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Maybe this...

Posted by tketcham (A) on May 24, 2012 at 10:14:58

In Reply to: SME 309 tonearm - website discrepancy? posted by misterharrison on May 23, 2012 at 02:58:30:

Hi, Mr.Harrison,
The descrepancies between the specifications (also found in the manual) and the dimensions for effective length may be an artifact from when SME changed the geometry of the 309 after some people complained about unacceptable inner groove distortion with the previous design. SME changed the 309 alignment geometry from the Lofgren A (Baerwald) geometry to a modified geometry about half way between the Lofgren A and Stevenson alignments.

The stated effective length of 232.32mm provides a matching linear offset of 91.54mm, which ends up with tracking distortion curves as shown in the link to the Vinyl Engine aligment calculator provided below.

This post I made back in Feb 2010 may be of interest:

"I read Keith Howard's article "Arc Angles" in the March 2010 issue of Stereophile magazine and found his recommendations for null point alignment parameters especially interesting. He uses inner groove radii of 56mm or 58mm and an outer groove radius of 146.3mm for the playing surface of a typical LP. His suggested optimal zero-tracking-error radii are 61.6mm and 118.4mm and 63.6mm and 119.6mm respectively. Plugging the latter groove radii values of 58mm and 146.3mm into his ArcGeometer utility plus the effective length of the SME 309 tone arm (232.2mm), the resulting calculations for optimal overhang of 17.0mm and offset angle of 23.2mm are exactly the parameters that SME uses for the 309.

Wonder if Mr. Howard was consulted when SME decided to modify the geometry of the 309?"

Regards,
Tom

• SME 309 alignment   (Open in New Window)

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