The question asked:
"I accept that the majority of tonearms in use today are 'fixed' to their turntables and cannot be adjusted.
The accuracy of the positioning of these tonearms vis-a-vis the spindle to pivot distance is often not the greatest and I appreciate that the UNIprotractor [a fancy alignment device] achieves the correct cartridge alignment regardless of the accuracy of the tonearm placement.
Having said that however, I believe that the cost of the UNIprotractor can best be amortized by someone possessing multiple arms and often in such a situation, some of those tonearms will have removable headshells?
If each tonearm is 'inaccurately' positioned in terms of S to P distance, the swapping of headshells+cartridges between arms, will result in multiple accumulating errors......no?
Additionally, if one is using fixed headshell/cartridge combinations such as the FR-7 series or some EMTs, adjustments within the headshell/cartridge are simply not possible and correct spindle to pivot dimension is essential to extract the best performance n'est pas?
It is obviously ideal to have the tonearm set at the correct S to P dimension and an accurate method for achieving this is desirable. "
My answer:
I haven't used it, but I'm sure you are correct in assuming a different principle [to the Uniprotractor] for the Feickert [another expensive set up tool]. If it has the facility to set or measure mounting distance, it is because it uses arcs which have to be referenced to a known pivot point.
You are correct in saying that headshell type integrated cartridges [like Ortofon SPU or Fidelity Research FR7] having no facility for adjustment need to have the effective length of the arm with the cartridge fitted known or measured. Also the true cartridge offset angle (which will depend on the arm/cartridge combination) also known or measured. Then an appropriate mounting distance can be calculated, and precisely set.
Alternatively, using an arm with a known alignment, you can use (if it has the same alignment) a two point protractor to set up. By sliding the base the offset will be given automatically. Swapping different kinds of these cartridges would depend on them having the same headshell mounting collar to stylus distance [usually stated as 50mm but not necessarily].
You are also right in saying that swapping headshells could be problematic. It looks to me that swapping headshells between arms would require them to be set up initially with one headshell/ cartridge combination which is non adjustable [ eg SPU] which remained set once the first arm was done, and which was then used to set the remaining arms with adjustment achieved at the bases of the others, assuming they were different arms.
The alignment chosen would have to also be the same for each set up, irrespective of the arm design, but the arms could be different lengths.
Then any cartridge/ headshell combination could be set up to suit these alignments without moving the bases. This would depend on the headshells having slots, but the initial headshell could have holes or be unadjustable as long as it was used in an arm designed to the chosen alignment.
I think that covers most of the difficulties..... please feel free to point out a simpler way, any mistakes or anything I've overlooked by email.
