PT magnetic field

[hylaphone]

Regarding the magnetic field of a power transformer, is it attributed more to the low voltage, high current windings? Or vice versa?

If best orientation of the PT and OT isn't enough to quiet down the hum, would I be better off trying a toroid for the filament supply or HT supply?

[Helmholtz]

I guess the outermost winding (whichever this is) contributes most to close-distance outer magnetic field depending on orientation.
But even an unloaded PT has a leakage field.

What transformers, what orientation and distance?
Are you sure the hum is from transformer coupling?

[xtian]

Are you asking a theoretical question, or do you actually have transformers in your hand?

When energized by mains AC, a PT radiates EM at idle--doesn't even need to be passing current. I have built about 60 custom amps over 15 years, and almost every one of them was a custom or experimental design, so I positioned the PT/OT/choke and listened using the headphone trick to help arrange each project. I could find a good null point for each one, even with PT and OT adjacent to each other.

[hylaphone]

Interesting. This is more theoretical as I approach a new project of a 200W bass amp...
Thanks for the insight!

[xtian]

In that case, go for a toroid! No radiated EM, a bit lighter in weight.

[R.G.]

The magnetic field is nearly all inside the iron, and it's generated by the primary winding. There is some magnetic field that never goes into the iron; we call this field "leakage inductance". Some of the field in the iron escapes at gaps.

Leakage inductance flux escapes mostly along the outsides of the winding, and especially at the ends of the coil in non-toroidal transformers. In toroidals, it escapes anywhere along the coil, and is much smaller because toroids don't have gaps to "resist" the internal M-field. In E-I cores, field escapes at the Es where they run into the Is. The tighter the E-I stack is jogged and jiggled together, the less escapes at the junctions because the gaps are smaller.

The primary is responsible for all of the magnetic field inside the transformer. It sets up a "reservoir" of M-field, paid for as the magnetizing (no load) primary current. Secondaries suck out some of this field, which is instantly replaced by current from the primary to keep the inside field full.

You can 3-D map the leakage field around a transformer with an oscilloscope probe with a small loop of wire on the end.