Hey Idreyn,
the pHat DAC has some male headers next to the RCA jack to get the signals.
Picture: https://cdn.shopify.com/s/files/1/1004/5324/products/Pi-DAC_3_of_3_1024x1024.jpeg?v=1445586297
at the bottom there are 2x4 male headers. You just have to connect to Pin "L" (or R) and to Pin "0V" to get your signal for the op amp.
It's an unbalanced signal, only 2 wires per mono channel.
Happy Hacking,
Steffen