"Alan WA4SCA" wrote ...
One has to start somewhere, and the presence of ground loops can, and
have, resulted in RF and not just audio problems. FWIW.

This is a FAQ in the audio world. It is a very common problem
with getting audio into and out of laptops. Their "brick" power
supplies are notoriously noisy. At AF frequencies, and likely RF,
as well. In the audio world, we use full transformer isolation in
and out. (Or else use an inexpensive USB-powered audio interface.
Likely cheaper than trying to isolate the PSK31 interface.)