Here is a post that I got from another website, which is written by Jon Risch. It seems to be very informative about this subject.
SRA vs. VTA
Enough is enough! Here is a summary of the differences between SRA and VTA.
VTA or vertical tracking angle is the angle the cantilever makes with the surface of the record. Typically in the range from 10 to 30 degrees for cartridges, this should be matched to the VTA of the cutter head cutting stylus's pivot angle as it cuts the record, which can be likened to a defined length and pivot point, ala the phono cartridge.
The cutter head swing arm angle is a relatively fixed value for the various brands of cutter heads, and unless adjusted in the field, should remain very consistent in the field for all cutter heads of similar make and model. The range of modern VTA is approx. 24 degrees to 18 degrees, with most cutter heads being adjusted to around 22 degrees.
If the VTA of a phone cartridge is off, it will generate FM distortion (2nd order), and in almost all cases, will not be audible unless the cartridge is more than about 4-5 degrees off, and then only under ideal conditions of perfect alignment otherwise, and a highly resolving system.
One factor that CANNOT be matched by the user is if the arc length of the cutter head's effective swing arm length matches the cartridge cantilever length from pivot point to stylus tip. This mismatch will generate 3rd order FM distortion products, but again, has to be fairly far off to cause an audible problem.
SRA is stylus rake angle, or the angle the contact edge on the side of the stylus makes with the record groove. It is usually expressed with respect to the surface of the record, with straight up and down being zero degrees. For spherical sytlii, there is no SRA, only VTA, and then you can adjust for VTA without regard to the stylus angle. For eliptical and fine line stylii, there is a defined and relevant SRA.
Getting the stylus contact line to line up with the HF modulations of the groove wall is similar to aligning a tape haed to the recorded waves on the magnetic tape: you want them to be totally parallel with one another. When a fine line stylus is not aligned with the groove wall in terms of matching the SRA to the record walls groove angle as cut by the cutting stylus, then the footprint of the stylus will be riding over more than a single HF groove wiggle at a time. This results in a loss of HF's, and a blurring in time of the recovered signal, just like on a tape deck. If that was all that occurred, then incorrect SRA would be rather benign.
However, the situation for the groove wall is not like that of the tape deck, the groove wall and stylus are a mechanical interface whereby the groove wall modulations can torque on the stylus edge as it passes over the modulations at a rake angle that is not the same. This tends to generate spurious signals that are not harmonically related to the original signal, and the torquing tends to cause the intrisnic cartridge cantilever/moving system resonances to be excited and stimulated. The result: hash and HF frazzle that reaches surprisingly low in the audio band due to intermodulation with the signals being recovered from the record groove.
The distortion levels for SRA are much higher, and much more irritating than for the VTA distortion. This is because they are aharmonic in many cases, and the IM reaches down into the midband.
So when adjusting the angle of the cartridge/tonearm system, SRA should be the parameter being adjusted for, not VTA.
If you have any doubts about how to set this via my info, follow your cartridge manufacturer's recommendations. They usually specify that the top of the cartridge body be parallel to the records surface at the nominal recommended tracking force. However, if you use a tracking weight different than the middle of the tracking range, or the "recommended" weight, then the angle will be slightly off.
You can actually see the angle of the diamond chip that is the stylus, as most stylii have the diamond chip shank axis the same as the contact line axis. One of the notable exceptions was the old Shibata style stylus, which had the angle of the stylus rake different from the angle of the whole diamond chip/shank, and the contact line was actually slightly curved in the vertical. Using a penlight or maglight flashlight, have someone illuminate the stylus tip from behind (center of record) while you look at the cartridge from the side while it is in an outer groove on a flat non-warped record. With either a bit of squinting or the aid of a magnifying glass, you should be able to see the diamond stylus shank extending down into the groove. Really, all you will be able to see is the part that sticks up from the groove, and through the cantilever on the top side.
After studying the range of modern cutting lathes, it was determined via measurement and listening tests that the optimum SRA was a value that would provide decent playback on most records, without resetting the SRA for every individual record, was approx. 1 degree forward (top away from the tone arm pivot).
If you back illuminate the stylus as related above, you should be able to see the diamond chip's angle, which corresponds to the SRA. It should definately not be tilted back (the shank top toward the tone arm pivot).
If you make up an index card with a straight up line and a line 1 degree off of that tilted in the proper direction, you can use such a card as a visual aid in aligning the stylus chip angle.
This set it and forget it SRA alignment will playback 80% of all records with a minimum of average SRA error, usually within a degree or two of the record groove walls rake angle. Anything less than 0.5 degrees off of exact alignment for a particular combination of groove wall rake angle and SRA is not very audible, although an error in the direction of too high at the tonearm pivot (shank top more away from the tone arm pivot) will tend to sound worse. That is why this average SRA angle is biased toward a slightly higher average error in the benign direction, and not the other way.
This adjustment will at least get you in the ball park for a starting point. It helps to remember that if you adjust the SRA by ear for a particular record, you may be as much as 5 degrees or more off for another record.
For the published version of this, see:
November 1980 Popular Electronics article titled: "Phonograph
Playback: It's better than you think." by myself and Dr. Maier, p. 48.
AND
High Fidelity in the March, 1981 issue on page 31.
AND
Audio magazine, March 1981, page 21, "More Than One VTA",
again by Dr. Maier and myself.
Jon Risch
