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R107/2 Head Crossovers MF/HF EQ.

 
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velo5338
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Location: Augusta GA

PostPosted: Mon Mar 14, 2011 9:41 pm    Post subject: R107/2 Head Crossovers MF/HF EQ. Reply with quote

This question is directed towards the ex-Kef guys who may have details in their memory concerning the R107/2 head (MF/HF) crossovers.

I am part way through converting my 107s to 107/2s with a set of Kef parts I bought some years ago. I have refoamed the bass drivers and installed the big crossovers in the bass cabs. I managed to work out the terminal assignations by tracing the circuits on the boards.

The question:
Examining the head crossover I can see that there is some kind of L-pad built into the boards. There is an array of 8 resistors arranged in pairs across a common line whose function seems to be to vary the output of the T33 HF unit. Does anyone remember how these were connected? There are pins soldered into the board in such a configuration as to suggest a (3 pin ?) jumper might have been used to select an appropriate pair of resistors.
Or was it more complicated than that - perhaps the resistors might have been connected in parallel on either side of the common line yielding a much larger number of possible combinations?

if no one can remember how they were originally connected then I guess I will solder a wire to each of the pins (a piece of CAT 5 cable will probably work) and bring it out through the hole under the badge and do some listening tests connecting the resistors in different combinations.

If there's any interest I can take some photos of the old 107 and new 107/2 crossover boards so others more skilled than me can establish the actual circuits.

Tried to post this on the other (newer) Kef board but kept getting a 'wordsearch' error message.

Thanks
Mike
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Mike

107s, 105/3s, 104/2s, Quad 57s
Quad 33, mono blocked 303s, Quad IIs, QC24 etc.
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SaSi
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PostPosted: Sat Jun 18, 2011 11:21 am    Post subject: Reply with quote

I'm afraid I can't offer you help with your question but I can't resist asking you something.

How did you take the 107 heads apart? I have a pair (what survived from a pair of 107s) and removed all visible screws from the back but no luck. I'm not sure if the two halves are simply stuck together or if there's more to it.
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speakerguru
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PostPosted: Sat Jun 18, 2011 5:43 pm    Post subject: Reply with quote

My only official involvement with the 107|2 was to bring my bass guitar into work to play through the bass cabinet for cone strength testing of the 10" bass drivers. The first prototypes failed miserably at the cone neck.

I can't help with the HF attenuator section except to say that if they used the scheme Dave Smith and I used for the LS3/5A redesign then you should be able to work out how to wire the taps by the silk screened marking on the pcb. They should have -0.5dB, -1.0dB, etc., marked on there. No?

btw: did you get the metal dome T33s to go with the new XOs?
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dsmith
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PostPosted: Mon Jul 18, 2011 2:36 pm    Post subject: Reply with quote

speakerguru wrote:

I can't help with the HF attenuator section except to say that if they used the scheme Dave Smith and I used for the LS3/5A redesign then you should be able to work out how to wire the taps by the silk screened marking on the pcb. They should have -0.5dB, -1.0dB, etc., marked on there. No?



I remember your guitar being in the lab for occaisional torture testing. Didn't Phil play the accordian? Thankfully no one was into the bagpipes!

I remember that the revised 3/5a had the stepped L pads. We didn't want to trouble with the tapped inductors. I don't remember the exact topology but just that we needed to strap in two connections for every step. You can't let all the L sections hang across the network (and just select the output) because the multiple L's would load the network to a low impedance. Proabably the L's had one common connection (perhaps ground) and the connections to input and output would be chosen based on computer sorting of the tweeters. (Actually, the nominal sensitivity difference between tweeter and woofer).

Did you work on the 3/5a too? I remember pestering Laurie for the project and taking a few trips to the BBC for meetings. It took a fair amount of effort to figure out what a "nominal" older version was, there was so much variation.

David S.
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speakerguru
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PostPosted: Fri Jul 22, 2011 9:16 am    Post subject: Reply with quote

Hi Dave,

I think I worked on the new vc for the B110. Wasn't it 5 Ohms? I also laid out the pcb for you, probably with tape. I think that was pre CAD.
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