I work on a project where i need to combine sound that comes from audio jack input (let say from tablet) with a sound we create from an MCU with DAC. So it basically means I need to sum the sound of : right speaker + left speaker + DAC.

Please see examples of what we did so far (notice we didn't yet chose the resistors for the op-amp):

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How do you recommend to do it? Is op-amp is the right way? Do I need an amplifier at the output before I connect the it to a speaker? Can you advise on values for resistor/any other component/other solution?

I don't have much experiment in that area.


A summing opamp is the correct way to do this. However, your current configuration is problematic: you have the positive input connected to ground, as well as the negative supply of the opamp. This means that your signals need to be ground referenced (have a center point around 0v), but the opamp can't output voltages below 0v, so half your waveforms will be clipped off.

To fix this, either connect the opamp's negative supply to a negative voltage rail, or bias your signals to VCC/2, and connect the opamp's positive input to a resistor divider that also provides VCC/2.

Whether or not you need another amplifier before a speaker depends on the speaker; a small speaker is likely within the capabilities of your opamp, though it may not behave ideally. If in doubt, find a suitable audio amplifier IC and connect it on the output.

  • \$\begingroup\$ Thank you. that was very helpful. Now I can see in the scope the output but I'm not sure how can i connect this output to a speaker. I try to do it with 8ohm 2W speaker and when I connect the output to one of the sizes of the speaker + pad and the second pad (-) to GND, than it shortened my output to GND. Do I need a special amplifier? How do you advise to connect it? thank you. \$\endgroup\$ – Dudi Nov 24 '15 at 13:20
  • \$\begingroup\$ @Dudi The opamp you're using is designed for high precision applications, not for output driving, and can only source 40mA, which works out to about 0.01 watts across an 8 ohm load. You should either swap out for an opamp that can drive more current, or use an audio amp IC. \$\endgroup\$ – Nick Johnson Nov 24 '15 at 14:11

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