REVIEW | MARTIN COLLOMS

KEF’s smaller, smarter sub
AT AROUND 25CM ON EACH SIDE, THE LATEST KEF ACTIVE SUBWOOFER IS TRULY TINY,
BUT MARTIN COLLOMS DISCOVERS THAT ITS COMPACT DIMENSIONS CONTAINSOMETHING REALLY RATHER SPECIAL
Without question, at £1399 KEF’s attractively compact KC62 subwoofer is a most clever box of tricks,
but is it just too small for its own good? Are two reaction-cancelled 6.5in/14cm bass units enough,whether they be long throw high excursion types or not? An indication of just how hard they mightneed to labour is the provision of 500 watts for each driver of necessarily efficient D Class amplification.
Such massive available power poses all kinds of questions, including just how intelligently this design
manages programme demands, in respect of its own natural physical limits for acoustic output overfrequency. Certainly, physics will not help it get from a mid-bass frequency of 50Hz at a steady soundlevel down to a genuinely low 25 Hz. This seemingly simple requirement actually requires that the two
pistonic bass driver elements move four times as much by 25Hz.
It is worth noting that his seemingly small loweringin numeric frequency comprises a complete,potentially informative musical octave, andshould not be dismissed as just ‘25Hz extension’of a nominal 20,000Hz audible spectrum. And
helpfully, that theoretically required increase incone movement will tempered by a measure ofroom power gain, the latter generally increasing
with reducing frequency, thus ameliorating that

otherwise daunting theoretical acoustic power
demand which is defined for ‘free space’ operation– no room and thus no boundaries.
At this point we raise a question about purpose:
do you want a floor-shaking kick-in- the-chest‘thumper’ to rock your boat for home theatre, orsomething more subtle and sophisticated, aiming
to improve the overall sound quality of your system,
perhaps to include classic stereo? For a home

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“REVIEW

With low distortion
bass on offer there
is no ‘harmonicky’
accompaniment.
We are talking low
coloration here,
an almost purebass output

theatre set up the low frequency ‘effects’ feed, theLFE, is a processed percussive ‘signal’ designed tosurprise and shock, a key part of the scripted drama.
But where a sub is intended as an enhancement toa higher quality audio system, it needs to be goodenough in terms of dynamic range and distortion
not to degrade the inherent sound quality of themain stereo pair.
Extant published reviews have concentrated onjust how loud the compact KC62 design will play forits size, but seem not to be so concerned as to what
contribution it could make to system sound quality.
So many sub woofers add noise harmonic distortionand coloration to their low frequency output, andcan bounce and vibrate the floor – some so muchso that they may ‘walk’ across the room much likean out of balance washing machine. Such selfgenerated and unwanted noise is just another kindof distortion, and who wants a subwoofer exhibiting
such behaviour?
Low noise, low distortion
By contrast, key aspects of the KC62 design are
directed to low noise and low distortion, and fora start this quality makes the audible location of
the sub almost undetectable. Good quality low
frequencies are felt as more of pressure filling thespace, rather than freely propagating soundwaves
which might be located by ear. That short distance
between our ears is a limiting factor in this low
frequency ranging ability which progressively
reduces below 500Hz.
However as higher frequencies are reached, andwhere some upper harmonics and distortion might
be present in a hard working sub’s output, then its
location may no longer be indeterminate. Stereo
imaging may be affected depending on whether
such ‘noise’ may be located aurally, marring theimage from the main stereo pair, never mind theadded coloration.

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Fortunately the dynamically balanced, opposed
drivers of the KC62 deliver minimal mechanicalreaction forces, and thus suffer from negligible
vibration. In addition, the KEF distortion control
technology suggests that higher harmonics will
hardly be present, and thus the low frequency
timbre for the system as a whole should remain
neutral. As such, if the goal that the sub cannot belocated by ear alone is achieved, then the primarystereo image will remain undisturbed.
Now we are in a position to consider potential
audiophile benefits, which rely absolutely on the
sub not increasing noise and distortion, with its
inclusion aimed at improving the overall uniformityof power over frequency and thus the quality
envelope in the room. By adding a firmer low
frequency foundation to the music, even for a stringorchestra, there may be a better sense of scale, andfrequently better bass speed and tune playing, even
if that augmentation is so subtle that it does notseem directly audible.
All of that might be expected of a modern‘intelligent’ sub such as this dense, compact cube,
approximately 10in/25cm on each side. It hasversatile connectivity, and is configured in what I
consider an ideal formation: push-push, back-toback, force cancelling and thus anti-vibration, andanti-rocking. A vast range of install possibilities,
voicing and matching adjustments are possible withthis subwoofer to help optimise the performance ina given loudspeaker installation, both with KEF andother loudspeaker designs. And of course, there isradiation coupling gain with this summed pair ofdrivers, so the equivalent unit driver size is actuallyrather closer to a long throw 10inch 250mm driver.
So, it is not quite so ‘small’ after all.
And there’s more. Belying its modest size andcost, this sub is nonetheless highly sophisticated,
employing many advanced technologies to deliver
a performance well beyond its size and price. It will
be seen as a natural pairing for the LSX and LS50
series including the LS50W II also reviewed in thisissue – and indeed there’s even an optional wirelessconnection module for use with these speakers, theKW1, selling for £180 – but it will also find service inmany other situations.
This is a sealed box design with low self-noise,
this helping to make its more aurally invisible,
an important factor. At such low frequencies it istotally non-directional thus easing placement andorientation. With a total of a 1000W of efficientdrive power, amplifier clipping is avoided, withintelligent overload control to avoid significant
audible distortion). Bass driver motion is mappedelectronically, over power and frequency, such that
audible overload cannot be reached even withheavy input signal overdrive. If it were present,

REVIEW

significant distortion can mask deeper bass tones,
even to the point of an audible droning coloration.
Low distortion bass may be less obvious, but when
you hear it you soon appreciate how musically rightit really is. And in this design the overload problem
is circumvented.
With such low distortion bass on offer there
is no ‘harmonicky’ accompaniment, but instead,an almost pure bass output. Certainly boomy
exaggerated low frequencies would disfigure andmask the output from a good stereo loudspeaker
pair. In addition to the heroic magnet design ofthe KC62 which helps to control distortion, thevoice coil current is monitored and then correctedin advance by using an electronic map of thedriver’s mechanical performance limits. Where
necessary the amplifier output is then dynamicallypreconditioned to minimise distortion.
5.0% distortion and more is common withsubwoofers and as such is mildly audible. This isbecause the resulting harmonics are present in thehigher frequency range and are thus more aurally
obvious. KEF engineers have fashioned their active
distortion control so well that with the absence of
higher frequency signals, the pure low frequency
bass tones are in the famous tradition, ‘felt rather
than heard’. In this case the physical source of suchpure, long wavelength, low frequency bass cannot
be identified, while the resulting pressure field
drives the room space, aiding that sense of power,
envelopment and excitement.
Seemingly impossible considering its small
size, the free field low frequency limit is claimed
as an improbably low 11Hz -3dB. We will find out
whether this is true, but before you look forward toearthquake style bass observe that the purpose ofthis design limit is more subtle than it looks. Firstly
there will be lowered power needs with reducing
frequency, both from the naturally reducing
power envelope of programme, to the increasing
room gain with frequency from pressure loading:that 11Hz limit will hardly be exercised at highpower. And you still get the bonus of improved
time coherence from the increased low frequency
bandwidth for the connected system, even if thissub is not intended to produce full ‘Home Theatre’
power at the lowest frequencies.
Installation
The KC62 has comprehensive facilities to match
almost any conceivable setting and installation. On
the control panel there is the expansion socket (forfitting the optional palm sized wireless module forremote connection and control, also effective for theconnection and the coordination of multiple subs),
and then we have a miniature array of four switcheslabelled ‘HPF’ high pass filter’, to set the low frequency

limit. Using these you might wish to optimise thesub for best mid-bass ‘punch’ for a movie installation,rather than for the deepest audiophile bass.
Alternatively set it for best frequency extension andphase response at more modest sound level e.g. for aclassical music application or jazz.
The control, is for absolute phase: ie ‘correct’ or‘inverted’ (-180deg), to help integration with thephase of the low frequency range of the partnering
audio installation, which could be bass reflex
or sealed box – either of which might suggest a
different phase setting according to the crossover
frequency best suited to that installation.
‘EQ’ : KEF offers five preset equalisations,captioned room, wall, corner, cabinet, andapartment, three of them offer shelving rather thana continued rolloff. These comprise:‘Room’: Here KEF indicates placement near
the TV, somewhat off-centre between the stereo
loudspeaker pair near the front wall. This is foran essentially neutral or ‘flat’ setting, where thefrequency response at normal volume settings is flat
across the working range, extending down to 10Hz
-3dB at normal sound levels.
‘Wall’: Designed for subwoofer placement ona side wall of the room, 1/3 along. Now the low
frequencies are stepped down, shelved below 50 Hz
to -3dB at 20Hz.

KEF’s Jack Oclee-Brown on
using multiple KC62s
Noting Floyd Toole’s inclusionof multiple smaller subwoofer
arrangements to achieve
more uniform bass in theroom space in his book
Sound Reproduction, KEF
scientist Jack Oclee-Browncommented. ‘Martin, I’m glad
you picked up on the ideaof using these small subs
in “Welti/ Devantier/ Toole”
configurations.’
Jack continued,”Multiple
subwoofer configurations
were absolutely one of thereasons that we started
to consider doing a verysmall but genuinely highperformance subwoofer. I
think that the KC62 could
open the possibility ofmultiple configurations:or even the improvement
offered by two as stereo
subwoofers, particularly forpeople who couldn’t fit intypically sized subwoofers”.
REF: Low-Frequency
Optimization Using Multiple
Subwoofers: Todd Welti,
Allan Devantier, published2006: Journal of The AudioEngineering Society)

The KEF preset equalisations

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REVIEW‘Corner’: when placed in a rear corner behind theseating, or, for a diagonal TV arrangement placed in
the front corner behind the AV set up. Here the bass
shelves down 6dB by 20Hz
‘Cabinet’: similar to Room but where this small
is sub located on the A/V shelf unit. Now its output
is cut 9dB by 20Hz in view of the nominal 3dB gain
expected per additional boundary‘Apartment’: here the sub ‘impact’ is intendedly
moderated because low frequency sound can travel
through walls and be heard in adjacent rooms.
Now the -6dB point is set a bit higher at 30Hz, while
the lower frequencies are no longer shelved, but
continue to roll off more steeply to -24 dB by 10Hz.
LFE, crossover and level controls
Further controls include selection of LFE mode forhome theatre (Low Frequency Effects: e.g. for a mono
feed from a receiver, of summed and conditioned
bass signals for movie set-ups). This line level signal
will be preconditioned to Dolby specifications andneeds no further frequency tailoring.
Then a calibrated rotary control sets thefrequency range of the sub , which dial ranges
upwards from a nominal 20Hz. You can set theworking upper limit continuously from 40Hz up
to 140Hz, the scale calibrated at nominal 20 Hzintervals. Finally this upper row is completed by
the volume/level control for the sub and somecalibrations would have been helpful here forsetting up and for revisiting settings.
The lower row comprises the three pin IEC 13A
socket for power, a rocker switch for power ‘on’, asmall two-channel wire clip input for speaker level
feed, in conjunction with a matching plug – which
if used needs care so as not to leave bare wires toavoid shorting the supplying stereo power amplifier.
Now, on the home straight at last, we have four
phono sockets, the first two being line-level inputs
for left and right channels, and the second pair the‘high pass filtered’ line out, left and right, for the

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stereo amplifier feeding the main stereo speakers.
Here the lowest frequency signals are filteredout while the stereo speakers now become the‘satellites‘. With the bass workload taken from thesatellites there will be a consequential improvementin power handling, distortion and maximum soundlevel. In the case of pairing with the LS50 WII, thelatter has its own sophisticated electronic controls
for the high-pass filters which aid best matching
to the sub, potentially raising the combined
performance to a new level.
A subwoofer installation needs planning forthe best results: first for the sub or multiple sub
location(s), and to facilitate the cable routing,noting that there are several ways of installing a
versatile sub such as this one in a given system.
Several trials may be required.
The stereo loudspeaker system, if active as in
the case of the LS50W pairing, may already havematching settings for using a subwoofer so thatno further cable links or settings are required todefine low and high pass matching. For placement,
this sub has a resilient, bonded anti-vibration padon which it sits, so carpet, hard floors or shelves
are suitable, and of course the non-directionalradiation of low frequency bass means the sub’s
physical orientation is essentially uncritical.
Technology
The aluminium alloy casing is hard anodised, ‘silver’
for our sample, or alternatively available in ‘carbon
black’, and machined from a sturdy curvilinear
aluminium extrusion, making it almost impervious
to flexure or resonance. Yes, you can hide it away,
but the tidy industrial design allows it to be placedin plain sight if desired.
Despite the compact dimensions useful deep
bass is available thanks to four design features,
here beginning with the double cone, backto-back unitary bass driver assembly, which isreaction-force-cancelled, and built on a massive
die-cast alloy frame. This Uni-Core™ technology ispatent-pending and employs almost concentriccoaxial motor coils, folded into a high flux double
element ceramic magnet system, this reduction indrive unit size allowing the enclosure volume to be
reduced by a third.
The subject of a further patent application, thebass drivers’ novel ‘P-Flex’ surround has a complex
geometry with two benefits: a longer throwcompared with the usual ‘half-roll’, and better
behaviour in the face of the exceptionally highback pressure which occurs within this enclosure.
This is achieved with a unique pleated design
inspired by origami paper folding techniques,
and even that’s not the end of the proprietary
technology: yet another patent application in

REVIEW

progress covers the constant measurement of theKC62 drivers’ voice coil currents, facilitating thecorrection of non-linear distortions by computed
comparison with the ‘perfect’ drive signal. This formof motional feedback substantially reduces the total
harmonic distortion (THD) at higher sound levels
and lowers the colouration which results from such
distortion into the bargain.
Mapping the driver limits
Further signal processing is present in the form ofdynamic performance mapping of driver limits witha subtle level control, providing momentary soft
limiting of maximum bass power over frequency.
This prevents excess driver excursion, thus generally
avoiding audible overload events.
The final piece of this size/performance puzzle
is the amplification: so how much power do you
need to get a quart of bass out of a pint pot? Herethe KC62 has power in abundance, the drivers eachenjoying a rated short term 500W RMS of customdesigned high-efficiency Class D power.
Sound quality
The KC62’s colouration and distortion levels, within
the sensible working dynamic range, were so
low that I could try it out in number of situations.
Perhaps the most improbable was with my massive
Magico S5 II floor standers. These are closed box,
all-aluminium cased and costly powerhouses with
twin 10inch, 250mm bass drivers a side, so I might
have might have expected that, at just a fifteenth
the size of one Magico, the KC62 would have almost
no perceptible effect.
It did: applied in small doses it added definitionand clarity, better tune playing and a useful‘direct coupled’ percussive crispness. At such low
frequencies sound power from several sources
sums well in a room, and until I set the volume
near maximum for this powerful audio system as awhole, it was surprising that the KEF kept pace.
Given that, you might speculate that two or
more KC62s, well located, would be even better atsmoothing the usual room dips at low frequencies.
It seemed that it was not that the bass definition
was better than the Magico alone but rather
that this precision sub was able to contributeconstructively to the room drive party. For realistic
home theatre effects in such a large room you
would need several KC62, and logically would
then purchase much larger subs. But for precisionstereo replay of high-quality music feed this was aninteresting and musically rewarding experiment inan otherwise incompatible high-end system.		
Used in a more typical set up, with the KEF LS50W
II, the connectivity was straightforward. Trying thereliable wired connection – the accessory wirelessadaptor wasn’t supplied – the sub-augmented
LS50W II took on the scale and bandwidth of amuch larger loudspeaker design.
Considering the relatively small bass-mid
driver of the LS50II W, which is neverthelesssurprisingly effective, diverting the lowest
frequencies to the sub frees up the stereo pair tosound faster, more dynamic and more expressive,
belying their compact visual footprint. This was
powerful sound on a larger scale, combined withhigh quality upbeat bass: there is a particularquality to a bass driver when direct coupled
to a power amplifier, as in this case, and it
unquestionably adds value.
Conclusion
Here KEF engineers have mastered the subtle
balancing of features, engineering and soundquality to deliver substantially clean extended
bass from an improbably compact, versatile andattractive high tech enclosure. The KC62 subwoofer
is a Best Buy.
Lab report
Because bass is omnidirectional at such low
frequencies the measurement mic can be close,essentially avoiding the influence of room
reflections on the measurements. There was ample
input sensitivity for almost any set up.
At moderate loudness nearfield measurement
resulted in the following set of responses with level,
showing the nominal ‘11Hz’ limit, seen here as -10dB
relative to the passband level at 50 Hz, but still aremarkable bandwidth. With the level raised 20dB,
there is the beginning of curtailment at 10Hz, now
reading -14 dB at 11 Hz, as the operational safe limit
envelope is in control
Distortion was surprisingly low regardless ofthe size. Flat out at 30 Hz it was clean subjectively
with a fine -27dB of 2nd and 3rd with 4th at-39dB.
Further increase engages the auto limiter. The
level was refenced to 96dB, 0.5m floor plane. It willplay 11Hz, essentially silent pressure waves only
revealed by a gentle hum from the faintly audible
5th harmonic at 55Hz! It can be driven to full
excursion limit at 11Hz, so I regard the seemingly
impossible claim as largely met.
Flat out at 11Hz, second and third harmonics
were fine at about -22dB, and by 20 Hz – here atfull power – it was coming on song with negligible
distortion, audibly silent 2nd –28dB, 3rd -27 dB 4th-40dB etc. And at 100Hz the were no worries about
distortion encroaching on the bass lines from thestereo pair, 2nd was just 0.35% , third 0.28%. Clearly
the advanced distortion control mechanisms are
effective, resulting in one of the lowest distortion
subwoofers anywhere near this size and price.

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HIFICRITIC Review
Specifications
KEF KC62 sub-woofer
Design Uni-Core force cancelled		 combination drive unit with		
2 x 140mm alloy piston		radiators____________________________
Frequency response		
11Hz – 200Hz (±3dB)
		nominally -3dB at 11Hz____________________________Max output 105dB/m (free field,		
mid band)____________________________
Amplifier type Class D Amplifier,with active driver performanceenvelope control and feedback____________________________
Power amplifiers 500W RMS per		bass driver, built in____________________________
Variable low pass filter		 40Hz – 140Hz, plus pre-set
		room EQ curves____________________________
Inputs
LFE Input, RCA phono		sockets, Speaker Level inputs____________________________
Line output RCA phono sockets,		
also high pass filter____________________________High pass filter
40Hz – 120Hz,		
plus bypass____________________________
Power requirements
universal
		 100-240 V ~ 50/60 Hz 13A IEC____________________________
Power consumption 1000W max,		
typical 5-10W____________________________
Weight14kg(30.9 lbs)____________________________
Dimensions (H x W x D) with rear
panel and feet:		
24.6 x 25.6 x 24.8 cm
		(9.68 x 10.07 x 9.76 inches)____________________________
ColourAnodised alloy,
		silver or black satin finish____________________________
Wireless connection interface
Optional KW1 kit£180____________________________
Price
£1399
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