Simple Headphone Mixer

I was having a hard time finding a way to connect several audio sources into one pair of headphones. Using splitters can cause damage, passive mixers would have poor audio quality, and real mixers are overkill and expensive. I feel an electronics project coming on...



I'm planning to fit it in an Altoids tin.  It will be my first project using SMD instead of through-hole.  It's using single-supply op-amps, and I'm planning to filter the power down to 4V, so it won't be able to drive especially high-impedance headphones, but I'm hoping it will have good noise immunity and quality. I will configure it with unity gain, so ideally the volume will be the same as whatever the inputs are. It has jumpers so that you can use either two regular op-amps, or one op-amp and one special headphone amp, and also so you can bypass the voltage regulator for a bit more power (at the expense of noise).