After some superficialy study of WM8978 I have ruled against it. It is very complex and setting it up is a huge pain in the arse. Worst of all, it requires too many pins to communicate. Pins, especially 3.3V-compatible pins, are a precious commodity on Tang Nano 9K. Looks like this neat little codec creates more problems than it solves, so it's out to the parts bin.

I might want a little builtin speaker some time later, but for now I just wanted a nice buffered output where I could plug my active speakers without fearing too much of sparkles going up delicate 1.8V I/O lines. So I came up with this fairly basic circuit.

The input is 1.8V GPIO (pins 80, 79 on Tang Nano 9K).

The opamp is LM358. There's a passive filter that doesn't pass high frequencies into the amplifier. The gain is set experimentally, I'm still undecided on the value of feedback resistor. Currently it's 220K and it seems to produce levels comparable to what my laptop outputs, albeit a bit quieter. At the same time, they swing wider than what "line out" standard dictates. At the same time, 1 + 220/47 = 5.6, seems that the gain here is a bit high.

My USB hub that I tend to use for debugging seems to add a horrible bit of extra noise in the power lines, so I have a basic 20 ohm / 100uF filter on the 5V line. Not the ideal way of dealing with this, but it removes a lot of annoying hum.

The signal part of the circuit is repeated twice for stereo output. AY mix used in vector06cc is L=C+B R=A+B. The 8253 and beeper output are always mono. Stereo AY sounds neat as it turns out.

I still have many questions and I may need to revisit this circuit later.