How critical is the exact placement of the tonearm

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

Thom- Unless I totally have the whole conceptual model of tone arm-cartridge-stylus geometry wrong (which could easily be the case) your comment about the Schroeder does not appear to apply to the M2, in which the cartridge carrier can move back and forth (and also pivot, damn it!). Pls confirm if you get a chance.

You are absolutely correct, Swampwalker. The Model-2 has a slot in the headshell which allows you to adjust the effective length (pivot to stylus distance) of the tonearm. The DPS and the Reference are fixed in a hole which allows the cartridge to pivot but not slide back and forth. My guess is that Frank didn't like the structural integrity of a slot milled into the hardwood headshells.

Playing with the Ellison spreadsheet will help you get the grok on all of this.

As an aside, I was pointed to a compendium of tonearm specifications (not the Vinyl Engine website). I don't have the link handy, but if ddriveman is following this thread, I'm sure he'll provide it.

Anyhow, the number of alignments chosen by the various manufacturers is astounding, and by no means limited to Baerwaald, Loefgren and Stevenson. In that document you'll find null points placed all over the map.

Many highly regarded arms (Dynavector, et. al) have specified some unknown geometery. When I get a chance, I'm going to draw some arc protractors up for some of these geometries - to the extent that I have a tonearm on hand which I can "force" into these alignments.

One of these arms is the Artisan that I carry. Way back when Ralph (Artisan) and I were evaluating the arm, we decided that we needed to have an easy means of providing the end-user with a reliable means of producing a known geometry.

Ralph and I felt that an arc-style protractor would better help the user visualize and set up their cartridge.

The visualization problem arises because the manufacturer's chosen alignment specifies an effective length that is almost 3mm short of what the 214 mm pivot to spindle distance would predict for Baerwaald. It also specifies an offset angle of about 1 degree less than what Baerwaald predicts.

To achieve a Baerwaald alignment, a cartridge of average stylus position relative to mounting bolts, with a perfectly aligned cantilever would be some 2.8mm forward of the center of the headshell slots and rotated clockwise by about 1.25 degrees from being parallel to the headshell.

All of these differences would likely confuse the user if they use their two-point Baerwaald protractor and begin the process by placing the cartridge bolts in the middle of the headshell slots and line up the front of the cartridge body so that it is parallel to the front of the headshell.

I should most definitely return to this alternate geometry exercise to understand the intent of the manufacturer. There's been no rush, since "nuts on" Baerwaald has been very, very nice.

Cheers,
Thom @ Galibier

Speaking of adjustable, SME tonearms allow adjustments of pivot to spindle distance. I have such an arm (a Pluto -- not an SME).
The funny part is, unless someone gives you the exact pivot to spindle distance, you're stumped if you move the base of the arm...
To make things even more exciting, my arm's instructions, for example, spec 219mm pivot to spindle & 23degrees offset angle... Well, it's either one or the other -- but not both (per Baerwald).
What shoudl I choose???

Yes, Greg ...

With the SME architecture you're "stuck" with a fixed offset angle (to the extent that your cartridge screws have some play in the headshell holes).

This essentially means that the cartridge manufacturer's production tolerances dictate the offset angle, as well as the tonearm's effective length (due to the position of the stylus relative to the mounting bolt holes).

The fact that you can nicely alter the pivot to spindle distance on their way-cool adjustment track means that you can set the pivot to spindle distance to agree with this effective length which is dictated by the stylus position.

This leaves us with one "uncontrollable" parameter - offset angle.

I need to do some empirical reserach on the importance of offset angle. Different offset angles move your null points, but given that we're talking about perhaps +/- a degree (due to manufacturing tolerances), this may not be such a big deal.

We've certainly learned that tracing a perfect arc (getting pivot to spindle and effective length to agree with each other) are critical, and the SME design provides you an excellent means of achieving this.

This offset angle thing is an area I plan to investigate, along with all of those alternate proposed geometries.

Cheers,
Thom @ Galibier