Audio mixer_adder

 
Because tb does not have such equipment or it is very expensive, I drew one.
It has been verified to solve the problem of one headset for multiple devices.
Note:
It is best to use a rail-to-rail op amp, so that you can use a 3.3V ldo and have a better power supply rejection ratio.
         Otherwise, use 4.5v lp5907 or directly connect the flying wire to the 5v power supply.
Choose a good USB power supply with low ripple, so that the power supply noise will not be heard. Of course, the power supply pin header is also reserved, so you can use other power supply solutions.
For others, see the schematic diagram annotations.
It doesn’t matter what op amp is used (on the basis of satisfying the power supply in front), believe me, you can’t hear it. Dual op amps are enough, and the cheaper one can use TL072/4558. The former has a lower power rail voltage difference, but it is not rail-to-rail. Feel free to make it more expensive if you want, or as long as it meets the usage requirements mentioned above.
If you do not use the MicroUSB interface, use the second version of the PCB. This version removes the microUSB to prevent soldering difficulties.
The resistor is a direct plug resistor, but the package of the painted patch requires twisting the legs to attach it.
The input and output coupling capacitors are shown in the schematic diagram. In fact, it has little effect on what you use (I am not responsible for this sentence, because I have not tried all types, just speaking from experience, but generally speaking, in addition to the selection in the picture, MLCC and aluminum electrolytic capacitors have no problem in terms of subjective hearing when used in this situation)
It is not a big problem whether to use a case (the opening of the case is drawn crookedly, and I don’t remember where to put the source file)
p.s. If you have any questions, please post in the comment area, please do not send a private message