Though I don't have solutions, I do have a few thoughts.
* Is your audio coming through clean, but also has this noise, or is the audio itself distorted?
* Can you describe your external interface situation / what is the outboard mixer?
* What are you runnning your pre-amp module into / at what point are you introducing the balanced cables?
* Do you have a recording of the sound? Maybe one of the more keen-eared electrically minded folks here can identify it.
* Output module - I'm not familiar with your output module, and I certainly don't mean to bad-mouth a company I'm not familiar with, but perhaps it's introducing some noise? Also, it's description on the manufacturing page says that it's main use is to go from line level (significantly lower signal) to eurorack level (significantly higher signal). If you're already putting a eurorack level signal in there and adding gain, I'd imagine some noise or distortion would be introduced.

-- HGsynth

Hey, thank you very much for your answer.

About your questions :
- My audio is clean, I don't think it's a distortion of the sound. It's just that the more I increase the gain, the more noise I get.
- I recently bought this mixer (https://www.thomann.de/fr/the_t.mix_xmix_1202_fx_usb.htm) but i don't think this is the source of the problem because i don't have this issue with others synths that i use. My soundcard is a Komplete audio 2 by Native instruments
- I don't use a preamplifier module as an output but simply to send the audio from my Analog rytm MK2 into the modular system. It's a Sovage engineering Duo stereo. Do you think I should use it as an output?
- I'll try to record a sound soon and send it to you. (Sorry for taking so long to reply, I'm very very busy at the moment)
- I don't think this module is the cause of the noise because I already had this problem before I bought it. I thought that this converter module would allow me to convert the level of the modular to the correct level for my mixer and then allow me to record on Ableton.

In any case, thank you for all your advice. I'm taking note of the modules you've recommended, and I may buy one of them later if I don't find a solution to my problem.