Sonus Faber Extrema loudspeaker Page 2
A key element in the Extrema design is the use of one of the world's best tweeters, Dynaudio's 28mm soft-dome Esotar T330. Costly though it may be, this unit offers not only an unrivaled combination of bandwidth and power handling, but also low-frequency characteristics considered ideal for this type of speaker design. With the system crossover set at a relatively low 2kHz (most systems in this class have a crossover in the 3-4kHz range, up to an octave higher), this high-frequency unit's low fundamental resonance of 500Hz and high damping (Q=0.2) are considerable advantages. This assembly has a shaped, vented center pole and a large, damped acoustic chamber behind the dome. Fitted with a massive ring magnet, the metal faceplate allows firm fixing, while the high-density coil uses a dense "Hexacoil" wind.
"Something special" is a good description for the unusual bass/mid driver, including an exceptionally large "Hexacoil" motor coil for a nominal 220mm framed driver of 145mm active diameter. This 75mm-diameter coil has a medium-term thermal rating of around 200W, but will sustain 2kW in doses of 10ms duration. Most 190mm bass/mid drivers have a much smaller 25 or 33mm coil, and only a 50W rating.
Virtually hand-built to Sonus Faber's specification by Skaanings Audio Technology in Denmark, the modular construction comprises the cone and motor assembly, a large frontplate, and a system of bolts to secure the magnet and pole system. For the Extrema, the almost rigid cone is made of magnesium oxide-loaded polypropylene reinforced at the center by a concave polypropylene cap. The long-throw cone's textured coating of a heavy carbon-loaded vinyl acrylate provides further resonance-mode control. The front suspension is a tough, partially foamed Neoprene of good mechanical "Q" to help preserve low-frequency dynamics. The magnet is an "SD" type, with a high-conductivity pole cap to short-circuit eddy currents and thus reduce inductance and inductance-modulation effects, including third-harmonic distortion. Such techniques are only viable where the system designer is allowed significant budget for drive-unit R&D.
Moving down into the low-frequency range, the Extrema is a 20-liter bass-reflex design with a difference. Belonging to the ABR, or "auxiliary bass radiator" class, the moving plug of air in a conventional reflex port or duct has been replaced by a moving solid diaphragm, in this case the extra-solid polystyrene-wedge cone of the trusty KEF B139 bass unit (footnote 2). This has been seen before in ABR service, for example in many of KEF's own systems and the Ensemble series from Switzerland. If the ABR has good excursion capability, which the B139 does, as well as a useful radiating area (equivalent to a 10"/240mm driver), then good levels of low-distortion bass are achievable without the "chuffing" or "wind" noises heard from a small port. In addition, the acoustic opacity of the wedge diaphragm ideally will block the egress of the enclosure's potentially harmful acoustic standing-wave modes.
The Extrema adds a neat twist to the tale: here the B139 is not just a diaphragm but the complete bass driver complete with motor coil and magnet. While careful schemes could be devised for bass damping using acoustically resistive apertures, Sonus Faber has exploited electromagnetic damping. They place a desired resistor across the auxiliary woofer's motor coil; damping energy appears in the moving-coil and is dissipated in the resistor.
Sonus Faber has gone even farther in their exploitation of this principle, providing user-variable bass damping in the ABR range, effective over an octave centered on 50Hz. No less than five switch-selected positions are available. This is only too easy to accomplish once the working principle has been adopted. The step resolution is fair enough at 1.5dB once the ABR output is integrated into the main response. A mass-loading cylinder has been applied to the B139's diaphragm to lower the resonant frequency.
The substantially built Extrema rivals the Wilson WATT in the quality of its construction, weighing 40kg (nearly 90 lbs each). The black satin central section of the enclosure is built of a patented sandwich construction—seven vertical slabs in all—bolted together to form a highly rigid, nonresonant structure. The outermost slabs are made of solid walnut 1.25" thick, beautifully finished and lock-mitered in a series of vertical steps to produce a warp-free assembly.
The front of the enclosure is heavily sculpted and beveled, both for reasons of style and, more important, to reduce diffraction effects. Asymmetrical enclosure boundaries help, as do curved surfaces, while low diffraction considerably helps stereo focus. The sides are parallel, but front and back panels are strongly sloped; this, together with the tilted top surface, strongly dilutes standing-wave effects inside the enclosure. The tilted front corrects time alignment between the two drivers.
Sonus Faber provides a utilitarian, 55cm-high, all-steel speaker stand with a matte crackle finish in black enamel, and thick top and bottom plates. The top plate, rather small given the Extrema's footprint, suggests no coupling technologies bar the obvious use of mastic pads at the corners. The larger bottom plate is threaded to take floor spikes, in this instance terminated with small, radiused balls. I used my own sharp spikes to ensure a good key to the floor through my carpet. The stand has six large rectangular pillars, vertically oriented, and comes ready-filled with lead shot and sand. The stands, extremely heavy and super-rigid, are devoid of audible resonance.
I also tried the Extremas on a pair of Acoustic Energy AE-2 stands, which did not look as good. Furthermore, the height was too great for the time-aligned mid/treble axis to be presented to a seated listener's ear level. With a speaker of this caliber, optimal stands are crucial; the Extrema stands worked well.
Extremas are burned in for 60 hours at the factory before shipping; mine sounded fine straight out of the box.
Hearing is believing. From the beginning, the Sonus Faber sounded very good, proving relatively uncritical of the matching equipment, its location, or listener position. That's not to say such matters were unimportant, but simply that the speaker's design is fundamentally correct: its balanced authority dominated the proceedings from the start.
Footnote 2: KEF ceased B139 production this Spring. Sonus Faber Extremas manufactured from June '92 use the very similar bi-radial 3021GT03 unit from TDL.—John Atkinson