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In the Forum: Analog Playback
In the Thread: Copper Mat on a Micro Seiki Gun Metal Platter
Post Subject: One more thing...Posted by Wellington on: 7/22/2017
Anyone contemplating buying a CU-180 should know that it must be dead flat. Even the slightest departure from being flat will negate the benefit of intimate contact across the entire surface of the platter.
How flat? Flat enough that the weight of the CU-180 alone is sufficient to overcome the normal microscopic deviations from perfect flatness in both the platter and CU-180. In other words any minute departures from perfect flatness caused by machining, too small to see, will be overcome by the draping flexure caused by its own weight.
How can I test for this? Two ways. Visibly sighting is not enough. If it is visibly bent it is already an expensive (and deadly) Frisbee. Put the CU-180 on the platter without a record and tap all around the playing surface region with finger tips or knuckles. Check around the whole circumference. Rotate the CU-180 relative to the platter in increments and tap again. Try the rotation around all 360 degrees. You should feel and hear a "granite mountain" at all times. If you hear any tapping or clicking sound or buzzing, any at all, the CU-180 is slightly bent. Reject it. I am assuming the use of a massive Micro Seiki platter here, and assuming that that platter is flat, which it will be unless it was dropped. Another test is to lay the CU-180 onto a heavy glass table or a granite countertop used as a truly flat reference plane. Try to see/hear if there is even the slightlest play. Repeat the tap test.
What could cause a non-flat CU-180? Being dropped, just once, even onto a carpeted floor. Or getting flexed in shipping with inadequate packing.
Can't I just bend it back? No. At least almost certainly not. Once bent it will be hard to get it back into true flatness and small voids will occur from stresses remaining in the alloy. Think of unfolding a paper clip into a "straight" wire. How straight is it? No amount of fidgeting with it can ever make it truly straight again.
Buying? I have seen a few CU-180s for sale that say "slightly bent". A 1% bend does not decrease value by only 1%; it decreases value by 100%. Think paper weight. Get assurances that a return is allowed.
But a vinyl record is never perfectly flat. True of course, but we have to get the interface, the constrained layer, between the platter and CU-180 correct first. Then we can use a record weight or clamp to mash the record flat (as flat as possible) directly onto the clean CU-180.
Isn't this a pain in the ass? Yes, it is. But if you want to hear what your Micro Seiki is truly capable of, it's worth seeking out a flat CU-180.
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