Gramophone Dreams #49: KEF KC62 subwoofer
We have inherited an infinitely vast library of recorded musical art, the majority of which is well-recorded but has yet to be fully and completely reproduced. Countless times, I’ve played an album and thought, am I the first person ever to hear this recording sound this clear and microscopically detailed? Audiophiles understand that in order to be fully enjoyed, great recordings need the finest possible audio reproduction. Reciprocally, the finest audio systems are best enjoyed when playing great recordings. It’s a horse and carriage thing.
One of my favorite great recordings is 1994’s Madar (24/44.1 MQA FLAC ECM/Tidal), recorded by ECM conjurer Manfred Eicher and performed by saxophonist Jan Garbarek, Tunisian oudist Anour Brahem, and tabla player/percussionist Ustad Shaukat Hussain. When I listen to Madar with my eyes closed, its complex palette of tones, exotic aural textures, and morphing tempos dance vividly before me. Its sounds convert to visual and tactile associations, which then morph into inspired reverie. How completely and effectively this well-recorded album presents itself is a good indicator of how accurate and well-balanced my sound system is.
The above musings are intended to prepare you for my latest confession: Every loudspeaker I’ve ever owned failed to present music in a complete and properly balanced manner. Why? Because they rolled off too quickly in the lower frequencies. This includes the several pairs of Quad ESL-57 electrostatics; they made more bass than I thought they would and yet few audiophiles would describe the original Quads as full-range. It also includes my large Altec Lansing A5 Voice of the Theatre loudspeakers, which rolled off sharply at 60Hz, and my long-term reference Altec 604Es in Altec 612 cabinets, which rolled off lower but not low enough.
Then, of course, there is my three-decade love affair with the BBC LS3/5a. Every time I listen with these speakers, I pause to admire their midrange beauty and resolve, then pause a bit longer and sigh, wishing for more focused, structured sound below that exquisite midrange.
The first time I experimented with high-quality commercial subwoofers, I was using stacked Quad ESL-57s on Arcici stands. I don’t remember the subwoofer brand, but I do remember that those two used subwoofers cost more than my two pairs of Quads. I had high hopes for this system, but I never got those large, expensive subs to blend with the Quads in a satisfying manner. I tried passive and active low- and high-pass filters, but the woofers always soiled the midrange.
For a bunch of years, I experimented with Ted Jordan’s full-range aluminum-cone modules on open baffles. I added Panasonic ribbon tweeters and various Focal or Dynaudio woofers (in sealed, aperiodic, or transmission-line enclosures) and never once found a combination I could live with. But what I learned was invaluable. I discovered that, in order to achieve a correctly balanced character, a loudspeaker’s high frequencies must not be more extended than its low frequencies, and vice versa. Every loudspeaker has what I call a geometric center to its power bandwidth; adding extension to the top or bottom tends to throw that power center off, making reproduction feel unbalanced.
The KEF KC62
My lifelong focus on midrange beauty forced me to accept some amount of bass deficiency and to stop thinking about low-frequency augmentation—until just a few months ago, when I received a press release for the new $1500 KEF KC62 subwoofer (footnote 1). Intuitively, I felt this little 10″ cube (KEF calls it “football-sized”) was scaled and designed just right for my listening room. Before the KEF sub arrived, I knew exactly where I would put it: out from the wall, centered between my speakers.
A cut-away diagram from KEF shows the KC62’s force-canceling, 6.5″ Uni-Core drivers with their concentrically sleeved voice-coils sharing a common pole-piece. The substantial magnets use the KC62’s extruded aluminum enclosure as a heatsink.
Besides limiting undesirable cabinet movement, the KC62’s bipole radiators double a single woofer’s radiating area without demanding a larger box. According to KEF’s website, the KC62’s magnetically unified drivers employ a current-sensing feedback loop that counteracts voice-coil nonlinearities, “distortion reducing” P-Flex folded-cone-surrounds, and a variation on their new, much-discussed Meta-material Absorption Technology. Like KEF’s LS50 Metas, the KC62 comes in either Carbon Black or Mineral White, both matte finishes.
When it arrived, I set the dense, 30.9lb KC62 on my desk so I could study it as I read the owner’s manual. On its side-mounted control panel, I was impressed to see a single mechanical switch for choosing between five DSP room-placement profiles: Room (for positioning away from walls), Wall (for next to a wall but not in a corner), Corner, Cabinet (near a corner, inside a cabinet), and Apartment (which reduces the level of the KC62’s lowest frequencies).
The KC62 includes line-level (RCA) and speaker-level inputs (Phoenix connector) and a variable (40Hz–120Hz) high-pass–filtered (HPF) line-level (RCA) output; the main speakers, then, can be driven directly (full-range) or first passed through the KC62 via the line-level inputs and outputs to roll off the low end to blend with the sub. These high-pass turnover points are set, between 40Hz and 120Hz (or bypass), via four panel-mounted DIP switches. The woofer’s output level and low-pass turnover frequency are set with matching, calibrated knobs. There is also a 0°/180° phase-inversion switch and an expansion port that accepts KEF’s (optional) KW1 wireless adapter ($200).
The main thing to remember about the KEF KC62 is that it is a 10″ cube, which is really small, and really small is easier to position than big or—especially—really big.
If I had two KC62s I would have placed them firing sideways near the walls (but away from the corners) directly behind my speakers. In that position I would expect them to disappear entirely, leaving me to experience some increase in soundstage dimension and ambience retrieval.
But I had only one sub. Earlier, I wrote that I knew where I’d put it, but before I put it there, I wanted to assess its power in other locations. First, I put it in a corner, then behind one speaker, then against a wall next to my listening position (which sounded quite good). I ended up with it centered between my speakers about 3′ from the front wall. I kept the DSP room placement switch on Room.
Placing a subwoofer in a corner maximizes room gain thereby minimizing cone excursion for a given volume, potentially lowering distortion. Unfortunately, all of my experiments with corner placement had unruly out-comes. Corner placements seemed to expose or exacerbate formerly hidden room issues.
With Falcon LS3/5a
The first speakers I tried were my (new old) reference Falcon LS3/5a (Gold Badge Edition). I wanted to orchestrate their satellite-to-sub merger as smoothly and inaudibly as possible, so before I listened, I did some measuring.
In my room, the Falcon LS3/5a’s sit on 24″ Sound Anchor stands, 70″ apart with 13″ between their cabinet backs and the front wall. In that position, their midrange tone is superb, and they measure about equal in level at 100Hz, 1kHz, and 10kHz. I used that level as my measurement baseline. The Falcons begin rolling off sharply at 80Hz and show a narrow 6dB dip at 160Hz. I was curious to see if the new KEF sub could pull the dip up while flattening the 40Hz to 200Hz range.
My method was to patiently adjust the KC62’s output and low-pass turnover point while plotting frequency response graphs using 1/3-octave warble tones from Stereophile‘s Editor’s Choice test CD and the excellent but now-discontinued “Decibel: dB Meter” app for the iPhone. My measurements were taken from chest level in my normal listening position about 7′ from both speakers. I dot-plotted my with-sub measurements on log frequency and linear (decibel) amplitude graphs, directly over my measurements made without a subwoofer.
Footnote 1: KEF, GP Acoustics (UK) Ltd., US distributor: GP Acoustics (US) Inc. 10 Timber Lane, Marlboro, NJ 07746. Tel: (732) 683-2356. Web: kef.com
Published at Tue, 18 May 2021 15:51:45 +0000