The Allo Digione provides galvanic isolation from the RPI and delivers a very good S/PDIF output over coaxial cable (there is no optical) to a DAC. It has a regular RCA connector and a BNC connected (supposedly less jitter). They are both mounted in an Allo perspex case.
I have had it for several years and it is in very good condition and full working order - but I now have an Asus Tinkerboard and use USB, so it is no longer needed.
I originally paid £150 for the Allo DigiOne, RPI 3B+, and the Perspex Case, and am selling the set for £95.
I can provide a 8GB micro-SD card with either Volumio or PiCorePlayer on it (please let me know).
If you need a S/PDIF cable I have ~1.5m Belden cable with RCA connectors or a ~0.5m Van Damme with a BNC connector at one end and RCA at the other end. Personally, I found the longer cable slightly better but it is fairly stiff. Please let me know which you would prefer.
**There is no PSU for the RPI3+**. I use a battery and connect it using a USB Type-A to micro-USB cable.