I did some things this morning in the university laboratory with an oscilloscope, I tried using a offset of 2v, and now I see that im getting 300mv on the input (just noise, unplugged cable) and in the output 3,50V.. so now its centered at 2V nut its losing a little bit. I used a 100k resistor between the ground and the hot leg of arduino as you said. Today I havent had time to try som stuffs and measure from guitar and so..but I will have the whole morning tomorrow to test inputs and outputs...
thanks so!