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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?
SukkoPera
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  • This won't be a very effective mixer. you need to use a proper inverting mixer to do the job correctly and more accurately. Se this: https://electronics.stackexchange.com/questions/268547/inverting-summing-amplifier-vs-non-inverting-summing-amplfier/268558#268558 – Andy aka Aug 10 '23 at 18:48
  • Thanks, but note that my sources are fixed, they cannot be disconnected. – SukkoPera Aug 10 '23 at 18:55
  • Shouldn't be the RV's connected the other way around? – Status Re Aug 11 '23 at 11:13

2 Answers2

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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.

Andy aka
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  • Thanks, I've edited the original question with an update circuit, could you please have a look? – SukkoPera Aug 10 '23 at 21:44
  • Yes that's the best way forward but, for audio you don't need an extra inverting stage. – Andy aka Aug 10 '23 at 22:29
  • Ok, but the package has 2 amps anyway, I just thought I'd put the 2nd one to some use. Thanks! – SukkoPera Aug 11 '23 at 05:59
  • @SukkoPera if we are done here, please take note of this: [What should I do when someone answers my question](https://electronics.stackexchange.com/help/someone-answers). If you are still confused about something then leave a comment to request further clarification. – Andy aka Aug 11 '23 at 08:48
  • If you can please have a look at questions 8, 9 and 10 I guess we're done :) – SukkoPera Aug 11 '23 at 09:58
  • @SukkoPera added to answer – Andy aka Aug 11 '23 at 12:12
  • Thanks again. Just to be clear about question 10, do you mean it's ok to ground it? – SukkoPera Aug 11 '23 at 12:37
  • @SukkoPera no it isn't. You need to have a dual supply that provides positive and negative voltages to make this work. I'll edit my answer. – Andy aka Aug 11 '23 at 13:48
  • Thanks, do I need the caps in series with the wipers even if I already have series caps before the inputs shown here? – SukkoPera Aug 11 '23 at 14:57
  • @SukkoPera yes because the potentiometer provides a resistance to ground/0 volts. – Andy aka Aug 11 '23 at 15:34
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The datasheets for the TL072 recommends a power supply that is minimum 10V total because the input must never be within 4V from the negative supply. Usually a positive 5V and a negative 5V is used with the inputs biased at 0V.

Your 220uF output capacitor is calculated for a 32 ohms speaker. The TL072 cannot drive a speaker so a 1uF capacitor should be used. The 220uF capacitor will take 10 seconds to charge.

Audioguru
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  • Thanks, but I will be using a TLC272, not a TL072. – SukkoPera Aug 11 '23 at 18:40
  • I have never used a TLC272 because it is Cmos with a high noise level. With a +5V supply, the +inputs of both opamps must be biased from a filtered +2V because the inputs on most of them do not work if higher than 4V. – Audioguru Aug 12 '23 at 01:09
  • I see, is there another opamp that would work with a single 5V supply that you'd recommend for this circuit? What about OPA2134PA for instance? – SukkoPera Aug 12 '23 at 07:04
  • The OPA134 works very well with a supply that is 30V. As most opamps, with a 5V supply the max output level (about 1V RMS) will be very low nit then it an drive a power amplifier that is using a higher supply voltage. – Audioguru Aug 26 '23 at 03:35