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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
martykl OP
3,563 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
6,770 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.
nrenter's avatar
nrenter
691 posts
 
If you really want to get into the details of a Baerwald geometry, you could
check out his paper on "Analytic Treatment of Tracking Error and notes
on Optimal Pick-up Design" over at
http://www.helices.org/auDio/turnTable/baerwald.pdf.

Yeah..I'm not really looking for that level of detail, either. But there are some
great resources over at AudioAsylum in the FAQ:

http://www.audioasylum.com/cgi/d.mpl?audio/faq.html

In particular, check out John Elison's downloadable Excel spreadsheet for
calculating horizontal tracking error and distortion, with graphs. Very useful.
atmasphere's avatar
atmasphere
7,068 posts
 
Marty, if you want a short answer, for the location you are asking about, you can set it up with 2 null points, but you will have a much larger tracking error at all the other points on the LP. By locating the radial arm at the end of the LTT these out-of-null points have a much-reduced error.

If you were to draw some circles representing the platter, and impose the radii of the two arm locations, you will see immediately what the problem is.
martykl OP
3,563 posts
 
Atma,

For any length tube it would seem that the geometry is the same (save the headshell offset), no? Now, to accomodate any "normal" tube length the pivot might have to be moved back some, but if I've (finally) got Baerwald figured out, it shouldn't matter which radius you're tracking: the radius nearer the operator as usually tracked by a pivoting arm or the radius perpendicular to the "rest" position of a pivoted arm, as tracked by an LTT. Or - alas - am I missing something again?

Thanx,

Marty
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