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RE: Null points for 7" discs?

Posted by John Elison (A) on July 8, 2017 at 15:31:58

In Reply to: RE: Null points for 7" discs? posted by flood2 on July 8, 2017 at 02:42:06:

> Consequently it is of greater importance to minimize the distortion at the innermost grooves

You used the plural form of "grooves" meaning multiple grooves. In other words, you seem to be talking about minimizing distortion across the inner groove area and not just at the innermost groove. You can accomplish this more effectively by placing the inner null-point in the middle of the inner groove area rather than at either end of it. This is what Lofgren's alignment does. This is why Lofgren's "A" alignment has the lowest distortion possible in-between a given modulated groove envelope whereas your alignment with the inner null-point placed exactly on the innermost groove will have greater RMS distortion throughout the inner groove area as well as throughout the entire modulated groove envelope.

On the other hand, it probably doesn't make that much difference since the modulated groove area on a 7-inch record is so small anyway. When you compare the distortion curves for a 12-inch record with a 7-inch record, you can see that a 7-inch record has much lower tracking error distortion as result of its smaller size. However, Lofgren's "A" solution for a 7-inch record still has slightly lower distortion than your solution.

Lofgren's "A" Tonearm Alignment for a 12-inch Record

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Lofgren's "A" Tonearm Alignment for a 7-inch Record

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Flood2's Recommended Alignment for a 7-inch Record

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