A Mighty Mini
Sound & Vision|April - May 2021
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!”
By Daniel Kumin

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

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