The Audio Injector Octo sound card for the Raspberry Pi - differential powered breakouts. Find more information at : http://audioinjector.net
These KiCad (http://kicad-pcb.org/) files give both the schematic and PCB gerber layouts for the RCA breakouts for the Audio Injector Octo sound card.
These balanced breakout boards are based on Sallen-Key filters.
Matt Flax flatmax.com