Sometimes this means we have to go into uncharted territory, and someone needs to draw a map.
I'll take this analysis to the extreme, and if I get sucked into a black hole ... well ... at least we'll know where the event horizon is. Consider me, your cartologist.
Here There be Dragons
The Plan Unfolds
Based on our observed "stability" of a 92 degree SRA, we decided to take a series of azimuth readings with our Feickert Adjust + software at 3 SRA settings: 91, 92, and 93 degrees. These extremes of 91 and 93 seem to be valid boundary conditions, based on the small variance from 92 degrees we've observed to be necessary during fine tuning.
The attached azimuth plots show the subtle shift over these rather large SRA changes. Click the images to expand them.
Observation and Interpretation
Note that a negative angle on the horizontal axis represents the headshell rotating counterclockwise from horizonal when viewed from the front. Likewise a positive angle represents a clockwise deviation from horizontal.
For phase angle, the L/R curves intersect as follows (approximate):
- SRA = 91: 1.2 degrees CCW
- SRA = 92: 1.4 degrees CCW
- SRA = 93: 1.6 degrees CCW
The crosstalk intersections similarly drift in a counterclockwise (negative) direction as we move from 91 to 93 degrees.
Analysis and Conclusion
The big takeaway is that an SRA change of 1/4 degree will require a minor azimuth change. You can think of this as the "fine tune by listening". if you're a truly tortured soul, you can break out your Feickert or AnalogfMagik sofdtware, oe your mono summing cables, and have at it.
With a line contact stylus, you will hear a 0.1 degree difference, but the good news is that you can play with this as the urge strikes. Your setup won't fall off a knife edge if you raise your bearing pivot by 1/4 degree (1mm on a 9" arm).
The key to this involves establishing a high degree of confidence in your SRA setting. This is where danger of falling into the false nulls trap I discuss in our setup reports lies. I wish I could advise you otherwise, but I've yet to find a better tool than a USB microscope. Perhaps Version 2 of AnalogMagik will unlock the puzzle.
One point I would make relates to Jonathan Carr's comments about stylus variance of plus/minus 1.5 degrees. Assume this to be the case if you're adjusting by ear. For a 9" tonearm, plus 1.5 degrees translates to raising the arm at the bearing by 6mm from the starting position of a level headshell. You likely wouldn't be inclined to experiment to such an extreme, but perhaps this example will encourage you to do so.
One thing I would emphasize is to somehow, keep track of your baseline settings so you can return to them should you get off track. For SRA, this depends on your tonearm. A Vernier scale like the one on TriPlanars or some Kuzma 4Poinrs is nice, but not necessary. For "simpler" arms, a spacer like the one shown here will work. Absent feeler gauges and such, a stack of business cards will work. It's a good idea to keep track of the measured thickness, using a dial indicator.
Returning to an azimuth baseline can be a bit more tricky. Even with our azimuth gauge, we can only re-establish azimuth to about .25 degree accuracy. This is the best we can do, absent firing up our Feickert Adjust + software. One thing I must warn you about: even with the most sophisticated azimuth adjustment we've encountered (the Kuzma 4Points and Stogi Reference), you will be amazed at how little movement results in a 1/4 degree change, and we're after much smaller changes than that.