KEF Blade loudspeaker review

One of Britain’s oldest loudspeaker companies, KEF, founded back in 1961 by Raymond Cook, celebrated its 50th anniversary with the release of the unique Blade loudspeakers, which were the result of a full five years of research and experimentation.

The heirs to the Cook legacy know the law: to succeed you have to keep evolving, trying new things, finding untrodden paths. It’s always been that way with KEF. For example, the completion of the Austin project, which the company worked on in the early 2000s, first led to an update of the reference lineup of KEF, then some of Austin technologies were passed on to junior ranges, improving the entire lineup of the company. This was followed by the Muon project, which introduced new technical achievements of KEF. As long as this development cycle continues, the company will move forward the Blade project is a new challenge along the way. It’s been three years of experimenting with new technologies and design ideas, only to find out for yourself at the 2009 Munich High-End Show. After enjoying the furore at the show, KEF engineers spent the next two years perfecting the Blade concepts.

Design Features

Key idea in the new concept – Single Apparent Source, i.e. the technology of imaginary point source of sound. KEF has been very active in this area before. The same proprietary Uni-Q module, which combines on one axis tweeter and midrange driver, creates a seemingly point source of sound in the MF/HF range. But the British went even further in Blade, combining all ranges of the sound system in one point, thereby automatically avoiding the phase matching of speakers at the level of separation filters, which require complicated circuits, use of expensive components and in any case not always work correctly in the end. In Blade, coherence is achieved in the most direct way possible: the Uni-Q front unit is flanked by two pairs of woofers, symmetrically placed at the side of the speaker above and below the coaxial unit. The bass drivers in each pair integrate into a single unit, thanks to bonded magnets and three metal rods running the width of the cabinet to connect opposing driver cones. This not only adds stiffness to the Blade cabinet but when the bass drivers are phased together they cancel out each other’s influence on the cabinet and eliminate the accompanying vibrations and parasitic sound from the side panels Force Cancelling technology . Bass quality is further enhanced by the use of isolated chambers for each pair of bass drivers, with acoustic inverters with ports on the back of the cabinet.

The Blade’s four bass drivers come with 225mm polymer cones held in place by concave semi-circular surrounds. Huge magnets and oversized voice coils drive the speakers without distortion, at any power handling capability, which incidentally maxes out at 400W RMS . Of course, operation in such modes involves high heat dissipation and air compression from the back of the cones. Given the paired positioning of the bass drivers in the cabinet and the use of Force Cancelling technology, the usual path of ducting through the magnet system is closed. The designers got out of the situation by equipping the bass drivers with venting holes, ingeniously placed in the gap between the voice coil and the diaphragm. Another interesting feature of the Blade bass drivers is the viscous-elastic connection between the voice coil and the cone. So they built a mechanical filter to filter out high frequencies that would reach the resonance value of the diaphragm, causing audio distortion. Sure, you could have used a higher-order electrical filter for that purpose, but, as we’ve already mentioned, the Blade designers intentionally avoided that. The Single Apparent Source concept itself involves using the simplest possible crossover to optimise phase coherence across the driver array. Separation filter circuits utilize hand-assembled select elements on the board using hinged mounting. The Blade’s wiring panel features audiophile-quality WBT terminals, the number of which allows Bi-Wiring/Bi-Amping connections.

The Uni-Q module, now in its 10th generation, uses a 25mm tweeter equipped with a rear damping chamber and a double titanium diaphragm composed of nested spherical and elliptical domes. The unique diaphragm construction greatly increases the ruggedness and resonance resistance of the diaphragm, especially on the edges where the weak point of the single dome is located. The tweeter is built into the proprietary Tangerine acoustic lens, featuring an ingenious multi-blade design that simultaneously increases sensitivity and smoothness of response in the upper octaves. Extended the tweeter’s working range up to 35kHz +/-3dB and 45kHz at +/-6dB by doing away with the need for a hypertweeter.

To optimize the Uni-Q’s midrange coaxial driver, British engineers developed a new hybrid membrane and ran rigorous computer simulations of the mechanics of the speaker using finite and boundary element methods. The 125mm diameter diaphragm, made from an alloy of lithium, magnesium and aluminum, is reinforced on the inside by a Liquid Crystal Polymer skeleton, an extremely high stiffness to density ratio with superior damping properties that noticeably improves the quality of the metal diaphragm with very little added weight. In addition, in order to move the resonances even further into the high frequencies, the midrange diaphragm profile has been given a special notch and the voice coil is fixed to the cone at the node point corresponding to the first resonance frequency, smoothing out its influence. All these measures have resulted in a dramatic widening of the frequency range, where the midrange works almost like a perfect piston. Distortion is further reduced by the optimised neodymium magnet system. KEF engineers admit they have never matched tweeter and midrange in a Uni-Q unit more perfectly, which is particularly important because the midrange driver cone is partially a horn to the tweeter dome, so any uncontrolled resonances of the former will immediately influence the sound of the latter. One of the reasons KEF rejected the use of a cone shaped cone midrange driver in the Austin project, replacing it with a curved diaphragm on an almost flat suspension, which in the Blade transitions to an oversized polymer overlay – an “acoustic lens” framing the Uni-Q. It also involved a lot of calculation and experimentation, but it did achieve an impressive level of off-axis tweeter response, better than a coaxial module plugged into the flat faceplate of a conventional speaker.

Design considerations

The distinctive visual identity of Blade was created by the designers headed by Erick Chan at ECCO Design in New York City. As a result the choice of a slender and airy silhouette of Blade was influenced by the series of works by modernist sculptor Brancisi, “Bird in Space”, in which the author expressed the idea of movement and flight in abstract terms. This futuristic image suited Blade perfectly, both in terms of visual effect and in terms of functionality and acoustic feasibility.

So it is no coincidence that the Blade cabinet looks like a modern art exhibit, with its flowing curves, parabolic façade profile and sleek metal stand at the base. The floor connection is made by means of spikes of adjustable height, for convenience the spikes are equipped with toothed adjusting wheels. The latter won’t hurt, because with the weight a little less than 60 kg the speakers will be hard to tilt back and forth. Good-looking forms are also acoustically very useful, as an enclosure without parallel surfaces produces no standing waves, and the curved facade minimises sound reflection from the baffle towards the listener. Rugged and acoustically inert cabinet made of a composite material with a corkwood core and woven carbon fiber outer layers. The shape and structure of the cabinet, additionally reinforced on the inside by numerous struts, make it exceptionally resistant to various vibrations. The final touch to the exterior of the Blade is an abundance of multi-colored finishes to suit every taste. In addition to the standard glossy black and the same white finish, the speakers are available in a choice of twelve additional finishes.

TECHNICAL SPECIFICATIONS

KEF Blade

Bassreflex Floor Type, Single Apparent Source

No. of Bands 3

Speakers mm HF 25

SCH 125

4×225 woofer

Recommended amp power W 50-400

Frequency range Hz 40-35k +/-3dB ,

28-45k +/-6dB

Crossover frequencies Hz 350/3k

No magnetic shielding

Sensitivity dB 90

Impedance ohms 4

Two-cable connection yes

HxHxD mm 1590x363x540

Weight kg 57.2

1 000 000 Dollars.