The circuit I designed is a buffer amplifier type design intended to take two channels of audio input from a passive monitor controller, send to either the resistor circuit or low-pass filter (as seen in the schematic), and then to the second op-amp (which has a trimpot to control DC offset for each channel), and then send as an audio output at unity gain (or near unity gain).
The circuit is powered by +/-15 VDC, taking a 36 VDC power supply and sent through a virtual ground type divider and regulated with 7815 and 7915 voltage regulators (although I’ve tried disconnecting the power section and using a bench power supply setup to provide + and – voltages with the same problems).
I designed the circuit using the Falstad applet and I’ll attach the text for the circuit so you can see for yourself how I imagined it would work, and I’m attaching images of my schematic, screenshots of my oscilloscope, and images of my soldered perfboard circuit.
When I try to input a 120 Hz, +/- 2V sine wave with my function generator, all I get from the output is a mostly flat voltage. It wiggles and shows occasional noise and whatnot when I try tuning the trimpot, but what you see in my image is basically the gist of it. In addition, the opamps (OPA2604APs) get a little warm, which I don’t think is a huge problem, but the 7815 gets pretty hot.
(The question editor is now telling me that I need more "reputation" to post more links. You can find all the images at the imgur URL that I posted at the end of my description.)
The following are URLs for the relevant datasheets: