KEF Reference 207 loudspeaker
Soon after my review of the 107, KEF introduced its Uni-Q driver: a tweeter powered by a powerful but tiny neodymium magnet that enables it to perch on the front of the midrange unit's pole-piece, the midrange cone forming a waveguide for the high frequencies. The advantage of this is that the dual driver's dispersion remains constant with frequency. The disadvantage is that a waveguide vibrating at low frequencies is not necessarily the optimal environment for a tweeter, while reflections of the tweeter's output from the termination of the midrange cone can affect its response. If you look, for example, at Tom Norton's March 1996 review of an earlier KEF Reference design, the Model 4, you can see some treble peculiarities that result from the tweeter's non-optimal acoustic environment.
KEF's engineering team, led these days by Dr. Andrew Watson, has used the latest design technologies such as Finite Element Analysis and Laser Vibrometry, to optimize the drive-units and their environments to enable the company's new flagship, the Reference 207, to be based on a Uni-Q module.
The handsome Reference 207 consists of the gloss-finished, cast-aluminum pod that carries the new Uni-Q drive-unit, mounted atop a wood-veneered cabinet with an elliptical plan section and complex internal bracing to minimize cabinet resonances. The coaxial tweeter uses a titanium dome with an elliptical profile. The profile of the Uni-Q pod continues the flare of the upper-midrange cone via a unique flat surround to optimize dispersion and minimize the production of standing waves in the cone. The supertweeter is mounted in a chrome-plated steel "bullet" mounted atop the pod and covers the range above 15kHz. Basically a scaled-down version of the Uni-Q tweeter, it too uses a low-mass titanium dome with an elliptical profile. It's claimed to have a flat response up to 50kHz and useful output up to 80kHz!
To avoid excessive motion of the Uni-Q midrange cone and to maximize the system's dynamic range, a separate lower-midrange driver is used in its own sub-enclosure. Covering just the two octaves where instruments and voices have their fundamental energy, this 10" unit has been engineered for low distortion. It uses a low-mass paper cone with a central phase plug instead of a dustcap, and its motor is of the short-coil/long-gap topology, which, in conjunction with twin shorting rings on the voice-coil former, should give very linear behavior. Each of the two 10" woofers is mounted in its own sub-enclosure and has its own flared reflex port. One port is mounted on the front baffle beneath the lower woofer; the other, for the upper woofer, vents upward from the top panel behind the Uni-Q pod. The woofers again use linearizing Faraday Rings but this time with a long coil mounted in a short gap to give constant drive force regardless of cone position. The reinforced paper cone is terminated in a half-roll rubber surround.
The speaker's fit'n'finish are superb. The story of the Reference 207's design is far too long to do justice to in the space available in this review. Those interested in learning more can download a White Paper from the KEF website. Suffice it to say that the 207 appears to embody everything the English company has learned about drive-unit and speaker-system design in the 40 years since it was founded by the late Raymond Cooke.