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tonearm geometry question

I've followed the linear vs pivoted thread with some interest. Itt raises a question that someone with greater technical expertise may be able to clarify for me.

At rest, both a pivoted arm tube and an LTT tube share a common position tangent to the platter ( call it the CP line) and a common anchor ( or pivot) point (call it CAP). From there, a pivoted arm tube defines an arc across the record, while the LTT tube slides on its anchor point from the CAP along a line perpendicular to the CP line and tangent to the platter until it hits the inner groove. Call this the LTT anchor journey.

My question: Why is the pivot point on a pivoted arm not located halfway along the LTT anchor journey. Wouldn't this reduce the pivoted arm's error by half? Surely loading/removing the record can't be the reason. What am I missing?

Thanks in advance.

Marty
martykl
nrenter's avatar
nrenter
765 posts
 
I think I understand what you are asking, and the answer is simple, but the mathematics behind the answer are not. If you use the geometry defined by Baerwald, there are actually 2 points along the arc that provide 0 tracking error. These are your null points. What I believe you propose is a single null point smack dab in the middle of the LP. This would greatly increase your tracking error. I'm posting this response from my phone...I'll try to provide more detail later.
martykl OP
3,578 posts
 
Nrenter

I believe that am proposing a null at dead center. My mathematical training is limited, but intuitively, a center null would seem to minimize overall deviation relative to the radius described by a stylus moving across the record on an LTT. I would add that you it seems to me you'd also need to hold the length of the arm tube constant for a comparison of pivot placements.

BTW, if a null is a point on the radius I outlined above, how can a pivoted arm decribing an arc centered outside the platter (ie your standard pivoted arm) cross that radius line twice. I don't understand Baerwald.

Any clarification is appreciated.

Thanx,

Marty
martykl OP
3,578 posts
 
Upon reflection, I can visualize 2 nulls across the radius (baerwald), but it's not intuitive that this would minimize total deviation from the desired radius - the calculus is beyond me. But, the same logic could be applied to create 2 nulls with the central pivot I describe - the tube must be longer and the pivot further away from the radius. Either way, for any given tube length. it seems to me that a cenetred pivot would reduce overall deviation.
martykl OP
3,578 posts
 
Sorry to bother everyone, but I think the bulb just came on:

Any tube length defines an arc of a fixed degree of curvature. So long as that arc crosses any radius on the LP at the 1/3 and 2/3 points of the section of radius being tracked, total deviation should be the same. My scheme merely fixes the radius in question at the same place an LTT would.

Thanks again,

Marty
lewm
12,487 posts
 
For a third option, between conventional pivoted tonearms and LT arms, try the 47 Labs RS-A1 tonearm or the headshell which can be purchased separately.
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