This is an excellent usb device. It’s so good, I have four. But they are not all created equal and the best ones I have found are from a seller called DIYINHK on Ebay. However, to add spdif, I had to add a circuit to drop the CM6631 3.3V output to 0.5v spdif level. So there is a 1nF series capacitor followed by a potential divider of 300R and 100R.
I added a PE-65612 pulse transformer to isolate the output.
Here is the circuit :
Here is the cheapest one that does I2S, but it has a noisy regulator, cheap 12mhz oscillator, and the firmware limits it to 24 bit. In this thread you can read about changing it to 32bit, thanks to the helpful people at Diyaudio.com.
I replaced the noisy AMS1117 3.3V regulator with an ADP151, and fitted components for an spdif output.
The spdif is not going into a BNC but into an SRC4392, so the circuit is a little different from above.
Other receivers I have are Ti PCM2707 (several), Tenor TE7022L ( Teradak Teralink X2 and others), and XMOS. In order of sound quality : 1) XMOS 2) CM6631A 3) TE7022L 4) PCM2707. 1 and 2 are similar, 3 and 4 are similar, so the CM6631A is the best value for money.