Ground hum through speaker

[johnmath]

The point where a winding ends and wires are brought to the outside is a strong source of stray field a

Thanks for pointing this out. I did post "rotating the transformer to move the wires away from the circuit board could help" not realising the significance of field leakage.

 

[as_audio]

In fact this is a significant cause of noise in some applications
and methods exist to optimise toroidal transformer coil winding,
but it is not always easy to teach your local winder to do so ..