Try using a signal divider of two resistors on each input. 1k between(+-) the input and ground (on the board) and a 1k-4.7k series resistor between the board's input(+) and the signal source. By selecting resistors, you set the desired output level.
Use a high-quality power source no more than 16V for a current of at least 3-5A.
Use a high-quality power source no more than 16V for a current of at least 3-5A.