KEF Home THX Loudspeaker System
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.)
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-passfiltered 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.
There's only one THX/Pro-Logic input, which is configured on the assumption that it will be fed from the low-passfiltered 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.
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 speakerabout 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 40Hzright 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.