DAC gets hot with headphone (mini jack) connected [solved]

#1

So I'm having this weird problem where the audio section gets very hot when anything is connected via the mini jack. It starts in the DAC and then spreads west from there. No problems on 1/4" output and it doesn't happen if the other end of the cable is left floating, only when it's actually connected to another device.

Now, there is a problem with this board: one of the electrolytics, C46, is missing (probably my fault — I have two, and this is the board I am not careful with.) Given its position I can imagine all kinds of stuff going wrong, but I don't quite have the chops to figure out whether it is involved here and what the impact would be.

So, I guess that's my question: would this be causing the DAC to pass a lot of current or something like that? If not, any ideas about what could cause the problem I've described? (I did also solder a header along the GPIO, and I don't see any bridging, but are there any risks there?)

Finally, I don't have any SMD components (nor the patience to solder them) but is there any reason I shouldn't drop a 10µ through-hole electrolytic on there? The leads will be parasitic and I'm not sure I have any low-ESRs around... my instinct is to yolo it but I figured I'd ask since I'm posting.

TIA for any help.

#2

I'm not sure about the technical reasons but I remember reading that the headphones out must be connected to headphones only, nothing else. I think it must be something regarding the grounding...

#3

Yes, the 'ground' on the headphone output is not connected to 0V (like the ground on the 1/4" connector), but something a bit above. I can't remember the exact figure, a volt and a half or something like that. The reason is to avoid large capacitors on the headphone output of the audio codec. The downside of course is that the headphone output must not be connected to anything which has a ground reference to the Axoloti in any way, or a large current will flow between the audio codec headphone ground output and whatever is connected to the headphone connector.

Hence the limitation only to use the 3.5mm connector for headphones.

#4

Thanks, I have a pretty good picture of what's going on now. The good news is the only thing wrong with my board seems to be decoupling on the 1/4" input. I'll solder something on there before I do any FX patches on this one.