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I am mixing two stereo audio signals together and would like to control the output volume using the (0-3.3v) DAC on my microcontroller. I'm wondering if there is a simple modification I can do to this circuit so that it accepts a 0-3.3v reference for volume or would I need an additional opamp stage? If additional stage is needed, what would you recommend as it's configuration?

Mixer

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  • \$\begingroup\$ Link to datasheet? Hit the edit link under your question ... \$\endgroup\$
    – Transistor
    Commented Apr 18, 2019 at 17:40
  • \$\begingroup\$ The OPA1662 is a pair of op-amps. There is no simple modification to do what you want. \$\endgroup\$
    – Transistor
    Commented Apr 18, 2019 at 17:44
  • \$\begingroup\$ Your circuit isn't going to work anyway without some definite bias on +In_A and +In_B. \$\endgroup\$
    – Dave Tweed
    Commented Apr 18, 2019 at 18:02
  • \$\begingroup\$ So you would add an additional stage that multiples by the reference? Can suggest a configuration that takes DAC output as a reference? \$\endgroup\$
    – physiii
    Commented Apr 18, 2019 at 18:02
  • \$\begingroup\$ Thanks Dave, adjusted schematic accordingly. \$\endgroup\$
    – physiii
    Commented Apr 18, 2019 at 18:15

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While you could, with some effort, build a voltage-controlled amplifier, let me suggest a somewhat different way to accomplish volume control + mixing. IC manufacturers provide a rich set of devices for this function as some form of volume control is required in just about every consumer audio product out there.

The Texas Instruments LM1971 is a digitally-controlled audio attenuator. So instead of using your microcontroller's DAC, you would hookup digital signals to the IC's LOAD, DATA and CLOCK pins.

There is no need for your OPA1662 input stage. Simply connect the left and right audio signals to the point labelled Audio Input with 1K to 5K resistors. I'm assuming your audio signals can drive that impedance. You'll still need the 1uF cap.

enter image description here

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  • \$\begingroup\$ Great...I'll get the 2 channel version: LM1972 . I2C would be nice so I wouldn't use up 3 IO pins. \$\endgroup\$
    – physiii
    Commented Apr 18, 2019 at 21:53
  • \$\begingroup\$ Hi Randy - I'm using this in an automotive application. Looking at pin descriptions on page 4 of LM1972's datasheet. Can I tie AC ground to car chassis ground or should I go to Vdd? \$\endgroup\$
    – physiii
    Commented Apr 19, 2019 at 21:57
  • \$\begingroup\$ The data sheet says the AC GND pins are not connected to the die in any way (no bond wires). If it were me, I would just connect them to GND. \$\endgroup\$
    – Randy Nuss
    Commented Apr 20, 2019 at 3:07

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