KEF C75
Speaker Rescue - Rejigging the KEF C75
I've just been dudded by the ebay/paypal system, which didn't back up the buyer (me), when I bought a pair of KEFs which turned out to have stuffed midranges. I already owned a pair of C55 and found them pretty good, so reckoned a pair of C75, which are bigger, should be OK too.
So. What to do with a pair of KEF C75 floor standers that have both midranges seized up? Not easy to find replacements, but they might turn up one day, with a bit of luck. But these are old late 1970s speakers, and require an exact replacement, so it could take a while. The concentric arrangement which KEF used (then and now) means you can't just stick another midrange driver of about the same size and character in there.
The speakers still work, but are more recessed than they should be, and missing the midrange emphasis that imparts life to so many instruments and voices.
So, here's what I have done - and again, as with the Sonab OA6 project, everything is reversible if the right drivers are found one day. Step one was to find a midrange that sounded ok in this setup, so I tried several that I had lying around.
This was done by simply attaching some wires to the midrange feeder wires and listening to the whole speaker as best I could, with part of it hanging out! I tried a Coral unit, but it seemed a bit too loud and not blending in. The selected one is a Morse (Japan) 6" AlNiCo, paper cone, ostensibly full-range as it has the whizzer cone as well. If I find something better later, I can replace it, but it has been a bit of a fiddle and the results are already pretty good.
Where to put this driver? I can't put it in the box, there's no room, since we're leaving the Tweeter-Mid exactly where they are. So, it's going on top. How to mount it and how to feed it a signal are the essential issues. I decided to make a demi-cabinet for it to be mounted in. I thought being open-backed would be ok, as I have experienced several good speaker with this characteristic.
I started by making these demi-cabs up from some offcuts of 18mm MDF, using a jigsaw. The front baffle has a hole cust to the right size, then a curved top for décor purposes, and two side panels for support and stability.
Once these pieces were cut out and screwed together, the drivers were mounted in them, and we were nearly ready to test the theory. But at this point I had to open up the C75s again and re-route the midrange wires. I disconnected them from the seized mids, and connected jumper leads. These new leads exited the cabinet through a very small hole in the back (which I will caulk to ensure air-tight, but not much air is going to move through there anyway, as the cable pretty much fills it).
I crimped some spade lugs to the new leads, and we were ready to fire it up! At first I had some doubts, but as time went by the sound got better, right up until the final stage, covering the demi-cabs with black fabric "socks". I sprayed the MDF black before cutting out a pattern of fabric ($8 from Lincraft) which I pinned together and Barbara sewed up to my directions on her machine.
Essentially the front part is the same shape as the front baffle, then a tube of fabric goes back and is drawn in with elastic at the back. This tightens it all up so it clings to the shape of the demi-cabinet.
The sound is now very enjoyable, lively and all that I require. Of course the sound varies with amplifer and room, but interestingly I get very good results from the old Sonos Z100 amp down in the theatre room, which has good acoustics, or from the Dynaco 120 power amp (60w x 2) or from the McIntosh vintage MC50 monoblocks, which are only rated at 50w each, but sound a lot stronger - just like most McIntosh's I've had experience of. I will soon be taking delivery of a MC2100 stereo power amp (100w x 2) from the USA, which will be interesting to try. Considering how old the amps I've played around with to date are, they really impress.
So there you have it. Be sure to let me know if you see a set of the drivers or a cheap set of KEF C75s anywhere. But for now, I'm enjoying the ride. Speaker Rescue strikes again!
