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Setting Cartridge alignment using a Two Point Protractor

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The following is a method for setting cartridge alignment using a two point protractor.

This method is applicable to tone-arms with slots for positioning the cartridge fixing bolts. For tone-arms with adjustable bases, and fixed head-shells (such as SME arms), a different procedure is required. This article is an update of the instructions I posted last year on the same subject. The old instructions were a little loose in terminology and contained some errors. In particular, I have changed the procedure in step 6 slightly. I hope it is more clear now. The old article can be found at:

http://www.AudioAsylum.com/audio/vinyl/messages/38880.html

Preamble.

A two point protractor is a device which fits over the spindle; two grids are marked on the surface of the protractor, and these grids are positioned at different radii from the spindle. The distances from the centres of the grids, to the spindle are referred to as the null radii. When alignment is achieved, using a two point protractor, the tracking error will be zero at both null radii. A two point protractor will, in principle, give optimum alignment, as it sets the cartridge so that the tracking error of the stylus is zero at two points directly. There are many other protractors and jigs which use indirect method to set alignment. Examples include, the MoFi Geodisk, or proprietary jigs which are attached to the head-shell, and require alignment of the stylus relative to some reference point in the jig. For those who favour the indirect methods (which are often 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 issue. The user can select the null radii of their choice. In particular, the user may choose null radii which are different to those selected by the tone-arm designer. In this case, the result, after alignment, may be that the cartridge is skewed in the head-shell. Alternatively the user may wish to set the preferred alignment of the tone-arm designer, in that case it is necessary to find out the chosen null radii of the tone-arm, and then to get a two point protractor with grids at these distances.

A commonly chosen pair of null radii are 66.00 mm and 120.89 mm. These values are based on minimising the maximum distortion between the IEC standard groove radii of 60.325mm and 146.05mm. Regardless of the choice of null radii, the alignment method is the same.

As an example, I have used the numbers 66.0 mm 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 tone-arm pivot to the stylus tip.

Offset angle is the angle between a projection of the stylus cantilever on the record surface, and a line from the stylus tip to the centre of the arm pivot.

Tracking error, is the angle between a projection of the stylus cantilever on the record surface and a tangent to the record groove at that point on the record surface.

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

Method.

1Initially set the cartridge, so that its fixing bolts are roughly in the centre of the head-shell 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 head-shell until the cartridge is squared up on the grid. The cantilever should be parallel to the centre line on the grid.

5Move the tone-arm so that the stylus is at the centre of the inner alignment point (at the centre of the inner grid at 66.0 mm). It is necessary to adjust the angle of the protractor on the turntable to do this. 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 (when viewed from above) in order to achieve alignment on the inner grid. Do not make any adjustment at this point. 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.

6Case 1:

Increase the effective length of the arm, by sliding the cartridge forwards in the head-shell. The correct effective length can be set by sliding the cartridge forwards by twice the apparent correction necessary to align the cartridge on the inner grid. For example, if sliding the cartridge forward by 1mm in the head-shell, will give alignment on the inner grid, at this stage, then the correct adjustment is to slide the cartridge forwards by 2mm. If possible do not adjust the angle of the cartridge in the head-shell while adjusting the effective length. After completing this step, the tracking error should be about the same as before, but with the cartridge skewed in the opposite direction.

Case 2:

Decrease the effective length of the arm, by sliding the cartridge backwards in the head-shell. The correct effective length can be set by sliding the cartridge backwards by twice the apparent correction required to align the cartridge on the inner grid. For example if sliding the cartridge back by 1mm, gives alignment on the inner grid, then the correct adjustment, at this stage, is to slide the cartridge backwards by 2mm. If possible do not adjust the angle of the cartridge in the head-shell while adjusting the effective length. After completing this step, the tracking error should be about the same as before, but with the cartridge skewed in the opposite direction.

7Move the tone-arm back and reposition the protractor, so that the stylus falls on 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 and 7 until alignment is achieved at both points.

Notes

The above method should achieve alignment in a few iterations of steps 5,6,7, and 8. When the procedure is complete, the cartridge and cantilever should be squared up with the grid, when the stylus is positioned at either alignment point.

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 apparent at the inner grid.

It is best to disengage the drive belt, and if possible to lock the suspension of floating sub-chassis turntables.

With a conventional two point protractor, it is always necessary to adjust the position of the protractor on the platter when switching from the outer grid to the inner grid.

If the cartridge cannot be aligned on both grids, due to insufficient movement in the head-shell, then the best compromise is to slide the cartridge as far as possible in the right direction, and set the angle on the inner grid. In this case reduced tracking distortion will occur at the inner grooves, in exchange for increased tracking distortion at the outer grooves. It is the opinion of the author, that increased tracking distortion at the outer grooves is a better compromise, as other distortions are more significant at the inner grooves.

After completing a successful alignment once, the previous alignment can be used to set the effective length when changing to a new cartridge. With the old cartridge still in place, position the protractor on the platter and line the stylus tip up at the centre of the outer grid. Then clamp the platter and protractor in place. Set the new cartridge so that it is aligned with the either grid; alignment at the other grid is guaranteed.


Brian Kearns.



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Topic - Setting Cartridge alignment using a Two Point Protractor - bkearns 03:58:40 02/07/01 (19)


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