How critical is the exact placement of the tonearm

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

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?