I built a solid-state alternative to the TPA’s Sidecar.

Its main features are:

• Switching between I2S and S/PDIF with bus switches (solid state devices).
• Support for two I2S inputs, with source selection.
• On-board LDO regulator with 4.9V output for the Buffalo III’s 4-input S/PDIF board.
• Drop-in replacement for the Sidecar.

Schematic, PCB, etc. in its dedicated page: Super Solid-state Sidecar for Buffalo III