KEF KM-1
The Ultimate
listening Experience
KEF's KM-1
is a remarkable loudspeakers. Indeed, KM-1 could fairly lay claim to being the
Ultimate Loudspeakers.
To see KM-1
for the first time is midly awe-inspiring. To hart it is to be immediatelly
struck by a sense of sureness, ease and authority in the reproduction of music,
and indeed by how quiet they sound, despite the system's enormous loudness
capability. For KM-1 is an exercise in low distortion and headroom KM-1 can
'cruise' when playing at very high volume levels, with power to spare, and yet
achieve a degree of dynamic resolution unknow to virtually any other
conventional loudspeaker system.
Dynamic
Resolution
itis not
sufficient a loudspeaker to be capable of handling the full dynamic range
between loud and soft, it must be able also to resolve fine, low level musical
detail whilst simultaneously reproducing complex loud pasages of music. It can
only achieve this if the system has extremely low levels of colouration. This
was a crucial element in the KM-1 design rief.
Originally
developed for the British Broadcasting Corporation as a high-level pop studio
monitoring loudspeaker, they have been in use at the BBC's Maida Vale music
studios in London since the spring of 1982 with extremely satisfactory results.
In the
development of professional monitoring loudspeakers the demand for higher and
higher sond pressure levels has tended to obscure the pursuit of sound quality.
There are
many loudspeakers available which produce accurate smooth sound but which are
too fragile to be used at the very high continuos levels required in recording
studios. On the other hand there are numerous loudspeakers capable of going
very loud, but which have irregular frequency response, poor transient
behaviour, considerable colouration and severe non-linear distortion.
KM-1 is
therefore ideally suited to satisfy the professional recording engineer or the
disceming audiophile with a large musicroom, in search of a system without
compromise. KM-1 will satisfy the professional musican, perhaps with recording
facilities in his home, who wishes to combine the finest possible domestic
sound reproduction with his creative activities. KM-1 also makes an ideal
auditorium loudspeaker for small to medium-sized halls and theatres.
Immutable
physical laws - elegant enginering solutions.
KEF
engineers have merely accepted the basic limitations of dynamic loudspeaker
engineering - and then applied a variety of ingenious engineering techniques to
push them harder thananyone has managed hitherto. The target for KM-1 was 123
dB spl, which compares with the 112 dB maximum of Model 104/2 and 108 dB max
for Model 105.2. Considering that 6 dB represents a doubling of sound pressure
and quadrupling of sound power, and that 10 dB involes ten times the sound
power, and task was by no means easy. To achieve these design parameters, it
was decided to use active driv, whereby each drive unit is powered by its own
amplifier.
Multiple
bass units can be used without prejudicing stereo performance, so four 300 mm
units, similar to those used in Model 105.2 are each driven by a 100W
amplifier, each operating into its own separate enclosure. The midrange
presents unique and conflicting problems. These arise from the needto achieve
high output lewels with low coloeuation, whilst at the same time maintaining
good dispersion which necessitates the use of a small drive unit.
The KM-1
solution is to use two units a vertical array to create good stereo imaging,
and then to drive each unit with a 350 watt amplifier to achieve the required
levels.
B110 units
with specially modified polipropylene diaphragms cope easily with the high
g-forces involved. Fluid cooling conducts heat from the coils to the magnet
structures and thence via 64 mm diameter metal bars to a massive heatsink
covering the entire rear face of the mid and high-frequency enclosure.
A single
tweeter is necessary to obtain the best stereo performance so a very special
version of the T52 was developed. Efficiency is quadrupled by a massive 21,000
gauss magnet system.
Cooling of
any high output system should be both efficient and quiet. Cooling by fan can
be distracting, switching in as it often does, in a quiet passage following a
period of high-level running. KM-1's amplifiers and midrange heat sinks are
cooled quietly and efficiently by air covenction in a centrally situated
'chimney' arrangement. The emphasis on cooling arrangements in KM-1's mid-and
high-frequenc section is essential. It ensures that the three units voice coils
run cool, preventing compression of high level signals.
In KM-1 the
use of ferrofluids, coupled with the elaborate heat dissipation arrangements,
mean that thermal compression effects are reduced to an insignficant level -
less than 0,5 dB over the whole frequency range, even at very high continuous
levels of playback.
KM-1 is
fitted with a comprehensive but entirely practical protection system. This
system gets much closer to the heart of this problem area than any hitherto
devised. Real time information on the voice coil and heat sink temperatures is
provided by onboard analogue computers whilst the system is playing. Whwn these
temperatures approach predeterminated thresholds the playback levels is reduced
until conditions return to normal.
The
construction of the system is modular. A metal frame centre section carries
MF/HF enclosure together with the amplifier and power supply. The two bass
'wings' bolt onto the sides of the frame thus forming the central chimney. The
MF/HF and amplifier sections are mounted in two filing cabine style slides
which may be pulled forward for easy servicing
KM-1's
elegant design and immaculata finish allow it to blend equally well into a
high-tech studio environment or a home music room. The swept back frontal
aspect permits stereo imaging of astonishing accuracy.
Specifications
Frequency
Response: 38 Hz - 23 kHz ±2 dB (-6 dB at 34 Hz and 25 kHz)
Directional
Characteristics: Within 1.5 dB of
reference axis response for ±7.5º vertically, to 20 kHz
and for ±15º horizontally, to 8 kHz
Maximum
Output: At least 120 dB each third octave from 60 Hz - 20 kHz free field from
one system at 1metre, or 2 systems at 2 metres.
Thermal
Volume Compression: Typically less than
0.5 dB at any frequency and at any level up to maximum output under normal
programme conditions.
Distortion:
Second and third harmonic less than 3.2% at 110 dB, less than 1% at 90 dB from
60Hz to 20kHz measured at 1m in anechoic conditions.
Input
Impedance: 22 k Ω - floating
Input
Sensitivity: Full output for nominal 0
dBm input (0.774V) adjustable in 3dB steps from - 21dBm to + 12dBm.
Signal-to-Noise
Ratio: 105 dB
Driver
complement:
LF 4 x 30 mm
Bextrene direct radiator,
MF 2 x 110
mm polypropylene direct radiator,
HF 1 x 50 mm
Mylar dome radiator.
Dimensions
(H x W x D): 763 x 1315 x 665 mm; (30 x 51¾ x 26 1/8
in.)
Weight:
140kg (308 lbs)
