Balancing! All your lines should be balanced. What this does it splits the signal in two and flips one half of it. Then the side that is flipped is flipped back at the end, and added to the first one, therefore cancelling the noise induced in the cable out. You need a DI box, especially since your cables are long and then something to unbalance the signal at ...
There are all kinds of specific technical answers that you might receive. However, the one general rule that I have found to be true with audio workstation setup is this: some system configurations will work, and some will not work, period.
You are putting together a house of cards made of computer hardware, computer software, audio subsystem architecture, ...
There are several things that can be causing the problem, go through the list below to see if you can narrow down the cause of the issue. It may be that that issue is always there, and that you can only hear it when using headphones, or it maybe that the headphones themselves are to blame:
Try a different pair of headphones to see if it's the heaphones ...
If you're using a hard wired setup and you get crackling noise/noise reduction it could very well be that something in your chain line needs to be repaired. The cord may be old or rusty.
Tell me if I have this right:
Synth -> Input (software mixer) -> Output(Speaker)
Thanks for your support guys. The noise actually comes from the X2U adapter, at merely 50dB it couldn't handle the low signal of the SM7B. Now with Cloudlifter the signal is very clean. Naturally after equalizing and compressing some noise might come out but it can be easily fixed with a Izotope Denoiser.
Well. Static electricity can also affect sound. Might be that you must keep the mic absolutely steady. I bought röde m3 - which crackles if you move it around. They know that and send a rubber ring so the connection wont start causing noises. I just keep the mic fixed and the cable as well. I don't, however, do on-stage performances.