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general tonearm design question

Many popular tonearms are designed with with the fulcrum for the vertical axis at ~70 degrees (e.g. Rega, SME), rather than perpendicular (e.g Origin Live).

Doesn't the former design cause the needle to track to the outer groove as the counter weight swings downward?

...or does the cartidge/tube somehow counter this?

Would a counter weight mounted at 90 degrees to the fulcrum, yet the arm tube set at 70 degrees be the solution to this, or am I wrong?

(The reason I started wondering about this was due to the varous Rega counter-weight mods. I would think that a lower center of gravity would exacerbate this problem. Furthermore, wouldn't a lower center of gravity only be effective on a uni-pivot design?)

your thoughts?
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Showing 3 responses by nsgarch

nsgarch's avatar
nsgarch
2,524 posts
 
The lower (than the vertical bearing) CG gives an arm more stability when tracking warped records. The angle of the vertical bearing is not necessarily 70 degrees but is set to match the headshell's angle to the armwand. This insures that when the arm (actually the stylus/cartridge) move up and down when (again) tracking warped records, that the azimuth of the stulus remains vertical to the record. If the bearing were perpendicular to the arm and the headshell angled, then (looking at the stylus head-on) the stylus would describe an arc as it rose up. (Was that as clear as mud?! ;--)
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nsgarch's avatar
nsgarch
2,524 posts
 
David, I can't explain OL's design, it doesn't make sense to me. Maybe Doug has some thoughts. . . . . .

I mean one can always make excuses like: if a record is sooo warped that if the height variation (from flat) would significantly throw the azimuth off, you couldn't play the record anyway, etc., etc. But I put my trust in the wisdom of the designers like SME and others. I'm not really sure what happens to unipivot arms when the stylus hits a bump. Anyone?
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nsgarch's avatar
nsgarch
2,524 posts
 
You could test unipivots for azimuth stability by throwing on (just said that to make Doug cringe ;--) an SQ Quadraphonic record. Its groove not only deflects the stylus left-right-up-down, it TWISTS it CW and CCW!
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