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

[stefanovesa]

Hello,

I try to bring UP the topic

To summarize, I suspect that MY 7055-C isolator doesn't work.
As tested, it seems that is not providing galvanic isolation.
The "tingle" on MacBook chassis, when charging with two prongs charger, is not there, as if the ground of the laptop and the DAC were connected and not separated. At the contrary 7054 seems to work as expected.
Further, my 7055-C doesn't work with iPhone USB-C port. 7054 is working.
Do anyone has tested the same, or I need to fill a RMA for possible defective unit?
Many thanks for assisting.

Cheers,
Stefano

[Daniel (Intona)]

I don't think that the isolation of your 7055-C is broken. There are two differences between 7055-C and 7054 that affect your issue.

1. The 7055-C has a direct capacitive coupling between the two grounds while the 7054 has only parasitic capacitive coupling which is far less. The coupling is not only needed technically for USB 3.0, but it is forced by law to satisfy electomagnetic regulatory. It's still galvanic isolation. The lower capacitance is apparently the reason for your "current feeling" as there is no defined AC path. From technical perspective, both versions are totally safe because the current is in the nano- to microamps range.

2. The power requirement of the 7055-C is quite higher than of the 7054. Together with the end decive, this is more than the USB-C port of the iPhone will provide. Although it would be capable but this would require a MFi certified cable plus USB PD handshaking which is to my knowledge impossible to meet with the 7055-C.

[stefanovesa]

Hello Daniel, thanks for the reply,

I've just filled a RMA form for the unit, but at this point you can ignore it.
I'm sorry I was feeling a bit apprehensive about the "issue".

Cheers,
Stefano