Why ring core signal transformers may not even need a magnetic shield

At your risk. Take all safety precautions.


On so many Pleiades microphone, input transformer, one or more active device amplifier experiments usually a microphone is connected to an input transformer. The input transformer may be capcitively coupled to an electron tube control grid. An output transformer then feeds an ordinary microphone amplifier.


Such tremendous detail amplifying arrangement with so much total gain might be expected to have a huge magnetic or electromagnetic pickup interference susceptibility.


Yet output is in most cases clean of such man made artifacts even if the front end input transformer is magnetically unsealded.


Why is this so?


One reason is the use of batteries (in series with a fuse for fire hazard safety). Batteries do not radiate hum as power transformers, chokes, rectifier arrangements do.


Other reason is that low cut filter coils or mic transormer coils are wound on ring or toroidal cores. The toroidal shape is possibly the most magnetically efficient shape (in the universe?). And if a core of high permeability is used almost none of the internally generated field leaks out and none of the external man made fields can get in.


But even if no core is used, ie winding toroidally on air, still most of the interference is out.
The reason seems to be that the ring core winding shape is hum bucking at any point of the circle circumference.


That is for any one turn of wire there is one on opposite side of the ring core circle circumference (at 180 degrees) of opposite winding direction. Since they are in series what they pick up or what they would radiate cancel.