KLS9 loudspeaker - page 2

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Analysis of the HM210ZO's frequency response by our B&K measuring microphone and Hewlett Packard 3561A FFT spectrum analyser showed it rolls off above 3kHz and, as expected from an optimally damped and consistent synthetic material, doesn't break up badly at high frequencies. It  looked suitable for a slow-ish roll off Butterworth (or thereabouts, damping wise) second-order filter response, but in the end I found a first-order (-6dB/octave) low pass filter was sufficient, as it should be with a good driver. The 1.2mH inductor has a ferrite core, which allows a low DCR of 0.65Ω to be achieved, maximising sensitivity and leaving driver/box Q unaffected.




The high pass section for the tweeter was engineered to make it flat in response terms and a good phase match with the bass/midrange unit. The first-order low pass filter combines with the HM210ZO in a complex fashion, because the drive unit itself rolls off above 3kHz. Also, the drive units are horizontally displaced by 30mm, which amounts to 90degrees of phase shift at 3kHz. Experiment showed that best phase matching was obtained with a first order filter feeding the tweeter, making the crossover network very simple. I usually expect to use second or third order networks for the tweeter, but the slow roll off of the HM210ZO allows the tweeter to be rolled in high up the audio band, keeping low frequency energy out, and with this arrangement the phase matching proved excellent.


First order filters don't provide much flexibility in the all-important response tweaking department but in this design it didn't matter. Frequency response was as I prefer it, with a slow downward trend, no crossover suckout or dip around 3kHz at all, and no phase errors in this region to put in an off-axis suckout or dip. Keeping the midrange flat ensures good detailing and vocal projection are achieved, without artificiality, like brightness.


Talking of which, I never allow a tweeter to peak. The TWO25MO response is tailored by the series high-pass capacitor so it exhibits a) no peaking and b) it rolls off slowly toward 20kHz. This ensures there's no sting in the treble. Another subjective disaster is to allow treble output to rise toward 20kHz; it gives painfully incisive treble.


In the end, KLS9 turned out to need a minimal crossover, much simpler than any of our earlier designs. That makes it even easier to build, of course, and less expensive too.


I expected fairly benign load characteristics as a result of this simplicity and was not surprised to see a gently undulating impedance curve, as our analysis shows. This means there is little reactance in the load, always a good thing, since energy storage is kept to a minimum. The curve sits high up, meaning overall impedance is high, because of the HM210ZO's intrinsic 6.3Ω DC resistance, which sets minimum impedance.


Overall impedance measured 12Ω, meaning the speaker draws little current, so it will not stress amplifiers. When impedance is high, sensitivity usually suffers. In this case, it turned out to be a respectable 88dB SPL (Sound Pressure Level) from one nominal watt of input (2.84V), so amplifiers of 30-80watts are suitable, according to room size.



The cabinets should be built from 18mm MDF according to our cutting plan. The most important parameter is cabinet volume, but it is not so crucial that dimensions must be exact. Glue the cabinet with Evode Resin W, or similar. It can be screwed as well, if wished - use coarse thread screws made for MDF. Note that we put in a bracing panel, with cutout in it, two-thirds of the way down. This is to brace the side panels to avoid boxiness.


The assembly sequence is -


1) Glue top, bottom and side panels together, then screw/pin on the front panel to hold them in shape whilst the glue dries. Do not fix the rear panel.


2) Glue and screw in the internal bracing panel.


3) Cut driver cutouts on front panel.


4) Glue damping pads of heavy, natural fibre carpet felt, onto every internal panel. An alternative is bituminous felt used to damp car panels.


5) Cut terminal panel and port cutouts on rear panel. Fix port into cutout and glue in place.


6) Screw and glue rear panel into place.


The cabinet can be accessed internally through the bass/midrange cutout at front and the terminal panel cutout at rear. Internal damping wool can be put in through the front opening.


7) Solder wires onto drive units and lead them out through the terminal panel cutout when screwing drivers into place.

* Note polarity - the narrow pin on both drive units is positive (+, marked red on woofer). Also, the tweeter has reversed phase, so connect +ve from the crossover to the tweeter's -ve (wide) terminal.


8) Solder crossover components onto terminal panel. Connect drivers to crossover then screw the terminal panel into place.


We placed the terminal panel right behind the bass/midrange unit so we could fix it using nuts and bolts, if need be. A 7mm recess had to be routed out for the chassis front face, leaving 11mm for screw thread. We were worried they might not hold after repeated use, but proved OK in practice.

kls9 internal-2


The cabinet was proportioned so that a point midway between the bass/midrange 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. Wide fronted loudspeakers image messily, so beware if you are thinking of altering this dimension. You will notice that the cabinets are 'handed' by offsetting the tweeter, so there are right and left hand speakers. This places the tweeter asymmetrically, dispersing phase and diffraction effects, and it reduces the path length to the cabinets inside face, pushing effects up-band. Bear this in mind when cutting the front panels, since they must be different. The tweeter sits closest to the inside panel of the speaker, when viewed from front as a stereo pair.



The inside view of the cabinet shows the bracing panel and damping pads. The cutout size is not critical, but should be big.




We used a router to inset the chassis of the bass/midrange unit, making for a flush baffle in order to minimise destructive reflections. The tweeter has been mounted over the edge of the bass/midrange chassis, to get it as close as possible; it is one wavelength away at 3kHz. I did not want this distance to be exceeded, since I rate good imaging very highly. For this reason, all the cabinet front edges were routed to have a smooth radius on them, to avoid edge-diffraction. This makes the cabinet difficult to veneer, so we applied Unibond as a sealant and then eggshell black paint.


The crossover is so simple it can be soldered directly onto the input terminal panel. Glue the crossover components on with a hot melt glue gun or, better, drill the panel and bolt on two 3-way tag strips. The capacitor, inductor and resistor can then be soldered on. Keep the inductor and capacitor apart and at mutual right angles to minimise inductive coupling ('cos a capacitor is a coil of foil with inductance).


We suggest you use Solen audio quality polypropylene capacitors. The inductor is ferrite cored for lowest DCR, measuring 0.65Ω. These properties must be replicated, or performance will change. The resistor will not get hot; make it 5W-9W. We used a high temperature wirewound, but really this resistor is best made up from 1W carbon film types, because of their neutral sound. The terminal panel we show is a bi-wire type and the 'speaker is bi-wirable of course.


Because the panel is positioned directly behind the bass/midrange unit, a pad of carpet felt should be placed over it to eliminate reflections from the rear of the bass/midrange driver.


The reflex port is not too critical. To keep distortion down, it must be made fairly large. Our port was made 8cms diameter and 4cms deep. You might like to try experimenting with these dimensions. The port is placed close to the floor on the rear baffle. Since it works low, around 35Hz, it is close enough to the main driver to avoid phase problems. Positioning is not too critical.


Ideally, the base should have floor spikes fitted. The cabinet side panels could well be braced further by a 1in. dowel or similar, because this 'speaker produces a lot of bass energy.


And finally, to the matter of internal damping. Regular readers will know we recommend long haired wool. It must be teased out and should be supported internally on threads to prevent settling over time. The bracing panel is useful for this purpose. Put an open gauze over it and wool on top, as well as in the bottom chamber. Don't over stuff the 'speaker or it will sound lifeless and the port will not work properly. Reflex cabinets must not be too heavily damped for this reason. The amount of damping used is a matter for experiment, being sufficient to suppress cabinet boom but not enough to create a lifeless sound.


Most people like to finish their cabinets with a good veneer. To retain bevelled edges, I suggest hardwood 1/4 rounds are routed into the edges, or stained softwood 1/4 rounds. Veneer up to these edgings. Bear in mind that all around the tweeter, up to 12in away, there should be no protuberances, especially ridges. The surface acoustic wave it produces must not be obstructed, nor should it be caused to diffract off sharp surface discontinuities. Attention to detail here, together with good drive units, narrow front baffle and accurate phase matching between drivers yields very sharp, clean edged non-ambivalent images, plus well embodied cymbals made of solid metal, rather than the sort of mellifluous representation that arises as a result of phase errors and anomalies.


The cost of these speakers is low relative to the specification. The Audax drive units total £150, wood around £30, terminals, wool, components, wire, ports, etc around £70 (for two speakers). That's £230 in all. For this you get a high technology loudspeaker with superbly even and well damped bass that goes very low. It also has a clean, clear mid-band and is easy to drive. Have fun!




KLS9 COMPONENT LIST (per loudspeaker)


Drive Units

Bass/midrange   Audax HM210ZO

Treble   Audax TWO25MO



L1 1.2mH, 0.65Ω, ferrite cored

C1 4µF, 50V wkg. min. Solen

R1 3.3Ω resistor, 4W min.


Input bi-wire input terminal panel

Component mounting 3-way tagstrips

Damping long haired wool

Panel damping carpet felt

Wood glue Evode Resin W

Panel damping Evostick

Wood 18mm MDF

Wire silver plated copper, single strand

Port 7cm-8cm diameter, 4cms long



Input bi-wire input terminal panel

Damping long haired wool (1 lb)

Wire silver plated copper, single strand


Port 2 x 65mm ID, 40mm long

Wire PTFE silver plated copper

Long haired wool 1lb




Output is strong down to 50Hz in our room. The response characteristic falls slowly and smoothly toward high frequencies, by about 3dB overall. This is deliberate, to ensure the 'speaker sounds smooth but full bodied in its rendition.




Overall impedance measured 12Ω, with a pink noise test using an rms reading meter. The twin peaks are typical of a reflex, where the port resonates out of phase against the box, at 36Hz here. Above 200Hz impedance rises smoothly, due to voice coil inductance, then levels out as the tweeter comes in. KLS9 is an easy load for an amplifier, drawing little current and possessing little reactance.



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