction, engineering, and finish, and a helpful operating manual is supplied.
It so happened that I first tried this speaker with a fine, budget-priced, integrated amplifier, the Audiolab 8000A, and achieved remarkable results for the total cost of the system. The R107 immediately provided the rewarding experience of a big, focused stereo soundstage offering good depth and perspective. Resolution of fine detail was most promising, the tonal balance was essentially neutral, and the bass extension and power placed it in a rarefied league commensurate with some of the largest and most costly models in the business. Outweighing the importance of these primary qualities, however, was a sense of reserve and ease pervading the subjective dynamics. Even with this modest amplifier (typically 70W/channel), the volume control was set well down, and the maximum level attainable seemed unbounded.
Old warhorse demonstration records were brought out and belted for all they were worth, to see just how well the '107 could rattle the windows! I had no doubts, even at this early stage in the auditioning, that here was a formidable product. Other amps—a pair of Krell KMA-100s on temporary loan, as well as my Audio Research M100s driven by an SP-11—were pressed into service. These provided more performance in quality terms, but the improvement was not as great as anticipated. The small integrated amplifier—admittedly a good one—had, in fact, taken the R107 close to the limit of its performance, and in the main, larger amplifiers only showed the 3-4dB maximum-level improvement that was possible with their higher voltage drive.
The various KUBE settings were the subject of much experiment, though ultimately the manufacturer's suggested adjustments of 20Hz bandwidth, Q of 0.4-0.5, were preferred, in this instance with a touch (+1dB) of contour lift.
Various room positions were also tried: while the sound was clearly optimal in a free-space position, 0.9m from the rear and 1.1m from the side walls and floor-spiked, this speaker could also survive a position close to the back wall surprisingly well. Some bass rolloff (30Hz), a reduced Q (0.3-0.4), and some contour cut (-1.5dB) helped it to balance pretty well against the wall, though there was a noted impairment in the stereo focus and depth, with some increased wall-reflection coloration also apparent.
As listening proceeded, it became clear that this big KEF was fundamentally "correct" in a manner more reminiscent of the R105 II than the more exuberant R104/2. Checking my notes on the R105, I found the R107 to be much more sensitive and capable of maximum sound levels some 5dB higher, as well as producing still better stereo imaging and a faster, yet more extended, bass.
Taken to the limit, it did prove possible to fault the '107 in relatively minor areas. There was a hint of an "enclosed" quality in the high treble, like a dulling of "air." The midrange could sound a touch lean at times, this characteristic also noted with church choir, where upper-range voices could mix and harden together. Care was taken to ensure that a similar effect in the ear itself, resulting from excessive level, was not occurring.
The bass had a quick, fluid quality and appeared to pump effortlessly from the large port. The upper bass was clean and articulate, setting high standards, but ultimately the low bass was a little overblown, as if the 20-30Hz band was "heavy." On some material, it produced a sort of low rumble, an exaggeration of program faults sufficient to require resetting the bandwidth control to 40Hz. The bass balance was fine down to 40Hz, but the extra plumpness below this range could not be ameliorated without drying up the next immediate curve.
A final and still moderate criticism concerns two other areas—the treble and general transparency. In general terms, the R107 sounded clear and open but did not fully resolve detail far in back in the depth plane, despite a decent retrieval of the recorded acoustic. This was a broad-band effect which was a little poorer in the treble. Good forward treble detail was available, but there was also muted "grain," with a loss of real transparency; just how much this matters will depend on the quality of the program source and, of course, on the transparency of the matching electronics.
Next, we removed the KUBE from the chain. Yes, we were aware that the treble would then be 2dB brighter, the output peaking a little at the upper crossover point, and the bass not optimally aligned. Nevertheless, the "naked" speaker sounded quite well balanced, while the change in bass performance was less than might have been expected. Although this was admittedly an academic exercise, the sound quality was briefly evaluated under these conditions: subjective dynamics (a sense of "liveness") and transparency were both significantly improved, and rather more than any superficial change in response might have suggested. The KUBE electronics are appropriate and well matched to the basic R107, but appear to bound its performance, preempting its potential challenge to the next rank of quality. This might otherwise have been achieved via significant expenditure on the matching power amplifier.
This remarkable speaker represents a challenge to the increasing performance standards being set by large panel speakers, matching the best of them in terms of both bass power and extension, while at the same time conceding little in terms of coloration levels. In fact, the KEF beats a number of the panel models in such fundamental areas as tonal balance and response uniformity.
The R107 is a refined system, with no obvious flaws or rough edges. In no way does it represent a "first prototype"—its designer is not going to have second thoughts during the next six or twelve months! The R107 is clearly a bighearted speaker offering good engineering value and a high overall performance. The bass extension is remarkable, with a genuine 18Hz possible at respectable levels, as well as the ability to produce respectably high maximum sound levels appropriate to larger rooms. Its KUBE provides control of the bass and tonal balance to suit a wide range of locations and specific acoustics; the extended bass is allied to a high standard of sound quality throughout, and has not been obtained at the expense of the mid or treble quality.
The R107 is clearly a most effective flagship for KEF, and shows off the company's engineering strengths to the full.—Martin Colloms