Uni-Protractor Set tonearm alignment

Pgtaylor, Dertonearm …

It goes without saying that we're venturing into the fine " ether" of tonearm setup. With respect to SME's and the upper end Schroeder tonearms, the effective length changes require a bit of trigonometry, or a CAD program to determine, and producing a protractor dedicated to a particular cartridge needs to take this into account.

Follow me …

1. The effective length is the STRAIGHT LINE distance from the stylus tip to the tonearm bearing.
2. The cartridge is offset, per Baerwald/Loefgren/Stevenson geometry

So, merely adding the effective length difference will not give you the correct results (close, but no cigar). A cartridge whose stylus is 1mm further forward than a statistically "normal" cartridge increases the effective length by less than 1mm.

I just modeled a Tri-Planar on my CAD tool. I chose this arm because I had the numbers handy: effective length = 250mm, offset angle = 21.949 for Baerwaald. I assumed a 40mm long headhell (the offset component) for this exercise.

A cartridge with a stylus positioned 1mm forward of the statistical norm increases the effective length by.9275mm, for a net effective length of 250.9275mm. Simple addition would predict 251mm, or a variance of .0725mm (.0028"),

.0028" is within the threshold of audibility for a high performance analog rig, which is why everyone on this thread has justifiably been whipped into a frenzy over a better tool.

The takeaway from this is that any protractor generated for arms like the SME V, and the upper end Schroeders, needs to take the effective length, the length of the ofset section of the arm wand, and the proposed geometry into consideration.

I posted the drawing to the following URL for anyone who's interested: http://www.galibierdesign.com/images/forum/Eff_length_Model.pdf.

Cheers,
Thom @ Galibier

Tom Mackris.
Of course you are right when you say that the extra stylus tip-mounting holes distance cannot simply added to the manufacturer's stated effective length for the tonearm. In fact I was aware of that. What I was trying to illustrate was a way to measure the actual effective length (rather than calculating it) by effectively drawing a line (but in fact using the Feickert trammel to simulate it) from the stylus point to the arm pivot 'as the crow flies' (and then measuring it). Surely this (leaving aside for the moment any practical difficulties in achieving it) would give the correct measurement (?).
regards.

I understand what you were trying to accomplish, Pgtaylor. This was an illustrative example for those who haven't worked through the math.

BTW, do you really think that the Feickert has the precision to measure to .003"? If you can do this with that tool, you're a better man than I am.

Cheers,
Thom @ Galibier (the "h" is silent)

Dear Thuchan: I like almost all use audio after market items, I own no least than 9 different protractors but you know why?: because my ignorance level over time.
Fortunately that level improve over the years.

Regards and enjoy the music,
Raul.

Dear Pgtaylor, the SME V's alignment is indeed a very special problem. The Schroeder is different and offers the user a wider range of options.
But back to the SME V, which was when introduced anticipated like no other tonearm ever before or ever after.
The SME V is unique in the sense that it's offset and effective length (at least it’s designers thought so and intended it to be that way...) are fixed and pre-determined. Problem is, that SME Ltd. took for granted that each and every cartridge manufacturer would strictly follow IEC standards regarding stylus-mounting slots distance. Which of course they did not.
Now there is the legendary SME slide base to allow sliding the whole tonearm back and forth. That way the arm kind of "moves to the wanted alignment spot".
In theory....
The fact that the fixed offset angle of the fixed headshell isn't really a feature which eases things in any way did not really appeal to the SME engineers in their strive for setting the technical frontier in tonearm design.
Now can one align the SME V precisely with ANY cartridge mounted ?
Yes.
As long as the protractor reacts to a change in P2S as well as to a change in effective length automatically.
The UNI-Pro individual templates do offer for a given tonearm (and there is a individual UNI-Pro template for the SME V of course) an optimized spot of alignment - the corresponding effective length and ( in the case of the SME V ) the P2S are direct results of the alignment and are automatically generated.
Without any calculation required by the user.
In the case of the SME V (which really is a very special issue) there is however way less options for variations in alignment/calculation curves.
The SME V is a super strict 9"/Baerwald IEC-standard tonearm.
You can't really align him a Loefgren or Stevenson curve with good results.
I hope this answers your question, but you are welcome to get in touch directly in case you have further questions regarding your SME V.
Cheers,
D.