You will not be suprised to hear that the H9 was designed to be a guitar pedal.
That said, many people use it with a mixer. It can handle a line level which is not too hot.
If you feed it to or from a balanced line (XLR or stereo jack) you will want to unbalance the line, usually by connecting one of pins 2 or 3 to ground.
If it still does not work, you will need to have a tech look at your setup.
