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KEF Home THX Loudspeaker System

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KEF’s Home THX speaker system is somewhat unusual in that it includes an active subwoofer. (While most Home Theater subs are powered types; it’s just that few THX models are.) Although powered speakers have never enjoyed much popularity with American audiophiles, they can yield better results than the mix’n'match approach because each amplifier/driver combination can be optimized.

Any combination of a THX amplifier and THX loudspeaker will have about the same sensitivity as any other such combination. The reason each channel of a THX-certified amplifier is required to have an output capability of 150W (into 6 ohms) is because one or another of them may be called on to drive a subwoofer of unpredictable impedance. (In fact, THX subsequently modified its amplifier power standard to accommodate receivers, whose loudspeaker outputs have clearly defined functions. Only the receiver’s subwoofer outputs are required to deliver the full 150W.)

Description
The front KEF AV3 speakers are typically THX in design, in that they have dual midrange/woofers and three tweeters, with all five units mounted vertically in-line. The AV2 surround speakers are also typically THX, in that they are dipoles. Cabinet finish and construction appeared to be very good, although my rubber-mallet test (less painful than the equivalent knuckle and fist tests) indicated a slight resonance around 500Hz from two spots on the AV3′s back panel. (The subwoofer enclosure was resolutely inert.)

The KEF AV1 subwoofer’s amplifier is mounted inside the subwoofer enclosure on standard rack mounts. It is removable, allowing it to be optionally installed in a separate 19″ rack. I opted not to do this, which proved to be a bit of a mistake (see below).

The subwoofer amplifier can accept balanced or unbalanced inputs, be fed from the loudspeaker outputs of a Home Theater receiver or the line outputs of a stand-alone surround decoder. It can be operated both as a universal subwoofer (“stereo” mode) or as a THX subwoofer.

When the Stereo/THX switch is in Stereo mode, the Level control can be used for setting LF balance, and the (crossover) Frequency control on the back of the amplifier can be used to change the subwoofer’s low-pass inflection point between 50Hz to 150Hz. In THX mode, the level and crossover controls are deactivated; both the sensitivity and the crossover frequency are fixed according to THX specs.

The subwoofer amp’s two sets of inputs are marked Conventional Stereo and Home Theater THX/Pro-Logic. The Conventional Stereo inputs accept full-range left- and right-channel inputs from a stereo preamp or receiver, feed a low-passed blend of the L and R bass to the subwoofer amp, and return the signals to output receptacles for connection to an external power amplifier. The unbalanced outputs can be full-range, for a typical stereo pair of loudspeakers, or high-pass–filtered below 100Hz for use with satellites of limited bass-handling capability. Conventional Stereo also has XLR pass-throughs that draw off the bass signals and feed them (blended) to the subwoofer while outputting full-range L and R signals to the main stereo amplifier. No high-pass filtered outputs are available when using the XLR connectors.

Also among the Conventional Stereo group are two pairs of 5-way binding posts, to be used when you want to augment the low end of a system that has no line-level outputs. Each pair is internally strapped, providing the option of passing the stereo signals through the AV1 and on to the stereo speakers, or of simply drawing off signal from the receiver’s outputs. If the pass-throughs are used, the cables to and from the AV1 should be of fairly heavy gauge; otherwise, 16-gauge zip cord will do jes’ fine.

kef-speaker

There’s only one THX/Pro-Logic input, which is configured on the assumption that it will be fed from the low-pass–filtered Subwoofer output of a surround decoder; it has no internal filter. A single output, which is simply a hard-wired feedthrough from the input, can be used to drive another power amp and subwoofer. (If the decoder has stereo subwoofer outputs, these can be summed by connecting them to both THX receptacles, or can be used to drive a pair of AV1s.)

The “Blend” control is an interesting innovation, but I’m not sure what it does. The manual seems to imply that it’s a phase control, to adjust the timing of the subwoofer relative to the other speakers. Indeed, the control is marked with a + range and a – range, which would support the idea that it varies woofer phase. But this is impossible! There’s simply no way the subwoofer could deliver output before the signal reaches its amplifier input. Advancing the woofer’s phase ahead of that of the other speakers could only be done by delaying the other speakers’ signals and operating the sub in real time. Perhaps KEF can explain what’s going on here.

Installation
The THX spec calls for very narrow vertical dispersion from the front speakers, which means they must either be raised to ear height or tilted upward by an appropriate amount. (Unless the center speaker is behind an acoustically transparent screen, it must either go over or under the screen.) Most “universal” speaker stands can get the speakers to ear height, but few of them allow you to adjust their vertical tilt. KEF makes optional stands for their LCR speakers, but their 36″ height puts the speakers’ tweeters about a foot above ear height, and there’s no provision for tilting them. I used Neoprene doorstop wedges (available from any hardware store) under the rear of each speaker to aim them downward.

The stereo pair was placed flanking the on-wall screen and about 26″ in front of it, and spaced so as to form an angle of about 50° with respect to the center of the listening area. The center speaker was located the same measured distance from the center listening seat as the left and right speakers, but on a 10″ stand I had, and propped up with wedges under the front to aim them upward. The subwoofer was placed at the left of the left-channel speaker—about a third of the room width from the side wall. (In my listening room, one or two subs in that 1/3-width location gives the smoothest low end; 1/4 or 1/2 width causes a deep dip at around 40Hz—right in the middle of the bass drum, double bass, and exploding-Death-Star range.)

The surround speakers went atop a pair of industrial tripod-base public-address speaker stands, close to the wall on each side of the row of audience seats. (This is a disadvantage of dipole surrounds; only one row of seats can be in their null zone, which is where you’re supposed to be; anyone who isn’t will hear a higher surround level than they should.)

Since everything in the system (except the signal sources) was THX-specified, there wasn’t much need for calibration. The front-channel levels were right on the nose, as was the subwoofer level. A few input levels had to be reset to accommodate different-level sources, and the surround channels had to be raised by about 2dB in order to meet spec, but that was it

Devout subjectivists may scoff at the idea of setting subwoofer level by measurement, but the results I’ve gotten by metering it have been consistently spot-on, as long as the spl meter is placed at head height in the main listening seat. With that setting, program material has varied from somewhat lean to slightly bass-heavy, almost exactly according to expectation. Old LPs sound fat because of their LP equalization (which required low-end boost that flattened out at 100 rather than 50Hz), early stereo LPs are usually a bit lean, and recent audiophile recordings (Dorians, Telarcs, References) are as fat-sounding as one would expect from the use of wide-range omnidirectional mikes for their main pickups. Movie soundtracks followed the same pattern: Early stereo LDs are more or less bass-shy, and later ones increasingly have thunderous bass, often far beyond the point of wretched excess. (Give the public what it wants: bass, bass, and more bass!)

Misgivings
Shortly before KEF was to ship their THX system to me, a devastating review of it appeared in Home Theater Technology magazine. The reviewer, Stereophile expatriate Corey Greenberg, described the system as being in-your-face aggressive and excessively bright. The bullet-to-the-head was a succinct three words: “We hated it.” Barely a day after I read that, I got a call from KEF informing me that delivery of my review system would be “slightly delayed.” Seems it had gone back to the drawing board for a few last-minute modifications.

Suddenly, I felt a great unease. I’ve seen (or rather, heard) what usually happens when a manufacturer reworks a product in response to a bad review. What usually happens is that the “correction” overshoots the mark, resulting in a sound that’s flawed in the opposite direction. So I asked KEF if I could also borrow a set of the speakers that got panned, for comparison with the new, ostensibly improved ones. I was told they didn’t have any of them left. Not a single one. Nada. And this was before the new ones were even in production!

I had a vivid mental picture of men in white coats taking sledgehammers to KEF’s inventory of the tainted speakers, like a farmer culling plague-infected chickens. (More likely they were stripping the old crossover networks out of them in preparation for the new ones.) So, while I wasn’t exactly expecting this system to sound mediocre—after all, KEF has a long track record for making very respectable speakers—I must admit that I approached these speakers with some misgivings.

Listening
First off, these speakers proved to be very easy to listen to, which would imply that they are very different from the ones reviewed in HTT. They sounded silky-smooth, relatively free from colorations, impressively authoritative, and they imaged like gangbusters! Center bunching from mono sources was very tight, even with the center channel turned off, and specificity from stereo sources was outstandingly good. Between-the-speaker images were stable and unambiguously placed.

These are great Home Theater speakers, with a low end that never once complained, no matter how much cinematic violence I threw at them. Bass was very deep, solid, and well-behaved, but only moderately well-defined. The lower midrange seemed a little elevated, producing a vaguely chesty quality that was consistent from recording to recording, and there was a slight “aw” coloration.

kef-subwoofer

Highs were sweet to the point of dullness, despite a slight complementary accent from the two sets of power amps I tried. Detail was okay but nothing to celebrate, while the midrange was more laid-back than that of any THX system I’ve heard, which is (in my book) not a strong endorsement. With the speakers driven by the Boulder amplifiers, the system sounded a little less laid-back, but not by much of a margin.

With neither the Boulder amplifier nor the Parasound, did the KEFs have the immediacy necessary to elicit startle responses. This didn’t seem as much related to frequency response as to speed; they lacked what I call “snap” and Martin Colloms calls “pace.” It sounded as though the speakers hesitated very briefly before reproducing a sharp attack. They also seemed to darken timbres, as though the instruments making the sounds were larger than life. The effect was similar to playing an LP at reduced platter speed, except that there was no accompanying pitch change. These qualities were far less noticeable when viewing films than when listening to music, because the picture occupies much of the listener’s attention, but they were quite noticeable when that distraction was removed.

Dynamic range was good, with less spurious compression than average, but more than I’ve heard from some speakers. (Notable for their lack of compression were a pair of powered monitors from an English firm called ATC, the original Fosgate Home THX speakers, and the massive Dunlavy SC-VIs, footnote 1). But while the KEFs were nonetheless quite capable of producing prodigious volume levels (THX calls for a max headroom of up to 105dB), they showed some evidence of stress—a slight coarsening of textures—at levels above about 95dB.

To say that the KEFs sounded less like any Home THX speakers I’ve ever heard is not to say they were better music reproducers than the others. They were just quite different. All the others had several things in common: They reproduced the core of the sound—the musical and extra-musical midrange—with a high degree of accuracy and realism, they could play at very high levels without strain, and their low end was astonishingly good. But all have lacked the suavity, delicacy, refinement, and resolution of the legendary high-end speakers. Maybe there are some THX systems out there that have it all, but I haven’t heard one.

But I don’t believe this is a result of constraints imposed by the THX requirements. They are, after all, only minimum performance requirements, which define a level of acceptability below which a product cannot receive THX certification. There’s nothing in the THX specs that says a speaker can’t exceed those minima.

I think, rather, that the reason for THX speaker shortcomings is that some high-end manufacturers who sign on to the program do so more for its promotional value than out of a conviction that the THX specs have any relevance for music reproduction. For example,even though KEF signed up for the THX program, some statements in their instruction manuals suggest they haven’t really signed on to it. For example, instead of recommending THX-certified amplifiers for driving their THX AV2s and ’3s, they provide helpful suggestions for choosing appropriate amplifiers. They don’t even acknowledge that THX amplifiers exist! The clincher, though, was their suggestion that their THX system should “supplement” one’s music system rather than replace it.

JGH Concludes
So, did I “hate ‘em?” Nope, I just didn’t like ‘em very much. As impressive as the KEF THX speakers were with noisy movie soundtracks, I found them to be unexciting music reproducers. They never gave me goosebumps, they never made me sit up and take notice, they never drew me irresistibly into the music, and they never inspired an orgy of revisiting all my favorite recordings. The best I can say for them is that—like a bland personality—they never irritated or offended me.

Sorry, KEF, but I think you overdid the course correction. I know you can do better with these, even within the “constraints” imposed by THX, because I’ve heard KEF speakers that were better. But until that happens, I can’t recommend this system for music listening.

Review by : stereophile

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