I don't have many restrictions on what I can do. 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 think I should use an op-amp, but I am not sure which configuration is the best. Using sufficiently large resistors I can get past this, but the volume is too low to be usable. 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. Ideally, it would have an input 3.5mm stereo jack and an output jack that delivers the converted mono sound. I'd like to build him a compact stereo to mono converter. When he listens to music with headphones or earbuds, he can't hear one of the channels which degrades his listening experience. I have a friend who is permanently deaf in one ear but has normal hearing ability in his other ear.
