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Mu Metal

I think you might be referring to 3M Ultraperm 80. I've used that myself, but it's only moderately helpful. Like you say, it's thin (.004"), and that just isn't enough to be very effective at 60Hz. The last time I had a coupling problem, I wrapped the output choke (under a cathode follower) with Ultraperm. It attenuated 60Hz at the output about 6dB. That wasn't enough, so I removed it and installed an aluminum plate instead. That reduced the hum about 10dB to 12dB. I didn't try using both, but probably should have.

I suspect mu metal needs to be at least .020" to .030" thick to provide good shielding at these frequencies. I once worked on a RF project where we made the entire chassis and all the sub-compartments from mu metal. The goal was to push unwanted EMI (including 60Hz and 120Hz) into the noise level, which was about -170dBC. IIRC, the panels were about .080" thick. The boxes weighed a ton, but the shielding was excellent.



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