Two things you could do about the magnet influence on the toroid:
Get some mu metal foil or sheet and place a shield below the speaker magnet,
about the diameter of the speaker.
You could also determine the effect of the lid on and off, and then offset
the cal with lid off, to make it match what you want with lid on. A pain,
but maybe the mu metal shield is the best thing to do. You might try a
plain steel shield and see if it will be sufficient to block the field from
the magnet as well. Worth a try to use a piece of tin can rolled out flat.
73,
Stuart K5KVH
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