The situation is this: in order to isolate electrically the computer and acoustics acquired audiokonverter SPDIF-RCA (computer connected to it by an optical patchcords). It turned out to untie the background of 50 Hz is gone, but there's a new nuisance. It turns out this audioconverter in the absence of input signal (off comp or just a patch cord is not connected to the input), apparently for reasons of saving energy, something there in his disables and as a consequence the output in the speakers there is noise. The noise is more like "white" and not on the crosstalk of 50 Hz.
Ordered here for such a Converter
Actually, the question(s). How to get rid of this noise? What to do with the Converter? Can advise to use some other Converter? What?