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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 3 responses by peterayer

peterayer OP
1,382 posts
 
Thanks Pcosta, I thought of that. If I don't tighten the clamp at all, the LP is flat but raised entirely off the platter by the thickness of the washer. If I tighten the clamp just enough so that the outer edge of the LP touches the platter, the LP is indeed pressed into a convex shape. My suspicion is that the supplied washer is simply too thick to allow the LP to both lie flat on the platter and maintain complete and good contact with the micro-grooved platter surface.
peterayer OP
1,382 posts
 
This occurs with all of my records, any thickness. It is the original black SME washer that came with the table which I bought new from a dealer. I don't know the exact thickness or diameter of the washer, but I think it is just over an inch in diameter. I will measure it later. I do tap the edge, or check it visually, to make sure there is contact all around the perimeter of the LP and tighten it only enough to achieve this contact and no more.

Having thought more about this phenomenon, I think VTA is constantly changing because the tonearm angle is going up as the stylus is riding up hill slightly with each revolution as it travels toward the center of the LP. Perhaps I should contact SME or Sumiko.
peterayer OP
1,382 posts
 
Thanks for everone's response. I bought another washer which is half the thickness of the SME washer and did some listening tests. With the new washer, the LP was essentially flat on the platter and tightening the clamp slightly did create good contact between the platter and LP.

For reasons I can't explain, the thicker SME washer produced slightly better sound than the new, thinner washer and also considerably better sound than with no washer or clamp. I heard a little more detail and darker backgrounds during very quiet passages of solo instruments and the overall sound was richer and fuller, especially with full orchestras. In a word, everything became slightly more three-dimensional. But the improvement was very subtle over the thinner washer.

Has anyone else had a similar experience?
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