Oh cool - thank you so much for the detailed explanation! That makes lots of sense (though as I suspected it doesn’t shed any light on my troubleshooting process).
There have been a few times that I thought an unbalanced headphone jack would be convenient (one less adapter to pull out of the drawer 
I’m sure I’m not the only one who’s thought that…
Just performed one more impedance test that solidly puts the issue onto the ZynADAC soundcard.
With the card removed and totally disconnected, I measured resistance between pin 14 (ground) and pins 17-20. 17 and 18 read ~4.5 Ohms, as did pin 19, but pin 20 measured infinite. Same as I found in the tip to sleeve test on my balanced audio cable.
I still don’t see anything visible soldering issues but I supposed it could just be a dry joint on pin 20 of the header?
Your description seems to have changed. You were saying the resistance between tip and sleeve was zero and now are saying it is infinite. Do you have different readings under different scenarios?
@riban wow, thanks for pointing that out. I’ll need to reassemble my Zynthian to re-run that first test to be sure, but my guess is that I mistakenly reported zero in the first test when I should have said infinite. I will definitely re-check that soon.
While I did take some physics courses on electricity and magnetism I’m still quite a novice with the multimeter, so please forgive my mistake! (0.0 ≠ 0.L)
I just reconnected everything (except the power supply) and here are the readings:
Ω
Ω
Ω
I’m not sure why but in my initial readings both poles of the right channel measured ~7.5 Ω and now they are down to 4.6.
Anyway, this measurement matches what I read directly on the ZynADAC soundcard when it was completely disconnected.
It seems your zynADAC is broken. I will send you a replament ASAP.
Best Regards
Thank you @jofemodo! I am hoping the broken card can be repaired and reused. I know they are short supply components. Can I send it back to you, or to someone who is willing to take a shot at fixing it?
if it’s possible, I might be interested in trying! just hit me up @jofemodo, I’ve been trying to get one but it seems the shortage makes it hard to get the components