I tried it with the Coax input with RCA out to the amplifier, it has the exact same problem.
Its not a problem of the Amplilfier or the source, its a problem of the Fiio BTA 30 Pro. I tried it with different devices, inputs, cables, usb ports, amps, sample rates, etc, the Audio cutoff always happens, its 100% a problem with the Fiio BTA 30 Pro.
For what it's worth, I successfully replicated your issue on my BTA30 Pro. Realtek optical output -> BTA30 Pro, the sound gets cut off exactly as you say. It's worth noting that the same issue does NOT happen when the same Realtek source is used with a FiiO K5 ESS.
However, I can tell you that the BTA30 Pro isn't the only device that has this annoying behavior with the Realtek optical output. I have experienced the issue before, just can't remember what the DAC was.
I think what we're seeing here is a combination of two issues:
- The Realtek optical port goes into standby and stops transmitting when no audio is playing. Some devices don't do this and continue transmitting a dummy signal even when there is silence, which prevents the issue.
- The BTA30 Pro goes into partial standby when it doesn't detect any signal on its input, and takes an annoyingly long moment to wake up.
If FiiO could disable this behavior so that the BTA30 Pro behaves more like the K5, that would be great. Not to mention the even more obnoxious noise gate feature that affects the bluetooth TX mode - seemingly another case of a power saving feature leading to a ruined user experience.