Transformer Shielding Tapes

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

rackmonkey

Well-known member
GDIY Supporter
Joined
Sep 20, 2016
Messages
969
Location
Texas
I think this subject is worthy of its own thread and I don't want to continue to hijack Alexc's post about eBay transformer cans, so I'm moving those posts from that thread into this new one.

The tape topic started off with this comment:

Quote:
"I’ve tried one of the 65x65x80mm steel cans from eBay you speak of. Quick and dirty test showed about 6dB noise attenuation. I lined the inside with 3M magnetic shielding tape, and subsequently measured 34dB attenuation. I ended up actually using that setup on the input of an AM-864 point to point build with an Edcor XSM 600:10k (wax potting)..."

AusTex64 asked what tape I used, and I replied that is was 3M CN-3190.

There are tapes that appear on paper at least to be better than the CN-3190 I ended up with. I bought that product because I found a deal I couldn't pass up. The stuff is marketed as a "conductive fabric" in the charts here, but in the datasheet, 3M describes it as EMI shielding tape with conductive adhesive. The datasheet claims ~70 dB attenuation for 3190, so it was good enough to my thinking.

Obviously, my measurements didn't get close to -70dB,  but the tests I did were as close to "real world" for pro audio as I could get. And I only used a single layer of tape.

Take a  look at the "EMI shielding tapes" chart below. You might want to look into those first. Or not.

4CnK1RI.png


eFljTbL.png


sNig4Bm.png
 
Jakob raised the point in Alexc's thread:

Quote:
"But - isn't all these tapes just electrostatic shielding?

What we really need for audio transformers is the magnetic shielding.

For calming sensitive stuff, I sometimes use use shields like e.g. https://www.aliexpress.com/item/32811373222.html

Jakob E."

My reply:

"Yeah,  good point Jakob. Most of those are copper only or copper/tin, etc. Won’t work for the low freq stuff we’re trying to shield from. The nickel based tape, however - in combination with the steel can - did provide attenuation of the power transformer noise to the levels I mention above in my tests. You’d expect that from a permeable material like nickel, I believe. Anyway, I’m sure I’ll buy more when I run out because it works..."

 
I'm always looking for shielding tapes because they're easy to use, and because I can line any old can I may want to use for a project with them to make them effective. When a can isn't required for aesthetic reasons, you can also just put tape around the coil of the transformer (if you have room between the windings and the lams).

Here's some russian material I just bought a couple of days ago that looks promising:

https://www.ebay.com/itm/Magnetic-shield-tape-permalloy-79HM-for-CRT-clock-defense-or-other-DIY-10x38cm/113300894060?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160727114228%26meid%3D9edcf041b77b4b11abf4e102e2227479%26pid%3D100290%26rk%3D1%26rkt%3D4%26sd%3D113300894060%26itm%3D113300894060%26pmt%3D1%26noa%3D1%26pg%3D2060778&_trksid=p2060778.c100290.m3507

Tape for magnetic defense of CRT tubes, its a sheet of permalloy 10x38cm by default, can be shorter or longer if you need(send me message if so)
Thickness - 0.1 mm
Permalloy parameters

79HM
manganese - 0,836%
iron - 17,244%
nickel - 78,443%
molybdenum - 3,478%
 
Another interesting one.  I’ve seen this stuff at several magnetics companies’ sites, on Amazon and eBay. Main trade name is Yshield, but if you search for “MCF5 shielding”, you’ll find it. It’s an amorphous cobalt alloy. Manufacturer claims it provides similar shielding of LF magnetic fields as mu metal, but isn’t sensitive to deformation or drops. It’s not super cheap at about $17 for a 5cm x 26cm strip, but you’d be able to shield several transformers with a strip that size. So it may be pretty economical, all told.

https://www.ebay.com/itm/YSHIELD-Mumetall-shielding-film-MCF5-MF-Width-5-cm-1-meter-Electrosmo-/172148816584

Link above is just for the picture. shipping is atrocious on that listing. I’ll edit with the lowest total cost source I can find.

Cheapest I’ve seen for US buyers is on Amazon. 1.96” (5cm) x 3 linear feet (roughly 95cm) for $30 with free shipping (Prime deal, so might be a shipping charge w/o Prime). Probably also available outside the US, but not sure what Amazon charges.

https://www.amazon.com/Magnetic-shielding-film-MCF5-Linear/dp/B00H4D8GZ2

Part Number: MCF5
Item Weight: 3 ounces
Product Dimensions: 3.8 x 3.4 x 1.4 inches
Item Package Quantity: 1
Batteries Included?: No
Batteries Required?: No
Shipping Weight: 3 ounces
Date First Available: December 5, 2013
Product Features

Length 1 foot ; Width: 1.96 inch
Attenuation LF magnetic field: 30 dB (97 %) ; Attenuation HF: 40 dB at 1 GHz
Weight: 180 g/m2; Material thickness: 0.02 mm; Color: Silver
Permeability: 4 = 25,000; max. = 100,000; Saturation polarization: 0.55 T
Composition: Co69, Fe4, Mo4, Nb1, Si16, B7
 
The marketing material has me skeptic. Somehow, I have a feeling that there is some over-selling going on in this topic:

The magnetic shielding property of a ferromagnetic material is not acting by preventing magnetic fields from penetrating (or shorting them like with electrostatic shielding), but by redirecting or "bending" them by providing them with a suggestion of path-of-lower-magnetic-resistance.

This redirection means that the material must be able to "contain" as much as possible of the fields, which again means that shielding efficiency is directly proportional to the amount of material / thickness of shielding.

And seen from a different angle, if you can really have 10-12dB of magnetic shielding @60Hz per layer of thin tape, why would any transformer company ever chose use heavy, large and hugely expensive mu-metal cans..?

Yes, I'd love for this magshield tape to exist - but I really don't think it does. At least not as described. I bet it's great for electrostatic shielding though, but so is every conductive material.

Jakob E.

 
It’s a good point, Jakob, and worth digging into.

There’s more reason for suspicion than just material thickness. We also know that bending/deforming reduces mu metal’s shielding effectiveness, which is by nature what you do with tape. (That’s one reason that cobalt tape caught my eye).

That said, tape is economical enough and convenient enough to make it worth some testing here to see just what can be accomplished.

I’ve used the tape I’ve bought in conjunction with iron/steel cans and had better results than with the can alone. And that’s the primary way I’ve used it/thought folks could benefit from it. Think of it this way: the nested (expensive) cans used by the transformer companies (at least the classic ones like UTC etc) created alternating layers of copper, mu metal, and sometimes iron. An air gap between each layer of shielding improves effectiveness, and they used that approach. The copper gives you electrostatic/RFI protection, and the mu/iron give you LF interference protection. This is the basic effect I was going for by lining an iron cam with the stuff. If you were to build up layers, alternating between good old copper foil and this mu tape - maybe with some thin masking tape between each layer to create a gap between shielding layers, you could create a very effective shield. (I think your skepticism gets to the point of “okay, how MUCH would you need?”)

I’ve also theorized that one could get adequate if not spectacular LF attenuation using tape alone, wrapping around the bobbin and the parts of the lams adjacent to the channel formed by the bobbin. (I’d said that Edcor uses that technique, which they do. But looking again at what they do, they are just using copper tape, which helps w/RFI but won’t help with LF noise.) I wonder again if you could use that technique and alternate a layer or two of mu tape with copper - maybe some thin masking tape in between. (It wouldn’t take long to get thick enough to where you might create mounting challenges, but there’s a middle ground here).

Maybe let’s test all this and see what we can do with it once and for all. I’ll set up the test conditions I used earlier to test the 3M stuff and document it/take a picture. Folks can poke holes in the methodology and I’ll tweak to address them. I’ll take screen shots of the scope/FFT with each test. We can then either put this idea to bed or people can run with it, but at least they’ll know about what they can expect before they drop the cash on tape.
 
I remember one time working on a walkman cassette player I owned , I had removed the band of magnetic  material around the motor , the amount of motor noise and harmonics picked up by the playback head dramatically increased , so I do believe magnetic tapes can be effective .

It does provide fair warning  about handleing the razor sharp  Ysheild foil in the product description ,worth remembering to save 'claret on chrome' and your paws in bandages  :D
 
In the pic below you can see the band of 'mag metal' secured by the label ,
Maybe a  mabuchi cassette motor would make a neat transformer screening can with rotor and magnets removed , the metal strip could be put inside the can and extra circles of mu-material cut for top and end , they come in a variety of sizes appropriate for input transformers . Knocking out the magnets without deforming the housing could be tricky, in any case flame and extreme temperature will break the glue's bond long before the metal looses integrity .
I see examples of said Mabuchi's with sheild foil  around the $2.50 mark off SqueeeBay ,
'Poor Man's Can' you could call it  ;)
 

Attachments

  • Mabuchi.jpg
    Mabuchi.jpg
    79.5 KB · Views: 24
That pic reminds me of those super quality pots  that have each have their own shield can.

I believe it was the uber-engineered 'Fairman Channel' from Denmark in which I saw them.

Real craftsmanship and investment for the professionals.

 
Very interesting thread..
Has anyone tried any of the conductive paints?
i.e
MG Chem 841AR
https://uk.farnell.com/mg-chemicals/841ar-15ml/nickel-conductive-coating-12ml/dp/2811941

Maybe we could buy cheap plastic cases and coat? Film cases etc..?
 
Back
Top