Hi Don,
Thank you for the quick reply.
Yes, that's strange.
As for the ground loop, I've isolated the whole pcb, by removing the source and replacing it through a 470 ohm resistor. The recordings are quiet.
I thought that if the ground loop is within the pcb, it should show up anyway, right?
The problem arises whenever I connect my (low Z) generator.
I also tried with different cables with different lenghts.
No luck.
The cable is a RG 223 U with double shield.
The only practical cure is adding a resistance in series.
:(