Thank you for your answer. you refer to the background noise known as "the noise floor". I am aware of this fact, but the problem is another: the noise present on all the channels is perceived too high, especially considering the technical specifications presonus mic preamps, much praised as very clean. The problem also occurs by disconnecting all the cables and leaving only the power and headphone cable that is used to monitor. I did all the tests but the result does not change. I also asked friends who have sound cards from other brands, which confirm a very low background noise, much less than my study 68. I think my sound card is faulty, I'll wait a few days to confirm some things and then I will change it. I have also opened a ticket in the presonus support and I am also waiting for their response.
I'm sorry for my english, but I used google translator.
Thanks for your help.