since there is reason to believe the ®´s j-fet input stage rather attenuates than amplifies
the E - field variations i set up a simple band-pass based 1 ::
this might still require extra input stage (for deep buried wires)
+ experiment with the layout , sensor , 3-sholds´ sensitivities before we can say if it´s for any good
updated an old Sq.-wave oscillator to a voltage controlled one -
to be mixed with prev. ~50Hz output ::
it is good to pre-test and choose + adj. the optos to the same transfer ratio ,
everything else shouldn´t be too much trouble ?