Spring Line Hum

Martin Czech czech at Micronas.Com
Wed Jul 19 17:20:34 CEST 2000

(perhaps) no problem here, but:
I read somewhere that a double shield (iron and then high mu material)
is recommended, because the high mu stuff gets easily into saturation,
which is when the shielding is not working any more.

It is very easy to shield high frequency magnetic fields by some
conductive containment (copper or even paint), no mu needed here, because of
strong interaction of H and E-field, ie. the H-field will induce
large eddy currents in the shielding which cause another H-field
of opposite polarity, thus considerably weekening the overall field.

For low frequency this doesn't work very well, as E and H-field
get more and more seperated, in this case shielding will rely
on "magneto-optics", ie. field guidance by surfaces of different
permeability, similar to the well known laws of optics.

This means: high mu material (expensive), massive walls
and soft corners (ie. a ball shaped containment will be best,
a cubic will have stray field in the corners).

You may buy high mu caps for audio transformers at Farnell,
may be a transducer enclosure can be made out of this.

There are also metal caps for torroid transformers, but I
have no source for that.

Best is to keep any magnetic field sources away in the first place.
Anyone seen a Waldorf I from the backside? The transformer
is OUTSIDE the enclosure, good for the internal stuff,
but maybe not so good for third party, like Juergen said...


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