Mono plug in output jack

[ReneMadeira]

Hello, Randall,

I was wondering what would happen if a mono plug is inserted in the output jack.
The output #2 will be shorted to ground, is there no risk of the audio board's output chip getting damaged?

Regards,

[Randall (Admin)]

Good point and question.

Ideally, there would a series resistor on each output line to prevent the output driver from working hard (or potential damage) when the output is shorted to ground.  There unfortunately is not on the AudioInjector card or the pi-Stomp board.

The AudioInjector does include a 10uF series capacitor, so at least DC won't be shunted to ground.  The outputs of the Wolfson chip on the AudioInjector are meant to drive headphones as low as 16 ohms.  So, we can probably assume if our "short to ground" is not much less than half of that, we should be fine.  Fortunately, we also have a volume pot before the output jack.  Pots are not perfect.  Most never short the wiper completely to the outside terminal.  I just tested 5 of the dual gang Alps pots I use for pi-Stomp kits.  When "cranked" all the way up, they ranged from 9 to 7 ohms.  So 7 would be worst case in that sample.

I did a Spice simulation and found that with full scale sine-wave input, and a 7 ohm load, we could expect current peaks of around 20mA.  That would be 2.8mW.  Significant, but probably not an issue for the card.  And it's doubtful that your unused output would have a signal driving it that hard. 

If you're concerned about harming the card, you could:
1) Use a TRS cable.  Possible but highly unlikely that anything on the other end of the cable (ie. an "input") would short the ring to ground.
2) Avoid routing audio to output2 (so there's no current to shunt)
3) Keep the volume control just a few degrees from "cranked".  That should yield a load resistance of 20+ ohms.
4) Upgrade to an IQAudio Codec Zero card which does include a 450 ohm series resistor on each output (as part of a low pass filter).  FWIW, future pi-Stomp kits will include that card instead of the AudioInjector.  It's a superior card IMO, but unfortunately didn't exist until the end of 2021.  How to do that: https://www.treefallsound.com/wiki/doku.php?id=customization_guide#swapping_out_the_audio_card

[ReneMadeira]

Hello, Randall,

Thanks for your detailed response.
In my case with the Pi 4B and the 1590D enclosure, I'll not solder the jack directly to the board, and so I'll install two output connectors, as I have plenty of space.
In fact, the reason of my question was to decide if I'd route the output #2 to the ring of jack #1 in case there is nothing connected to jack #2.
It would give more flexibility, but I think I'm just leaving it out for the moment, as I don't have a use case in mind. If I need it in the future, it's a quick mod to make it with a small resistor.

BTW, about using the Pi 4B, it has different power requirements than the 3B and 3A. Despite the current being 3A, I measured the consumption and it never passes 1.2A with pi-stomp (and also a mic preamp). Even though the PSU supplied has 2A, the cpu was getting throttled by low voltage.
I researched a little and discovered that the Pi 4B requires 5,1V. I tweaked a 4B psu with a compatible power jack and now the throttling is gone.
I'm just letting you know because I believe it would be good if it was noted on the hardware customization guide, in the pistomp wiki.

Regards,

[Randall (Admin)]

Thanks for figuring that out.  I will post a warning on the wiki regarding the pi4.  Might need to do some experiments with various supplies to see if throttling is an issue and can be prevented.