How critical is the exact placement of the tonearm

In the above arc protractor thread, Richard posts a link to a set of 3 loupes sold at Edumnd Scientific Supply ... for about $5.00.

Cheers,
Thom

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

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

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

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

Agreed that getting it "right" is the goal, Greg. It's fascinating to me that through the years, so many different geometries have been proposed as being right.

Cheers,
Thom @ Galibier

Hi Radrog,

I think that everyone is in agreement that you can get there from here with any tool. The issue is (a) how an individual interacts with the tool - how intuitive it is to use, and (b) how much effort it takes.

To date however, I have to add that everyone who has tried an arc-style protractor will never go back. I'm sure that there will be an exception to this at some point.

Hats off to Wally for his out of the box thinking years ago.

Cheers,
Thom