Here is a simple stereo headphone mixer that can be used for monitoring and mixing 2 sources. A very helpful device for online streaming. How? For example you are playing a keyboard on a band in an online stream and using a headphone as monitor. You can use this to mix the headphone output of you keyboard and headphone output of the streaming mixer so you can hear more of your keyboard.
About the circuit
The circuit is simple and only uses 1 opamp IC. It is operating with 5V that is why we have a 5v regulator so you can power it from 7V up to 24V.
They layout is obviously using as SMD which is not easy for DIY. I will not be publishing the PCB layout but you can request to me if you need it.