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Can anyone explain why SME's website shows two different figures for the pivot to stylus distance?
Under "Dimensions" it says 232.32mm.
Under "Specifications" it says 232.2mm
I know it's only 0.1mm difference, but it seems very unlike SME to have that kind of discrepancy on their website?
And which one is correct?
Follow Ups:
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
Hi Tom,I didn't realize that SME had changed the design of the 309. Are you sure about that? Of course, I don't really know, so you could be right.
I did hear something about inner groove distortion influencing SME's decision to use a different geometry for the 300 series than was used for the SME V and IV, but I thought the inner groove distortion compliant was directed at the SME V and SME IV. The SME V was introduced first, then came the SME IV, and finally the 300 series was introduced. Both the SME V and SME IV are based on Löfgren "A" geometry with inner groove radius at 60.325-mm and outer groove radius at 146.05-mm. All 300 series tonearm are also based on Löfgren "A" geometry but with inner groove radius of 58.0-mm and outer groove radius of 146.0-mm.
It appears to me that the effective length of 232.2-mm and corresponding mounting distance of 213.4-mm are simply mistakes on the SME website and also in the manual. Perhaps I'm wrong, but if you look at the SME 310 and SME 312, the same mistakes do not exist. Moreover, those numbers do not correlate with any of the other parameters listed above them for offset angle, linear offset and overhang. Do you know what linear offset would be compatible with an effective length of 232.2-mm and mounting distance of 213.4-mm? It is definitely not 91.54-mm because that would provide a totally absurd alignment. See the graph below.
Best regards,
John Elison
Edits: 05/25/12
Hi, John,
I had read a few posts (but could not come up with them in a quick search tonight) about the history of the SME 309 and it was Keith Howards' article in Stereophile that got me to considering the merits of the revised 309 geometry. It was a post by Brian K. that I referred to in a couple of my posts on the SME 309 but I could not find the original thread. I may try to find the reference tomorrow while enjoying a cup of early morning coffee.
Here are two posts that may be of interest if you haven't seen them yet:
B.K. post
Klaus post
Regards,
Tom
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?
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.
Well, it just strikes me as odd that a precision engineering company like SME would have such an error on their website.
And the line from the manual that says "it will be necessary to view slightly to the left or right of the centre line" hardly fills me with confidence. How much exactly is "slightly"?
You just need the outline of the tonearm on the protractor to be exactly parallel with the tonearm itself in order to achieve a perfect alignment. The outline does not need to be directly below the tonearm. It will be directly below the tonearm for a 9.5-mm MHST distance, but it will be slightly off to one side or the other for a different MHST distance. As long as it is perfectly parallel with the tonearm you will get a very accurate alignment with this type of protractor regardless of MHST distance.
The two most important points of any alignment are the alignment null-points. The SME protractor will get you there as accurately as any other kind of protractor. Of course, you are aligning the tonearm only. Therefore, you still need to be sure the cartridge is aligned perfectly in the headshell and that the cantilever is perfectly aligned in the cartridge body. If you are unsure of either and you want to align the cantilever, you need to use a different type of protractor.
Best regards,
John Elison
Thanks for all your advice, John.
A man could lose his mind fiddling with his cartridge alignment!
I'll have another fiddle tonight...
I'm pretty sure the specifications of 232.2-mm effective length and 213.4-mm mounting distance are wrong because they don't equate to a reasonable alignment when combined with a linear offset of 91.54-mm. It is my understanding that all 300 series tonearms are designed for Löfgren's "A" alignment based on an innermost groove radius of 58-mm and an outermost groove radius of 146-mm. This produces alignment null-point radii of 63.6-mm and 119.5-mm with a linear offset of 91.54-mm for all 300 series tonearms regardless of effective length.Best regards,
John Elison
Edits: 05/24/12
And don't get me started on Mounting Hole to Stylus Tip (MHST) distances! Not all cartridges are the same in this dimension. As I understand it, the SME alignment protractor is designed to work with cartridges that have a MHST distance of 9.5mm
So what do you do if you have a cartridge that is different?
And why don't all cartridge manufacturers list this dimension in their specs?
Grrrr.
PS Great arm though!
The SME alignment protractor will work with any mounting-hole-to-stylus-tip (MHST) distance. It is based on a specific alignment null-point and a specific linear offset. SME explains the procedure for alignment in the manual on page 19. However, if the MHST distance is different from 9.5-mm, effective length, mounting distance and offset angle will also be different from the specified parameters. Linear offset and null-point radii will always be the same, though.
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