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I have tried two different power supplies with same result. One of them is original Apple and the other one is cheap China model.... Will check with third one when I get the chance 
I always use external power for Fireface 400(Because I use it as a stand alone mixer)
I just made a small recording of connecting and disconneting the axoloti plus I took notes of noisefloor of axoloti when connected with power supply and not.
Maybe I am not using right name... There is low hum and hi pitched noise that might be mistaken for crackles.... l
With out power supply noisefloor is really good around -90 db, like G2, Blofeld, Micro q etc. Super great
When the power supply is connected to the laptop the noise/hum floor rises to around -45db.
Heere is a link to a recording of Axoloti being connected to a computer WITH power supply connected which comes in at around -45db.... Dont know if you can make anything out of it.. Anyways here it is
https://mega.nz/#!6IYQEaKA!BGSKb7pRI7jVVz_KIXOH2SsEWQnysaFdD9YGyyJs0fo
Ground loops can convert electrical activity (power consumption spikes) into audio noise. Sometimes you can hear disk activity, or screen updates for instance when dragging a window around. Without digital systems involved that 'd be almost only line frequency, 50 or 60Hz depending on region, and their harmonics.
if i connect my novation lauch control...after a while there is a hum
maybe a fault of novation ?
I tried with same result; around -45db noise/hum.
I havent got another power supply for the FF400. Will look for one and get back to you. Probably going to be a while....
I've experienced some 'cpu' noise noise when I was using the headphone out at the beginning, but now I'm using line out and a Y cable connected to my mixer (mono) and it's fine
i was using the headphone out first, but one chip on the axoloti got really hot + cpu noise
The headphone output is really intended for headphones. Its ground pin is biased at 1.65V and it does not not have a ground loop break circuit.
I seem to have a noise problem as well. Axoloti is connected with USB to my Mac Pro, stereo out to mixing desk. Noise still exists when I plug in a power supply in the Axoloti. Audio interface is connected to play and record via the mixing desk.
I don't have the problem when I connect the Axoloti with USB to my laptop and feed it into my mixing desk.
After i Got new laptop i havent had Any issues With Noise. I Think The cheap china power supplys that you Can get on eBay are also to blame.
Also a friend of mine have just Got a new octatrack. And when he connect his axoloti to The octatrack he gets Noise. Even though he uses same cable as he used when connecting Directly to audio interface, which worked
Noise problem is solved. My Axoloti was plugged into a powered USB hub, and the hub had a flakey power supply. Works fine when using the Axoloti on the same hub without the power supply. Thanks @johannes for helping me fix this situation.
Is it possible to get ground loop noise with only a DIN MIDI cable conencted to a laptop USB-MIDI interface?
I would think not, since the midi is opto-isolated, right?
All quality MIDI interfaces have an opto-isolated input and so does Axoloti. But I've seen reports of cheap chinese usb-midi interfaces without opto-isolated input - beware! A crappy (non galvanic isolated) power supply could also still cause trouble.
Dear Johannes.
Sorry for coming back to this after so long but I actually have the problem that I hear all this stuff while an extra power supply is connected!
I can, with or wihtout the extra power supply connected, actually see the laptops drive make noise in conjunction with the drive indicator light and even hear it when I move my pen over the connected drawing pad without actually touching it! So I connected a good seperate power supply to the Axoloti but that does not help.
This seems to suggest that the external power supply I use does actually not make the Axoloti switch away form the USB power line!
Only when I disconnect the USB cable does ther noise polution disappear. So don't connect it during recording is what you might say but the problem is that I (want to) use my Axoloti in "workstation mode", so constantly connected LIVE to the PC.
OK, my laptop is not top of the bill (Packard Bell) but I do not want to buy a whole new laptop or brand new battery pack for this single problem.
Do you have any suggestions? Could it fro instance be that the 1A output level of my power supply is to low?
Marc
Maybe you have a ground loop. The more you connect power sources and devices, the more it is likely to happen.
You can look for a "ground loop isolator"
As has already been established earlier in this thread this does not seem to be a simple ground loop problem. It's about laptop processing noise getting into the Axoloti outputs even when a propper independant power supply is used.
I had a very similar problem with my nord modular G2 engine (computer noise when the USB cable was connected), i solved this by using a ground loop isolator. But your problem is certainly different.
Did you try to put your Axoloti in a shielded and grounded enclosure ?