Permanent 50hz

If there is nothing connected to the speaker except power cord, and we are assuming it's a powered speaker, (no RCA or XLR signal cables) -- if you are still getting 50hz buzz, then I agree with Eric that the power supply is failing (probably a bad capacitor).  The amp/ps board in the speaker likely needs to be replaced.

actually the noise is quiet..i need to put my ear very close to the mid unit
from 5 cm I can't hear the rrrrrrr
i don't know if it's normal or not

Not normal and Why would a 50 Hz tone be coming from the mid range driver. The cross over should have taken that out. What kind of speakers are they?

If you have to put your ear to the speaker to hear any artifact its quite literally no concern. Almost any amplifier will have some audible artifact in that situation.
Don' worry'bout it.

It actually sounds like a minor ground loop hum.  If you have to get within 5cm of the speaker to actually hear it, then don't worry about it.  My system has the same thing.  Sometimes this will be very noticeable in systems and people can have a very hard time getting rid of ground loop humm.  Humbuster may or may not fix this.  Sometimes, you'll need to get the Granite Ground Zero to resolve this ($500+)