KLS 9

THE DESIGN

To achieve reasonable power handling as well as deep bass with real punch, an 8in. cone is called for. To maintain quality above 2kHz, I chose a High Definition Aerogel (HDA) driver from Audax, the HM210ZO. HDA is a composite material comprising Kevlar and Carbon fibres, which together form an extremely light yet rigid cone with optimised internal damping. The tweeter I chose was the tried and tested Audax TWO25MO. This fabric-dome device reaches down far enough to meet the mid/bass unit at around 3kHz and has sufficient sensitivity to make a good match. With the HM210ZO, on paper a box volume of 2.6cu. ft. (0.075 cubic metres) produces bass down to 36Hz (-3dB), which is low. You will find our cabinet is, in fact, 0.072 cubic metres, but lining the cabinet internally with sound absorbent makes the cabinet appear bigger as far as the drive unit is concerned. The cabinet was proportioned so that a point half way between the mid/bass and the tweeter would be 2ft. 6in. from the floor - ear-height in a normal seat. The front was deliberately kept narrow for best imaging. The reflex port itself is not too critical, although to keep distortion down, it must be made fairly large. In terms of positioning, it should be placed close to the floor on the rear baffle. Ideally the base should have floor spikes fitted. The cabinet side panels would benefit from bracing with 1in. dowel too, since this speaker produces a lot of bass energy.

THE CROSSOVER

KLS9 turned out to require only a minimal crossover, which makes it even easier to build and less expensive. I found a first-order (-6dB/octave) low-pass filter was sufficient, as it should be with a well-engineered driver. The high-pass section for the tweeter was calculated to make it flat in response terms and a close phase match with the mid/bass unit.
And the final result? Frequency response was as I prefer it, with a slow downward trend at high frequencies, no crossover suck-out around 3kHz at all, and no phase errors in this region to put in an off-axis dip. Keeping the midrange flat like this ensures good detailing and vocal projection. The crossover is so simple it can be soldered directly onto the input terminal panel. We used Solen audio-grade polypropylene capacitors along with a ferrite-cored inductor for the lowest DCR.