How critical is mu metal shielding in mic transformer?

I have these, 1:10 transformer, supposedly made under license from Jensen (per the guy who sold them), but the company has no information about them at all.

For mic transformers, mu metal shielding is essential.

Cheers

Ian

If you go 1000' south of my house into the woods, you may not need magnetic shielding on a mic transformer.

If you go 1000' north of my house into the woods, you WILL need magnetic shielding, for the field from the transmission lines.

Almost any place you want to record (near power) you will need shielding. I have got-by with steel utility box for low-quality audio where a little hum was accepted, but not good audio.

Mu is not essential. 3 or 4 "russian doll" nesting steel boxes, not touching each other, may be as good as 1 or 2 layers of Mu.

PRR said:

If you go 1000' south of my house into the woods, you may not need magnetic shielding on a mic transformer.

If you go 1000' north of my house into the woods, you WILL not need magnetic shielding, for the field from the transmission lines.

Almost any place you want to record (near power) you will need shielding. I have got-by with steel utility box for low-quality audio where a little hum was accepted, but not good audio.

Mu is not essential. 3 or 4 "russian doll" nesting steel boxes, not touching each other, may be as good as 1 or 2 layers of Mu.

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Mmmm...when you say "not touching each other," - do you mean not touching but individually grounded or some / all 'floating' ...? Cheers.

Newmarket said:

Mmmm...when you say "not touching each other," - do you mean not touching but individually grounded or some / all 'floating' ...? Cheers.

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Magnetic shields don't need to be grounded to work. I think PRR means held apart by non-ferric means (like plastic or brass standoffs).

Cheers

Ian

ruffrecords said:

Magnetic shields don't need to be grounded to work. I think PRR means held apart by non-ferric means (like plastic or brass standoffs).

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Yes. You're right but I sort of forgot that because we usually ground them anyway so they function as a normal emi shield too (although it'd be better with, say, copper).
I didn't imagine the advice was that they float physically  but it kinda reads like I might have.

As PRR shared, distance matters. Back in the 90s I designed some fixed install (background music/paging) amplifiers with mic transformers and 70-100V audio output transformers for constant voltage systems inside one chassis. Inside one small 3-5W unit the mic transformer and 100v output transformer was literally inches apart out of necessity.

That mic input transformer used mu metal and absolutely needed it.

JR

pot them in a copper end cap.  We did that back in the day at a place I used to work at.  We had the transformers custom made locally and then potted them like you would a 990/2520, etc for shielding purposes in a copper end cap. For a while we had people asking us about nibco transformers. Nibco is the company that would machine the copper end caps we picked up at Home Depot.

So your copper worked well, Pucho? For EMI shielding, it's my understanding that magnetic permeability matters most in the shielding material.  Copper is pretty low in that regard. I would think it would work well for RFI shielding, but less so for induced magnetic interference.

you can get a Jensen 1:10 for about $70 with can,

DI box has no can on xfmr, but it is inside a case,

Thanks everyone. So it sounds like getting a copper end cap and keeping power transformers out of the same chassis is a compromise solution?

@pucho812 what did you use for potting?

I woukd be concerned with the power supplies if units above and below it in the rack as well.

rackmonkey said:

So your copper worked well, Pucho? For EMI shielding, it's my understanding that magnetic permeability matters most in the shielding material.  Copper is pretty low in that regard. I would think it would work well for RFI shielding, but less so for induced magnetic interference.

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well EMI is ElectroMagnetic Interference so the terms covers it all. H field and E field.  But the (relatively) low frequency fields you get from transformers are more H field than E field so yes you need magnetic permeability in any shielding material of realistic dimensions.

You can buy adhesive backed mu-metal to wrap a shielding can. Works very well. Copper on it's own will not do much for magnetic interference since it does not have a high magnetic permeability.

I made a post about this for a V76 build with some info:
https://groupdiy.com/index.php?topic=68498.msg871733#msg871733

As other's have posted, shielding is important in a input transformer because any induced hum on the transformer is amplified by the preamp circuit.  Shielding is typically measured in dB, i.e. -20 dB shielding which means the induced hum is reduced by a factor of 10.

Mu-metal has a permeability more than 10 times steel.
https://en.wikipedia.org/wiki/Permeability_(electromagnetism)

Right. I share my approach in this thread. Links to some good research on shielding and materials:

https://groupdiy.com/index.php?topic=66718.msg845727#msg845727

ruffrecords said:

For mic transformers, mu metal shielding is essential.

Cheers

Ian

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Agree 100 per cent...but RF is sneaky...depending on the intensity, audio rectification can occur in the gain stages with resulting degradation...

rmburrow said:

Agree 100 per cent...but RF is sneaky...depending on the intensity, audio rectification can occur in the gain stages with resulting degradation...

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But... transformers are bandpass filters that generally scrub off excessive RF energy making rectification in later active gain stages less likely.

JR

pucho812 said:

pot them in a copper end cap.  We did that back in the day at a place I used to work at.  We had the transformers custom made locally and then potted them like you would a 990/2520, etc for shielding purposes in a copper end cap. For a while we had people asking us about nibco transformers. Nibco is the company that would machine the copper end caps we picked up at Home Depot.

Click to expand...

Copper will do nothing to stop EMF interference. It is used to combat RF, a whole different animal. You could easily prove this using one of the copper shielding kits for guitar cavities. Mu metal in it's various forms is what is needed to stop EMF (hum) problems. It acts as a magnetic shunt, "short circuiting" the magnetic field away from or around the coils of the input transformer. If you look at almost any one of the microphone preamp input transformers in any of the classic (and not so classic!) consoles, you will find them in that mu metal casing. Not copper.

What about power transformers? What's the best material to shield them inside the rack?

warpie said:

What about power transformers? What's the best material to shield them inside the rack?

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Depends on how much you want to reduce their stray field. Better to start with a low extraneous filed type like a toroid rather than an EI type. Then place it inside a steel box. I have found this combination is good enough to keep hum out of a mu metal screened mic pre transformer with 70dB of gain in the pre.

Cheers

Ian