Thanks for the fast answer.
Another fact : It does exactly the same when I just use headphones plugged into the soundboard (without speakers)
The Pedal is brand new I just tested it before I bought it, and now I remember that I tested it on a guitar amp !
Oh and might be wrong, I actually use monitoring speakers, M-audio bx5a.
Those ones : http://www.idjnow.com/ProductImagesLarge/BX5ADELUXE.jpg
