KEF R700 loudspeaker
A match made in Maidstone
Only a few attempts at using coincident drive-units in reference-quality speakers have been successful. Among KEF's UK competition, Tannoy's well-regarded coincident-driver technology has been around for decades. On the other side of the planet, and more recently, TAD of Japan uses coincident drivers in its high-end speakers designed by Andrew Jones. (Jones also designed a line of Pioneer speakers that employed a coincident drive-unit inspired by the TAD designs.) SEAS, a Norwegian company, also produces coincident drivers, though it's best known as a supplier of quality drivers to other manufacturers and the DIY market rather than for producing its own, finished speakers.
Beyond that, however, the pickings are slim. I use the term coincident here to refer to a dual-element drive-unit that positions the tweeter deep in the center of a bass/midrange (or dedicated midrange) driver. A more common term for such a driver is coaxial, but to me that's more appropriately used to refer to a dual-element driver that simply puts a tweeter in front of the woofer cone. Terms to clearly distinguish the two have not been standardized, but the coaxial arrangement invariably interferes with the radiation from the woofer cone and colors the midrange, and is not commonly found in serious loudspeaker designs.
Positioning the tweeter inside the throat of the midrange or mid/woofer cone has advantages and disadvantages. The advantage is a concentric radiation pattern that should be uniform in all directions, without the usual comb-filtering interference common to all multiway speakers with separately situated drivers. In the latter category, which includes the vast majority of speakers, the drivers are generally positioned vertically. This restricts the inevitable driver-to-driver interference primarily to the vertical axis, though such speakers remain more or less sensitive to listening height.
Positioning the tweeter at the apex of a waveguide (a short horn) also offers the advantage of better matching the radiation patterns of the woofer and tweeter at the crossover frequencyan issue known to speaker designers but not always well addressed. But in the Uni-Q the waveguide for the tweeter is the midrange cone. If not well-designed, this can have adverse effects on the smoothness of tweeter's response. However, in 25 years of consistent refinement, including the so-called "Tangerine" waveguide positioned in front of the tweeter dome, KEF has largely eliminated this issue.
East meets West
KEF's R series of speakers, designed in the UK but manufactured in the company's factory in China, includes two bookshelf models and three floorstanders. For home theaters, there are also a surround speaker, a subwoofer, and two center-channel models. A coincident driver is ideal for use in a horizontally configured center speaker, as it largely eliminates the comb-filtering dips that plague such designs for listeners sitting off-center. But there's no reason why the R-series center designs can't be upended and set on suitable stands, for audiophiles who prefer that arrangement, and used for two-channel stereo. The main downside to that approach will be less bass extension from enclosures smaller than those of their floorstanding cousins. The R700 is specified down to 37Hz (6dB), while the R600c center speaker, also with two 6.5" woofers, is rated only to 50Hz (6dB).
The R700, my subject here, is the middle model of the R-series floorstanders. It consists of a 5" Uni-Q driver specifically designed for midrange and treble duties, together with a pair of concave-coned, 6.5" woofers in a ported enclosure covering the range below 500Hz. All of the cones, and the tweeter dome, are aluminum. The Uni-Q is positioned between the two woofers in a vertical D'Appolito configuration.
Around back are two ports; KEF provides adjustable foam bungs that can be used to fully or partially block one or both ports, as needed. Dual terminals are also included for biwiring or biamping, if desired. Instead of the usual straps that connect such terminals for single-wiring, there's a knob on the terminal plate that either straps the terminals together or disconnects them, depending on whether it's turned fully clockwise or counterclockwise. If you biamp the speakers, however, be careful to check how this knob is set; without visible connecting straps, you can't tell by looking if the terminals are unlinked. Tying the outputs of two amps in parallel can ruin your day.
The R700's cabinet appears to be very well built, and comes beautifully finished in one of four options. Two of these are in a satin-finished, real-wood veneer rather than the glossy look common to many Asian-built speakers. Our Rosewood samples were flawless, and so smoothly finished I could feel no texture on them at all. The speakers also come with metal feet that not only provide additional stability but also offer easy adjustment of the provided spikes.
Room and Setup
I've been using the same listening room for nearly 14 years nowan eternity in audiophile yearsso have become fairly familiar with its sonic signature. Measuring about 27' by 15.5' (at its widest) by 8', depending on how balky my laser measure is on any given day, it was designed to be a combined living and dining space. But apart from living with music and movies, the room is used for neither living nor dining. I've blocked all the windows with lightweight fiberboard, which likely has some acoustic effect apart from its intended purpose of blocking light for daytime video evaluations. It certainly cuts down possible reflections from the roughly 12' expanse of windows on one of the long walls. Some day I'll take down the fiberboard just to see if any critters have taken up residence there in the decade and a half since its installation.
In any event, the basic setup of that system was determined back in those dim, Neanderthal days of 2000, when $25,000/pair speakers were merely visions of sugarplums in the collective minds of high-end audio marketing departments, when CRT projectors and standard-definition DVDs passed for awesome on the video side, home-theater magazines were 150 pages, and we all thought the world would end at midnight (EST, of course) on December 31, 1999.
The very first products I reviewed in this room were a Madrigal projector and the first generation of Revel's Performa speakers, anchored at the front by the F30 floorstanders. At that time I determined the best positions for the speakers, allowing only for the width required by the projection screen. The latter has subsequently been supplemented by a wider screen more appropriate for HD video and today's brighter digital projectors. But my original, preferred speaker positions, adjusting only for the somewhat wider spacing required by that somewhat wider screen, remain, since for my home-theater work the speakers must support the screens, which are suspended from the ceiling and can't be moved. (I can retract them when the main course is music.)
The plane described by the baffles of the front speakers is about 7' out from the wall behind them. The left speaker is about 4' from the left wall, the right speaker about 3' from the right. A carpet covers most of the solid-oak floor, which is suspended above a crawl space, not a concrete slab. With the carpet, the soundboard, and a few acoustic panels, the acoustic of the room is well damped. There are also several storage shelves loaded with CDs, SACDs, BDs, DVDs, LPs (not much used by me at present, alas), and even a few remaining laserdiscs (not used at all!). There are also a few built-in cabinets, which provide useful diffusion. The walls are lath and plaster, not drywall. There's an open doorway to the kitchen abeam the left speaker; two other room doors are generally left closed during listening.
I initially placed the KEFs in the positions described above, driven full-range with two-channel sources by my resident Integra DTC-9.8 surround processor used strictly as a 2.0-channel digital preamp. The amp used throughout the review was the Parasound Halo A 51, again using only two of its five channels. The source was a Marantz UD7006 universal player, with a coaxial digital link between the player and the Integra. Though most of my listening was to CDs, the Marantz was also supplemented by an Oppo BDP-105D universal player for SACD and DVD-Audio playback.
The first listening test went only well. The sound was clean and smooth, but almost too smooth. The bass was fairly extended but a bit uneven. Many of the speakersincluding KEF's larger R900sthat I've reviewed in these positions do reasonably well, but without the help of a subwoofer or two, only the largest speakers I've tested there have been truly impressive in the bottom end.
Since there was no need in this review for the speakers to be locked in to my favored positions in order to support a video screen, I was able to move them closer to the front wall. I also moved the listening chair a bit closer. While I couldn't find perfect positions (in-room measurements at the new locations showed bass extension to the mid-30Hz with both ports open), but apart from a significant dip around 60Hz, the balance was much better. At that point the R700s were just under 8' apart, 9' from my listening chair, and 4.5' from the wall behind them, with all ports fully open. The speakers were angled in toward the listening seat, their cabinets tilted slightly back to compensate for the tweeters being about 5" below the height of my ears when I'm seated. The latter had more to do with being closer to the tweeter axis than with any midrange/tweeter interference, the latter clearly not a factor with a coincident driver.