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Setting tangental alignment using a two point protractor

210.250.76.20

Posted by bkearns on October 17, 2000 at 20:19:12

The following is a method by which to set cartridge alignment using a two point protractor.

Preamble.

There has been a lot of discussion on this subject here, and it has been proposed that a two point protractor is inaccurate, as it does not set effective length. This is incorrect. A two point protractor will, in principle, give the best alignment, as it sets the cartridge so that it is tangential to the record directly. Other methods which involve setting effective length and offset angle will also give the correct result, however they do not set alignment directly. So there is no way to check if there is some error in the technique. For those who favour the Geodisk and Dennesen methods (which are much faster than a two point protractor), they should check at least once, that their technique is correct using a two point protractor.

This note, does not make any recommendation on the optimum null radii for minimising tracking distortion. There is a lot of data on this subject, however it is a separate subject. The user can select the null radii of their chioce. But whatever the choice the alignment method is the same. In particular if one opts for the commonly chosen null radii at 66mm and 120.9 mm then that may disagree with the preferred alignment of your tonearm designer. So the result, after alignment, may be that the cartridge is not parallel with the headshell. Again this is a separate issue. If the user wishes to set the preferred alignment of the tonearm designer, then it is necessary to find out the chosen null radii of the tonearm, and then get a two point protractor with grids at these distances from the centre of the record.

For the sake of an example I have used the numbers 66mm and 120.9 mm for the distances of the grids from the record spindle in the text which follows.

Definitions

Effective length is the distance from the centre of the tonearm pivot to the stylus tip.

Offset angle is the angle between a projection of the stylus cantilever on the record surface, and a the effective length.

Tangential angle, is the angle between a projection of the stylus cantilever on the record surface and a tangent to the record surface.

For more info on terms and definitions, seehttp://www.AudioAsylum.com/audio/vinyl/messages/37478.html

Method.

1Initially set the cartridge, so that its screws are roughly in the centre of the headshell slots.

2Place the protractor over the platter spindle.

3Position the stylus on the outer alignment point (at the centre of the outer grid at 120.9mm).

4Adjust the angle of the cartridge in the headshell until the cartridge is squared up on the grid.

5Move the tonearm so that the stylus is at the centre of the inner alignment point (at the centre of the inner grid at 66mm). If the cartridge is squared up with the grid then the alignment is complete. However if the cartridge is not squared up with the inner grid then determine which direction the cartridge should be rotated in order to achieve alignment on the inner grid. Do not adjust the angle of the cartridge. Case 1: if correct alignment requires an anti-clockwise rotation of the cartridge, then the effective length is too short. Case 2: if correct alignment requires a clockwise rotation of the cartridge, then the effective length is too long.

6

Case 1:

Increase the effective length of the arm by sliding the cartridge forward in the headshell. The correct effective length can be set by sliding the cartridge forward so that the tangential angle is off by about half as much in the opposite direction. In other words, if the required adjustment to align the cartridge at the inner grid is a rotation of 5 degrees, anti-clockwise, then slide the cartridge forward until the required adjustment for alignment is a 2.5 degrees clockwise rotation. Do not rotate the cartridge if possible while adjusting the effective length.

Case 2:

Decrease the effective length of the arm by sliding the cartridge backwards in the headshell. The correct effective length can be set by sliding the cartridge backwards so that the tangential angle is off by about half as much in the opposite direction. In other words, if the required adjustment to align the cartridge at the inner grid is a rotation of 5 degrees, clockwise, then slide the cartridge backwards until the required adjustment for alignment is a 2.5 degrees anti-clockwise rotation. Do not rotate the cartridge if possible while adjusting the effective length.

7Move the tonearm back to the outer alignment grid. Adjust the angle of the cartridge again so that the cartridge is squared up on the grid.

8Repeat steps 5, 6 7 and 8 until alignment is achieved at both points.

Notes

The above method should achieve alignment in three iterations of steps 5,6,7, and 8. With experience you might be able to do it in one iteration.

This method of setting offset angle at the outer point, and effective length at the inner point is best, as errors in effective length are more critical at the inner grid. An error of 1.25mm in the effective length will give a tangential alignment error of 1 degrees at the inner grid, and 0.6 degrees at the outer grid.

The above point explains why the two point protractor sets both angle and effective length. An error in effective length gives a different tangential alignment error at both points, so correct alignment at both points guarantees the correct effective length and the correct offset angle.

While it might be stating the obvious - here goes. After completing a successful alignment once, it is not necessary to set effective length when changing to a new cartridge. Simply place the protractor on the platter and line the stylus tip up at the centre of the outer grid before changing the cartridge. Then clamp the platter and protractor in place. Set the new cartridge so that it is aligned with the outer grid, and inner grid alignment is guaranteed.

Regards,

Brian.

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