Torsional vibration; Gimballed vs Unipivot

[IslandPink]

OK Folks,

I had a moment of clarity the other night while listening to some music . This is not the normal state of mind I seek at this time, but you never know when it's going to hit you I suppose .

I was musing about the transmission of vibration through a tonearm to its support, wondering about why this interface is still important musically .

I think I now understand why there is STILL vibration that has to be resisted by the arm-pillar, for a gimballed arm, and why unipivots differ ( crucially ) in this respect .

From what I can see , the gimballed arm is decoupled for two important motions ONLY - the side to side pull caused by lateral stylus ( music ) motion, and the up and down vibration caused by vertical music modulation and record warps . These motions are then mainly resisted by the counterweight, which is the preferrable state of affairs as far as I can see - a short and relatively 'clean' mechanical interaction . There is at least one important motion that is NOT effectively decoupled by the bearings . This is the torsional ( twisting ) motion - caused by lateral stylus motion and the moment of the stylus around the arm-tube centre axis . This is mentioned in
Mark Baker's woffle on his Origin Live website and promo material, with reference to submarines attacked by depth-charges and suchlike .

There are other effects , maybe significant or not - a push-pull effect along the length of the arm, for instance . I don't know how big these effects are, and I don't think they differ from gimballed to unipivot arms .

In any case, if you think about the torsional mode , this one ( high-frequency, as usual, due to RIAA ) goes down the arm and may cause chatter in one or both of the bearings , but even if it doesn't , it ends up as
a bending motion on the arm pillar or on the arm mounting flange and on any interface between arm pillar/flange and the plinth . Any difference in hardness here then causes a reflection of energy back ; any looseness causes 'noise'
to be generated which also feeds back . In either case, this junk signal has no chance of coming back in phase with any musical signal and must cause distortion or clouding of the original music .

What's very interesting to me , is that this mode of vibration, and the problems that can be caused, do NOT happen with a unipivot - this torsional mode is decoupled at the unipivot , and is resisted solely by the counterweight and any outriggers , together with the basic stiffness of the arm tube . This seems preferable .

I can already feel JamesD chuckling to himself, thinking how each new bunch of audiophiles has to rediscover these audio truths for themselves and present them as 'new insight' ... !
Hope this is of interest .

Mark

[Clive]

Mark,

I wish you'd stop trying to get me to spend more money on a Hadcock/Morsiani etc.

Your arguement seems logical and quite obvious....now that you've made the point. Wish I'd had the clarity of thought.

Clive

[Black Stuart]

Well thought out,
and as someone pointed out on another thread - why did the belt drive take over so completely from the idler - cheaper/quicker and simpler to produce - ergo more profit.

Black Stuart

[James D]

Mark,



Very nicely thought through. Now are you and Clive going to make your own unipivots? It really is the only way to go...

James

[IslandPink]

Hmmm,
Yes, usually have some form of 'clarity of thought' after I've spent a lot of money . 'Welcome to the next level' as JC Morrison said , this time in analogue gear .

Now then James, where were those DIY links - there was a nice-looking acrylic one I saw somewhere .

You must understand this project has to take its place after the SE amp ( end of this week ?! ) and the phono mods , then the subwoofer ...

Mark

[bornin50]

Hi All,

If this is crazy let me know.

This decoupling of the unipivot idea, do you think it also applies to knife-edge designs, a bit? And could similar effects explain the positive user feedback for the Rega vtaff, which decouples the arm from the fixing?


Regards

Colin

[IslandPink]

I think knife-edges are interesting, and have some of the unipivot 'flow' which must come from the low friction ( or stiction ) of the small bearing area . I got some feeling of this from initial use of my SME 3012 . The wiring needs to be improved on this, though, to compare properly to the other arm .
This torsional vibration problem is not improved with the knife-edge of course, but there is some advantage over normal gimballed arms in some respects .

I haven't come across the Rega vtaff before , I'll look into that .

Mark

[James D]

Ok . Diy unipivot links:

diyaudio thread
this one contains lots of further links. Including the Altman link

Thread on EC8010s unipivot

A dynamically balanced tonearm

The Boozhoundslabs tonearm

Marco Sandrin design

A nice bloodwood 'altman'

These are all standard unipivots. There are several Schroeder style thread unipivits around as well but that is for another day.

What can be used for the bearing? Almost anything that comes to a point. So kniting needle, sewing needle, dart tip, ballpoint pen end, machined point on a screw, sharp screw or nail, sewing machine needle, gramaphone needle.

One I'm thinking of trying is a sapphire rod supporting a ruby ring bearing. The parts are available online in the states via swiss jewel.

But then I read this account of pivot friction in clocks... umm maybe turned steel and minature ball bearings is the right way to go... In which case smallparts is a good source. No doubt there is a UK source but I don't know them...

Ohh for Mark here is the acrylic one

James

[JerryT]

Lovely stuff!

Comment 1:

The problem with the pivot friction in clocks article is that the pivot is not a small (> 0.1 mm), very hard point on a hardened and polished surface. Knitting needles - quite deliberately - aren't made with very fine hard points (and are usually made from springy steel, not the brittle stuff that hardens so well. Perhaps making the pivot isn't as easy as it seems.

comment 2:

Do we want the arm to oscillate for ever - or is damping a good thing? My Mission 774 employs silicon fluid to achieve damping - as did the arm onthe Rock turntable and assorted SME arms. Don't we want low initial resistance but then the original oscillation to decay rapidly rather than interact with subsequent large amplitude disturbances?

Don't know if any of this helps. But I'm fascinated by unipivots!

Jerry

[NealG]

A little larger!

http://www.scheu-analogue.com/Bilder/cantus1.jpg

http://www.scheu-analogue.com/Bilder/cantus2.jpg

http://www.scheu-analogue.com/Bilder/cantus3.jpg

http://www.scheu-analogue.com/Bilder/cantus4.jpg

http://www.scheu-analogue.com/Bilder/cantus5.jpg

http://www.scheu-analogue.com/Bilder/cantus6.jpg