Added - with positive signals only

Adding works with a noninverting stage too. The difference is that more of the signals get back-fed into the other channel. If that's a problem, you can bias up an inverting amp to do the adding, then another one to undo the inversion.

Cheers

Phil Hobbs

Add them inverting, pivoting off of some pseudo ground, and then invert again.

Or if they're high-impedance enough so they won't talk to each other, connect each one of them through a resistor to the non-inverting input of an op-amp, and gain the result up however you want.

First solution shown on this page...

...Jim Thompson

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Weighted-resistive sum, then go into a classic opamp non-inverting gain stage.

I'd draw it but got to go to Safeway now.

average them using a even resistive divider and then use the op-amp to double that.