KEF R107 loudspeaker Thomas J. Norton 1995
It's a little mind-bending to recall that KEF first introduced their Model 107 loudspeaker in 1986. KEF, like most British loudspeaker manufacturers, tends to be conservative when it comes to its high-end models. Why mess with a good thing? The Model 107 has never been a radical design—that wouldn't be KEF's style. But the original 107 built on a solid background of innovation from KEF products in the immediately preceding years.
Take the twin coupled-cavity enclosure. First used by KEF in 1984 for the Model R104/2 and generically known as a bandpass enclosure, the concept was originally patented by Andre d'Alton in 1934. Interest in the technology has exploded in the past 15 years, arguably triggered by Laurie Fincham's (then of KEF) "A Bandpass Loudspeaker Enclosure" paper, which was presented at the 63rd AES convention in 1979.
Simply described, a bandpass enclosure has two separate compartments, with the driver mounted on the internal wall that divides them. The compartment behind the driver acts more or less as a conventional cabinet, bass-loading the woofer. It may be sealed or ported. (The Bose Corporation owns a 1985 patent on the front- and rear-ported bandpass enclosure.) The enclosure in front of the driver is always ported by a large duct to the exterior of the cabinet. One important characteristic that results from this arrangement is that the high-frequency output of the driver that reaches the outside is rolled-off acoustically. This greatly simplifies the design of (or even eliminates the need for) a conventional electrical low-pass filter for the woofer.
While it's not without problems (notably the potential for resonances to find their way out of the port and into the audible output of the loudspeaker), proponents of the system argue that its positive attributes greatly outweigh the drawbacks. Claimed gains include all the usual suspects: higher efficiency, greater extension into the deep bass, lowered cone excursion for a given sound pressure level, and improved power-handling.
In the 107's implementation of the bandpass system, two 10" woofers are each mounted in their own separate, sealed chambers. The front of each driver feeds into a third chamber, which is in turn vented to the outside at the top of the enclosure, immediately adjacent to the head-unit that contains the midrange unit and tweeter. A rigid rod—which runs through the voice-coil centers—links the two woofers to cancel any differential movement between their frames.
Two other features of the Model 104/2 found their way into the original 107: conjugate load-matching and electronic bass equalization. The former is a crossover configuration that results in a load that remains very close to the target impedance of a resistive 4 ohms across the frequency range; the latter uses an outboard, powered network (the KUBE) to tailor and extend the low-frequency response. Two controls—one for bass, the other for treble—were provided on the KUBE as a supplement to its fixed-response contouring. These were included to provide for a limited degree of room compensation. The KUBE was designed to be placed either in the tape loop (an alternate tape loop was provided on the KUBE itself) or between the pre- and power amplifiers.
The 107's 5" polypropylene-cone midrange and 1" dome tweeter were mounted in a separate enclosure atop the woofer cabinet. This small enclosure, contoured to reduce diffraction and damped with a mineral-loaded polymer, could be rotated separately from the bass cabinet to facilitate proper setup.
Updated in 1990 to Model 107/2 status, with a revised crossover, a metal-dome tweeter, and biwiring terminals, the 107 sailed into the mid-'90s facing increasingly stiff competition—not only from other manufacturers but also from other KEF models, such as the new Reference Model Four. Termination (or, as the British might put it, redundancy) seemed an imminent possibility.
But with further refinements to the midrange and tweeter balance, along with changes in the head-assembly damping, KEF readied the 107—now known as the Reference Series Model 107/2 Raymond Cooke Special Edition—to do battle with the forces of 1995. Is the result as impressive as the length of its model name? Does it do justice to the name it bears—that of KEF's late founder and president, Raymond Cooke? And does it justify the KEF's $5900/pair price tag?
I set the 107/2s up in my 18' by 26' by 11' listening room, well out from the walls and firing down the long dimension of the room. The speakers come with large grillecloth-covered hoods that may be used to cosmetically conceal the mid-tweeter modules; I used them grille-less. Before doing any listening, I ran them in for a long weekend with pink noise played at moderate levels.
My first listen wasn't too encouraging. The sound was rather bloated and forward, with little transparency. I fiddled with the KUBE controls, but they didn't solve the problem. The timbre seemed reasonably consistent from left to right (the importance of this observation will soon be clear), but a day or two of listening convinced me that I needed to try a change of positioning.
This wasn't the first time a loudspeaker has been unhappy with this room arrangement, though my recent experiences with the Thiel CS7 (reviewed elsewhere in this issue) confirmed that it could work well with the right loudspeaker. The next day I put the 107s in the cross-room (diagonal) position that has worked well for me with numerous other loudspeakers. This arrangement has proven to slightly lean-out the mid- and upper bass—a balance change that seemed to be what the KEFs required.