A set of Harman Kardon Soundsticks are obsolete with most modern computers. They originally took digital audio via USB output from the computer and fed it to a multi-channel audio amplifier chip which drove 2 desktop speakers and a sit-on-the-floor subwoofer. USB drivers are no longer available.
I'd like to make these work again by connecting the audio-out jack from the computer to the appropriate points on the Soundsticks' PCB.
The main component on the PCB is the amplifier:
My question is basic: how do I connect to both stereo inputs (inputs 1 and 2) and at the same time connect to the subwoofer input (inputs 3 and 4)?
I presume the answer is a simple resistor network mixing the 2 signals for input to 3/4?
And yes, there's filter networks there, too, and it is important to know the appropriate "injection" point, which will take some time.