SpeakerTalk

[Sparks12kv]

Greetings KEF lovers and experts. I'm a long time KEF lover and a keeper of broken toys. I have a little project I would like to explain to the group and would very much appreciate any feedback about its feasibility.

A few years ago I found a pair of beautiful 107/2 bass units in rosewood. Woofers need to be refoamed, but I've done two sets of 104/2 speakers and I understand they are similar coupled woofers, no problem redoing those.

I've tried in vain for years to find a pair of 107/2 heads that don't cost as much as a complete pair of speakers, they normally sell for a fortune. Yesterday I came across an interesting possibility, a very abused pair of 105/4's. They are missing one crossover in one bass unit, and the cabinets are beyond repair. Very sad. But the heads are intact and by swapping them both onto the one complete bass bin, I know they are both working.

As you might have surmised, the question is how best to run the heads from the 105/4 on the 107/2 bases. I know they won't ever be original, but I still want to try. I suppose I could separately amplify both, but the real goal would be to truly integrate the two. I'm not very optimistic about finding a pair of 107/2 head crossovers to swap for the ones currently in the 105/4 heads (unless anyone here has a spare set of those precious boards.) I'm more inclined to try to use the output from the 107/2 three pin umbilical and feed that to the six pin connector for the 105/4 heads if possible.

So there's the story. I'm all ears about what could be done. Thanks in advance for listening and hopefully chiming in. Mark

[speakerguru]

The head housing/mouldings were the same for all "headed" KEF reference series, 105|2, 105|4, 107 and 107|2. The exception was early 105s which used wooden fabricated heads.
That's the good news. I can't help with any circuit details about the crossovers. Having said that, the crossover frequency for a B110 mid and all those bass sections would have been at around 150Hz. So, an active filter crossover at 150Hz, bi-amped to allow for sensitivity adjustment between bass and mid, would be a good starting point.
Active single op-amp filters being one of my specialities, I guess it's no surprise that would be my approach .

[Sparks12kv]

Understood, it would be possible and probably easier to move to an external crossover for this project. But, I'm trying to work with the equipment I have.

Does anyone on the forum have any information about the signal in the umbilical cables for either the 105/4 or 107/2? Is the signal out of the 107/2 a full range and does the crossover in the 107/2 do all of the signal modification? What are the inputs to the 105/4 head, or even what are the color coding of the wiring at the 6 pin connector? Hopefully someone on this forum can chime in. Thanks in advance.

[speakerguru]

You’ll have to do your best to trace the wiring. All I can say is that there will be sense lines from each drive unit for the protection circuit, in the head, to model the voice coil temperatures.

[exkefman]

Another tip:

Bear in mind that the pin on the underside of the head assembly and the socket (on the top of the LF enclosure) were to a degree "matched" so that:

a) they would actually fit together as some pins were oversized (not on purpose !!) compared to the socket and hence might not fit - this was sorted out in production so that the heads and bass cabs in a pair would physically fit together.

and b) that they wouldn't wobble in use.

So, some careful fettling might be required to make a larger pin fit a smaller socket or vice versa, a very, very thin piece of paper might be required to prevent wobble.
_________________
regards
Tim

(I used to work for KEF from 1988-1995 - you can see my "profile" here: http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1706)

[Sparks12kv]

So I'm getting back into this project, since I'm forced to stay home like everyone else. What about testing the pins on the head units using very low power full range input with bass blockers inline. Does anyone really know which pins feed what in the heads? I don't want to burn out the tweeters but I don't have any information or schematics to use to figure it out. Anyone?

[speakerguru]

I don't remember specifics, but there was a general scheme across all the models.

Starting from the Concerto 3-way which used red for LF, green for MF and blue for HF. Like the light spectrum; red if low frequency blue is high. Black was used for all the common returns ("ground"), white for full range input signal.

Later models got more complicated and needed to "un-common" the return. The wiring then went:
IP: +white, -black
LF: +red, -purple
MF: +green, -yellow
HF: +blue, -brown

Also beware in two woofer coupled cavity systems where one woofer is facing the opposite way mechanically, then it will be wired in opposite electrical polarity.

hth

[SaSi]

I'm not sure it's feasible, at least using the passive crossover.

I have a pair of 105 (first version with the wooden heads) and reverse engineered a pair of 107.1 with the two heads and main crossovers at hand (I rebuild the bass cabinets as close as possible using reference from drawings and used B200 SP1014 in them. BTW, they sound terrific, much better than the 105).

The problem I see is that the 107 is a 4Ohm system and so the B110 and T33 are 4Ohm drivers, where the 105.4 heads have 8 ohm drivers. So the loading from the mid and tweeter will not be right. And as the 107 crossover does go to extreme lengths to create a resistive 4Ohm load to the amplifier, I suspect the different drivers will disrupt that, along with relative levels.

[Parts]

The 105.4 head has a tilt mechanism, and a different pin assembly than 107.

It also uses different tweeters and midrange units, and has an S stop circuit on board, no hf/mf crossover as I remember.

I have an old set of 105.4 in the loft, someone painted them gloss black, still to work out how to remove household gloss black paint from them, they had been stripped, hardwired, and S-stop removed, hf/mf was from main crossover directly to a 3 pin xlr retrofitted where s-stop dial was.

Kef 105.4 was an 8ohm speaker, Kef 107 was a 4ohm. There was a partial crossover in head but I was sure someone told me some of the head crossover network was in the main body?

I dont remember ever having that verified. But do know modified tr wire configuration on original 107 was all done on the main crossover, therefor we would need to verify what signal went from main crossover to crossover in the original 107 head.

107.2 was differennt again, still 4ohm, but biwirable witth different tweeters therefor crossover network.