Bottlehead Forum
General Category => Technical topics => Topic started by: vetmed on February 11, 2012, 05:49:10 PM
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I built a MC step-up using some UTC "ouncer" transformers. Seemed to work well but a little bit of residual hum. This evening I was moving it about and the hum increased or decreased according to where I put it, sometimes it could be very loud and annoying. What puzzles me is that the transformers are fully enclosed in a cast aluminum enclosure. I thought this sort of thing was supposed to shield against EMI and/orRFI interference. Any opinions would be greatly appreciated as they sound good when the hum is absent, and the positioning seems to work best exactly where they can't be placed i.e. in midair between the TT and the phono amp!
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Any metal will shield from electric fields, though the higher the conductivity the better. But only high-permeability magnetic materials will shield from magnetic fields, such as those that leak from power transformers and motors.
The best-shielded transformers typically use alternating mu-metal and copper cans, that nest inside each other. Typically 30dB per mumetal can. You can use soft steel, but it is less effective and must be much thicker - I've heard 1/4" is as reasonable as it gets.
Mu-metal itself is very sensitive to mechanical shock, and must be annealed after forming. Magnequest Mike says if you drop it on a concrete floor, just throw it away, it's lost its magnetic properties. But there are high-perm tapes that are more tolerant, such as the molyperm used on the PT-1 to reduce its stay field. I don't know where to buy it though.
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One place to buy mu metal, co-netic and netic materials is:
Magnetic Shield Corporation www.magnetic-shield.com
Look at the LK-110 LAB KIT
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TI shield has worked well for me, It's clad instead of an alloy. Unlike MU-metal you can alter it mechanically without destroying it's shielding properties. You can get small pieces from Michael Percy audio.
Cheers,
Shawn
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Thanks everyone for clarifying that. It seems the culprit is the Origin Live motor controller box since the only other piece of equipment in the immediate vicinity is my Seduction phono amp which I believe is heavily shielded/protected against such unwanted energy.
You can often find mu metal cans to shield UTC transformers, there are several on eBay right now. I'll probably go that route.