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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
thom_at_galibier_design's avatar
thom_at_galibier_design
729 posts
 
Walk around your turntable (closer to your left speaker) and look at it again.

Cheers,
Thom @ Galibier
martykl OP
3,578 posts
 
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's avatar
nrenter
768 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.
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