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SME record clamp causing convex playing surface

I have an SME Model 10 turntable. I recently noticed that by following the manual's recomended procedure for playing records, ie. placing the included washer under the record label, placing the LP on the platter and then screwing down the record clamp, the LP's surface becomes convex. The center of the LP is raised higher than its perimeter forcing the stylus to run uphill as the record plays. I confirmed this conditon with a bubble-level. I also presume this causes there to be a space between the underside of the LP and the platter surface, greatest in the center and decreasing until the vinyl meets the platter near the perimeter, which can't be good. Does this condition, presumably on all SME tables, cause problems with channel balance, pitch, record/stylus wear? It seems to me that the ideal playing surface would be completely flat. Has anyone else made this observation?
peterayer

Showing 2 responses by nsgarch

nsgarch's avatar
nsgarch
2,524 posts
 
The famous SOTA reflex clamp comes with a washer and does the same thing. The idea is to force the outside edge of the record tight to the platter, even if it isn't perfectly flat. If your records are pretty flat anyway, just use a thinner washer; I wouldn't recommend 'no washer' because then you might end up raising the outside edge of the record and you don't want that.

Don't worry about the stylus travelling uphill. It won't even notice it, just reduce the anti-skate a little bit ;-) My Goldmund platter is slightly concave, and the result is about the same when the clamp is set.
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nsgarch's avatar
nsgarch
2,524 posts
 
Peter, when I used my SOTA clamp, I kept about 3 different thickness washers handy and woud use the thinnest one I could to bring the outer edge of the record down to the platter (which you can never do 100% -- even 50% closure is enough to damp out resonances.

A medium mat (ie harder than felt) can help, just remember the washer still has to sit directly on the hard platter surface for the clamp to be effective. So you'll need to find a washer that's the thickness of the mat (cut a hole in the mat to acommodate it) and leave it as long as you're using the mat. Then stack an appropriate thickness washer on top of that depending on the flatness of the record.
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