The PH1 500 Series module is a device input for a smartphone, tablet, or laptop. It allows you to patch the headphone output of the device directly into you studio system so you can listen to a clients music on the control room monitors. The PH1 takes the unbalanced input & converts it to a balanced audio signal with the correct impedance for your console or monitor controller. Since it's in the 500 rack it's always connected and ready for a quick reference check.