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Old 27th November 2008, 08:04 AM   #1
VHF man is offline VHF man  Australia
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Default Magnetic Shielding Foils

I recently purchased some samples from Lessemf
http://www.lessemf.com/mag-shld.html

I'm wondering if anyone has used any of these materials or similar in the construction of power amplifiers to reduce field radiation from high current supply leads and large electrolytic capacitors etc.
I have a spare cassette tape deck head and I'm proposing to use it to measure the field radiation at various point around my DIY amplifier before and after supply and electro shielding.

Any other thoughts on how else to measure the field strength
are welcome.

VHF
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Old 27th November 2008, 08:36 AM   #2
PMA is offline PMA  Europe
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Non-linear material.
Regarding measurement - make a coil with 1 or more turns; you can calculate induced voltage according to elementary formulas, v depends on dB/dt.
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Old 27th November 2008, 10:46 AM   #3
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Default Re: Magnetic Shielding Foils

Quote:
Originally posted by VHF man
I recently purchased some samples from Lessemf
http://www.lessemf.com/mag-shld.html

I'm wondering if anyone has used any of these materials or similar in the construction of power amplifiers to reduce field radiation from high current supply leads and large electrolytic capacitors etc.
I have a spare cassette tape deck head and I'm proposing to use it to measure the field radiation at various point around my DIY amplifier before and after supply and electro shielding.

Any other thoughts on how else to measure the field strength
are welcome.

VHF

Cassette head has a nearly closed magnetic circuit, and will be very hard to get quantitative results from. A small air cored coil, with known area and number of turns is much better for accurate measurements - as suggested by previous poster.

I have used shielding materials for low level measurements in the physics lab, measuring nanovolt to microvolt sized signals on superconducting circuits. Most audio applications (except for phono stages) are at high enough level that shielding should not be needed if good layout is observed. If you do want to use shielding, I would suggest simply dividing power amp into two sections, with the PSU in one, and the amp board in the other, with a metal divider seperating them.

If you want to shield wires, don't attempt to shield an individual wire, but instead shield a circuit carrying opposing currents, preferably tightly twisted or bundled. The shield then only has to deal with residual flux .
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