“Adding” 4 audio signals?

[240RS]

I have 4 microphones with 4 audio signals that I expect to sum op to zero. Ultimately I want to detect whether the sum of the signals is below a threshold.

I ran a test with two signals of which I inverted one. I fed those into an opamp and was very pleased to get the difference (which is the sum in reality). Believing I had found the way. But then got stuck extending that idea to 4 signals.

Which made me wonder: is there a standard way to add two and more signals? My signals are between -1 and 1V, 40 to around 1kHz.

Thanks!!

[Berni]

https://www.google.com/search?q=op+amp+sum

Works for any number of inputs, just add resistors.

[Jan Audio]

You wanto hear the audio ?,
then you need to use half the volume when mixing more harmonics.
You only detect the volume ?, then distortion dont matters, go to the limits.

[Terry Bites]

https://www.electronics-tutorials.ws/opamp/opamp_4.html

For least phase error use DC coupling where possible. The majority of modern opamps won't work as comparators. A TL07x?
Between the summer and the comparator you are going to need to turn the signal to DC. The Simplified Peak Reading Detector is the thing https://sound-au.com/appnotes/an012.htm

[240RS]

Wow! Why had I missed that basic circuit. Thanks! And issue more than solved. No more inverting.