How critical is the exact placement of the tonearm

Thom, I am not sure whether I under stand this correct though:

If I read the post on vinylengine it seems that the problem of spindle to pivot distance is only a problem with arc protractors (if the spindle to pivot distance is wrong one gets the wrong arc across the record, the wrong null points and higher distortion.

What I don't understand: If I use one of the standard two point protractors to adjust the nullpoints correctly, am I not close? Of course if my spindle to pivot is slightly off, I may move the cartridge in the slots and the only effect is that the effective arm length has changed slightly. There really is no preferred effective arm length and and the distortion curve is pretty close since it is fixed by the null points and the variation in effective length is small.

What am I missing in the geometry? Why no align then for the two null points correctly. Is it just that an with arc protractor, it is easier to see any alignment errors? But then you can never determine the effective length more accurately than spindle to pivot distance anyway. Both methods seem to have similar errors built in.

Thanks for all your clarifications!

Cheers,

Rene

Hello All,
There is a minefield of information on this thread for newbies. It does explain however, that when I put a different type of allignment tool on my Rega P3 many years ago, I couldn't get the geomtry right.

Anyhow, that arc protractor sounds helpful. Does anyone have a link for that Gent in Hong Kong who makes one for under $100?

Also, where does that old Mobile Fidelity alignment disc (the one that was the size and shape of an LP) fit into all this? Is the Mobile Fidelity disc Baerwald geometry? Is that disc an accurate enough tool to use in the correct alignment of arm and cartridge?

Hi Rene,

You didn't miss anything. You are correct that there is no preferred effective length (as long as you are not using an arc style protractor which is drawn for one and only one set of parameters).

The benefit of a two point protractor is that with enough diligence, flexibility in the headshell slots, etc. you can get a perfect alignment irrespective of your pivot to spindle distance being off - as long as you have enough movement in the headshell slots to adjust the effective length and offset angle to match your "wrong" pivot to spindle distance.

With an arc style protractor, the price you pay for ease of visualizing and solving the problem is that the pivot to spindle distance must be very close to perfect, because the arc is drawn from the theoretically correct arm mounting (pivot) point.

Dsa - A few posts up, I have a link to an arc protractor thread. In that thread, Richard posted a link to the protractor fellow. He loves it even more than his "original" arc protractor.

Cheers,
Thom

Hong kong guy Arc protractor link http://www.mintlp.com

Very easy to deal with, fast turn-around. Not sure I've got the alignment right yet; its pretty tough when you wear progressives and are really far-sighted, but it appears to be well made and the instructions are not bad.

Hi Swampwalker- thanks for the link. Looking at the info on their site- the customer sends their tonearm and turntable brands (and also cartridge?) and they make a tractor specifically for that set-up?