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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
atmasphere's avatar
atmasphere
12,012 posts
 
Well, if you think about it, as you optimize the 'new' position you will see that it has a lot in common with the 'old' position...
04rdking's avatar
04rdking
178 posts
 
Marty,
"For any length tube it would seem that the geometry is the same (save the headshell offset), no?"

I don't follow that. A longer arm will have a larger radius. The larger the radius, the less the tracking error/distortion between the null points.
martykl OP
3,578 posts
 
Atma,

You needn't optimize the pivot point - I'm saying you optimize the tube length to the chosen point.

I was only trying to explain that the (silly) flaw in my reasoning was that the dual point null (Baerwald) eluded me as I was focused on the particular radius tracked by an LTT. By way of explaining my (bad ) logic, I suggested an "apples to apples" (fix the tube length) comparison to illustrate that I missed a key fact:

Any radius is a radius! Or, as you say, they all start to look the same. By this logic, a 9" tube (or 10" or 12") on a pivot on an LTT designed for a straight tube of the same length should operate identically to one that is traditionally placed.

04 RDKing - I trust the above clarifies your point - I omitted the word "given".

Anyway you slice it, I just didn't get Baerwald and couldn't visualize how a pivoted arm actually traces a record.

Thanks again, guys,

Marty
04rdking's avatar
04rdking
178 posts
 
Marty,
I believe you're missing the forest because of the trees. A radius is not a radius. A 9" radius is not the same as a 12" radius. They are both arc's, but not the same. You keep saying "the particular radius tracked by an LTT". A LTT has no radius. It is linear.

Point is. If one were to put the pivot point in the center of the null points on a fixed head/arm, the tracking error would be gratly increased. In fact, I believe with this configuration, one would only be able to achieve one null point....... Stevenson geometery.
martykl OP
3,578 posts
 
The perimeter of an LP is a circle. The radius I refer to is the portion of the radius of that circle (that portion between the first groove and last groove) on the LP that any tonearm will ideally track. An LTT tracks a (portion of) a particular radius that is perpendicular to the "at rest" position of a typical pivoting arm. A pivoting arm tracks a different (portion of) radius of the LP. The error of a 9" tube is constant (i think) so long as the pivot point allows 2 Baerwal nulls from any radius. This may seem painfully obvious, but I missed it anyway! My radius is the path on the LP that is ideally tracked by any arm.

You are refering to the radius defined by the length of a pivoting arm which determines the cumulative deviation from the radius on the LP to which I referred. The longer the tube of a pivoted arm, the larger your radius, the less the cumulative error - from any of my radii on the LP - so long as the pivot is optimally placed for that particular tracking path (my LP radius).. If the tube is long enough and th pivot point optimized, 2 nulls can be acheived on any radius.

Clear as mud?

Marty
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