@Photojoe4 Maybe there could be some exposed pads for I2S we could access with pogo-pins through a modified case?
The Allwinner A64 has I2S on either pins PB3-PB7 or pins PB3 + PG10-PG13 according to the datasheet and according to the feature matrix on the SUNXI wiki, I2S should be supported in mainline Linux since 4.17.
Unfortunately the first channel (on the PB pin block) is shared with the SIM card pins if we use the A64 for that. Even worse PB3 (I2S0_MCLK, I2S master clock) is shared with SIM_VPPPP ("SIM Card Power Voltage"). However, the second channel (on the PG pin block) could potentially be of use if we can find another way to solve SIM_VPPPP. I'll leave it up to the PINE64 team to answer that one since this is pretty much all the information I can figure out by myself from the SoC documentation. Maybe we don't even use the built in SIM functionality?
Anyway it looks like the I2S hardware supports up to 192kHz sample rate and 32 bits depth. This is detailed in chapter 2.1.8 in the A64 manual.
The built in DAC+AMP is specified to "100dB SNR@A-weight", which means you're looking at somewhere around 90dB SNR without an A-weighted spectrum. Obviously that doesn't hold true for the whole range of 20Hz-20kHz when using the dB(A) scale, but I'd say the difference between the built in 100dB(A) SNR DAC+AMP and a high end external 120dB SNR DAC+AMP is immeasurable when you're anywhere but in a professional studio or measuring the theoretical response on an oscilloscope.
Unfortunately when I skimmed through the manual I couldn't find anything about the rated power or design impedance, but unless you had some really high impedance headphones to take out with you I'd say the built in audio path is sufficient for high quality playback. You might be glad to hear the A64 does support true line level outputs on C16/D16 in addition to the amplified headphone output on C13/C12 but I don't know if we'll get those routed on the board. Users would probably confuse the line out connector for an extra headphone connector so I would probably use a different connector for the line out anyway.
This is all very much overkill considering (1) the priority right now is to get the first phone out of the door and (2) the built in DAC+AMP combo is way better than it needs to be for outdoor use. Maybe we could have line out and/or a nicer amplifier on PinePhone 2? To be honest I'd be more interested in the former if you want to use a high quality external amplifier, just cut the internal amplifier out of the signal path entirely and use a 1:1 isolation transformer to get differential outputs.
This got a lot longer than I anticipated when I started writing an "I'll just copy some stuff from the datasheet" post but I hope I helped answer at least some of your concerns.
Edit: The dB/dB(A) comments are for the SNR but I had omitted those three very important letters