Audio Op Amp

[jheatac]

I'm learning...

Car audio amplifier. Stereo in - mono out

The left and right low audio inputs go to a dual op amp on the + input of each. The - input is tied directly to the output. 15v rails.

Leaving the output of each they are tied together through a resister.

Is the purpose of those just to isolate the input signal?

[Benta]

The right term would be "buffering". As you don't know the input signal characteristics (output impedance from the source, etc.), the buffers make certain that the dignal summing is under control at all times.

[jheatac]

Thank you. I'm starting to figure this out

[jheatac]

Just to confirm. In this application, the output is all but the same as the input?

[Benta]

The opamps are configured for x1 voltage gain, yes.

[BeBuLamar]

If you want stereo in and mono out it's best to configure the OP amp as a summing amp.

[jheatac]

I am walking myself through the audio path. I think the summing amp may be next

[TimFox]

The simplest way to add two analog channels (L & R) together is with two resistors (say, 10 k each) to the mono output, but that increases the output impedance driving the next stage.
A single op amp can either buffer the output of the two resistors (non-inverting) or sum the two resistors into a virtual ground (inverting);  both choices give a normal low impedance to drive the next stage.