KEF KM-1

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)