> *6. Will the circuit work as a whole?*

You will get severe volume interactions between the two channels when you adjust their individual pots. For instance if adc_out is not required to me in the mix then you would adjust RV1 to ground the wiper but, unfortunately, this will also ground the wanted mixed_mono signal. Result: total interaction and not really a mixer.

> *3. Will setting RV1 or RV2 to zero cause a "jump" in the volume of the
> other source? If yes, can i put a small resistor in series with the
> pots in order to avoid this effect? How small?*

It may need to be as high as 100 kΩ to overcome the problem and, you might start to hear a little resistor noise.

> *1. Is the stereo-to-mono "circuit" correct?*

Yes, that will suffice.

> *Are the values of R8, R9, RV2 ok?*

They are OK.

> *Is R27 needed?*



If you didn't have it then when RV1 volume was turned fully up it would kill-off the mixed_mono signal.

I strongly suggest that you to use a proper mixer circuit using an inverting op-amp and forget this cranky method.


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> *8. Can R25 be safely removed?*

> *9. Is 470k a sensible value for R28?*

> *10. I am powering the opamp with a single 5V supply, is it correct to ground the positive input or do I have to bias it at 2.5V?*

Yes to questions 8 and 9 but, for question 10, you need a dual rail supply for the op-amp or, use a 2.5 volt bias and add coupling capacitors connected in series with the potentiometer wipers.