a continuation of DIY TT bearing...But now Tonearms!

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Frank, Nocdplz, and all

I have a question for you guys.

I think the topology of Alto tone arm generates stronger skating force than regular tone arm.

Here is my analysis. My analysis is based upon Kogen’s article. Here is the link.

http://www.audiomods.co.uk/papers/kogen_skatingforce.PDF

Please correct me if I am wrong.

Please see the drawing. You can see that SF2 is larger than SF1. In other words, skating force under the topology of Alto arm is larger than the skating force under regular pivot arm.

Jim
 

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Hi Jim,
You are overlooking a key point. The offset angle cannot be left unchanged for such a comparison since you need to change the "appearent" offset, here the bend in your armwand(which doesn't equal the effective offset angle for the right arm).
In other words, for a given eff. length, all that matters is the offset angle vs. the pivot to stylus line. Where you place the offset(headshell, armwand) makes no difference.
In respect to your drawing, the right arm now features a larger offset angle(resulting in a higher skating force), but if the left arm's angle was chosen correctly, the right arm features too much offset(and vice versa). So you'd need to reduce the "bend" in the right arm's drawing for a proper comparison. Which will then show no difference in skating force...
What DOES happen is an increase in eff. mass(all other things equal) for any offset bearing topology(albeit very small).

Cheers,

Frank
 
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To put it another way, there is only the cartridge location and the pivot point. How you get from here to there doesn't matter. If you had mass-less material, the arm could turn left, go out the front door and come in the back, but the motion of the cartridge in relation to the pivot, and whatever forces are generated, would remain the same.

What's interesting to me is the moving pivot of the Schroder arm. There are an infinite number of paths one can move the pivot over to achieve zero tracking error, some quite short. They can be straight or curved. I believe his solutions are entirely passive. It might be possible to do an active system by passing a light beam (laser) through a moderately thick piece of plexi on the arm, measuring the angle with a PSD, and then moving the pivot to follow whatever path was needed for zero error. Offset angle and its resulting problems are eliminated, you get to use a well developed pivoting arm, maybe a unipivot, and have a shorter pivot path than a traditional linear tracker. Just thinking out loud.
 
Frank,

Thanks! I understand it now.

Conrad,

To put it another way, there is only the cartridge location and the pivot point. How you get from here to there doesn't matter. If you had mass-less material, the arm could turn left, go out the front door and come in the back, but the motion of the cartridge in relation to the pivot, and whatever forces are generated, would remain the same.

I don't think anyone can say it better than that. Thanks for input!

Jim
 
If you had mass-less material, the arm could turn left, go out the front door and come in the back, but the motion of the cartridge in relation to the pivot, and whatever forces are generated, would remain the same.

Amusing way to describe it! :D

Yes, all that matters is the two points (pivot & stylus) and the offset angle.

Welcome back Conrad! (Maybe you never left but we rarely see you on matters of tonearm.)


What's interesting to me is the moving pivot of the Schroder arm. There are an infinite number of paths one can move the pivot over to achieve zero tracking error, some quite short. They can be straight or curved. I believe his solutions are entirely passive.

I have to assume you are referring to the Schroeder LT arm. It is entirely passive and its guiding mechanism is magnetic. It has a non-contact magnetic path that forces the arm to move tangentially. Much of it has been discussed in my thread that is pertinent to that genre of arms with comments from Frank and other tangential minded members.


It might be possible to do an active system by passing a light beam (laser) through a moderately thick piece of plexi on the arm, measuring the angle with a PSD, and then moving the pivot to follow whatever path was needed for zero error. Offset angle and its resulting problems are eliminated, you get to use a well developed pivoting arm, maybe a unipivot, and have a shorter pivot path than a traditional linear tracker. Just thinking out loud.

I have come around to your idea, Conrad. I used to hate active "crabbing" arm but you are right that pivot arms have been highly developed and matured and all we need is a platform or island for it to move actively to track tangentially. Instead of the requisite 4 inches of lateral movement, I am thinking (out loud) of using the Thales or quasi-Thales concept of moving or pivoting the arm forward only 2 inches: kind of a like an active Schroeder LT arm cantilever moved by motors and sensors. It takes up less space and easier to install. Instead of having the entire arm assembly moving laterally 4 inches like Rabco or Goldmund, we can pivot or swing the assembly via a string or belt attached to a motor activated by sensors. In essence, an active Birch tonearm! I think it can be quite elegant. Just imagine anyone can install their favorite pivot arm (minus the offset angle of course) on this platform and still enjoy tangential tracking.

Just for yucks, here's a YouTube video of a fancy active linear arm. Totally over the top!
 
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