non-inverting summing opamp

[igor12]

What's the difference between using the non-inverting input or the inverting input for the summing section?  (besides the phase and resistor configuration)
-Igor

[amptramp]

If you use the inverting input, the non-inverting input is set to a fixed voltage and the op amp attempts to drive the inverting input to the same voltage.  This means, since the voltage at the op amp end of the input resistor is sitting at this voltage, it acts as a virtual ground for AC signals and none of the inputs interfere with each other.  This gives linear addition of the input signals, but the input impedance is the value of the input resistor.

The non-inverting input can be set to a high impedance, but the signals do not add linearly and there is no feedback to drive the input to a fixed level.  There is an input resistor to ground and input resistors from each channel, but instead of a virtual ground, the input goes to the resistor to ground.  If you make the resistor to ground large, mixing is non-linear.  If you make it small (with respect to the input resistors), the mixing is more linear, but there is a considerable signal loss.

For a mixer, I would use an op amp set as a unity-gain follower for high input impedance followed by the mixing stage with the inverting input.  You can add another inverting stage at the output if phase is important.

[PRR]

> What's the difference between using the non-inverting input or the inverting input for the summing section?

For permanently-connected inputs:

* polarity
* for given gain, you must calculate the non-inverting gain

No, the real advantage of virtual-ground mixing comes when you want to disconnect inputs.

For the passive mixer + NI booster, mix-loss varies with number of inputs. Break one source, the other sources get louder.

For the virtual-ground mixer, the gain stays constant for any number of inputs.

This gets very relevant if you must use a 16-input mixer on a string quartet. With all inputs connected to the mix network, but 12 of them grounded, you still have the mix-hiss of a 16-in mixer. If you disconnect inputs on the passive mixer but still have gain-of-15 in the booster, your output level may be way too hot. With virtual-ground mixing, disconnect 3/4 of the inputs, gain is unchanged, mix-hiss goes down 12dB.

In many smaller projects it is a toss-up. If polarity and minimum-parts are important, and you don't change the number of sources mid-tune, passive with booster may be best.