Designing an Audio Mixer Circuit with Individual Volume Control

audiomixeroperational-amplifierstereo

I'm trying to make an audio mixer circuit based on the amplifier circuit published by Scott Campbell.

I have no knowledge of opamps so I'm taking it mostly as-is but the input signal I am going to feed it with must be mixed from 2 different sources: the first one is a mono mixdown of a stereo source, while the second one is a mono source.

Circuit

I would like to be able to control the volume of the two sources separately (while the relative volumes of the stereo mixdown can be fixed), so my questions are:

  1. Is the stereo-to-mono "circuit" correct?
  2. Can I just put the RV2 pot like that? Are the values of R8, R9, RV2 ok?
  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?
  4. Is R27 needed?
  5. Do I need a series cap between the mixing point and the opamp positive input? (Note that the input signals already have a series cap each before they get to this circuit)

And, most importantly:
6. Will the circuit work as a whole?

Thanks in advance!

EDIT:

OK, I understand that the above circuit has issues, so I've switched to the following one, based on the TLC272 opamp, since I have some laying around:

New circuit

Does this make more sense? New questions:

  1. Would this work?
  2. Can R25 be safely removed?
  3. Is 470k a sensible value for R28?
  4. 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?

Best Answer

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.

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.

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