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In Reply to: RE: Background... posted by tketcham on May 25, 2012 at 18:30:22
Hi Tom:
Wow! Those reference posts are extremely technical and tedious. Yes, I've read them before and I am very familiar with the mathematics and methodology employed by Löfgren and Baerwald. Unfortunately, I don't see a connection between those posts and the question as to whether the numbers in your SME 309 manual are simply mistakes or valid parameters from a previous design. However, I have found a reasonable alignment that can be formed with those numbers rather than the absurd alignment I presented in my previous post.
It appears that all modern SME tonearms use Löfgren A (Baerwald) methodology to define their geometry. Although there are slight geometric differences in the 300 Series compared to the Series IV and V, the differences can be explained by the choice of innermost and outermost groove radii. For example, the IV and V use 60.325-mm and 146.05-mm for inner and outer groove radii whereas the 300 Series use 58.0-mm and 146.0-mm. Plugging those numbers into Löfgren A equations yield slightly different alignment null-point radii. Now, if we change the innermost groove radius from 58.0-mm to 62.975-mm, we can get an alignment with effective length of 232.2-mm and mounting distance of 213.4-mm. Below are graphs of this possible previous SME 309 alignment along with the current SME 309 alignment. It is clear that the possible previous alignment has greater inner groove distortion when the inner groove radius goes all the way in to 60-mm.
So, the question still remains: Was there a possible previous alignment or are those numbers simply mistakes?
Follow Ups:
Hi, John,
I figured you'd seen those posts. I only included them because they mention the atypical alignment geometry of the 309, which sets the inner null point closer to the end of the record. I did finally come up with one of the posts (link below) that I was referring to previously. (I was surprised to see that the post was in response to one of your questions.) There was another post, perhaps on Vinyl Engine, with similar information on the 309 but I cannot get a search to find that one.
It appears you've recreated a close approximation of the original (?) geometry of the 309. I've done something similar using the Vinyl Engine calculator/graphing feature. I could get the inner null to be very close to 66.0mm but the outer null was about 116mm if using an offset angle and overhang close to what is specified. I could get a better Lofgren A 66mm/120.9mm fit if the offset angle was pushed to 23.8 degrees and a corresponding 17.8mm overhang.
I guesss the thing to keep in mind is that all of this is speculation on our part. Only SME knows exactly why there are conflicting specifications on their web page and in their SME 309 brochure (EL=232.32mm) and manual (EL=232.2mm). But at least the speculation is based on relatively good evidence. :-)
Regards,
Tom
Well, Brian Kearns knows more about SME tonearms than any audiophile I know. He said he talked directly to an engineer at SME who told him they changed the alignment of the 300 Series because "a number of their users were reporting inner groove distortion with the series IV and series V tonearms." That is exactly what I suggested in my original response to your question above. Therefore, I'm convinced that those numbers you are concerned about are simply mistakes or typos. I don't believe there was a different design for the 300 Series that used those numbers. If anything, the original design for the 300 Series would have been based on the same geometry as the Series IV and V.
Thanks for finding Brian Kearns' post. I think it pretty much settles the matter for me.
Best regards,
John Elison
I wish I could find the other posts on the subject that had me thinking that SME had actually changed the design parameters of the 309 tonearm but your conclusion is a good one. The fact that the "mistake" shows up in more than one place still has me wondering. Oh, well, it's not important and I certainly don't lose sleep over the matter.
Have a great weekend, John.
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