SpeakerTalk

[SaSi]

Several years ago I got myself a pair of KEF107 heads along with the main crossovers. Everything else was a lost cause after a basement flooding and sadly the bass units were scrapped before I could lay my hands on them, even only to take exact measurements and copy them.

Anyway, the heads lay on their backs on a shelf in my own basement for all these years. As I was sorting out the chaos in there and trying things out to decide what to use, what to fix and what to get rid of, I thought I should give the head units a try. Until now I was putting it off because I had to fix XLR plugs to drive them.

So I terminated two lengths of speaker cable to XLR plugs and patched them to the workshop amplifier, a Technics SU-V6x. Listened to some 7.5ips reels on a B77 having the units resting on a roll of masking tape reel each, to my left and right, like computer speakers. Thought to do them justice since their size (without the bass units) was crying "nearfield".

I didn't expect to hear much low frequencies but what I wasn't prepared for was to feel them as well. It does feel punchy and precise and the B110s don't even seem to get worked up much.

I've been listening to these for the last two evenings and went through Supertramp, Dire Straits, Procol Harum and a few others.

After the initial wow feeling - and that was a strong one - it now feels like something is missing from the bottom end (both sonically and visually) - I would have felt strange if KEF did the bass units for nothing...

I now feel the urge to start a project and build something along the lines of R107 for these lovely heads. Thinking about going original as much as possible with dual SP1070s, or perhaps put one or two B139s or even go crazy and combine B139s with their PRs. I could try B300s but that would bring them back to R105 looks and the bass unit would not be a long tall plinth this way.

Or perhaps do nothing and mount them as surround speakers in my vintage 5.1 setup. The RL ones are R105s and I'm trying to figure out what to use for the rest.

Any thoughts?

[audiolabtower]

A couple of points from memory. The bass bin had lightweight 10 inch paper woofers. The best driver would be B300 but both that and B139 are likely to be less efficient, so may not be able to match the head in sensitivity without separate amps and volume controls.

I think the crossover point was around 150-160Hz, so the B110 crossover will have a roll off there. You could try taking the high pass components out to get more bass out of the head to use on its own (it should be slightly better bass than eg a LS3/5a) but I can't say what it would do to the impedance (or freq response).

Also most 107s were not flat and needed a kube to a) get a flat response, otherwise they were bright and b) equalise and boost the low bass to the correct level and tune to position in the room, otherwise they could be too bass light, or completely overpower a room in the bass without adjusting the contour (the kube gave around 20dB of boost at 10Hz).

[SaSi]

Thanks for the points.

I was thinking of SP1070 as they are paper coned and seem much more lightweight than - say - SP1075 that are most likely polypropylene.

I have tried the high frequency units without the main crossover (high pass filter). They sounded really good and I didn't expect such bass extension from a B110 (nor such punchy bass without reaching their excursion limits).

What I don't know is the impedance of the original R107 woofers. I am guessing they might be 8Ohm in parallel matching the 4Ohm system impedance.

Regarding balancing the drivers, I'm thinking of using an active crossover that also gives rudimentary equalization options that might - just might - compensate for the lack of the KUBE. Or - if that doesn't work out - introduce a parametric equalizer before the bass amplifier to try to do better.

[SaSi]

BTW, you mentioned removing the low pass section for the B110. I don't use the main crossover. I was thinking (because of the size) that this holds also the B110 low pass.

I may be wrong and the crossover on the heads may contain that as well. Do you have any idea? I haven't found any schematics on these.

When I got them, I tried to open the heads but even removing the screws didn't loosen anything and I abandoned the attempt in fear of breaking something.

Edit 1:
And an additional crazy idea...
I noticed that the original woofers in the 107 are 10". The B200 I was thinking of using are 8".

So, I thought... How about using 2 x SP1069 (16Ohm) in parallel and 1 x SP1070 (8Ohm) in parallel with the other two? And make this a 3 sealed cavity + 1 shared front cavity? Perhaps increase the size just a bit as each of the drivers seems to be comfortable with about 20lt in sealed configuration. Or reduce that a bit for a higher Q (aim for 15) and perhaps leave the overall dimensions about the same.

It would be nice if I could find the T/S parameters for these drivers but looks like I need to measure them if I want them. Or go the old fashioned way of a wild guess and cut and try from there (lol).

Edit 2:
OK, it's been a while since I searched the net for the crossover and another search brought it up on diyaudio.

Here it is for our benefit.
http://www.diyaudio.com/forums/attachments/multi-way/414671d1398420902-kef-reference-series-107-xover-schematic-kef107-mid-high-xover.pdf
Unfortunately, the guy that traced it was interested in capacitor faults so there's no value on the inductors. It seems I was wrong. There's certainly a second order highpass filter with the 20 and 4.2uF capacitors and then we have the more complicated section that I admit is beyond me to understand. That's why I prefer active crossovers. I'm more keen with active op-amp filters or even better adjusting a commercial active crossover...

It's interesting though, even with that in place, bass reproduction is pretty decent. Nothing to compare with the R104ab that I have hooked together and can compare A/B but decent.

And, btw, I haven't measured the overall load of the heads without the bass section but they seem to allow the amplifier (Technics SU-V6x) to drive them along with the 104s pretty well. Don't need to push the amp beyond 9 o'clock as the SPL gets pretty high already. The amp gets a bit warm to the touch but it's not an amplifier to drive heavy loads anyway.[url][/url][url][/url]

[audiolabtower]

Sorry, not familiar with the 107 internals. I just thought that since the 105 had the mid/top in the head unit, the 107 might be the same. Maybe it is all on one large board like the 104?

[SaSi]

I progressed with the project. Much to my surprise, the SP1014 appear to perform much better compared to the SP1070.

You can see more details and pictures here

[audiolabtower]

very impressive!