Stereo Amplifier

Review: KEF Reference 104 Speakers

Hence KEF have introduced the term 'Reference Series' to indicate a new line of loudspeaker enclosures capable of critical comparison with the original live sound, but which are not to be confused with genuine Monitor loudspeakers designed for broadcast and recording studio use. The Model 104 enclosure uses three units, each having a synthetic laminated diaphragm. The T27 HF radiator at the top left of the enclosure uses a one-inch Melinex moulded dome with an integral damped roll surround. It has an effective mass of only 350 milligrams and a fundamental resonance of 1,100Hz. The ceramic magnet is just under 3 inches in diameter and 1/2-inch thick. The front plate of the magnet system is extended to 4 inches approximately and acts as the mounting for the unit.

Below the HF radiator is the B200/SP1039 low frequency radiator which uses a newly developed visco-elastic damped Bextrene diaphragm, with a high temperature speech coil-magnet assembly. The outer roll surround is made of PVC and allows for a diaphragm excursion of approximately 1/4-inch. The ceramic magnet is sandwiched between heavy pole-pieces, the magnet being 5 inches in diameter and 3/4-inch thick, and is supported on a heavy, pressed steel, octagonal shaped chassis. With high power transistor amplifiers, temperature rise of the speech coil within the enclosed gap has been one of the reasons for unit breakdown. Considerable basic research was undertaken to reduce this contingency, for the temperature can rise to over 200°C when the speaker is used for highly modulated continuous passages, such as one meets in high level pop music. For example, a 60 watt amplifier will dissipate around 20 watts in the speech coil and, as it operates within a nearly sealed gap in the magnet system, heat is dissipated only via the mass of the magnet assembly. Normal speech coils are wound on paper or light card formers; the temperature rise will often distort the circular coil and either cause it to rub on the pole-pieces or burn out, or melt the leads soldered to the ends of the coil. To overcome this problem, the KEF unit uses a heat resistant voice coil which is joined to the diaphragm with an epoxy resin adhesive; the resultant assembly will withstand continuous operation at 180°C and short term overload up to at least 250°C.

The third unit is a tuned passive radiator measuring 13 x 9 inches and is similar in appearance to the KEF B139 bass radiator upon which the fortunes of the firm were originally founded. The difference between the B139 and the passive B139/SP1037 lies in the absence of a speech coil and magnet assembly. It has a similar fiat polystyrene diaphragm, faced with a metallic foil and using a roll surround. In place of the speech coil there is a metal ring, the mass of which is adjusted to augment bass radiation, increase efficiency and reduce harmonic distortion. The three units are mounted on the face of a 3/4-inch thick chipboard front baffle. Also on the baffle board is mounted a three-position rotary switch, named the acoustic contour control. This switch forms part of the crossover dividing network and has the effect of modifying the middle frequencies centred around 1.5 kHz by ±2 dB. The dividing network is constructed from three ferrite cored inductors and three reversible electrolytic capacitors mounted on a printed circuit board. This six-element network divides at 3,000 Hz and has a roll-off slope of 18 dB/octave on both sections. The specification indicates that, even at 50 watts, third harmonic distortion due to the dividing network does not reach 0.2 %.

The cabinet has a considerable effect on the acoustic performance of a speaker: complete rigidity and freedom from resonance are the main objectives. In the Model 104 cabinet, which has alternative external finishes in teak, walnut or white, the main structure is made from dense chipboard, veneered on both sides to prevent warping, and the internal surfaces are lined with heavy bituminous anti-resonant panels. A rigid cross bracing is located behind the LF radiator and, with the unit in position, the rear of the magnet contacts the bracing and stiffens the rear cabinet panel. The three units are recessed into the front panel, and the routing of the recesses is a piece of precision wood engineering. Synthetic sealing rings between the units and the front panel ensure complete air-sealing. In fact the sealing is so precise that, if one pushes in the slave B139 diaphragm, it pushes the diaphragm of the LF radiator outwards and it takes several seconds for the diaphragm to return to its neutral position.

To reduce internal reflections, the cabinet is filled with panels of polyurethane foam. Located behind the rear panel is the crossover network, and a recessed plastic moulding for external connections. As there are no standards for speaker connections, KEF wisely include alternative 4mm banana plug and DIN polarised sockets.

A loudspeaker grille can affect the acoustic performance, and should ideally be 100% acoustically transparent. All the KEF drive units are finished in black, and a black microcellular sculptured grille is used. This grille is 1-inch thick at the edges and tapered to 3/4-inch over most of the area, except for a visually interesting vertical trough on the right side. It is easily removable to gain access to the acoustic contour control, and is held in position by a new form of the well known Velcro 'hook', which is a plastic moulding with thousands of miniature "T" stems.

Transient response was of a high order, with no suspicion of exaggerated bass response. One of the best tests is with male speech, and the chestiness so often noticeable in badly designed units and cabinets was completely absent. Musical reproduction was very satisfying, being forward in tone and with complete integration of the stereo image. Due to the wide azimuthal coverage, it was not found necessary to sit at the precise centre between the enclosures to enjoy the best stereo reproduction. Bass response certainly benefits from the use of the passive radiator and was well maintained down to 50Hz. Although the signal gently rolls off below this frequency, there was still an audible output at 30Hz. Considering the modest dimensions of the enclosure, the KEF Model 104 has a well extended, uncoloured, low frequency performance.

Summing up, the Reference Series Model 104 can be regarded as an excellent speaker for domestic purposes. It is capable of handling volume levels far beyond that required domestically, and it is guaranteed for five years to the original purchaser against failure due to faulty manufacture.