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Why two null points (long but illustrated)

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In my picture, I’ve shown the outline of a 33 1/3 rpm record, the Null Points (NP122 and NP 66mm), and the arcuate path of my Kuzma Stogi S tonearm. At NP122 and NP66 I’ve shown tangents to the grooves (green lines) as one would find on a cart protractor. These tangents are what one aligns the cart to when using a protractor. Only one overhang distance will allow the cart to be parallel to both of these tangents. Note that a fixed angle is defined by the arc of the tonearm path and the orientation of the cartridge. All of the short red and green straight line segments in the photo have the same orientation relative to the path of the tonearm, and represent what would be the orientation of a properly aligned cart at those positions.

The red lines showing the orientation of the cart at the outer and inner edge of the “record” are visibly not tangential to the record groves. They are rotated counterclockwise relative to a the record grooves as viewed in the photo. Logically, there is an opposite (clockwise) rotation of cart relative to a the record grooves in the arc between the two Null Points (Zone B), with the maximum error somewhere near the middle. I have not attempted to demonstrate that opposite rotation in Zone B..

Why does this happen? Picture the sweep of the tonearm. Consider the orientation of the cart relative to a radii drawn from the spindle. As the tonearm sweeps to the right (towards the spindle), the orientation of the cart rotates clockwise. Now consider the orientation of the record grooves relative to the arcuate path of the tonearm. Relative to the path of the tonearm, the grooves also rotate clockwise as the tonearm sweeps to the right, as viewed in this picture. This rotation accelerates as the tonearm sweeps to the right

There are two null points because through the arc of the tonearm, there are changes in the relative rates of the rotations of the cart relative to the grooves, and of the grooves relative to the path of the tonearm. In Zone A, sweeping from the edge of the record towards NP122, the cart begins its journey misaligned, with a counterclockwise rotation relative the grooves. In Zone A, the rotation the cart relative to the grooves is quicker that the rotation of the grooves relative to the arc, and they become parallel a NP122. In Zone B, the rotation of the grooves relative to the arc increases, and that rotation eventually overtakes the rate of rotation of the cart, so that at NP66 the cart and grooves are again parallel. In Zone C, the rotation of the grooves relative to the path of the arm continues to accelerate, increasing the counterclockwise misalignment of the cart in the innermost grooves.

Considering my illustration, note that if an incorrect overhang increases the effective tonearm length, as would shift the path of the tonearm upwards in this picture, the misalignment in the inner grooves will become greater, and the misalignment at the outer grooves would be little changed.


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Topic - Why two null points (long but illustrated) - dpy 13:02:11 04/04/02 (9)


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