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B200 SP1039 rubber surround deformation

 
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SaSi
Senior Contributor 200+


Joined: 24 Aug 2008
Posts: 223

PostPosted: Thu Sep 29, 2011 2:28 pm    Post subject: B200 SP1039 rubber surround deformation Reply with quote

My 104 woofer surrounds have been deformed to the point where the surrounds are almost flat instead of the semi-circle they should normally look like.

I've seen many such woofers deformed like that - even the one in the picture gallery of the SP1039 here.

I have not seen this happen to any other driver of that vintage from KEF. Is there something different to the rubber surround of the 1039? Perhaps different material?

How succesful can a replacement with a generic rubber surround be?
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T.O. Chef
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Joined: 31 Aug 2007
Posts: 150
Location: Toronto Canada

PostPosted: Wed Oct 12, 2011 12:14 am    Post subject: Reply with quote

Love to hear any informed comments on this subject to SaSi.
Though all mine are still somewhat rounded there's a definite sinking appearance to them. Maybe because my favorite stands have a tilt back design?????
Wonder if the compliance of the material used for that particular B-200 was unique. Anyone got an opinion?
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SaSi
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Joined: 24 Aug 2008
Posts: 223

PostPosted: Wed Oct 12, 2011 4:08 pm    Post subject: Reply with quote

I recently received a few 8" rubber surrounds. They are a perfect match for dimensions.

I am going to replace the surround to one SP1039 that has the surround totally flat and also partly unglued from the cone. I will measure the T/S parameters after that and compare with the original ones and also with another driver with the existing, flattened surround.
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speakerguru
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Joined: 18 Nov 2005
Posts: 959
Location: Green Hut, Tovil

PostPosted: Thu Oct 13, 2011 9:46 pm    Post subject: Reply with quote

SP1039 had a vacuum formed Velbex surround which can recover back towards its original flat shape if conditions are unfavourable.
Velbex was a very difficult material to work with. The difference in temperature between it forming a nice shape on the mould and burning was very small. SP1039 surrounds were all formed on a hand operated non-automatic machine. I made all the prototype B300 surrounds on a similar machine myself. It was a pig.

Later B110s had the mould shape pre-distorted, overshaped so that the aged surround might tend to relax towards a semicircular shape.

In the 70s we would have loved to have found a rubber type which had anywhere near the same damping properties as Velbex. Dr Small had a research project to try to find a such a rubber replacement.

If you find one that doesn't produce a dip in the SP1039 response at the first cone resonance then you're a very lucky man. I suggest you buy a lottery ticket Wink
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