Stereo Headphone Mixer

https://www.electronics-tutorials.ws/opamp/opamp_4.html

How to build an audio mixer https://www.circuitlib.com/index.php/tutorials/product/39-how-to-build-an-audio-mixer

http://www.all-electric.com/schematic/simp_mix.htm

https://www.onsemi.com/pub/Collateral/AND8177-D.PDF

https://www.electronicdesign.com/resources/ideas-for-design/article/21799355/efficiently-design-an-opamp-summer-circuit

https://www.headphonesty.com/2018/03/1667/

Opamp Circuits https://learnabout-electronics.org/Amplifiers/amplifiers66.php

To invert or to not invert op-amp https://electronics.stackexchange.com/questions/294749/audio-summing-mixer-inverting-or-non-inverting-output

Digital Audio https://bunnystudio.com/blog/library/audio-ads/from-analog-to-digital-audio-what-you-need-to-know/

Audio DSP on STM32 - ADC DAC https://www.youtube.com/watch?v=lNBrGOk0XzE&ab_channel=YetAnotherElectronicsChannel

LM386 Amplifier Circuit https://www.youtube.com/watch?v=XHkKIEAo1vc&t=1615s&ab_channel=philomech