Founded in 1961, British speaker maker KEF pioneered concentric drivers with its Uni-Q technology in 1988. Since then, Uni-Q has been a hallmark of KEF speakers even to this day.
Uni-Core
Now, KEF has introduced another new technology, this time focused on subwoofers. Called Uni-Core, it places two drivers in a dual-opposing configuration—that is, mounted on opposite sides of a cabinet. They vibrate in phase with each other, moving inward and outward in unison.
By itself, that’s not new; many subwoofers use dual-opposing drivers—including the Sonos Sub—because the force imparted to the cabinet from each one cancels out the force from the other one. This greatly reduces cabinet resonance, which can distort the sound, and prevents smaller subs from “dancing” across the floor.
What makes Uni-Core unique are the voice coils of the dual-opposing drivers. One voice coil is smaller than the other and nestled within the larger one (see Fig. 1). The electrical current in each voice coil—the audio signal—generates an oscillating magnetic field that interacts with the field from a single, permanent neodymium magnet, causing the voice coils to vibrate in concert with the audio signal. Each voice coil is attached to a diaphragm, which vibrates accordingly.
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As you can see in this cutaway, the voice coil for the driver on the left is smaller than the voice coil for the right driver. As they vibrate, they actually overlap, which reduces the space they need without decreasing their excursion. Both interact with the same permanent magnet.
You might wonder, as I did, about the difference in size of the two voice coils and how that affects the magnetic fields they generate. Since they both interact with the same permanent magnet, their oscillating fields should be identical. According to KEF, that difference is electrically compensated for so that both present as electromagnetically equal.
The goal of Uni-Core is to enable high-quality performance while significantly reducing the size of the cabinet to make the subwoofer more aesthetically pleasing. With nested, overlapping voice coils, the drivers have much more excursion than a similar-sized dual-opposed, force-cancelling design, allowing much more output and low-frequency response from a much smaller cabinet.
The KEF KC62 subwoofer
KEFF
The KEF KC62 subwoofer is the first speaker to feature KEF’s Uni-Core technology.
KEF’s new KC62 compact subwoofer is the first product to implement Uni-Core. The sealed cabinet is roughly cubical measuring about 10 inches on a side, and the drivers are only 6.5 inches in diameter. Such a small subwoofer would normally poop out well above 20Hz, but the frequency response of the KC62 is specified to extend from 11Hz (!) to 200Hz (±3 dB). Of course, no one can hear 11Hz, but an extension that low does provide “footroom” so that the lowest audible frequencies do not overly tax the sub’s capabilities.
Another new technology used in the KC62 is what KEF calls a P-Flex Surround (aka Origami Surround). The surround, a flexible material that attaches the driver to the frame, employs a unique pleated design inspired by origami, the Japanese art of paper folding. This design resists acoustic pressure in the cabinet better than a conventional surround without adding mass and limiting sensitivity, allowing the driver to move more precisely, which is said to result in deeper extension and more accurate, detailed bass.
To reduce total harmonic distortion (THD), KEF has included its Smart Distortion Control Technology in the KC62. Many subwoofers use an electromechanical sensor to monitor the driver’s position relative to the waveform of the input signal. When non-linear discrepancies occur, which is common at high excursion, the driver’s motion is limited to reduce distortion. By contrast, Uni-Core’s low voice-coil inductance allows the KC62 to use an analog current sensor to directly and instantaneously measure the current in the voice coil, which is much more accurate than an electromechanical sensor, thereby reducing THD up to 75 percent.
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The 6.5-inch drivers on each side are inverted domes.
Additional fine tuning is provided by KEF’s Music Integrity Engine, a set of custom DSP (digital signal processing) algorithms. For example, iBX (Intelligent Bass Extension) uses digital EQ to extend the low range, while SmartLimiter constantly analyzes the signal to prevent clipping.
Then there’s the amplification—in this case, two specially designed Class D amps, each one pumping out 500 watts RMS to its corresponding driver. These amps are said to provide excellent control along with the ability to deliver sudden bursts of power when needed. In fact, the KC62’s maximum acoustic output is specified to be a whopping 105dB SPL.
Placement within a room is super flexible thanks to the cabinet’s sealed design and five Room Placement Equalization presets, which you select on the rear control panel. These presets adjust the EQ according to where you place the sub; options include away from the walls, up against a wall, in a corner, in a cabinet, and in a small apartment.
Speaking of the rear panel, the KC62 offers a nice variety of connections, including line-level inputs and outputs, and speaker-level inputs on a Phoenix connector, which is common in Europe. One of the line-level inputs is labeled “LFE,” which you would connect to the LFE (low-frequency effects) output from your AV receiver or preamp-processor using its internal low-pass crossover. The other line input is labeled Smart Connect, which automatically adjusts the gain of a non-LFE input.
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The rear panel holds all connections and controls, providing for flexible setup.
As with most subwoofers, the KC62 can accept full-range signals, and its Crossover dial sets the upper limit of the frequencies that are sent to the sub. Another control lets you specify the lower limit of the high-pass filter, which sends the mid and high frequencies in the input signal to the line-level outputs and on to powered satellite speakers or other devices. The HPF control consists of four DIP switches that would normally be used only by an installer; for most consumer installations, you would just leave those switches in their default positions.
Also on the rear panel is a connector labeled “Exp.” This is an expansion port that lets you attach the receiver module of the KEF KW1 Wireless Subwoofer Adapter Kit. Connect the transmitter to the output of your A/V receiver or pre-pro to establish a wireless connection to the sub.
Beauty as well as brawn
Finally, the KC62 offers a pleasing aesthetic. The curved cabinet is crafted from extruded aluminum, and it’s available in Carbon Black or Mineral White, allowing it to blend in with just about any décor. Plus, its diminutive size lets it fit just about anywhere.
With beauty, brains, and brawn in a surprisingly small package, the KC62 is sure to shake things up in the subwoofer world—and in your home—thanks to Uni-Core technology and KEF’s exceptional engineering. It is available now for a list price of $1,500. We can’t wait to take it for a spin!