So the ifi idefender3 is not safe?

Don't quite agree to run any amplifier without properly grounding it. Anyway this is especially true for a tube amp.

Thanks for your reply.

Yes head phones outputs still clear. I've tried with another amplifier, problem still there and not confined to the particular amplifier.  As I said simply switching off the DELL display port connected monitor (it's not necessary to pull off the thunderbolt/display port cable cable) by the apposite monitor button much of the audible noise disappears. Still have minor audible notebook CPU/GPU HDD noise coming out.

I've tried also to connect the dac, and amplifiers to different wall power socket with no success. Noise still there.

I've also investigated some other usb solutions. One cheaper solution over intona galvanic isolator (or ifi nano iGalvanic3), but expensive off the ifi idefender3, is this USB 3 optical cable reviewed here: http://archimago.blogspot.com/2015/05/m … tical.html

There are also other USB 3 optical solutions but the above is quite affordable since it is fully USB 2 (480mbits) compatible without any other support dongle. Some other solutions infact will require additional dongles for USB 2 compatibility connected at female end point.

Apparently this cable is still a hybrid solution. Not reallyt sure but the power rail is not cut along the entire cable length but seems still used only to power the female endpoint optical transceiver chip on the opposite side. That is enough for the reviewer to break his ground loop issues. More over that cable should have some how an "over current protection" built in (over USB 3/2 data rails on the female connector ???) as claimed in specifications sheet downloadable at the Corning home site.

But at this point my concerns is still there about the ifi idefender3 simple ground cutting the +5V and GND rail safeness (and obviously my home made usb cable solution!).ùBy the way the idefender 3 solution could be as cheaper as reliable. Having another 10meters cable upon the desk with the dac at only 0.5 meters by notebook still hurt my eyes as much as the noise is hurting my ears. But I still not want IFI to joke me and give them 60 euros to have the same result with an old inexpensive USB 2 modded cable by myself.

I could hear my CPU working on speakers, grounding was that bad. Even with RME unplugged from the power, some noise was leaking from PC. 
iFi iDefender 3 fixed that, speakers are now dead quiet when not playing anything. I almost returned RME (other DACs i had didn't have this problem!)

Had the same issue with my new ADI-2 DAC FS. Constant noise through the active monitor speakers, even worse when moving the mouse. So I went to eliminate influences one by one, until I found the actual culprit: a USB dock.

This is on a MBP which has 4 USB-C ports and no Ethernet port, so years ago I got a high-quality USB-C dock (from Lindy) to connect mouse/keyboard because at one point I had run out of USB ports.

To illustrate the connection setup:

MBP USB-C #1: Apple TB3 adapter -> Apple TB2 adapter -> Apple Ethernet dongle
MBP USB-C #2: Apple USB-A adapter -> RME ADI-2 DAC
MBP USB-C #3: DP dongle
MBP USB-C #4: Lindy USB-C dock -> USB-A -> Apple keyboard/mouse

So even though the ADI-2 was connected directly to its own, separate USB-C port, and the USB dock was connected to a different USB-C port on the other side of the MBP (which means it was connected to a different TB3 device internally), the USB dock was causing interference with the ADI-2 DAC.

Removing the USB dock made the whole setup dead silent.

Something to consider before buying expensive galvanic USB isolation.