Protractor User Guide
Phew, so many protractors in the set-up tools section !!!
Which one I should use?
In fact, it depends on the Turntable / Tonearm combo you’re using…
Some of the protractors posted on the Vinyl Engine are specific, some of them are generic. Some of them are Baerwald type, one is Loefgren and two are Stevenson
Baerwald? Loefgren? Stevenson? Which should I choose?
Which is the best?
I won’t tell you there is one better than the others. Make your own opinion.
Simply, the equation you need to solve to find optimal alignment is undetermined: there are too many unknown parameters. Baerwald, Loefgren and Stevenson apply different objective constraints to the problem to allow a solution:
- Baerwald The distortion at the beginning, the middle (where it’s high) and the end of the record must be equal
- Loefgren Alignment must minimize the distortion across the record
- Stevenson Distortion is more important at the end of the record so let’s minimize it by assuming a null point at the end of the record, i.e. at 60.325 mm (IEC standard end of the record)
What is a null point?
The path of the stylus follows an arc. As a matter of consequence, the stylus is not always orthogonal to the groove: it’s only orthogonal for two points named the null points. The main goal of alignment is to set up these null points at specific point’s dependant on the objective.
Conclusion: null point pairs (measured from the centre of the record) are different because of the added constraints:
- For Baerwald null points are 66.0 and 120.9 mm
- For Loefgren null points are 70.3 and 116.6 mm
- For Stevenson null points are 60.325 and 117.42 mm
When you align a cartridge (using a particular technique) by setting the null points, you’ll obtain the ‘correct’ effective length (distance between pivot and the stylus) according to your mounting distance, the ‘correct’ overhang (difference between the effective length and the mounting distance) and the ‘correct’ offset angle (the angle between a line going from the pivot of the tonearm to the stylus and the axis of the cantilever/cartridge body (assuming that the cartridge body is parallel to the cantilever).
A simple drawing will explain everything
Which protractor for my combo?
Baerwald is the more common alignment technique
Loefgren is less popular but it makes sense.
Now, Stevenson. (not very common)
To me (my personal opinion), the protractor offered by Rega with their TT and tonearms is the Stevenson type. The unique null point proposed by this protractor is very close to the 60.325mm stated by Stevenson. But aligning a cartridge perfectly a with a unique null point is almost impossible, except if you’re using the third hole of the headshell (the one in the front) with a Rega cartridge. In that case, you force the effective length to the correct value stated by Rega (to me, it’s 237.44 mm) and you only need one null point.
Now, with other cartridges, the second null point is necessary to obtain a good alignment (by the way, if you bought the turntable basic’s protractor, you may feel that this tool is not as precise as expected)
The first thing to do is to check if you can use a specific protractor since they are easier to use.
A specific protractor will only be good for a TT equipped with a specific tonearm:
TT Specific Protractors
Rega Baerwald and Stevenson protractors
The Rega Baerwald protractor and the Rega Stevenson protractor are only of use with Rega turntables equipped with Rega tonearms. It may be useful too if your arms mounting distance is EXACTLY 222 mm (as for every Rega turntable). For instance, the NAD 533 turntable can use this protractor since they are a copy of the Rega TT sold under another brand name.
Using these protractors is very simple: be sure that the stylus follows the arc and align the cartridge with one set of lines. Why one? Because you only need one.
What is the point of the other line? It’s just a way for you to check that your mounting distance is correct. If the stylus follows the arc, if the cartridge body is aligned with a set of lines, if your mounting distance is the one specified by the protractor, then, the cartridge body must be aligned with the second set of lines. If it’s not, there is a problem (and specially, if the mounting distance is not the one indicated, you won’t observe a perfect alignment using the second set of lines).
The Rega Stevenson protractor may be used by people who would like to align their cartridge the Rega way without using the protractor provided by Rega.
Thorens Baerwald arc protractor
The ‘Thorens Baerwald arc’ protractor is only for use with the following turntables equipped with stock arm:
Ambiance, TD 125-II, TD 125-II, TD 126-I, TD 126-I, TD 126-II, TD 126-III, TD 126-IV, TD 145, TD 145-II, TD 146, TD 146-II, TD 146-IV, TD 160-I, TD 160-I, TD 160-IV, TD 160-V, TD 165-I, TD 165-II, TD 166-I, TD 166-II, TD 166-V, TD 280, TD 280-II, TD 316, TD 316-II, TD 318, TD 318-II, TD 320, TD 520
Stock arm : TP11 I, II & III, TP16 I, II, III & IV, TP21 and TP28 mounted at 215.6mm from the spindle centre.
Again, the stylus must follow the arc and you must align the cartridge body with one set of lines. If it’s not aligned using the second set of lines, it’s because the mounting distance is not the one indicated here (215.6). In that case, use a generic protractor.
The “Stupid protractors” file proposes two generic protractors: One based on Baerwald theory and the other on Loefgren. The “Stevenson protractor” file proposes a generic Stevenson protractor.
A novice should go for the Baerwald version, people who know what they’re doing will choose the one they want (since they are big boys).
How to use this kind of protractor?
Two grids and a hole: the aim is to obtain a perfect alignment with the two sets of lines and the cartridge body.
First use the grid closest to the spindle hole. Put the cartridge in the middle of the headshell; twist the cartridge clockwise or anti-clockwise in order for you to observe the alignment with the grid. When it’s right, go to the second grid.
If the cartridge must be rotated clockwise (or anti-clockwise) to be aligned with the second grid, move the cartridge forwards (or backwards) in the headshell, then go back to the first grid. Align the cartridge and check with the second grid; repeat until alignment is achieved at both points.
Kearn’s universal arc protractor
Now for the “Kearns universal arc protractor”. Not for the novice! Here the trick is to ascertain which arc should be followed by the stylus. You must first know (measure) the mounting distance of your tonearm, check a table to know the effective Baerwald length and then choose which arc to follow and align the cartridge body with the grid. Once the set-up is complete, you can check with the B hole that you’re correct: using that B hole, the cartridge body must be aligned with the grid – if it’s not, you followed the wrong arc.’
Finally, the “stuff protractor”. When using a standard protractor we make the assumption that the cantilever is parallel to the cartridge body, that’s why we align the cartridge body with the grid. But the correct thing to do is to align the cantilever with the grid. Hmm, not that easy. The stuff protractor describes a way to align the cantilever using a mirror (you must be able to drill a mirror and to engrave it with lines). Again, not for the novice!
Please post in the forum if you have any questions.