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Old 22nd February 2012, 07:46 PM   #1
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Question Shielding a toroidal?

I'm a new DIY'er and need some advises on toroid's and how to properly shield them. I'm finishing off my first build which is an AMB M3 headphone amp. I want both M3 board, power supply and toroidal in the same case.

How do I properly shield a toroid? I've seen some people place the toroid in a case of it's own inside the main chassis. What's that toroid case made of?

Same question goes for the Lighter Note LDR pre-amp I'm thinking of building. It's supposedly also affected much by EMI from toroids, so I really want to spend a little $ preventing the toroids from being too noisy.

Thanks for helping out a newb.

/Greg
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Old 22nd February 2012, 08:05 PM   #2
jcx is online now jcx  United States
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toriods ideally don't leak any magnetic field

practically there is some from the wire lead outs, a little very close to the core from winding unevenness

air space is the cheapest "magnetic insulation" - magnetic fields are all dipole or higher order and fall off as the cube of distance or more

Iron "shields" by being a very good magnetic field conductor and "shorts out" the mag field - so it is best used surrounding the source, perforated low carbon steel sheet is the most economical, practical for home fab of power supply enclosures, lets air flow for cooling
since the lead out wiring, rectifier, ps cap loops may be bigger probelms I would build a sub box for the whole power supply if I found hum/buzz from the supply a problem

it is more likely that gnd and signal wiring common impedance coupling will be bigger issues

the ~10x lower line-to-secondary C of a split bobbin EI transformer can give lower case leakage current and less common impedance hum vs a toroid, if you put in a mag field leakage shield then I would go with an EI xfmr


I also think the M3 "3-channel" active gnd scheme is simply wrong - especially if you can have a dual supply with a "real" gnd

Last edited by jcx; 22nd February 2012 at 08:09 PM.
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