Pgtaylor, Dertonearm
It goes without saying that we're venturing into the fine " ether" of tonearm setup. With respect to SME's and the upper end Schroeder tonearms, the effective length changes require a bit of trigonometry, or a CAD program to determine, and producing a protractor dedicated to a particular cartridge needs to take this into account.
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1. The effective length is the STRAIGHT LINE distance from the stylus tip to the tonearm bearing.
2. The cartridge is offset, per Baerwald/Loefgren/Stevenson geometry
So, merely adding the effective length difference will not give you the correct results (close, but no cigar). A cartridge whose stylus is 1mm further forward than a statistically "normal" cartridge increases the effective length by less than 1mm.
I just modeled a Tri-Planar on my CAD tool. I chose this arm because I had the numbers handy: effective length = 250mm, offset angle = 21.949 for Baerwaald. I assumed a 40mm long headhell (the offset component) for this exercise.
A cartridge with a stylus positioned 1mm forward of the statistical norm increases the effective length by.9275mm, for a net effective length of 250.9275mm. Simple addition would predict 251mm, or a variance of .0725mm (.0028"),
.0028" is within the threshold of audibility for a high performance analog rig, which is why everyone on this thread has justifiably been whipped into a frenzy over a better tool.
The takeaway from this is that any protractor generated for arms like the SME V, and the upper end Schroeders, needs to take the effective length, the length of the ofset section of the arm wand, and the proposed geometry into consideration.
I posted the drawing to the following URL for anyone who's interested:
http://www.galibierdesign.com/images/forum/Eff_length_Model.pdf.
Cheers,
Thom @ Galibier