I have a friend who is permanently deaf in one ear but has normal hearing ability in his other ear.
When he listens to music with headphones or earbuds, he can't hear one of the channels which degrades his listening experience.
I'd like to build him a compact stereo to mono converter. Ideally, it would have an input 3.5mm stereo jack and an output jack that delivers the converted mono sound.
I've tried the simple circuit below using two jacks where I connected the channels using resistors, however, his devices detect the short circuit and won't play the sound. Using sufficiently large resistors I can get past this, but the volume is too low to be usable.
I think I should use an op-amp, but I am not sure which configuration is the best. Should I use a summing amplifier? Do I need a 9V battery to supply power, or could I run this on AA or AAA batteries using rail-to-rail op-amps? Which op-amps should I look for / avoid when it comes to audio quality?
I don't have many restrictions on what I can do. I'm an Engineering Physics student and I have access to nearly any circuit component, a 3D printer, a machine shop, and a PCB mill through my university.