FInally I have a noise issue

...all the above applies to laptops.  When it comes to desktops you need different solutions (e.g. isolation transformers) as, obviously, you cannot disconnect the mains supply to the computer.

Hi there,

I sometimes use an iPad as a cable-connected audio source, like I guess many people here do.  I found it is essential to be not charging at the same time.  You must have only the audio headphone/lineout wire connected.  Any other connection will cause two things  a hum loop (line frequency 50 or 60Hz tone) and the horrid switching noise and RF junk that comes out of all basic phone/usb chargers.  It can have a nasty subliminal burbling sound that is not completely audible most of the time but is nevertheless destroying many details in the sound.

With only the one connection from iPad ( or phone) to line-in I find the fidelity is not bad and the noise floor is pretty low.

Hope this helps,