Specific Op Amp explanation of circuit

[GamerAndds]

I don't know if this fits into the other section of the from, but I decided to post it here...

Here is a typical active 2/3 way xover (see attachment)... I understand all the op amp except one (it is marked with red pen). I'm also confused with the lines/wires feeding back to the inverting and non inverting inputs. Perhaps I'm not thinking the right way about it, but it is confusing me.


The circuit is 2 snippet from the behringer CX-4300 2/3 way active EQ.


My question is how does this work and output a high frequency out put? Can some one help me to understand its workings?

Thank you!

[ajb]

The op amp you've called out is in a summing configuration.  The way that multiple signals feed into it via separate resistors is typical of that configuration.   Conventionally all of the input signals would join up at either the inverting or the noninverting input, making the overall summing amplifier inverting or noninverting, but in this case a couple signals go to each, so it's a combined inverting and noninverting summing amp in a single stage.  Since the four following stages are all inverting, they alternate polarity relative to the initial input.  The stages with inverted outputs go to the noninverting input (so they get summed) and the stages with true outputs go to the inverting input (so they get subtracted).  Since four of the five summed signals are fed back from stages following the summer, this configuration keeps the feedback all negative, which is generally what you want with op amps.

[cvanc]

AJB's description is excellent.  It's a pretty clever circuit if you ask me (which nobody did ).