Android Pie seems to have fixed the digital gain defaulting to max. This means that the stock implementation is now optimal and using the gain control app is not needed (unless you want more voltage from a Dragonfly Red).
This was not announced anywhere to my knowledge, but is very pleasant surprise. I found it accidentally while testing the new dongle (above link).
As of the Nov patch, Pie on the Pixel has fixed both the line out and headset mode distortion banding when using the Google DAC.
When looking at the tracings, assume that stock will look as good as the optimized.
Made a simple spreadsheet with data so far:
The above numbers are using my gain control app, android volume at 25/25 to produce the best possible output signal
Note, the voltage at distortion threshold is the point where harmonic banding becomes >5 dB.
The THD is the harmonic distortion at 250 mV, this would be about 90 dB using full size cans like the Senn 650/800 that have a sensitivity of 103 dB/V.
Will work on uploading the exact images if anyone is interested.
Stepped sine tests at higher output (750mV) - An attempt to see how high impedance, lower sensitivity full size cans will effect distortion.
Edit: I updated the measurements and comments
Edit: Tested Apple USB C Dac
Edit: Tested the HTC dongle
Edit: Tested the Razer Dac
Edit: Tested the Realtec Dac
Edit: Tested the Geekria Dac
Edit: Tested the Audiolab USB device
Edit: Brief test of Dragonfly red. I would consider this to be the gold standard, very low impedance (<.1), minimal crosstalk, high volt out (not on stock), very low distortion.
Edit: There appears to be 3 (at least) "modes" that the Google dongle will do. Depending on the impedance of the headset/line out device attached, the output voltage varies quite a bit.
Low impedance - .36 Volt (high efficiency iem's)
High impedance - .9 Volt (full size headphones)
Very high impedance- 1.8 Volt (line out)
What gets interesting is the effects on testing. The "reference" sites like GSMarena, are testing with a Y cable setup (as I was) and will see high voltage out even with headphones attached since the use case was taken from the line out initially. What made me aware of this was that my app was not reading the Spl correctly, as if the max voltage was about 7 dB down (which it was). Also of note, the reduction in output is not due to a change in the Dac digital gain (the one that adjust from 1 to 175), so the distortion bands are just as bad since they kept that gain at 175.
Used both a Senn 800S and Shure 846 as load, as well as no load.
Note the flat FR on the SE 846 which verifies the low impedance above.
So superb impedance, great voltage out, pretty good crosstalk (with low impedance iems).
But, significant distortion using stock gain settings:
And it has 25 volume steps (each tick is 2.3 dB).