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I received an email through the Asylum from someone named James, but there was no return email address. James had a question about the following tonearm alignment equations, but I have no way of contacting him.The following equations are pretty straight forward and apply to pivotal tonearms with fixed offset alignments. The null-points are radial measurements from the center of the record to each null-point. I always use millimeters and degrees for my units of measurement.
Hey James, if you want additional information please send me your email address or else post your questions to the Vinyl Asylum forum.
Best regards,
John Elison
Edits: 04/16/17Follow Ups:
Hi John,
Looking at your equations, offset angle is a function of null points. But in reality, it is not possible to change the offset angle while changing null points for most of tone arms. Does this mean there are even more tracking errors?
Offset angle is a function of the sum of the null points divided by effective length. Therefore, the null-points can be changed without affecting offset angle as long as they add up to the same number, assuming effective length is fixed. In reality, the only parameter that is truly fixed on most tonearms is mounting distance. The vast majority of tonearms have slotted headshells so that effective length can be changed slightly and this also allows offset angle to be altered a few degrees in either direction by twisting the cartridge slightly before tightening the cartridge bolts.One thing everyone should know about offset angle is its definition. Brian Kearns stated it very eloquently in his Treatise on Cartridge Alignment, which can be found in the Vinyl FAQ :
Offset angle (theta): The angle between a projection of the cantilever on the record surface, and a line which passes through the centre of the tonearm pivot and the stylus tip.
This shows that offset angle is not the angle the headshell makes with the arm tube. It also shows that offset angle will change slightly by simply moving the cartridge fore and aft in the headshell slots without changing its alignment in the headshell. Therefore, simply moving the cartridge fore and aft in the headshell not only changes effective length, but it changes offset angle slightly, too.
Edits: 04/16/17
Thanks!
I understand it now.
It IS possible to change the offset angle. Almost all arms allow the cartridge to be twisted within the open installation channels. It is not necessary for the cartridge to be square to the front of the arm. Twisting the cartridge in order to have the cantilever tangent to the record grooves at two points is an important part of setting up a cartridge.
John, I may not always agree with your opinions, but your generosity in helping others out with technical questions in never in question.
Good on you!
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