Found it. The actual PC, bought fanless for physical quiet, generates a ton of electrical noise when “doing stuff” (it’s weird, not just video or disk or whatever, just activity in general) and passes it back to ground via the PSU. We’d never noticed it before because all the previous synths had a C7 power connector to an external power brick so were inherently detached from ground.
Currently it’s fixed by disconnecting the ground pin on the power cord, but that’s somewhat inadvisable in the long term…
The original post did not ask for troubleshooting advice, it asked for advice on a new audio interface.
Implicit in that seems to be that you have a new synth which has a 3-pin power cord.
Does the synth have balanced outputs, or only unbalanced outputs? If only unbalanced you may have to use a transformer isolation box to avoid noise currents flowing in the signal return.
Get an audio isolation transformer on the audio line. If the connections are balanced lift the audio ground (Lift the shield) as an alternative if you must (Isolation transformer still preferred).
NEVER LIFT AN AC GROUND!
My students do not want to be on the receiving end of me when I see people do things like this. Neither do the professionals I see do it on occasion I am in charge of.
Okay, the thing is I’m a software engineer by trade and a martial artist by passion so my knowledge of audio wiring is patchy (pun not intended) to say the least.
When I google “audio isolation transformer” I get stuff from tiny 3.5mm things which cost next to nothing up to units costing four figures and I don’t have the experience to tell which is suitable and what might actually do the job.
So, is this the kind of thing I’m looking for? I don’t have much more budget to throw at this at the moment. IMG Stageline FGA-202 – Thomann UK
Not that weird - every time a bit flips you get electrical noise of some kind (even to the point that in some cases it has been used as a side-channel attack to gain sensitive information about the system).
I have a UMC22 here which I bought as a basic I/O device for some testing a while back - It’s a bit (digitally) noisy, but it does have balanced I/O, and using that cleans it up quite a bit. Difficult to know if the item referenced will solve the problem, it looks a reasonable spec, but there can be a lot of variables.
(Also the ‘sound’ of a particular transformer is often cited as the - secret ingredient- in classic gear, from guitar amplifiers, to mixing consoles, and everything in between, so whether it solves the noise issue or not, that tells you it may affect the sound in some other, partly subjective, maybe good, or maybe not good way)
So in general you can use items that are lesser expensive. DI boxes are common for this and a variety of other reasons, so I would suggest starting there. While you can get more expensive DI boxes, a simpler DI box with ground lift can work fine as well. The reason I recommend DI boxes is not only can you do things like lift the audio ground easily but also it will provide isolation and protection for your keyboard in case, for instance, phantom power is applied on the lines. Most keyboards and particularly balanced inputs should handle this fine, but if you then adapt to 1/4" you can cause some issues on occasion plugging/unplugging as you short between pins (Why we like XLR instead of 1/4").
The device you linked to however should work fine for this purpose, it is just a couple of XLRs with 1:1 transformers inlines between them to allow for isolation and break the ground loop.
I have a Focusrite Scarlett 18i8 (3rd gen), a Zoom U-44 and a Behringer FLOW8 mixer. All three work flawlessly under Linux. The Focusrite seems to have the lowest latency.