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How to set SRA after determining true vertical?

Here is a picture of a stylus with zero rake angle:

http://members.cox.net/nsgarch/SRA@%200.0%20deg.jpg

Since all modern styli are symmetrical in the x and y plane about the verical z axis, the tapered stylus and its reflection will make a perfect "X" when vertical (z axis perpendicular to the groove) and viewed from the side.

This condition is established by raising or lowering the tonearm pivot post. Once you find this point, and assuming you have a typical 9" tonearm (about 230 mm from pivot to stylus) then each 4 mm you raise the post from the zero
SRA point will apply one degrewe of SRA to the stylus.

A test setup is shown in these two pics:

http://members.cox.net/nsgarch/SRA%20setup1.jpg
http://members.cox.net/nsgarch/SRA%20setup2.jpg

Equipment includes:

a mini Mag-Lite flashlite,
a first surface mirror from old SLR cameras -- easy to find at photo repair shop)
a 50X pocket microscope
bean bags

Don't forget to first remove antiskate and set VTF.

.
128x128nsgarch
eldarado
381 posts
 
Dan_ed,

I too am curious but, he is right. I drew it out in Corel Draw and he is right on the money.
richingoth
326 posts
 
Converting:

Angular Tilts To Linear Measurements

1 arc second = 0.0000048481232363502727814651700363688 inch per inch

= 0.000048 inch per 10 inches
= 0.000058 inch per foot

Linear Measurements To Angular Tilts

0.000001 inch = 0.20626480624702760022105769979869 arc seconds per inch

= 0.021 arc seconds per 10 inches
= 0.017 arc seconds per foot
ketchup
699 posts
 
Could you also set azimuth by looking for a perfect "X" with one of those microscopes? Of course the microscope would have to be placed in front of the cartridge. Could this be a better method than looking at the cantilever and it's reflection in a mirror?
nsgarch's avatar
nsgarch OP
2,524 posts
 
Ketchup, you are quite right, this is an excellent way to check azimuth. There is just one leeeetle setup problem you need to solve first:

If you just place the mirror on the platter, you'll never get a scope in there to look at the image because the platter is in the way. Here are two ways to overcome the problem:

1.) If your tonearm has the vertical bearing axis perpendicular to the headshell offset, then all you need to do is shim the mirror up on something about 3/16" thick like a piece of balsa wood or foam-core. Just high enough to let you rest the end of the scope on the platter and be able to see the stylus head-on.

2.) You could cantilever the mirror a half inch or so off the edge of the platter (taping it securely to the platter, AND taping the platter to the TT base so it can't rotate!) and this will also allow you to look at the stylus head-on.

Be very careful though! You might want to use a piece of masking tape to restrict horizontal movement of the tonearm.
dougdeacon
3,338 posts
 
Nsgarch,

Great geometry refresher, thanks! And yes, it was a compliment. As we probably all know, cutting engineers allowed a wide range of SRA's when cutting lacquers. If they'd all taken such care it would make accurate playback much simpler. Tonearm height adjustments could exactly = record thickness changes. That would be nice.

A naked-eye method for (fairly) precise SRA setting has been posted in the VA FAQ's for many years:

http://www.audioasylum.com/audio/vinyl/messages/1240.html

Jon Risch, the author, offers excellent technical explanations. After studying many different cutting lathes, he recommended that a good compromise SRA for "set and forget" rigs would be 2 degrees forward of vertical, ie, top of sylus pointed away from the tonearm pivot.

Before getting the TriPlanar I used Risch's method when setting up a new cartridge. It was an effective way to settle on a baseline arm height position. Now I'm spoiled. It's easier and more fun to twiddle while listening than to fuss with maglites and magnifiers.

Regards,
Doug
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