SME arm alignment

[Tony L]

Still not getting it. As it's possible to get the stylus/cartridge zenith perfectly zero at the desired null point by moving the arm pivot forwards or backwards what does the stylus tip distance have to do with it?

When designing an arm the offset angle is very tightly mathematically related to the pivot to spindle distance. The former can really only be calculated once the latter is defined. As such by moving the pivot to spindle distance, as the SME slide system does, can technically introduce an infinitesimally small ‘error’ in the offset angle at anything other than the nominal ideal position. In practice it is irrelevant as almost all cartridges are within just a mm of the same length and I’m sure SME went with such an easy to mount/easy to use system that even compensated for armbase mounting error. It was a remarkably clever engineering approach to the problem IMHO and in the vast majority of cases will end up with a cartridge in at least as good alignment as other arms given the “skillset” of most dealers and end-users.

PS When discussing this we can really only consider cartridges with absolutely square bodies, with cantilevers that are perfectly square to those bodies, and tips mounted in perfect alignment on those cantilevers. Anything other than this has almost certainly already introduced far more alignment error into the system by being impossible to sight accurately against a template. There is a special place in hell for the designers of rounded cartridge bodies!

 

[Paul R]

Still not getting it. As it's possible to get the stylus/cartridge zenith perfectly zero at the desired null point by moving the arm pivot forwards or backwards what does the stylus tip distance have to do with it?

There's a calculator at the Vinyl Engine that lets you model this. Overhang Shift Calculator - Vinyl Engine I've put the reported numbers for the SME 309 in and assumed a 1mm shorter cart. If you log in it will draw you a graph. But it gives the numbers.

I would probably make a protractor with the specified new inner and outer null radii and work to that. If I wanted to be as exact as possible. Although I would start with the SME tool and a squint and then check it against the suggestion. You can find web sites that will create a PDF for you that you can print. Laser printers are more than adequate to this task, IME. The simplest are probably those that draw the curves the stylus should follow as you swing the arm.

If you don't have access to a printer send me the PDF and I'll put a couple of print outs in the post.

For a superb overview of the history and mechanics of this process I suggest Keith Howard's article for Stereophile, Arc Angles: Optimizing Tonearm Geometry | Stereophile.com

 

[spxy]

Thanks Paul, this looks very useful!

 

[Rockmeister]

There's a calculator at the Vinyl Engine that lets you model this. Overhang Shift Calculator - Vinyl Engine I've put the reported numbers for the SME 309 in and assumed a 1mm shorter cart. If you log in it will draw you a graph. But it gives the numbers.

I would probably make a protractor with the specified new inner and outer null radii and work to that. If I wanted to be as exact as possible. Although I would start with the SME tool and a squint and then check it against the suggestion. You can find web sites that will create a PDF for you that you can print. Laser printers are more than adequate to this task, IME. The simplest are probably those that draw the curves the stylus should follow as you swing the arm.

If you don't have access to a printer send me the PDF and I'll put a couple of print outs in the post.

For a superb overview of the history and mechanics of this process I suggest Keith Howard's article for Stereophile, Arc Angles: Optimizing Tonearm Geometry | Stereophile.com

Interesting read, and making a clear point. You either do it the hard way and choose your compromises, or pick a deck and arm where a reputable and capable engineer has known this and produced a product after testing the compromises for sound. But my footnote says, be sure never to read any of this thread if you are in any way paranoid! You’ll never enjoy a record again

 

[spxy]

For what's its worth I bought an SME specific avid alignment protractor just to double check and the alignment and it is almost bang on on both null points when using just the supplied SME template with single null point to set up the arm. So the majority here are right there's not much in it. My next journey will be record cleaning machines.......

 

[Craig B]

My next journey will be record cleaning machines.......

Contamination is a whole other category of OCD. Good luck.