Johny B Good wrote:


[1] In my embarassed response to the mistake so eloquently picked up
by Phil, I overlooked his remarks about transformer shielding.

Mu-metal shielding _is_ sometimes employed, not internally but as a
total shroud around the transformer to screen magnetically sensitive
components from the inevitable stray leakage flux of classic E&I (T&U)
cored designs.


** Sometimes = extremely rarely, not in typical items of audio equipment and not available off the shelf. So your advice was pointless.


In this case, the flux levels are only a tiny fraction of that which
links the windings so doesn't represent a saturation risk to the
Mu-metal[2].

** Small ( like 5VA ) e-core transformers operate the core heavily into saturation - if you want a strong 50Hz AC field just pick up any plug-pak that uses one. Also, Mu-metal is ineffective at high flux levels while the other techniques I mentioned ( copper and steel bands) are very effective.


Toroidal transformers otoh, so effectively confine their magnetic
flux, they don't normally require any such screening, not even when
placed inside the confines of an IBM 14 inch CRT monitor originally
designed for 120v mains in order to convert it to 240v mains. A feat
not possible with a conventional transformer even when placed outside
of said monitor if not a good 60 cms or more away as I discovered two
decades back.

** Toroidals do have an external field and I often see steel bands wrapped around them to reduce it. These bands seem to be made from the same material as is used for the cores.

The magnitude of the field depends on the size of the transformer, any lack of symmetry in the windings - PLUS the level of magnetisation in the core. The biggest effect seems to be that toroidals are invariably run below core saturation while most e-cores are run well into core saturation.


[2] Is Mu-metal any worse than soft iron for self magnetisation? If
so, it seems a strange choice for the pole pieces of magnetic
replay/recording heads, especially since it has such poor wear
properties.

** Ever see a tape head demagnetiser ?

Every hi-fi shop sold them once.

I certainly do not want a large magnet anywhere near the mu-metal shield covering the tube inside my scope, as it would be a real bugger to demagnetise.


..... Phil


--
J B Good