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RE: Null points for 7" discs?

Posted by John Elison on July 5, 2017 at 06:53:41:
I just found a spec sheet that provides the minimum and maximum modulated groove diameters for a 7" disc. You can find it at the following link:
RIAA Dimensional Standards for Phonograph Records
The minimum and maximum groove diameters for a 7" disc translate into the following groove radii:
Innermost Groove Radius = 53.975-mm
Outermost Groove Radius = 84.1375-mm
The following Null-Point radii will provide the lowest tracking error distortion for a 7" disc:
Inner Null Radius = 56.97-mm
Outer Null Radius = 77.77-mm
The optimum null-point radii for any size phonograph record are based on its modulated groove envelope. You need to know the radius of the innermost modulated groove, r₁, and the radius of the outermost modulated groove, r₂. Then you can apply the following equations to find the null-point radii that will provide the lowest possible tracking error distortion within that modulated groove envelope.
Good luck,
John Elison