Gianluca, line voltage on the output of your pick up is paramount. Too much will drive the input of your recorder or amplifier as a distortion pedal would. If you notice that the sound improves as the battery weakens you will need to add or change your line voltage resistor to lower the amperage. I tried many systems until I finally came across the Myers pickups. I have been using Myers pickups for some years now and have recomemded them to many here on Melodicaworld. Thier systems are very clean and punchy. I have worked with Greg Myers on a few custom systems for my Diamante Wind Vox Melodicas, one (3) pickup exterior system and one (3) pickup interior system. The equalization range is great and no key clicking sound. Check out the website below and maybe consider one for your custom melodica inclosure. You can also contact Greg and talk to him about your needs and maybe he can design one that will work for your custom melodica. Also included a video of my Diamanté Wind Vox #002 showing the control box that Myers created for it.
click to go to Myers Pickup website then click product line top right
Diamante Wind Vox #002