Recently, I thought I might try to combine modern technology with the styling of 70s hi-fi by converting a pair of KEF Concord IIIs to work with DSP active crossovers, and also upgrade them from 2.5-way to 3-way with all-new drivers. The scheme is based on the same software and DAC that I used for my earlier DIY effort.
Some KEF Concord IIIs (not my particular pair) [nrpavs.co.nz]
I bought the KEFs a few years ago because I thought they looked fabulous, and I thought they would sound OK because they’re not tiny and contain two 8 inch drivers. I was wrong: to me they sounded weak and boxy, so it required no soul-searching for me to decide to modify them irreversibly.
I didn’t give my conversion project much planning. I already had some Peerless 8″ polypropylene drivers bought very cheap, which WinISD indicated were perfect for the enclosures, and I thought I could cross these over to 3″ drivers rather than the 4″ I used for my big speakers; I duly bought some Monacor SPH75/8 polypropylene mid-bass drivers. I thought about using 19mm tweeters, but in the end plumped for the same Monacor DT25 as I used in my main system because of their small size, particularly the front flange. All pretty cheap.
The KEFs are stylishly covered in a fabric ‘sock’ that was no doubt very cheap to make, but I think looks good. (There is even the possibility of commissioning the very talented mother-in-law to make new ones in funky colours).
I removed the small plinth at the base of the speaker (four long wood screws) and peeled back the ‘sock’ from there to reveal a rounded chipboard enclosure and the three drivers – the Concord is a 2.5-way system. I decided that I would replace one of the 8″ drivers with my mid and tweeter, and that I should therefore invert the enclosure in order to keep all three drivers close together with the tweeter close to the top of the enclosure. I removed the two 8″ drivers but left the original tweeter in position as a ‘plug’ for its hole.
I dusted off the router and made two 18mm MDF flanged discs to replace the 8″ drivers. I should have made the flanges wider because they’re not quite wide enough to take a screw head and clear the necessary foam gasket underneath, meaning I’ll have to clamp them externally. I painted them to seal in the sawdust.
The SPH75/8 is troublingly difficult to mount for a one-off hand-made ‘rapid prototype’: a virtually non-existent flange from the front or behind, and a magnet that is almost as wide as the driver, meaning that if you mount it from the front, there’s almost no gap for the air to flow around unless you widen out the area around the driver from behind. It’s squarish, so if you mount it from behind but don’t want the full thickness of the baffle in the way, you end up having to accommodate the corners, which is fiddly without machining a complex-shaped recess. I ended up mounting the driver from behind, shaping the corners with a chisel. Next time, I will definitely find a woodworking expert to make the ‘plugs’ to my CAD designs!
I needed to make a chamber for the SPH75/8. WinISD told me it should ideally be 3 litres or so – but probably not all that critical for the mid range. I figured the easiest way to do it would be some 110mm plastic piping from the local DIY shop which is quite thick and fairly ‘dead’ if you knock it. I could even buy a ready-made fitting to allow me to plug the end. I duly made an assembly and fastened it to the rear of the MDF ‘plug’ using some bent aluminium brackets. I stuffed it with speaker wadding. The volume works out at about 2 litres, so not far off ideal.
Mid range chamber made from 110 mm plastic pipe and end cap. Hopefully airtight by virtue of neoprene foam gaskets. It is stuffed with wadding .
Using self-adhesive neoprene foam and P-section draught excluder (this really does make a great seal), and plugging various holes, I rendered the mid range and bass enclosures pretty airtight. A top tip: hot melt glue is your friend. It plugs holes and gaps perfectly, and I have found that with a quick application it doesn’t seem to melt PVC cable insulation or ABS, so it’s ideal when you just want to feed cables through a hole in wood or plastic and seal the hole.
Crudely fastening it all together (it won’t matter how it looks when covered with the ‘sock’), I fired up one speaker this evening to have a quick listen using slightly modified settings from my big system. I found it really interesting and encouraging.
A modified KEF Concord. Those particular pieces of foam are just a temporary experiment, and would be too thick to fit under the fabric cover, anyway.
The mid and tweeter are going to sound spot on. The bass is not as deep as my big system (obviously) but reasonably adequate. Nevertheless, as an experiment, I tweaked the code to boost the bass in order to emulate a bigger driver and box. I am able to mute the individual drivers and so was able to listen to the woofer in isolation. The driver is clearly struggling! And so is the box (or is it a nearby piece of furniture ratting?). At (excessively?) higher levels I can hear some distortion. These bass drivers really were the cheapest possible, though. The enclosures are made from mere 15 mm chipboard and need bracing. So more work needed, but I will probably persevere because I really like the look of them (when covered). I imagine some reasonable woofers won’t cost much.
But the experiment possibly reinforces my notion that reducing the size of a speaker by a few percent really does make the job of getting it to work properly that much harder. If you want something that sounds astounding, without having to spend much money or effort, make it big.
Perhaps I was being a little too ambitious yesterday – trying to get too much bass out of the speaker. Also, I found a couple more existing holes in the box, and have plugged them with hot melt glue. I am pretty sure the seal is now good.
I have removed any bass EQ, and it sounds pretty good. Even though the settings are just guesswork variations on the ones for my big system, it is proper ‘hi-fi’. If I listen to the woofer in isolation, I can still tell it’s not absolutely top notch, but I may just not worry about it. Once I have the other speaker, I will be putting less power into it anyway.
The frustrating thing is that it will be a few days until I can do the other speaker, and hear them together…
Day 3 (elapsed time)
The second speaker has now been built up and I have endeavoured to set them up slightly more scientifically than before. I have measured them (woofer near field, and mid and tweeter far field ‘pseudo-anechoically’) and am applying roughly the appropriate correction to each driver (phase and frequency response, delay, gain) – but I still have ideas for refining this further. I have also re-implemented the EQ for the bass to aim for the same response as my big system(!). And you know what? It sounds fine! The bass really is there – not as ‘authoritative’ as my big system, but very deep nonetheless – I wish you could hear it. If the idea is that sealed woofers don’t go as deep as bass reflex or that an 8″ driver isn’t all that impressive, this might confound that notion. The cones do move quite a lot, though, and I know there is scope for better drivers.
‘Imaging’ is good, but I just don’t think that is all that difficult for an active three-way system. If I hadn’t heard my bigger system, I would think that these speakers were excellent – which is kind of interesting if you consider the way they have come into being.
[I just roughly re-fitted the fabric covers and put the speakers on their plinths. In dim light, they look like they will when finished properly. Of course they sound even better – you know it’s true…]
There is still scope for adding a foam anti-diffraction surround for the tweeters such as were fitted to the original speaker but have long since crumbled to dust (frequent hoovering was necessary when dismantling the speakers!). The enclosures could certainly do with some bracing, too.
And I still may try some different/better 8″ woofers.
I don’t actually know what I’m going to do with these speakers. If we ever build a house extension, I quite fancy a ’70s aesthetic’ and these speakers might even be allowed into the living room by the missus. But as an experiment they are very interesting. With my first ever speakers I think I hit the jackpot simply by making them big enough, with DSP active linear phase crossovers, three-way configuration and sealed woofers i.e. all the obvious, pragmatic features. Maybe the combination of wide baffle for the bass and narrower baffle for mid and tweeter also had something to do with the great sound. With these speakers I have made things slightly more difficult to start with which makes it more interesting perhaps.
Have raised the bass roll-off a bit (aiming for -3dB at 45 Hz, -12dB per octave) and temporarily added some pieces of foam around the baffle surround around the tweeter and mid. Crossover frequencies are 325 Hz and 4 kHz, both 4th order (actual filters are modified with driver correction). Now just listening to them, attempting to forget why I’m listening to them. And they’re pretty good!
At the next opportunity, I will maybe try replacing the woofers with the original driver as an experiment. An irritation is that I have filled up the original drivers’ mounting holes with glue.
[01/06/16] Just tried it, and found that they weren’t very impressive.
29/06/16 New bass drivers added. These are much better than the Peerless drivers. Corner frequency dropped to 38 Hz, and sounding good.
01/07/16 They’re sounding great.
15/07/16 Added some bracing to the most obviously flappy bits of the KEF Concord enclosures. Broom handle was much cheaper than dowel of the same diameter! The black square between dowel and enclosure is 1mm neoprene sheet. Dowel held in with countersunk wood screws from outside the enclosure.
Yes, the photos make it all look very ‘agricultural’, and the wide angle iPhone lens makes this bit of it look anything but square and perpendicular, but it is actually about right, and the speakers are solid, airtight etc. where it matters.
Has the bracing changed the sound? Can’t say, but it had to be done. I measured the driver in the near field again, and it hadn’t changed at all.
25/07/16 Restored the fabric ‘socks’ and they look good and wrinkle-free, much to my surprise. I really am quite excited by the fact that I may actually finish something for once.
A KEF Concord III with its fabric covering restored
As mentioned before, I ended up inverting the enclosures which meant that I had to remove the fabric ‘socks’ which were stapled very close to the ‘lip’ that is formed at the top of the enclosure. I was worried that I couldn’t find a staple gun that could get right into the corner of this lip, but in the end I found that an ordinary office stapler could do the job, which was fine. At the bottom of the enclosure, there are drawstrings which are pulled tight and tied off. The fabric stretches, so it forms a very flat covering.
The coverings are in pretty good condition for speakers over 36 years old, with just a couple of snags and small holes. The main problem is that they have faded from black to a very dark blue over the years which is only obvious if any of the non-faded material becomes visible through any slight misalignment. New coverings could be made in a variety of colours, but I think it would be preferable to retain the moderately coarse texture of the original material if possible.
Something that seems to have been an irritation to the previous owner is that the tops of the speakers are capped off with a square of hardboard covered with fabric, and over time these have warped, with the corners rising slightly. These have been re-applied by the previous owner using No-More-Nails or similar, to no avail. It was presumably an irritation to KEF as well because the Mark IV used plastic caps instead, but I am not convinced they looked as good. If I’m feeling creative I might try something different with black painted MDF caps, or aluminium sheet covered in fabric.
[Last edited 30/07/16]