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RE: Null points for 7" discs?

Posted by John Elison (A) on July 6, 2017 at 18:13:38

In Reply to: RE: Null points for 7" discs? posted by flood2 on July 6, 2017 at 16:20:50:

> For want of a suitable spec, I chose 215mm assuming a typical 9" arm such as a Technics arm.

Okay! That means effective length will be 225-mm instead of 230-mm. It increases my numbers only very slightly. If you align your Technics SL-1200 for a 7" record, it will have an effective length of 225.067-mm with an offset angle of 17.417-degrees. Here are the other specifications:

Innermost modulated groove radius = 53.975-mm
Outermost modulated groove radius = 84.1375-mm

Inner null-point = 56.966-mm
Outer null-point = 77.773-mm

RMS distortion within the modulated groove envelope = 0.110305%
Peak distortion in-between the two null-points = 0.162853%
Peak distortion at the outermost groove = 0.163073%
Peak distortion at the innermost groove = 0.163029%

The argument in favor of your universal alignment is not applicable here. Damian asked for an alignment optimized for 7" records and that's what I gave him. You can blow smoke all you want, but you can't have it both ways. All your previous arguments addressed the fact that your universal alignment was designed for different sized records. The alignment presented in the graph below is optimized for 7" records and it provides the lowest tracking error distortion possible in-between the green lines, which represent the modulated groove envelope for a 7" record.

Best regards,
John Elison

Edits: 07/07/17 07/07/17 07/07/17

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