Wrapping power cords and interconnects with copper foil.

You have discovered one reason why better cables matter. Proper shielding makes a big difference. I recently acquired a new to me power amplifier that only supports RCA. I tried several cables on hand, none expensive, one had hum, one was murky, the others okay. I plan to replace them with something commensurate with the rest of my cables. I would start with an investment in a good pair of interconnects. 

Yes it works. I did wrap my cheap cables many yrs ago. I have not tried it on my new cables.

@testpilot 

+1 ….. for quality build cables with proper shielding. Build quality matters …. Full stop.

Did you ground it?  basicly you are making a shielded cable fromm your apparently unshielded cables.   Shielding power cables prevents them from being a noise generator.  Shielding the interconnects is often not recommended and I do not use shielded interconnects since it can limit dynamics because of inductive effects.  that said, your shielding may be far enough from the conductors that inductive effects are minimized or even eliminated.  Inductive coupling goes as 1/d. 

So most people buy shielded power cables for this but your solution seems to work.  curious if you grounded it.

Jerry

How is aluminum foil? It is almost as good a conductor and quite a bit cheaper and easier to find.