Hello -
I'm planning to sum the headphone audio outputs of four Bela Minis, and send this signal to headphones. Also, I'd like to sum the line outputs of the same four Belas and send line out (via a 1/4 inch jack). Does the way I'm planning to do it as shown in the schematic look workable?
Thanks
You are probably better off using 10k resistors , or even 2k2, instead of the 1M you have there. Note that you won't be able to use the mixed headphone output for headphones. I mean, you could try by using 47 or 100 ohm resistors and see if you get decent performance there, but I wouldn't bet on it.
The line outs would be better off if you used a capacitor on each output from Bela (8 in total) instead of a global one.
Are you sure you need to mix line outs, when they have the same signal as the headphones? If the plan was to use the headphone mix as a headphone output, you may as well mix all the HP outputs to a single stereo pair, to use as the line out (this doesn't need capacitors), then use a dedicated headphone amplifier to get a proper headphone output.
giuliomoro thanks for the feedback!
ooops, yes the 1M was a typo 
yes, it makes sense to use the headphone outputs for both the line and headphone
how does this new schematic reflect your suggestion? any feedback is appreciated, thanks again
it doesn't look too bad. Your series resistors on the output are too large, they should be around 10 ohm unless you are driving high-impedance headphones.
that makes sense, thanks again!
