Two Protractors - Only One Aligns

@wlutke I get what you are saying, and agree that the arc should be flatter for a larger P2S (or a longer effective length).  However, if the null points are the same for both configurations, perhaps this doesn't matter.  By definition, the null points are the two points where the cantilever is tangent to the record groove, and this is not effected by the "curvedness" (or flatness) of the arc.  I think the difference is that a flatter arc will create less error at locations away from the null points. That is why a 12" tonearm is preferred over a 10" tonearm.

I found a good turntable geometry calculator on this website. This calculator shows that for any given P2S, the zero radii are the same.  What changes is the effective length and offset angle required to meet the null point requirements.  For example, calculating the zero radii for a 250mm and 290mm P2S, I get the same zero radii (63.32, 119.63) with the following differences:

P2S                        250              290
Eff. Length              264.79         302.84
Off. Angle                20.25           17.61

I guess the cool thing about my tonearm is that the offset angle is adjustable, because the headshell is attached at only one point and can be rotated.  Thus, if I am able to find a P2S that works for the effective length of the tonearm, I can adjust the offset angle accordingly. Early indications suggest that I can accomplish that with the MINTLP - but I haven't confirmed that yet.

Where I get confused is applying the protractors.  My MINT and TTB protractors both have the same zero radii (67 & 121 mm) which differ slightly from those calculated above (don't know why, maybe different calculation method).  But they are clearly offset from one another in a different manner.  So if I could lay them on top of one another, the two null points would not line up, even though they use the same zero radii. That probably reflects the difference in P2S, and the TTB must have some assumed value of P2S.  The problem is that both protractor manuals state that you should fix the protractor - so that the differing offset makes it either possible or impossible to match the two null points on the protractor. If the only requirement is that at each zero radius the cantilever is tangent with the grooves, couldn't I simply rotate the protractor and use the lines on the mirror to make sure that the angle is correct?

@lewm Is this to suggest that a dedicated protractor is needed for every possible effective length?  See my post above.  If that is the case, the TTB would be misleading.  Must have something to do with pointing the orientation line on the TTB to the pivot point.  But even when I come close to doing that, my alignment with the TTB sucks.

I think the unique thing about this "Musical Life Conductor SE" tonearm is that I can alter the P2S, the offset angle, and effective length (3% range).  My question stands: if I can match the two null points with the correct angle so the cantilever is tangent to the grooves (defined by the protractor grids), is it OK to rotate the protractor during alignment?

@lewm Sorry I missed your post at 8:05am before writing the one above.  I will need to see if the stylus follows the arc on my MINT - but it probably won't, because I'm using a 10" tonearm and the MINT was set up for a 12" tonearm.  The arc HAS to be different, and this is why less error occurs with a longer tonearm. But this doesn't mean that meeting the two null point requirements is not sufficient for getting proper alignment, right?  Although I know that I am close, I haven't yet fully confirmed that I can meet both null point requirements with the MINT without rotating the protractor.

This is why it’s good to have a single-point universal like the GeoDisc in your toolkit. Also I use the SMARTractor, another single-point true universal (with multiple curves to choose from) which is definitely more precise, but also much more expensive. And I’ve used a Mint LP (2-point) to good effect, but it’s frustrating to have to buy a new one for EACH setup (yes you really have to).

The simple GeoDisc should work quite well with your unusual tonearm, because the center pivot point is so easy to visually sight - when you can’t clearly "see" the pivot point, the GeoDisc doesn’t work very well. The GeoDisc is based on the Loefgren A IEC curve (i.e. Baerwald), which is quite common and generally works well. Your P2S distance should ideally be set based on the tonearm maker’s recommendation, but if it’s a little off in reality, a true universal like the GeoDisc or SMARTractor will compensate for that (as long as you have enough leeway in your headshell slots), whereas the Mint LP will give you an increasingly erroneous result as you stray further from what the Yip thought the P2S should be. With a Universal - if you find yourself "short" on the headshell slot length (towards the front end of the shell) then the P2S distance is probably too big, and vice-versa.

In theory the MintLP for a specific setup will be more precise than even a SMARTractor (and I must admit the Mint has FAR better lines for cantilever alignment), but in reality this advantage will often be negated by P2S distance not being in EXACT agreement (again, unlike a universal, the Mint will not adjust for discrepancies here). I don't think it's uncommon for the P2S distance to be off by 1mm (or more) from where you requested the holes drilled into a blank armboard - maybe with a fixed ecosystem like a Rega you can be more confident in its published P2S distance. And the back-and-forth iterations between the two alignment points is a bit more time consuming, and can be a tad frustrating at first.

peter s,

You said;
"By definition, the null points are the two points where the cantilever is tangent to the record groove, and this is not effected by the "curvedness" (or flatness) of the arc."
As I stated, the null point distance from the record center does not change. As the stylus traces the flatter arc, the stylus tracing angle changes less in relation to the grooves. The Mint inner and outer null alignment lines will be more near parallel to each other to compensate. Overhang will be different in order to compensate as well.