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back to alignment faq page

Universal Arc-Type Cartridge Alignment Protractor

by Brian Kearns

Hi there,

I developed a cartridge alignment protractor about 6 years ago, and I have been using it as a set-up tool for cartridge alignment for about the same length time.

I've used several protractors over the years, but I have to say (setting aside modesty form a moment) that I find this one to be the best.

I once had plans to try to develop this protractor as a commercial product, but now I realise that this will never happen, so I would like to donate the idea to posterity or to anyone with the wherewithal to have such a protractor produced.

I made my own prototype using a high resolution printer at my former place of work, and laminated the printed sheet. I cut the centre hole by hand

Essentially, it is an arc type protractor, however it is not customised to a particular tonearm length.

Background.
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When analysing tonearm geometry, I observed that the pivot of a tonearm of any length must fall along a line which has a perpendicular distance from the spindle which is equal to half the difference of the two null radii. So for alignment at null radii of 66.0 and 120.9mm, the line along which the tonearm pivot must fall has a perpendicular distance of 27.45mm from the spindle.

The above is well known, and is the basis of the Geodisc and the Dennison protractors, and more recently the Project CAT. Such protractors require that a sight-line intersect the tonearm pivot, and thus the location of the alignment grid is fixed on the platter; so once the sight-line is positioned to intersect the tonearm pivot, it is only a matter of squaring up the cartridge on the single grid for correct alignment. However, one problem with all of these protractors is that they are only as accurate as the users ability to point the sight line at the tonearm pivot.

The arc-type protractor is much easier to use, and any user who owns an arc-type protractor for their tonearm will testify how easy cartridge alignment becomes with such a protractor. However, arc-type protractors are customised to a particular tonearm mounting distance, and it is unrealistic to have a customised protractor for each toneram.

So what’s the idea.
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We know that all of the arcs scribed by a perfectly aligned tonearm are centered on a sight-line which has a perpendicular distance of 27.45mm from the platter spindle. But we also know that the alignment grid must be parallel to this line, and the center of the alignment grid must have a distance from the sight-line which is equal to the linear offset: half the sum of the null radii or 93.45mm for null radii at 66.0 and 120.9mm. For consistency therefore, all of the arcs scribed by a tonearm of any length must intersect the same grid.

Knowing this, the protractor design becomes trivial. The protractor comprises a series of arcs positioned along a sight-line where each arc intersects the same grid. And it gets better: by symmetry the same arcs must intersect another grid on the far side of the sight line. Now it can be seen that the two arcs fall at distances of 93.45mm from the sight line, and consequently the grids are respectively at distances of the inner null radius and the outer null radius from the spindle. There is one last step in the design: a second set of arcs can be drawn so as to create a second axis of symmetry in the protractor, this gives the option of checking alignment at the other grid by rotation the protractor on the platter through 180 degrees.

the result is a set-up tool which can lead to an accurately aligned tonearm in 20 or 30 minutes.

Here is a picture of the protractor.

Now, here is the interesting part.

It isn't necessary to know any of the parameters of your tonearm to use this protractor. There is only one valid line along that the stylus can track, so it is simply a matter of cueing the tonearm on the inner section of the arcs, and then on the outer section of the arcs, and adjusting the overhang until the stylus falls on the same arc at both places.

The next step is to cue a tonearm on the grid itself, and to set the offset of the cartridge in the headshell. Once this is done, the alignment is complete!

Here are some pictures of the protractor being used to align my recently acquired SME V tonearm.

(hint: click on thumbnail to view full size image.)

Tonearm cued on inner section of arcs.

Tonearm cued on outer section of arcs.

Tonearm cued on grid.

Verifying that the alignment is good.

Now sit back and enjoy the music.

There is one caveat: a critical point in the construction of any protractor is that the printing or etching of the lines must be very accurate, and the centre hole must be punched in the right location. Any significant errors in these will render the protractor completely useless.

Here are the protractor files ready to be copied to your hard drive.  Choose the version you prefer.  Personally, I like the color version.  The PDF files are in Adobe acrobat format.  The DXF files are a translation file that will work in most CAD programs. Ed.

How to copy from this page to your hard drive using Mosaic FireFox in MS Windows OS:

Other browsers and other operating systems will offer similar capabilities.

Reference Check to verify the accuracy of your print-out.  

Webmaster comments:

This looks really good so I thought I'd give it a tryout over here at The Analog Dept.  First you need to copy the file to your hard drive.  Then open it in Adobe Reader (pdf file). You need a hi-rez printer.  A photo quality ink-jet should do.  Make sure to print 'full-scale' or the protractor won't be accurate. *

Reference Check to verify the accuracy of your print-out.  

I chose to print out the color version so that it would be easier to pick out which arc the stylus is standing on.  I used a glossy heavyweight photo paper and then mounted (glued) the print-out onto some matte board to make it more rigid and permanent.  I used a pin to 'dimple' the stylus position of the alignment grids and also to mark the centers of the two spindle holes.  To cut the two spindle holes I used a drafting compass with the knife-edge attachment to make the initial circular cuts.  To finish off the holes I pushed through and trimmed with an X-Acto knife.  My newly cut holes are a tight fit on the spindle and don't appear to be off-center.

My newly printed, mounted and trimmed protractor ready for use on the Thorens.  But I cheated.  My tonearm and cartridge are already aligned and optimized.....I think.  I had previously used the 2-point protractor that comes with the HFNRR test record.  So this will be an exercise to double check the alignment made previously with a different protractor..... but with this new Universal Alignment Protractor from Brian Kearns.**  Besides, I like to verify the reading of one instrument from that of another as a means to verify the calibration the thing being calibrated.  And this Universal Arc Protractor checks the full length of the stylus path across the record.

Here I've laid the stylus down into the pin-hole on the grid.  In this photo I'm trying to establish that the cantilever is in alignment (parallel) with the grid.  Looks pretty good....as near as I can see.  It's dark under the custom Ebony body of the Denon DL-103R MC phono cartridge, but it looks like the cantilever is on-axis with the center grid line of the protractor.

Here's a long shot of the stylus at the inner end of the arc.   Note proximity of spindle pin.

Close up at inner end of arc.  The stylus is riding just ahead of the black mid-line between the green and orange arcs; indicating an effective length about midway between 230 and 240mm.  Close to 236mm.

Close up at outer end of arc. Still riding between the green and orange and just ahead of the black mid-line.  Say 236mm.  That's 1mm away from the manufacturers listing of 237mm effective length for any of their arms starting with the RB250 and those that came later.

Footnotes:

* For you pc newbies this means you can't use any of the "scale to fit" options that tend to deploy automatically every time you print something from your computer.  Look at your printing options within your printer driver software and make sure that, for this print, the size of the document won't be altered.

The protractor size is slightly larger than letter sized paper format.  I used 11 x 17inch (B plot sized) paper to print from my photo ink jet.  If your printer won't print that big, go ahead and use letter sized paper but accept that the very ends of the protractor will be truncated.  Still functional.

** Both alignment protractors in use for this exercise feature the Baerwald Theorum of overhang alignment geometry.  Hence, both protractors should, in theory, produce the same overhang alignment result.