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Will this circuit work? Any comments? Thanks!

Edit : by "work" I mean I can blend the 2 audio signals in any ratio with the pot;
it would be an OPA2132 opamp;
input would be audio signals in the range of 0-3.3v

my diagram

it seems even this circuit would work? enter image description here

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  • \$\begingroup\$ What is your definition of "work"? and what opamp are you using there? and what signals are on the input side? \$\endgroup\$ – PlasmaHH Oct 8 '15 at 10:39
  • \$\begingroup\$ @PlasmaHH Question edited. \$\endgroup\$ – Zilaihong Oct 8 '15 at 10:49
  • \$\begingroup\$ Please attantion to your diagram of output. You need dual supply here because its swing include negative cycles. \$\endgroup\$ – HOPE Oct 9 '15 at 16:09
  • \$\begingroup\$ I guess with both inputs swinging between 0-3.3v with roughly 1.6v center. The input of the final opamp would never be negative? \$\endgroup\$ – Zilaihong Oct 9 '15 at 16:31
  • \$\begingroup\$ If your sure about the input dc voltages then this circuit seems to work single supply reasonably. \$\endgroup\$ – HOPE Oct 9 '15 at 19:57
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There are simpler circuits like this one: -

enter image description here

And I've posted it for two reasons, 1) to show the decoupling capacitors that must be fitted and 2) to explain that your circuit has certain benefits that may not be that obvious.

You circuit does not rely on the signal sources being low impedance - if the pot is totally at one end of its travel it will 100% attenuate one signal whereas the simplified circuit won't do that if the output impedances of the signal sources are not zero.

If you are expecting unity gain, then you ought to figure out the proper value of feedback resistor - with your circuit's pot at one end, the other channel will suffer a fair amount of attenuation but this can be easily remedied however you need to state what value your pot has. I'd go for your circuit over my example.

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  • \$\begingroup\$ Thank you! I've edited my question post with a new diagram, please check it, I want to use a 10k pot. \$\endgroup\$ – Zilaihong Oct 8 '15 at 10:55
  • \$\begingroup\$ I wouldn't edit your question to reflect what answers have said - this makes my answer look stupid - please do consider rolling back your question to the original version that I answered. \$\endgroup\$ – Andy aka Oct 8 '15 at 11:01
  • \$\begingroup\$ If you are using a 10k pot then it would be sensible to consider a 33k feedback resistor. \$\endgroup\$ – Andy aka Oct 8 '15 at 11:02
  • \$\begingroup\$ Sorry I've rolled it back. 1 channel of signal is from an opamp buffer, the other directly from DAC, with a dual opamp chip I can buffer the DAC signal so I think your circuit would be a better option? \$\endgroup\$ – Zilaihong Oct 8 '15 at 11:05
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    \$\begingroup\$ @HOPE oh I see, it effectively changes the Rin on the opamp... thanks for notice! \$\endgroup\$ – Zilaihong Oct 8 '15 at 11:28
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The basic idea is OK, but the DC biasing is messed up. The DC level of R3 should be decoupled from the opamp input.

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I suggest you using such a circuit as below. first section is a inverting unity gain op-amp. then a voltage divider. and finally a voltage subtract circuit.

Vou = Input2 + (Input1/K)

which K is the constant of pot R3.

Advantages of this circuit :

  1. No Loading effect on the both input voltages.
  2. Control Gain of each Input voltage independently.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ thank you, but want I want to do is just crossfading between 2 inputs. and I guess your circuit would require split supply? \$\endgroup\$ – Zilaihong Oct 9 '15 at 3:34
  • \$\begingroup\$ Yes besides the exact controlling over gains, you need dual supply voltage sources like 7805 and 7905. \$\endgroup\$ – HOPE Oct 9 '15 at 5:59
  • \$\begingroup\$ thanks, but I only have positive supply so I would need a circuit to work with it. Please check this circuit : electronics.stackexchange.com/questions/194357/… \$\endgroup\$ – Zilaihong Oct 9 '15 at 8:05
  • \$\begingroup\$ Well I do, it seems an considerable circuit.But using an op-amp give you more S/N ratio than single transistors. If you prefer using single supply, then exact value of combination gain is loses. But I don't think it's too important to you. \$\endgroup\$ – HOPE Oct 9 '15 at 11:46
  • \$\begingroup\$ thanks! I have tried a very simple circuit with a simulation software and it seems it would work?! I have uploaded the pic in the question. \$\endgroup\$ – Zilaihong Oct 9 '15 at 16:00

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