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Old 10th July 2010, 12:55 PM   #21
phrarod is offline phrarod  United States
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You're educating me that's for sure. It might kill the project and I'll have to find a transformer source who already has them in a can
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Old 1st September 2010, 10:07 PM   #22
RCruz is offline RCruz  Switzerland
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I read this with much care and after some investigation I now understand Mumetal cans work not by diverting magnetic fields but by shunting them.... (to GND ?!)

Is it necessary to connect a mumetal cage to gnd in order to work properly ?

I have been experimenting with iron versus aluminium lids and found that a layer of iron near the magnetic source covered by another (alu) is better than the reverse (alu near the tx and iron on top). The gains are evident even without connecting the lids to gnd.

In the case of mumetal, can I expect a similar behaviour ?

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Ricardo
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Old 2nd September 2010, 12:30 PM   #23
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If you don't attach the shield to gnd you essentially have the shield as one plate of a capacitor and the ground as the other plate. Thus, there's an electrostatic potential between the two plates.
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Old 2nd September 2010, 12:53 PM   #24
RCruz is offline RCruz  Switzerland
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My best results are obtained with the plates connected to GND but I also get very good results just by covering the circuit with an iron plate.

I have read that severall thin plates work better than a thick one... these should be placed with a dielectric between them (air).... In this case I should connect all plates to gnd right ?
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Old 2nd September 2010, 02:28 PM   #25
qusp is offline qusp  Australia
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love the ZERO link! thats excellent. contemplating getting some of the mini enclosures and the deepdrawn cans. the mini flanged mumetal lids could be designed into a PCB with the mounting holes and grounding pads to cover a dac chip. I know the gains are unlikely to justify the means, but hell if nothing else it'll look sweet
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Old 3rd September 2010, 02:29 PM   #26
RCruz is offline RCruz  Switzerland
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Is Mumetal good for source shielding ( as well as target "sensible equipment" shielding) ?

I read that we need different materials to shield strong and weak magnetic fields.
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Old 3rd September 2010, 02:57 PM   #27
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The problem with wrapping things in sheet, or indeed making a DIY can from mumetal sheet, is that the permeability of mumetal drops massively after mechanical work. For high performance use, it is first made into the target shape, then given a special annealing cycle under a hydrogen atmosphere to restore the magnetic properties.

This is why decent shielding cans are so expensive. I used to work in a physics lab, and had mumetal shields for one of my experiments; they were very effective, but cost over 1000 - say $1500.

Second hand or surplus cans may be your best bet.

I know that Sowter transformers offer shielding options on some of their transformers, and will sell you empty cans.
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Old 3rd September 2010, 05:15 PM   #28
RCruz is offline RCruz  Switzerland
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Thank you.... I will look for Sowter TX
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Old 3rd September 2010, 05:40 PM   #29
jcx is offline jcx  United States
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low carbon steel/iron is cheap so you can use lots, heavy sections, air spaced layers and get good mag field attenuation practically and much cheaper than with exotics like mu metal

also you need to think of mag shielding as diverting the field - Fe is 100s-1000s of times more permeable than air so the mag field wants to "shortcut" through the iron


you could even use cast iron plumbing pipe fittings to make up shielding cans for small xfmrs - just don't try to take them on airplanes
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Old 3rd September 2010, 05:56 PM   #30
RCruz is offline RCruz  Switzerland
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No airplanes in my mind... yet

Just need to shield the IEC inlet in my phonos..... Too near to the right riaa board input load resistors.... picks up like an antena
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