James-F wrote:

Inside there are two isolation transformers with 1:1 turn ratio simply to galvanically isolate the interconnected equipment. The […]
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Inside there are two isolation transformers with 1:1 turn ratio simply to galvanically isolate the interconnected equipment.
The AC sound transferred magnetically through the transformer... like magic no wires.
The source of the hum/noise these isolation transformers fix is usually ground loops noises and it is not the kind of noise this thread is all about.
The cheap ones from ebay cut the bass below 50Hz and alter the phase response, I have a few myself and do not recommend for any serious audio application where sound quality matters.
Further reading: http://www.epanorama.net/newepa/2010/08/12/au … n-transformers/

Perfect answer, guys this is exactly what's happening. Galvanic isolation electrically separates devices and resolve ground loop, the (very) cheap ones are nothing but filters.

No need to apologize voodoo5_6k, isolation transformer is a viable solution and the first thing I try before lifting grounds (dangerous) of the interconnected audio equipment.
One of the things I do is to lift the ground of the CRT monitor but keep the PC grounded, this removes a lot of noise from the audio chain overall, especially when I connect the retro PC to my new PC for recording without audio isolation transformers.
This little box with audio isolation transformers actually works removing ground loop noises but it affects sound for the worse, so I risk lifting grounds for cleaner sound.
PS. These small audio transformers will saturate and distort especially with low impedance loads, once again not recommended.