Originally posted by peskywinnets
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This works, to the degree it does work, because the music fields are quite nonuniform, being concentrated near the magnets, so the inner coil develops more music voltage than the outer coil, so the music doesn't quite cancel.
Hum fields are pretty uniform, so both coils see the same density of hum flux, so if the area-turns products of the inner and outer coils are the same, the hum will cancel. At least in theory - alnico magnets have a permeability of about three, so the inner coil will get more hum than expected from the area-turns product, so some tweaking of area-turns products will be needed to achieve complete hum cancellation.
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