Stabilizing the JMW 10.5i arm with magnets?

ack

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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?

They told me Graham does it too... I am not in the mood of changing arms, and this is fun... and now for the details
 

ack

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May 6, 2010
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So the proof of concept worked, with attractive forces. In the first video below, you will notice how quickly the arm stabilizes; and without the mod, the arm would constantly wobble even worse with every LP all throughout the playback.

I used two 3mmx1.5mm (3/16" x 1/16") Neodymium N52 magnets in the arm and five layers of mu-metal tape that I use throughout for shielding, about 0.5mm total thickness. Any thicker and one of the sides would win and the arm would tilt permanently. Mu-metal is ever so slightly magnetic, unlike metallic rings that I also tried, and perfect for this application. The magnets are temporarily attached with blu-tack.


Here's is a video of the arm tower's underside with the mini magnets:


I can tell you, I have never seen this arm so stable before, LP after LP.

And a whole bunch of magnet sizes I had in hand to try:

magnets.jpg

The way the magnets are attached - perfectly along the horizontal pivot axis (but obviously away from it) - has had a very minimal impact on the tracking force, from about 2.250g to roughly 2.270g, really negligible, and closer really to the 2.3g I want to use on this cartridge, anyway.

Next, now that I know where the magnets need to go, I can order a custom ring magnet from K&J Magnetics (roughly 31cm in diameter) to replace the mu-metal, for repulsive-type stabilization, which should be a lot better still. And now I know it has to have a very weak magnetic field.

Listening impressions are underway, and for sure, substantial improvement in imaging, with sharper portrayal of instruments.


Finally, since I am into tweaking lately, if you are seriously into espresso making at home, here's the La Marzocco Strada 58mm non-pressurized (of course) basket I got for $30, and they have gone to extremes to make sure hole-size variations are extremely tight, and even include a histogram of the hole distributions of this very basket and an image of it - we are talking audiophile extremes here:

la-marzocco-strada.jpg

I hope you also enjoyed the 1kHz test tone - very stable, pun intended.

-ack
 

kleinbje

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Dec 20, 2012
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awesome, keep us posted. Although I must confess the wobble does bother me too much and my cart is very sensitive to azimuth setup DV-1 T. I would expect a need for decreased antiskate given opposing magnets. Did you need more with the numetal? Definitely want to try this.
 

ack

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awesome, keep us posted. Although I must confess the wobble does bother me too much and my cart is very sensitive to azimuth setup DV-1 T. I would expect a need for decreased antiskate given opposing magnets. Did you need more with the numetal? Definitely want to try this.

I didn't see the need for anti-skate. Let's see if K&J can make me the opposing ring magnet first - from the looks of it, their minimum thickness is 0.8mm which is still fine, however, it seems to me they only deal with neodymium magnets, and the force will probably be way too much. I am trying to locate other vendors for other types of custom magnets.
 

ack

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So while waiting for the custom-ring magnet quote, I have ordered even smaller 1/16" x 1/32" (1.5mm x 0.8mm) round magnets (half the size of the ones above), and opposing rectangular ones also 0.8mm thick, and the latter should cover the entire travel arc of the arm on the platter, if carefully positioned as I have with the mu-metal strips. The distance between these opposing magnets in this arm should be 4mm, which I think may be far enough for the force to be adequately small. These are also N42 grade (weaker field) and I can probably go lower if necessary, like N35.
 

ack

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So the proof of concept with the opposing rectangular magnets for repulsive stabilization "worked", and clearly demonstrates that stability is achieved must faster. But these are magnetized through-thickness and polarity is not uniform on the surfaces; actually, it feels like they are magnetized diametrically. So it "worked" as long as the arm doesn't move. Therefore, I took the rectangular magnets out, put back 4 layers of mu-metal (for attractive stabilization again) - and notice, this time it's 4 layers, not 5, so you have to experiment - until I receive my custom opposing ring magnet, which will be axially magnetized. For the ring magnet, I decided to drop to a much less powerful N35 grade, in order to control the repulsive force from the other side, with a variety of smaller magnets on the arm, and per the pictures above (and even smaller round magnets that I received since).

The specs for that ring magnet are as follows (and it's 1/16" thickness, the thinnest they can do, unfortunately; hence dropping to an N35 and me ordering even tinier round ones for the arm):

Custom NdFeB Ring Magnet
42.00mm (1.654in) O.D. x 21.00mm (0.827in) I.D. x 1.60mm (0.063in) thick
Grade N35, Nickel (Ni-Cu-Ni) Plated, Axially Magnetized

Delivery is expected in mid November, and minimum charge is $150, so I am getting a whole bunch of those.
 

ack

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A little update: I have been able to order 0.8mm-thickness ring magnets from Super Magnet Man, with otherwise the same specs, due to arrive end of October. Between the 0.8 and 1.6mm I should get the right effect.

Meantime, I've been listening extensively to the temporary solution of stabilizing the arm with attractive forces (magnets on the arm plus mu-metal on the opposite end), and the net effect is something short of dramatic. First of all, just playing that 1kHz test tone w/o the mod and standing right in between the speakers, the image constantly shifts, L/R/L/R/..., enough to give me a headache, so to speak. So image stability is a given with the mod. Next, timbral accuracy is improved significantly, as there appear to be phase shifts that the mod fixes. Finally, the bass has tightened up quite significantly, so much so that I have gone back to listening with the sub on all the time (crossed at 23Hz), and that is perhaps the most welcome improvement... imagine being able to hear a Bösendorfer's extended lower registers much more clearly. I can't believe it took me so long to get on this mod...
 

Audiophile Bill

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Mar 23, 2015
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Cool mods guys - I ditched my 10.5i because I found the cueing annoying with the wobble so this sounds great.
 

ack

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May 6, 2010
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So all ring magnets have arrived, both thicknesses (1/16" and 1/32") fit fine, and there are good news and bad news.

The bad news is that the ring magnets CANNOT easily be used, because the arm's stabilizing "ears" hanging off the sides (as shown below) are magnetic (some sort of steel, unlike the rest of the arm which is aluminum) and they interfere with the rings - basically, they are strongly attracted to the ring magnets and overpower any repulsive action of the small magnets under the arm. So to use these ring magnets one has to rip out those "ears", which I don't want to do (they are not screwed in, from what I can tell). I should have tested those "ears" for magnetic behavior before ordering the ring magnets.

The good news is that the simple original approach using attractive forces has worked really well all this time, though it does take about 5-10 seconds for the arm to stabilize. Below are a couple of pictures of the mu-metal strips (3 per side now), measuring 1.5cm x 3.5cm, and notice how on the right-hand set of strips I have intentionally [ever so slightly] pulled down the tip hanging out with some tape, because the mu-metal hanging in the air tends to tip up over time, changing the magnetic field. The mu-metal strips have to be perfectly flat before setting them on, so that the magnetic field is even. Lastly, there is a short video again of the channel stability.

Let me just repeat the final specs: 3mmx1.5mm (3/16" x 1/16") Neodymium N52 round magnets in the arm and three layers of mu-metal strips per side, each strip measuring 1.5cm X 3.5cm. The strips are far wider and longer than they need to be so that setting them *exactly squarely* on top of each other is not critical. For mu-metal, I use this thin product with adhesive in the back http://cdn.opentip.com/Tools-Home-Improvement/Admu-Mumetal-Magnetic-Shielding-p-3335624.html sold by the inch of length (fixed width).

All in all, the VPI arm should and could have been better designed; beyond the suggestion for removable stabilizing "ears", it is apparently fairly easy to stabilize; and with respect to repulsive forces, it looks like it could have been designed with ring magnets and opposing tiny ones, without really affecting tracking force or anything else, though high precision would be required. But I guess I could also be missing something.

vpi1.JPG

vpi2.JPG


-ack
 

ack

VIP/Donor & WBF Founding Member
May 6, 2010
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Well, if at first you don't succeed, try try again... I spec'ed out more ring magnets, this time with a smaller outer diameter plus much tighter inner for a snugly fit, thinking that they would not interfere with the "ears", and it turned out to be true; the arm is now fully stabilized using repulsive forces. The use of a high precision caliper helped a lot in this effort.

Specs: 3mmx1.5mm (3/16" x 1/16") N52 Neodymium round magnets in the arm, and two 20.7mm ID, 31mm OD, 0.8mm thick N35 Neodymium ring magnets

ring-magnets.JPG vpi-jmw-10i-with-magnets.JPG

Here's the latest video showing how quickly the arm stabilized once severely agitated (it took a lot longer with one ring magnet)


One of the additional benefits of using repulsive forces is how quickly VTF stabilizes on the gauge, something that is not true with the original arm. The obvious benefit is that the arm is extremely stable, much more than using attractive forces. The downside is that azimuth adjustment (by rotating the counterweight) is now more difficult, if you don't have a "perfectly" engineered cartridge like the A90 - one would need severe counterweight rotation to get even a small amount of adjustment; I did not have to adjust azimuth at all.

I used K&J Magnetics for the round magnets, and Super Magnet Man for the ring magnets. The tolerances are extremely tight, as verified with the caliper; for example, one of the ring magnets measures 31.03mm outer diameter and the other 31.04mm, and I bet this is just a small variation in the nickel plating, not the magnets' cores per se.

I would call this a happy end...
 
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