Going off tangent a bit, but field coils do open up a lot of possibilities that fixed magnets can't match. You could have active flux modulation compensation for example.
Power supplies have to be incredibly well regulated so that back EMF generated by the amp and voice coil don't cause the magnetic flux to wander.
Power supplies have to be incredibly well regulated so that back EMF generated by the amp and voice coil don't cause the magnetic flux to wander.