How critical is the exact placement of the tonearm

Good point. I am aware of the reasoning, but I just thought it was a point of interest. The RS-A1 is eccentric in many ways, but somewhere in there is a very good idea.

If you're close you can make up the difference with overhang adjustments most of the time.

Once I installed a RB300 Tonearm on a Linn LP 12. The armboard on the Linn was drilled for a Linn tonearm. (I only did this as a stopgap until my new Linn armboard came that was pre-drilled for the Rega tonearm by the way)

I think the two are different by 7 mm or so and the Linn arm hole is larger.

I moved the Rega tonearm around in the larger Linn armboard hole until I was about 3 to 4 mm off. Then set the overhang so that it hit both marks and to my surprise it played just fine.

No inner groove distortion, no odd sounding bass or high or mids or anything out of the ordinary.

About 2 days later my new armboard arrived and I quickly replaced armboards and set up my table again. I can't say that I heard any difference at all.

Hitting both marks on a two point protractor makes one very huge assumption. That is, the pivot to spindle is dead nuts on. If it is not, there is nothing you can do to correct for it as the tonearm swings the arc. Sure, you might hit it at one point and it might be close a little farther into the swing but at the opposite end you're going to be off. It is simple geometry.

All,

Dan and I have been hanging out elsewhere discussing the vitrues of arc-style protractors. I'm sorry to say that if you have not gotten your pivot to spindle distance error to well under .5 mm, that you've never heard your cartridge.

Furthermore, you have to be an absolute wizard to accomplish this correctly with a two-point protractor. Yes, there are those among us who can do this, but we are far and few between.

In that thread, there's a link to a provocative article on the Vinyl Engine site.

Also in that thread, our good friend Palasr discusses a fellow in Hong Kong who will actually deliver an arc style protractor to you in about 10 days' time !! Palasr is very happy with this protractor, and at $90, considers it to be a bargain.

It really doesn't make sense to cut and paste this volume of information here. If you're interested, click this link: arc style protractor thread.

Cheers,
Thom @ Galibier

One follow-up comment from Dan's 5/20 post ...

The nature of a 2-point protractor is such that all but the most tortured souls will get one null point aligned perfectly and consider "really good" on the 2nd null point to be good enough. It is not good enough, but you will lose a lot of hair by working essentially in the dark to get "perfect" alignment on both points.

In theory, with a two-point protractor, you can have a pivot to spindle distance error - as long as you have enough room in the headshell slots to compensate by changing the effective length of the arm along with the offset angle.

In compensating for an error in pivot to spindle distance (drilling error), by moving the cartridge in the headshell slots, you are altering the other two Baerwaald parameters (assuming a Baerwaald protractor) to bring them back into line.

Play with John Ellison's excellent spreadsheet ("Free Stuff" page on the Enjoy the Music website as well as a link to it on my Support page) and this will all begin to make sense to you.

Note that the spreadsheet is driven off effective length and derives the pivot to spindle and offset angle.

Depending on which parameter is known, you might have to run iterations to get your answer. I do this by setting the numeric precision to 4 decimal places (go to Format ==> Cells menu selection and select "number", set decimal places to 4).

This is admittedly overkill but it allowed me for example to find the effective length that yields exactly 222 mm pivot to spindle distance.

Now, there's a down side to arc protractors, and as Dan hinted, your pivot to spindle distance must match the one specified by your protractor.

This presents an interesting challenge for owners of SME tonearms (SME iV, etc.) as well as Schroeder DPS and References. The challenge lies in the fixed mounting position of the cartridge in these headshells (from an effective length perspective).

This is NOT a problem with the tonearms, but it does present a challenge for the user who wants to use an arc style protractor. For my Schroeder customers, I produce a series of 5 protractors of varying effective length in order to compensate.

Because there is no standard amongst cartridge manufacturers for the position of the stylus relative to the mounting bolts, different cartridges will produce a different effective length with these tonearms. It's discussed in detail on the thread I linked to above.

Cheers,
Thom @ Galibier