Stabilizing the JMW 10.5i arm with magnets?

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,201
20
38
Boston, MA
#1
I posted earlier on http://vpiindustries.com/forum/viewtopic.php?f=12&t=7884 the following:

Hi,

I want to stabilize my 10.5i arm because the wobble is sometimes too much to bear. For that, I was thinking of attaching two small neodymium 1mm-thick magnets at equidistant opposite ends around the pivot axis - like under the balancing "ears" of the wand, or even closer to the pivot axis, like under the round part of the wand - which would then equally attract either a washer (or any ring) glued or otherwise attached to the metallic base right under the magnets. I have measured the polished round base of the bearing itself to be 20mm, so I was hoping to find a 21mm inside-diameter with up to 1mm thickness magnetic ring or washer, but have had no luck with that type thickness or getting such a magnetic ring (on the other hand, 21mm inside-diameter is easily sourced from grainger.com).

Has anyone, or even VPI, done anything like this? I can easily source the small magnets (3mm round x 1mm thick) or even magnetic tape with adhesive, but I cannot find any magnetic ring or steel foil tape, though I can get foil tape any thickness I want from tapecase.com (but it's not magnetic), and I can only find stainless steel washers 3mm thick or more and I believe they are non-magnetic either.

There's got to be a solution to this???? There have to be 1mm thick magnetic washers somewhere, but everything I've found is some 300-series or similar austenite stainless steel version which are all non-magnetic????

Regards

-ack

PS: By "magnetic" I mean the ring/washer/foil would be attracted by the neodymium magnets
It just occurred to me that mu-metal, which I am using elsewhere for shielding, is partially magnetic and very thin, thus could do the job, and comes with an adhesive back... so stay tuned.

Meantime, if anyone else has any other suggestions, I am all ears.
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,201
20
38
Boston, MA
#4
Steve, it's a unipivot. @chop: I have seen Harry's mod, and he's just introducing a second pivot point with friction - not good. My approach is theoretically superior.

Ideally, I would be using a ring magnet, but they don't make them with the inner-diameter as wide as I want it. I suppose I could order one of these http://www.magnets365.com/hole-1-mm...50mm-x-2mm-hole-8mm-rare-earth-neodymium.html and machine the hole wider
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,201
20
38
Boston, MA
#6
I've seen many unipivots, none of which seem to wobble
Depends how they are stabilized, if at all. People hate unipivots because of this typical behavior.
 

JackD201

[WBF Founding Member]
Apr 21, 2010
11,031
22
38
Manila, Philippines
#9
I must admit I have never seen this. How does it affect the sound
vertically it sounds like wow and flutter, horizontally it makes images unstable and increases groove noise and wear, elliptically a combination of both :( Using the VPI outer ring will help the vertical, magnets could steady the horizontal theoretically. Magnets are really tricky though, they've got really small sweet spots. I shudder at the thought of the assembly lunging and snapping on and what that would do to a cantilever. Maybe neos aren't a good idea. Samarium Cobalt might be too much too. Something weaker perhaps? Alnico? Ceramic? Single digit max energy product types.
 

Steve Williams

Site Founder, Site Owner, Administrator
#10
IIRC several years ago when members were switching from horizontal trackers to unipivot with large arms, it was stated that these unipivots were as accurate or even more so than horizontal tracking arms. With that issue I fail to understand how that could be the case
 

JackD201

[WBF Founding Member]
Apr 21, 2010
11,031
22
38
Manila, Philippines
#12
Hi Steve

It's typically LP dependent. Horizontal is from off center holes and vertical from warpage. Put on a warped LP and turn off the VHD and you'll get a bit of an idea although the Graham is magnet stabilized so the change in direction won't be as severe. The point of unipivots is that supposedly can float and follow azimuth. The problem is how they change direction during the cycle.

Tangential arms have their own set of challenges but I personally wouldn't categorically state that one arm type is better than another. My own challenge when I had my JMW/Aries 2 was how the connecting wire on top would affect arm behavior. It drove the OCD side of me nuts. Not being as brave as ack to tweak I just went to a Graham when I got a chance to acquire an HRX for a good price with a Phantom as part of the package. All in all though, I think that the JMWs are great arms but they need more skill and dedication to set up than others. Especially with the mosquito proboscis of a catilever on the CA Titanium I had at the time. The drop counterweight for azimuth setting was a heck of a challenge as well. OCD and lazy, perhaps not the best combination of traits for that arm.
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,201
20
38
Boston, MA
#13
vertically it sounds like wow and flutter, horizontally it makes images unstable and increases groove noise and wear, elliptically a combination of both :( Using the VPI outer ring will help the vertical, magnets could steady the horizontal theoretically. Magnets are really tricky though, they've got really small sweet spots. I shudder at the thought of the assembly lunging and snapping on and what that would do to a cantilever. Maybe neos aren't a good idea. Samarium Cobalt might be too much too. Something weaker perhaps? Alnico? Ceramic? Single digit max energy product types.

The trick with what I am trying to do is to mount the magnets either exactly on the horizontal pivot axis, or behind it. That way, tracking force will not increase (if mounted on the axis), and might decrease (behind the axis) but then I can compensate; even if they are mounted in front of the axis towards the cartridge, I can still compensate. Either way, it's delicate work. Truth be told, I have tried something like this before with neodymium magnets, 1mmx1mmx1mm cube, top and bottom with opposing forces, but it was a b* to get them to align in an arc, never mind making sure the polarity is right, so I abandoned it. I hope this approach is better, and with 3mmx1mm round magnets I have in mind there isn't too much force yet, just enough to keep the arm stable... that's the thinking and hope.
 

JackD201

[WBF Founding Member]
Apr 21, 2010
11,031
22
38
Manila, Philippines
#14
Good luck ack, whether it works or not it will at least be fun to try right? No downside as they say. :)
 

garylkoh

WBF Technical Expert (Speakers & Audio Equipment)
Sep 6, 2010
5,416
1
36
Seattle, WA
www.genesisloudspeakers.com
#15
IIRC several years ago when members were switching from horizontal trackers to unipivot with large arms, it was stated that these unipivots were as accurate or even more so than horizontal tracking arms. With that issue I fail to understand how that could be the case
Steve, remember that discussion I had at your place after Newport one year? The scale of dimensions with respect to tracing the groove is tiny - and we are working on "best set of compromises" rather than "accuracy" or "perfection".

ack - you might want to try K.J. Magnetics. They manufacture custom magnets in the US. Whether you use an axially-magnetized or a diametrically-magnetized disc could make all the difference with your experiment.
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,201
20
38
Boston, MA
#16
Steve, remember that discussion I had at your place after Newport one year? The scale of dimensions with respect to tracing the groove is tiny - and we are working on "best set of compromises" rather than "accuracy" or "perfection".

ack - you might want to try K.J. Magnetics. They manufacture custom magnets in the US. Whether you use an axially-magnetized or a diametrically-magnetized disc could make all the difference with your experiment.
Thanks, I was actually just on the phone with them
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,201
20
38
Boston, MA
#17
BTW, here's how small the 1mm cubes I used before are

 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
5,201
20
38
Boston, MA
#19
If your objective is to stabilize the arm, opposing magnets might work as well as attracting magnets. You'll have to very precisely align them in either case, and opposing magnets would also reduce the pressure on the unipivot.

The problem is that there is magnetic flux gapping where adjoining magnets touch, so as the arm turns in an arc it might jump. It's one of the reasons I abandoned that approach, and am now looking to have just two magnets on one end (diametrically opposite of each other) and a wide static ferritic area on the other (underneath the magnets).
 

marty

Active Member
Apr 20, 2010
1,135
18
38
Far Hills, NJ
#20
I posted earlier on http://vpiindustries.com/forum/viewtopic.php?f=12&t=7884 the following:



It just occurred to me that mu-metal, which I am using elsewhere for shielding, is partially magnetic and very thin, thus could do the job, and comes with an adhesive back... so stay tuned.

Meantime, if anyone else has any other suggestions, I am all ears.
I tried doing this very thing about 30 years ago and had a lot of fun trying but eventually realized my efforts were a complete failure! However, rather than trying an re-invent the wheel now, aren't there arms made with magnetic stabilization. Reed comes to mind, no?
 

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