Angling for 90° - tangential pivot tonearms

[Cobra2]

Pic. from the net...somewhere..: Pioneer PL-L1000(A) arm

Arne K

[directdriver]

Active motorized servo tonearms are not the genre I am talking about in this post but they are still interesting.

I used to own several of those and I still have a couple spare arms laying around. Perhaps I should put them to good use. The arm's construction was over complicated and it's the only servo arm that the armtube moves laterally left AND right because the armbase is part of a linear motor; most other arms pivots and moves laterally to the left in only one direction. With so much tiny movement, the bearing quality was not that good. I took it apart just to salvage the motor and platter as stand alone direct drive turntable. It's a clever design but unfortunately the whole thing sits on a flimsy plastic wobbly suspended platform. It's certainly cheaper than a Goldmund T3 arm! I think the Yamaha PX-2 is probably another candidate for this kind of experiment.

Thanks for sharing. Here are the rest of the pictures in this forum post:







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[EC8010]

Originally Posted by directdriver

It's still more fruitful than talking about cables.

Absolutely. And you're providing huge entertainment with all those big pictures of unusual arms.

[Mark Kelly]

Arm design incorporating Burne Jones' Thales circle concept and Dynavector's split planes concept.

I actually built a prototype of this but didn't take any decent photos. This design fails for the usual reason: the complexity of the bearings creates excess friction or excess play (or both).

Currently working on a new version incorporating a new concept which seems to avoid this problem. Need more round tuits.

[bear]

Don't forget Rabco!

their arm was on a track run by a motor that was turned on via thin wire contacts but the arm still pivoted, thus the contacts. So it crabbed across the record.

My friend Al and I back in about 1973-4 built a radically upgraded modifcation of it - he still uses his. We built precision bearings and fabricated a lightweight arm for it. He still uses his, it works great.

I used a phototransistor and light source to trigger mine...

It was kewl.

Maybe I can get him to post a jpeg of his.

_-_-bear

[berlinta]

Hi!
Here's an arm I've built for my Neumann PA2(the original arm has a pivot to spindle distance of only 185mm) that fits this thread:

European Triode Festival 2009/hb/DSC_0501

Essentially based on the Thales circle but allowing for the headshell to rotate like the RS Labs(within the confines of the tensioning thread). Much less complex than the Thales arm and working extremely well. Skating compensation decreases with decreasing offset angle (when moving towards the spindle). Skating compensation is determined by the (lead shot)weight(s) at the end of the string.
I showed it publicly after getting permission from Micha Huber whose new arm is an exercise in combing mechanical complexity with elegant detail solutions(the skating compensation is waaay cool!)

Cheers,

Frank Schröder

[directdriver]

Mark Kelly: "Arm design incorporating Burne Jones' Thales circle concept and Dynavector's split planes concept. This design fails for the usual reason: the complexity of the bearings creates excess friction or excess play (or both). Currently working on a new version incorporating a new concept which seems to avoid this problem."

Thanks so much for sharing!! That's very close to what I have in mind, the split planes concept is the way to go. By separating the vertical and horizontal movements independently ease up the geometric issue considerably. My biggest problem with the Thales (Original model not the Simplicity) tonearm is that it has geometric issue on the vertical plane as the stylus angle is constantly changing in reference to the main arm (which is NOT part of the Thales triangle only compounds the problem) and not a good thing when encounter warped records. The split planes concept also helps not having a moving part right above the cartridge! The B&J design has too many pivot bearings and it's no surprise that it doesn't work well. I have abandoned the idea of using linear motion bearings in my previous posts. I believe it can be simpler and only having two pivots to make the geometry close to be tangential. Most arms of this genre requires at least 4 pivot points like a tetragon.

Thanks again for sharing and this is very encouraging. I look forward to the new design. The day we won't see air pumps will be soon.

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[directdriver]

berlinta: "Essentially based on the Thales circle but allowing for the headshell to rotate like the RS Labs(within the confines of the tensioning thread). Much less complex than the Thales arm and working extremely well. Skating compensation decreases with decreasing offset angle (when moving towards the spindle). Skating compensation is determined by the (lead shot)weight(s) at the end of the string."

Hi Frank, that's brilliant!!

It reminds me of that mysterious tonearm I posted earlier that uses two strings. But yours is even simpler by having a single constantly tensioned string that compensates the skating force. Two birds one stone!

I always wonder if the Thales guiding mechanism can be simpler. I thought of an idea by using long light round rod with teflon sleeve and it goes in between a vertical tuning fork like mechanism that can pivot so it guides and forces the cartridge to be inside the Thales circle and provides low friction. One can even add a u-groove bearing in between the tuning fork to ease the linear motion. But the mass of the rod will change at different points of the record and changing the effective mass. Hmm... no free lunch.

How does it sound compare to your regular arm?

I found more pictures of the arm and I hope you don't mind more people get to see them.

DSC_0295.jpg - & - DSC_0293.jpg - & - DSC_0210.jpg - & - DSC_0501.jpg

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[directdriver]

bear: "We built precision bearings and fabricated a lightweight arm for it. He still uses his, it works great. I used a phototransistor and light source to trigger mine..."

Of course, I haven't forgotten about the Rabco, as it is the granddaddy of motorized servo linear tonearm. I still have two faulty ones laying around. The Rabco arm works pretty well but is hindered by its bearing quality and flimsy armtube. I look forward to seeing pictures of you and your friend's modification. I want to salvage the motor and servo mechanism to make a sliding base so one can mount a conventional pivot arm like a Rega on it to improve the bearing. There's no need to reinvent the wheel on the tonearm part, we just need a suitable motorized gliding base with sensor for mounting an arm. Most arm of such genre cheap out on the arm part after they spent all the resource on the motor and electronics.

Once again, the genre of arms I am talking about is to exclude motors and air pumps, hence the title "pivot tangential arms." But I am still curious about the pictures.

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[directdriver]

Someone is thinking along similar lines in another forum. Very articulate and many good pictures, including showing how Frank's string tensioned pivoting headshell works and the making of a DIY pivot headshell on an SME Type3 arm.

Italian forum postings - ansaht.altervista.org/phpBB2/viewtopic.php?t=135 - "The brilliant Thales"

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