2. analog
  3. 1217633581

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
3,578 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
atmasphere's avatar
atmasphere
11,942 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.
More to discover