tonearm geometry question

Thom,

Thanx for the quick reply.

I understand that any pivot position that puts the tube tangent to the platter
yields the same geometry, but I asked a different question:

Why is the pivoted arm mounted so that it's tube is tangent to the platter,
rather than above it. I gather that headshell offset "fixes" the
geometry, but isn't tracking error minimized for any length tube if the pivot
were located halfway along the LLT's anchor point path?

Or. put yet another way - if you took an LTT and stopped it midway through
an LP, isn't this the ideal point for a pivoted arm to start since it is a 0 degree
error position for a straight tube (even one mounted on a pivot) and the error
induced between this start spot and the first groove (to one side of the pivot)
and last groove (on the other side of the pivot is smaller for any given tube
than is therwise the case?

For an arm mounted in this fashion, the arc defined by the stylus is split left/
right from rest, rather than exclusively right (as in the typical pivoted
mounting scheme viewed from behind and above) Viewed from a user's
persective - you slide the LP under the arm, move the arm to your right and
drop it onto the record to start play.

Sorry if I'm being dense here, but I'm sitting at a round kitchen table right
now and can't figure this one.

Thanks again,

Marty

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

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.

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

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

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

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