Need help analyzing a part of a circuit

[exztinct01]

I opened a small amp circuit out of curiosity as how these things are designed. Also, I want to see how it separated the signal to use for the low end driver. Speakers are only 3W stereo and 8W for the low end. Below is the schematic I drew from it. From preamp, it goes to volume then treble then goes to Con2 (Red & White). I don't understand how from Conn2, it passes through a dual opamp then to the bridged TEA2025. Can someone explain what's the purpose of that opamp circuit?

[phatt]

That opamp is just there to mix the two channels and send that mix through a low pass filter and onto the sub power chip, Input mix for sub is via R17 & R18.
Low pass filtering is done with C37 and also  C38 & C39, A bit of gain added at R23 to make up any loss. Well don't hold me to it all but I think that's close :tu:

Phil.

[exztinct01]

thanks Phil
But, can I use same concept for building a 2.1 system using 2 lm1875 for Left/Right channels and a single lm3886 for a sub?

I need a simple crossover for that project but even though it's not exactly a crossover as long as the sub power amp will only play low frequencies, then it's enough right?

[phatt]

Yes :tu:

Phil.