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Arm geometry and mounting distance

OK I know there have been dozens of posts on this subject but a few things are still a little unclear to me?
There is a clearly defined MOUNTING DISTANCE for every arm which dictates the pivot to spindle centre dimension.
There is a clearly defined EFFECTIVE LENGTH for every arm which dictates the stylus to pivot point dimension and thus the OVERHANG.

If we can accurately set the mounting distance correctly to the nearest +/- 0.2mm and then the overhang to +/-0.1mm, surely this must be as accurate as we can get to achieving the designed Baerwald or Lofgren geometry as long as the cartridge is aligned tangentially at the two relative 'null' points?

The problem actuality I believe, is achieving this degree of accuracy with the MOUNTING DISTANCE?

If your turntable was predrilled for the arm and mounted in the factory, you would imagine that the tolerances could be close to those specified but if a dealer drilled and mounted your arm or you yourself did so, I doubt that it would come within coo-ee of those sorts of tolerances?
Unless you have a machined metal template such as the Feickert Jig Feickert Jig to 'lock-in' the spindle centre, together with a machined and calibrated 'beam' to accurately span the distance to the arm pivot centre, it would be a fluke to achieve anything like the accuracy required.

Now I find little mention of the Feickert jig in all the discussions on tonearm geometry yet I find many references to the 'Wallytractor' (which I have), and also the 'Mint Arc Protractor'.

My question is this:-
If your tonearm MOUNTING DISTANCE is out by 5mm (1/4"), can you accurately align the arc using the Wally or Mint and thus all will be well?

I know that when I use the Feickert Jig and then run it over the WallyTractor I achieve perfect alignment whilst if I try to use the Wally first, it's impossible to achieve perfection?
halcro

Showing 2 responses by dertonarm

dertonarm
1,042 posts
 
Dear Henry,
I won't comment on the effective length in conjunction with manufacturer specified mounting distance, as this will no doubt (since it had done so in the not so far away past....) bring up all the tonearm geometry experts who do claim the opposite.
On to your question:
you can align ANY given pivot tonearm so some kind of geometrical "correct" geometry, even if his specified mounting distance is "off" by some millimeters.
However - it will ALWAYS alter the geometry - in other words: - you will NOT obtain the geometry the designer specified and optimized the tonearm for.
The location of the 2 zeros and the maximum errors of the tangential curve will be different and most likely not ideal.
Since opinion about where to specify the location of the 2 zeros do indeed differ and depends on the majority of records played with the tonearm in question this point is regarded by many audiophiles as being less important.
I however would always recommend holding exactly to the manufacturers specification (unless you are able to re-calculate a toenarm geometry yourself) and to align to a tangential curve with teh 2 zeros spread fairly wide and the 2nd close to the run-out grooves.
This will most likely give you the most stable soundstage AND the least inner groove distortion.
Cheers,
D.
dertonarm
1,042 posts
 
Dear Henry, you are an architect.
Let me try to illustrate the point in a language which is familiar to both of us.
As long as there is any overhang still possible with the tonearm you mount (even if the mounting distance is off according to the manufacturers specs) AND as long as you can get 2 zero point on the arc crossing the tangential line towards the spindle within the grooved area of a given record - as long as this is still the case and the mounting area of the tonearm allows for alignment of offset - you can adjust the tonearm.
It has a different effective length now, a different overhang and a different location of the 2 zero errors and most likely different (higher) maximum tangential errors.
The mounting distance does determine the geometry of a given tonearm following its designers intentions and calculations.
Thats all.
You an alter this geometry by changing the mounting distance and thereby altering all other parameters too.
Most likely it will not be a change to the better.
Cheers,
D.
It has a "new" geometry now and this geometry is most likely not perfect nor close to that.
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