KEF Reference Series Model Four loudspeaker
By Thomas J. Norton   •   March, 1996 The Model Four is the largest model in KEF's current Reference series of loudspeakers, discounting the R107/2 Raymond Cooke Special-Edition (reviewed in a follow-up in October '95). It's also the largest KEF model that uses their Uni-Q® loudspeaker configuration. When I visited the KEF factory last October with a group of audio journalists from the US, KEF emphasized the importance of Uni-Q technology to their future plans. They consider it proprietary, and intend to enforce the worldwide patents they hold on the design. One look at KEF's current line will be enough to tell you why they're so serious. Uni-Q drivers may be found not only in most of the Reference series, but in most of their other models as well. The most significant exceptions: the Raymond Cooke series, a few inexpensive models, and their THX-certified loudspeaker system.

The Uni-Q driver was originally developed by KEF for use in the Eureka Archimedes project, an ambitious research effort conducted by KEF and a number of other companies to investigate the interaction of loudspeakers and rooms. Uni-Q, for those unfamiliar with the design, positions the tweeter at the apex of the midrange cone, in the position normally occupied by a dust-cap. The advantages: a coherent radiator, free from the acoustic interactions present in all loudspeakers with physically offset drive-units. The disadvantages: the tweeter is now loaded, effectively, by a horn (the midrange cone). So we are trading one form of interference, perhaps, for another; hopefully we will gain more in the tradeoff than we lose.

Earlier versions of Uni-Q had shown that there are advantages to the design, though not without some sacrifice in top-end smoothness. But the Uni-Q technology is now in its fourth generation. My listening tests confirmed that it is definitely a different-sounding beast than earlier versions of the design. There's more to the Reference Four, however, than a...er...unique midrange-tweeter configuration. At the other end of the spectrum, KEF uses what is commonly called a bandpass enclosure for bass loading. In this type of design, the woofer is (or woofers are) mounted inside the cabinet. One side of the woofer is loaded into an internal chamber, which may be either sealed or ported. The other side loads into a second chamber, which is in turn vented to the outside. Properly designed, this type of system can provide extended bass, while the port to the outside acoustically rolls-off the top-end response of the internal driver(s)—simplifying the work of the system's crossover network.

In the Model Four, there are two 10" internal bass drivers. They each fire into their own individual, internal chambers. The other side of each driver loads into a third internal, common chamber. The latter—which is also connected via ports to the other two internal chambers—vents ultimately to the outside. The bottom line here is that the total output of the two internal woofers—below their 160Hz crossover—comes from the external port.

If all this sounds rather complex, it is. Any reflex design is inherently more difficult to design for optimal operation than a sealed cabinet; KEF's bandpass design, which they refer to as a "Coupled-Cavity," is even more so. And there's an added wrinkle. The internal drivers are connected back-to-back by a rigid rod to help cancel physical vibration. Vibration is further reduced by decoupling the front-mounted drivers from the cabinet by rubber bushings. The cabinet itself is well-damped; the knuckle-rap test indicates a damped cabinet of average thickness rather than the cast-from-concrete sensation typical of a cabinet with ultra-thick walls. Both approaches are valid design routes to solving the problem of cabinet vibration, each having strong advocates. Clearly, the Wilson WITT, which I reviewed in January (Vol.19 No.1, p.177), comes down on the other side of the fence.

The Model Four is much easier to move around the room than the WITT, though at over 100 lbs, the KEF itself is hardly svelte. Oxygen-free copper wiring is used for all internal connections in the Model Four, with different gauges for bass, midrange, and treble drivers. Knurled, gold-plated bi-wire terminals are provided. I don't like KEF's terminals. They're big, heavy-duty, and attractive—and will not accept most spade lugs unless you put one of the tines through the hole in the terminal and thereby mash it when (if you can) you tighten it down. But this is, I admit, a more irritating problem for a reviewer than for an owner, who's not likely to switch leads all that often.

The cabinet's base is made from a rigid, mineral-filled polymer (as is the front baffle), and can be filled with sand or lead shot if desired (I did not do so). Reversible, adjustable spikes are fitted; when the spikes are used, the gold-plated cabinet feet act as lock nuts for them. Recognizing the way the market is moving, KEF has provided magnetic shielding for all of the drivers in the Model Four, for use with video monitors if desired. While I have no objection to using a speaker like the Reference Four for combined audio/video duties, it would be a shame (from a stereo soundstage viewpoint) to place them close enough to a video monitor—and thus probably only 4-5 feet apart—that shielding might be a concern. Finally, all of KEF's Reference Series are matched to within 0.5dB of the standard for that model. And as I observed when I toured KEF's plant, each pair of Reference loudspeakers is hand-assembled to completion (starting with the finished drivers, finished cabinets, and assembled crossovers) by a single craftsman.