KEF KC62 SUBWOOFER

KEF'S Uni-core tech allows for dual woofers to generate deep bass from a compact encloure.

MANKIND HAS steadily sought to produce more and deeper bass from smaller and smaller enclosures ever since the first Neolithic audiophile blew through a conch shell and thought, “Damn, I wish this thing went lower, louder!”

Good luck with that. Hoffman’s Iron Law (that’s Josef Hoffman, the “H” in pioneering American hi-fi firm KLH) tells us, essentially, that amongst the three desirables of deep bass, lifelike loudness, and compact enclosure size, you can have any two but never all three from a single design.

It’s not really Joe’s law, of course, but the universe’s. High acoustical levels of truly low frequencies need a large vibrating surface moving a good distance in and out: it’s a function of wavelength, which increases rapidly as frequency decreases. And a big woofer needs a large box to baffle it properly and supply an adequate volume of air to achieve sufficiently low resonance—and, of course, to fit that big driver.

Beating these restrictions requires considerable electroacoustic guile, which is precisely what KEF— one of Britain’s oldest loudspeaker makers— has brought to bear on a new design it terms Uni- Core. Briefly, this is a clever, “force-cancelling” (back-to-back) double-woofer that backs two drivers onto a common magnet structure. One smaller-diameter

voice coil moves concentrically inside the other, sharing a common pole-piece, with an intervening aluminum spacer/ structure to hold the whole apparatus in alignment. This topology requires a balletic balancing of magnetic and electrical parameters like flux, resistance, reluctance, and inductance, such that two loudspeaker “motors” with necessarily very different voice-coil dimensions and air gaps maintain perfect electromagnetic balance over their full travels. While Uni- Core incorporates a deal of mechanical cleverness too, it is this high-wire act, no doubt achieved with considerable computer-simulation firepower, where the real Uni- Core magic happens.

Little bigger than a bowling ball, KEF’S KC62 begins, like all “mini” subwoofers, by trading woofer diameter for excursion: that is, in-and-out-ability. (The firm claims an innovative pleated surround increases Uni- Core’s linear throw.) Doing so sacrifices efficiency (in the physics sense) dramatically, which designers answer by applying more power: in the KEF’S case, 500 class-d watts for each driver— err, voice coil. Smart DSP equalization and low-frequency dynamic management do the rest, with the aid of another Uni- Core innovation: a current-sensing feedback loop from the actual voice-coils that feeds information about incipient distortion to the digital processing “brain.” Lastly, the KC62 sub's one-piece extruded aluminum enclosure’s thinner walls resist cabinet resonance and yield greater internal volume relative to external size.

SETUP

The KC62 might inspire the adjective “cute” if it wasn’t so incredibly dense— three times as heavy as the bowling ball it so readily calls to mind. Like every KEF product I’ve encountered over the years, it is gorgeously finished, with gratifying attention to detail of surfaces, textures, and tones. Two sides culminate in the opposing driver cones, while the bottom surface has a footing of some vibration-absorbing, wooferwalk-resisting stuff. A surprisingly elaborate control and input/output panel on the back has the usual (but unusually nicely detented) rotary volume and crossover knobs, 0/180 degrees phase and Lfe/normal toggles, plus a five-position EQ switch with Wall, Corner, and Room settings. Two additional settings, Cabinet and Apartment, are meant to limit deepbass output and thus vibrational transmission. Next is a four-place “DIP” switch set that can impose a high-pass filter on the KC62’S pass-through stereo line outputs, for a system lacking crossover facility in its amp or receiver. Finally, there’s an expansion port to accept KEF’S KW1 wireless adapter, an option that adds another $200 to the KC62’S already upmarket $1,499 price.

PERFORMANCE

The issues are relatively simple with a subwoofer. Does it go low enough? Does it go loud enough? Is it flat enough over its three-octave useful range to avoid booming or hooting? And is it acceptably low-distortion at its usable levels (remembering that we are much more tolerant of distortion at very low frequen

cies than at higher ones)? Most everything else, including my favorite oxymorons, “fast bass” and “tuneful bass,” is so much verbiage. (To whatever extent those are real, they say more about the low-frequency behavior of the listening room than of the woofer.)

Didactic digressions done, let’s address the little KEF.

Does it go low enough? Yes, indeed. I heard useful levels (caveats to come) of clean, un-doubled output down to 25Hz and even lower. Does it go loud enough? Yes. No. Maybe. (I’ll expand below, of course). Is it flat enough? Absolutely— especially given its flexible EQ options.

I started out with the KC62 in my desktop system, mated with IK Multimedia’s small but highly capable MTM powered monitors (see review on page 56). These have their own flexible controls, including a high-pass option at 60Hz. With this selected, and the sub connected to my DAC’S unbalanced left/right outputs and pushed under my desk well out of reach of my size

11s, I was rewarded with rich, fully extended sound requiring nothing more than nudging the KC62’S volume and crossover controls to balance up to near perfection. Fourplay’s smooth-jazz track “Café l’amour” (16-bit/44.1khz FLAC, Qobuz) has bassist Nathan East slapping a low E-flat-d vamp in spots, plumbing down near

35Hz. At my desktop levels— the MTMS can play clean and punchy to a solidly loud volume, but a bit shy of control-roommonitor level—the tiny KEF had no difficulty in “keeping up.” Deep bass stayed solid, dynamic, and undistorted, and the next octave displayed zero boom, thud, or unseemly whooshing.

Re-playing “Café l’amour,” I increased volume to the max the IK monitors could deliver, and then turned them off and auditioned the sub “naked.” I heard just what I expected: clean, solid bass. (Surprisingly, I found the KEF’S Crossover control, which I had as always dialed in by ear alone, was lined up bang on the 60Hz marking. That never happens!)

Staying with the same track, I gradually increased the sub’s volume control along with my DAC’S output-volume control and monitored the result. The little KEF got louder with no loss of bass content to perhaps 10db more than where I’d been at max with my desktop monitors; beyond that point, while mid-bass from the sub continued to rise, the deep bass stayed the same: bassdynamics limiting at work, as expected. But no discernible distortion or unseemly noises emanated from the KC62, until the last measure of gain from my DAC (its max output is well over 2 volts), with the sub’s volume wide open. Then, on loud kickdrum-bass unisons, the little woofers added a soft “phut” on each note as they found the limit of their travel. (Turning the MTM monitors back on at this level found them deep into their red zone, and indeed one quickly shut down.) Conclusion: KEF did an excellent job at balancing the KC62’S electroacoustic systems, extracting all the ultra-compact can give while preventing it from disgracing itself.

How would this tiny tot fare in my big system, supporting much larger, much higheroutput 6.5-inch three-way active speakers, and replacing an SVS cylinder subwoofer roughly 25 times the KC62’S cubic volume? Let’s find out.

Muscling the SVS aside, I placed the KEF in my established subwoofer spot, jacked in power and the long RCA cable running from my pre-pro’s sub out into the KEF’S unfiltered

LFE input, and performed a quick level-balance using the pre-pro’s match noise and my iphone’s Spl-meter app. With the preamp’s crossover set for 80Hz, I then played the same Fourplay track (via Roon) at a comfortable listening level, making small adjustments to level and crossover in my prepro’s setup menu. The result was solid, rich sound— just like my system always sounds. To confirm, I swapped back to the big sub a few times and heard no meaningful difference.

This was pretty much what I expected, but to an unjaundiced ear it should be little short of astonishing. At moderate listening levels (75-80db SPL average, - ish), the KC62 had no difficulty matching my big system’s extension, heft, or musicality—this is in a 22 x 16-foot room of roughly 2,800 cubic feet. Track after track yielded the same results. The opening of Prokofiev’s “Dance of the Knights” scene from Romeo & Juliet revealed rich, vibrant orchestral bass, with no hint of buzz or woofiness. The Eagles’ unplugged “Hotel California,” which has strong bass-drum content around 40Hz with “undertones” well below had high-impact, gutpunching slam.

For a natural-acoustic reference, I went back to the Prokofiev. At the moderately loud levels I had been listening at, the KEF did, as I said, excellent work. This was around 76-78db SPL average: about equivalent to what you might hear from a seat two-thirds back in Boston’s Symphony

Hall, the space with which I’m

most familiar. Adding 4 or 5db of level—about one-third-back— caused little change: maybe a slightly “lighter/brighter” overall balance, but there was still plenty of 30- 60Hz octave for full impact. Adding another 4 or 5db and we’re now in row three, nosing the stage. Here the difference became obvious, with the much brighter balance begging the question, “Where’d all that rich, deep bass go?”

It didn’t go anywhere, of course— it’s a balancing act. As my main speakers reached peak concert levels, the diminutive KEF simply ran out of lowest-octaves headroom, and its smart dynamic circuitry curtailed them to prevent audible distortion. Its uppermost half-octave (and the main speakers’ remaining 8 octaves), meanwhile continued to amp up. Result? The ear hears “bright,” and Prokofiev’s rich, chocolatey bassi/ contrabassoon/tuba unisons seem to nearly disappear— even though the sub is reproducing them just as loudly as it was at my initial listening level.

Much impressed, I now exercised the nuclear option, “Bass I Love You,” by electronica artist Bassotronics, a track with about as much 15-30Hz bass content as you’ll find. (All synthetic, of course; you simply won’t get this low and loud in nature without military-grade explosives.) At my established “modest” level, the KC62 produced just about all of the infra-bass—the super-low thud, the breathless sort of decompression between pulses, and the descending flutter of sub

25Hz content—with satisfying presence. Switching to my hulking SVS subwoofer again made surprisingly little difference, though the big bruiser was clearly just a bit stronger and more dynamic with this infra-bass stuff. In this case, however, “modest” was the KC62’S level limit. Asking even 2db more from it proved futile: the >100 Hz stuff got louder, but the <60 Hz stuff didn’t.

THE DOWNLOW

The fact that a literally breadbox-size subwoofer can produce actual 25 Hz-and-lower content at any musically useful level is newsworthy. And so is the KC62 as a whole: if you insist on breaking Hoffman’s Iron

Law, or at least bending it to its limits, KEF’S inaugural Uni- Core design does so as dramatically, and as classily, as any sub I’ve seen. Within the “iron” caveats of a small room and/or relatively modest level demands, it will match the performance of a well-engineered 12-inch sub in every important parameter except peak dynamics and level. There may be other minisubs, but none I know of go quite as low in quite as small and elegant a package as what KEF has managed with the KC62.