I think the best way to look at Torroid leakage is picture a javalin going through the center of the donut hole of the torroid. If you cap the hole, top and bottom, you have effectively shielded it. Here's a brief relevant blurb rom a Soundstage article:
http://www.soundstage.com/maxdb/maxdb091998.htmThe toroid, a round donut-looking transformer, has almost no magnetic field radiating out from it except along an almost straight line right through the hole in the center of the toroid. Within that center hole and extending up and down (toroids are almost always mounted flat because of this magnetic field) from it is one massively strong magnetic field. It has the same relative strength as the weaker but larger-sized field that radiates from the EI frame transformer, but it is so concentrated by the shape of the toroid transformer that that the density of the flux energy in the field radiating from a toroid is very high indeed. The larger the toroid, the stronger this radiated field. Imagine a miniature javelin going through the hole in the toroid transformer; this is a pretty good representation of the radiation pattern of the magnetic field (though the field spreads out a bit as you get farther away from the toroid). Placing some other component above or below an amplifier with a large toroidal transformer inside is asking for trouble. You could get hum or otherwise degrade the sound. Only a large amount of ferrous material, like cast iron, above and below a toroidal power transformer will block the magnetic field. The aluminum or thin steel chassis in most audio components is virtually transparent to the magnetic field from the transformer. So are wood or glass rack shelves. Even the second chassis of another nearby component won’t reduce the strength of the magnetic field too much. Other than an inch-thick cast iron plate, the best defense against these strong magnetic fields from toroidal transformers is distance or placing other components beside the component with the toroidal power transformer rather than above or below it.
Bob
