different behavior between 7055-C and 7054

[stefanovesa]

Hello,
I'm experimenting different isolation behaviour between 7055-C and 7054 (standard version).

The scenario:

MacBook Air battery powered or connected to power supply (not earthed power supply) --> USB-2 cable --> 7055-C or 7054 (standard version) --> RME ADI 2 DAC FS --> amplifier --> loudspeakers

case 1:

MacBook Air battery powered --> USB-2 cable --> 7054 (standard version) --> RME ADI 2 DAC FS --> amplifier --> loudspeakers

result: all is fine

case 2:

MacBook Air connected to power supply (not grounded power supply) --> USB-2 cable --> 7054 (standard version) --> RME ADI 2 DAC FS --> amplifier --> loudspeakers

result: if I put mi hand on the aluminium chassis of the laptop, I can feel a sort of current. See this post:

[External Link Removed for Guests]

I guess that 7054 completely isolate the source from the DAC, so I think all is fine and isolation is working. if I remove the Intona in fact the feeling of the current on the laptop chassis disappears because there is no isolation from source to DAC.

case 3, as case 2 but with 7055-C:

MacBook Air connected to power supply (not grounded power supply) --> USB-2 cable --> 7055-C --> RME ADI 2 DAC FS --> amplifier --> loudspeakers

if I put mi hand on the aluminium chassis of the laptop, this time I cannot feel any sort of current. Additionally, if I detach the power cord from the laptop, I can hear a sort of scratch noise from the loudspeakers when I detach it.
My concerns are that the isolation is working or not with 7055-C.
Sorry from the not-technical description, please let me know if the scenarios are clear or not.

Many Thanks,
Stefano