Inverting vs inverting op amp mixing

[Eddododo]

When combining two signals (in this case, blending two pickups), will using an inverting op amp to sum the signals be louder at 50/50 than a non-inverting stage? Put another way, is an inverting stage going to sum two pickups such that a 50/50 blend is louder than either side soloed, whereas a non-inverting stage would be ~ equal power across the mix?

As far as labeling it mixing vs blending, perhaps?

Appreciate any clarification !

[antonis]

It strictly depends on feedback/blend resistors ratio..

https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/virtual-earth-mixer-summing-amplifier.php

[ElectricDruid]

There's a good page about this here, with the equations for the two cases:

https://www.electronicshub.org/summing-amplifier/

In short, if the input resistors are the same for the two inputs, it doesn't make much odds. Both are a linear mix of the inputs.
The biggest difference is what happens if you use different input resistors for different inputs. For the inverting case, each one can be treated individually and has gain set as you'd expect. For the non-inverting case, *all* the input resistor interact and the overall effect is a complicated mess of the parallel resistances for the other inputs.

Equal power mixing is more complicated since it requires a sin/cos curve to be done properly. I've seen it faked using a piecewise linear approach, and I've seen a differential pair's "tanh" input response used to derive control signals for a pair of VCAs to do it.

For simple pickup mixing, the inverting op-amp would be the way to go since it's simpler to work with, you won't get interactions between the two pickups, and since you don't know what way up the signal coming off the pickup is anyway, an inversion at that stage causes no problems whatsoever.