Well, my desktop's USB of this high end ASUS P8B WS mainboard is very clean power and perfect wattage too.
However, the USB 2 clocks could be better, they have some drift. Unsuprisingly, USB 3 port does better still. Slower clock than expected, but very even.
Part of this stability is probably the high end PSU in it. (Seasonic. They do make some great cheap ones too.)
Not that any well implemented good DAC would really have issues with noisy power.

However, the best way is to connect the DAC via optical SPDIF (coax is acceptable too, but worse) and use a dedicated external USB power supply, avoiding all problems with drifty USB clocks and adaptive mode isochronous USB.
If you feel adventureous, build your own online power supply or buy a lab power supply. A very good lab supply for this kind of DC voltage and current costs some $60.
Do watch out for chinese air being sold instead - it may even deliver some power...

OR



Obviously if you are using a laptop, your choice is made for you.

I wouldn't say EMM doesn't understand USB (or that having an unisolated USB connection was a decision made out of ignorance of the tech), he made a calculated decision based on what he thought had more pros than cons.  Might not have been the right decision, but wasn't from lack of understanding the tech. 

The problem isn't USB really, but in general computer AND audio where more than one device is connected to mains via a SMPS (Switch Mode Power Supply) is what's giving the problems.
In this case one would have to isolate (break the ground loop).
For RFI, however, breaking a groundloop with an isolation transformer is not feasable as transformers show very little attenuation.
They do attenuate for the old fashioned hum though.

Consider each device connected to mains as an individual power source with each having a different (even when the same power supply is used) frequency spectrum, voltage level and currents.
To visualize: connect the + of a 12V battery to a + of a 6V battery while connecting the - of both batteries with say 50 Ohms.
In the wire connecting the 2 +'s there will flow (about) 120mA of current.
As the wire has a certain resistance a voltage a small voltage will fall over that wire.

Now change the batteries for audio equipment, the - contacts connected via a 50Ohm R to the safety ground or just simple the area (literally air) around you, substitute the current that will flow for Radio frequencies that ALSO visualize 2 audio devices which are 'connected' via ground, substitute the wire between the 2 +'s for the screen of an interlink + the all important PCB traces and signal routing in there. Note that digital works with edges (transitions) and levels (high-low) at very high frequencies.
Note that the SMPS garbage also works at very high frequencies in the same domain.

Now when you connect 2 of those power supplies (the one in the PC is HORRIBLE) and realise the ground/shield/common of that PC, (even when connected to safety ground !) will have a certain RF power and is connected to a source with another voltage/current frequency spectrum connected via the common of the USB cord you can understand that strange currents will flow and voltages will develop. (this all depends on IF the DAC is also mains fed or simply 'floating and USB fed)

For DIGITAL this MIGHT have some influence but would be limited but here's the thing... that USB ground is (in 95% of all DACs) also connected via interlinks to the amplifier.
The amplifier in itself also has a 'power supply' and the one from the PC/DAC is flowing in the ground/screen.
Depending on the contacts of the plugs, current and frequency spectrum of the RF signals, resistance (impedance in this case) of the screen, and HOW wires are connected in the amplifier section and the immunity for (common mode) garbage that is always present but mostly needs a minimal amount to be come audible.

Remember the cell phone in close proximity to audio/video equipment that emits 900MHZ or 1800MHz yet you still hear those familiar sounds... this is all due to rectifying/AM detection as in radio action of the semiconductors in the amplifiers themselves.

So there you have it... and this applies for only analog as well as soon as SMPS's are in use.
I am willing to bet people have experienced strange sounds or 'a strange pressure'  while listening to music they find hard to get rid off.
In the all analog days problems like hum would magically appear, now this can 'fog' audio.

One thing that is certain to break a (RFI) groundloop between the PC and audio is .... fibre-optics (think TOSLINK).
But having this between a DAC (with it's own SMPS) and an amplifier connected to the DAC also will show the problem but that horrible PC power supply is out of the loop so the levels might very well be below the 'detector' limit in the amplifier.
Other 'simple' measures can be taken to minimize garbage entering the audio chain and (possibly) clouding/fogging/interfering with audio.

Some other boring sources of info I've found:
http://www.audioasylum.com/cgi/vt.mpl?f=pcaudio&m=112754
http://www.murata.com/products/catalog/pdf/c35e.pdf

OK then, some questions I want to clarify:

Does "RFI" correlate to Radio Frequency Interference, EMI at very high frequencies?
So I reckon that both DC ripple and RFI are caused by the PC SMPS functional architecture, correct?
And any other non linear power supply and EMI being cauesd also by other sources, I reckon...
Can I minimize the EMI effects on a USB cable by shielding it with some material?

I have no idea what kind of PSU is inside my desktop PC, I think it's just a generic one probably SMPS.
But replacing the PC PSU is out of question to me, at this moment.
So what is that I could possibly do to minimize all the nasty effects on the following system?

PC USB - ODAC RCA - Violectric V100 - headphones.
The PC and V100 PSUs would be connected on the same power strip... is this elegible to create a ground loop?
It is mentioned that most USB interfaces are powered up by the PC USB bus, did I understand correctly?

I've seen several devices mentioned here to address common mode noise and ground loop but they're way to expensive.
How about something like this between the PC USB and the Dac?
http://www.circuitsathome.com/products-page/usb-interfaces
or this? https://www.olimex.com/Products/Modules/Interface/USB-ISO/

If I where to connect a portable battery pack to one of these isolators wouldn't I have a separate power supply for my USB Dac interface?
I see that one of these solutions does not garranty separate data and power lines, would it make no difference then?
Neither of these solutions supports High speed data rate(480Mbps) but I don't think that's very relevant for audio playback...

And what about Jitter?
"Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source." From Wikipedia, I love this website!

I've read that EMI can cause it, so I reckon that most USB ports from most PCs output data with some degree of jitter?
This seems to be an issue that derives from other external influences like the other issues mentioned above.
So it seems reasonable to me that by addressing the above issues first, I can prevent the formation of Jitter in the Data stream sent to the Dac... at least to some extent...
And even if there is still Jitter at the Dac input, it will be mitigated if the Dac has some internal reclocking feature built in... so I guesss that i shouldn't worry to much about Jitter with the ODAC... (Hope I'm making sense I have no idea what I'm talking about )