acoustic guitar hum

Hi folks,

I recently bought the Zoom R20 and I have a long list of questions but will try to address them on at a time in separate posts.  Hopefully I am in the right place.

Hey guys, I'm hoping someone can help here with fixing an earthing hum on my steel string acoustic guitar.

It has a Fishman infinity preamp and piezo under saddle pick up installed.

Observations : If I touch the metal end jack insert the hum goes away,
I put one hand on the mixing desk the hum is gone,