mu-metal shielding

I'm thinking of trying mu-metal to shield a couple transformers. One is a big toroid radiating noise into my rack. Another is a vintage mic input transformer with an open end on one side of its can that needs to be covered with a plate.

What's the difference between mu-metal and regular steel? Where does one find it? Will it help in the applications above or should I just use brass or steel or...?

Pint emoticons don't work with my old browser, so here's a virtual pint. Cheers.

Hi Tommy,

Try this search for the lowdown on mu-metal.

Justin

Tommy
long time no see
Steel will work ok try duct material for hvac if you can work
with it. Make sure it will stick to a magnet.

Mu metal costs much. Be sure to read handling issues as when you bend it
it no longer works as well.

Try steel two layers if you can do a quick one with tin snips
if you cannot get it low enough the hum.
Cut bait and order the mu-metal,

Try these guys.
http://www.magnetic-shield.com/

http://www.mushield.com/

Thanks for the links, guys.

AdrianH, good to hear from you. How are those mics you were hacking? That toroid I'm talking about is the one you saw on Kevin's bench (not his design!). It was not too noisy in testing at other locations, but once I got the modules set up at home I'm getting bad buzzing. The modules are close to the torroid.

I notice it changes in severity from time to time, and I know my place is prone to RFI on the AC. So I wonder if some of the noise is coming in that way. When I switch off the power the noise vanishes, while the pres continue to work for a few seconds in blissfully low noise floor. So it's either the toroid radiating, or noise in the AC getting through, or both.

I tried an AC 110 to 220 transformer plugged into a second 220 to 110 transformer to try and clean the dirty AC - poor man's experiment for an AC transformer - no difference.

I mounted a steel plate over the toroid and that helps about 20%. But I need to do more. I'll try a second plate or get mu-metal.

cheers,
tommypiper

Before you go there
can you mount the transformer in some external steel case?
Go to an electrical supply and ask to see the metal NEMA 1
boxes. Cool cheap and steel. But the vendor I like in in NC.
Try to see what you have out there.

I know moving it is a pain............But so is a bunch of mu metal.

Slow progress on mic spending too much time in bankers offices begging for money.

I sympathize. Good luck.

Thanks, yep, I've considered remote mounting the PSU. Funny, I had mentioned that the noise is different at times. Well, just tested it again and it's very quiet tonight, like was back east. So, I believe I've got some RFI issues going on that change with the weather (the sun?) and the zodiac and what I had for dinner, and the toroid is not in itself to blame as it seems to be simply transmitting whatever it's getting. Looks I should spend my time on the RFI in my AC instead.

Another funny thing, a couple different modules have different sensitivities to the noise. Kevin mentioned it's due to different component variations and subtle wiring differences. This pre rack is different every day on top of that. It's like we already know that no two mics sound the same, no two instruments sound the same, no two women are the same and no two pints taste the same... I guess we can be thankful for variety in life most of the time. Music and recording and gear usually leads to philosophy, sooner or later. There's nowhere else to go. Or to the bank manager.

cheers,
tommypiper
:guinness:

Two ways for shielding to be used:
1) to keep things out
2) to keep things in

Two different fields:
1) electric field
2) magnetic field

Two different frequency ranges:
1) dc to daylight
2) 50/60 cycle hum

Lets start with the power transformer:
You are trying to keep both magnetic and electrical fields in at a frequency range of 60 cycles.

Go to the Sowter website:
http://www.sowter.co.uk/

Click on the Alpha Index at the top of the page left.
The second article down, Soft Magnetic Materials, is an excellant source on transformers and metal.

On page 15 of the 19 page pdf, you will find a chart on metals and their shielding properties.

On the input transformer, you are trying to keep all fields out at all frequencies.
Mu metal with some copper foil backing would be good.
I think I can make you a Neve shield from some mu metal here.
I don't know if that was stock.
I will build it from my back plate off the Marainair.

Cheers!
cj