I found to be spending quite a lot of money in throw away batteries using my
Sony ICF-SW7600GR portable receiver. Thus I bought a filtered (stabilized?)
power adaptor (a generic one), it works well within many frequencies but on
some of them (always AM) there's background noise and it's impossible to
listen to LW, as it makes too much noise there.

I assume it has something to do with the mains, so my question is can I do
something to filter, even more, the power cord that goes into the receiver?

Thank you!