This project started from the idea/need to listen to my own music while playing a game called "Elite Dangerous" on my PS4. I could just use a CD / MP3 player but I wanted to have the background music mixed with the in-game sounds and be able to output everything on my headphones.
Plus I wanted to be able to balance the ratio between the two audio sources (my own music vs in-game music and sounds),
I used two stages of amplification but you can also use only one amp. stage, by ignoring the TDA7050 chip. Just don't solder it, and bridge pins 4-6 and pins 1-7 on it's PCB footprint. That will redirect the audio coming from the first amp. stage to the headphones connector.
The sound coming from the two audio sources is picked up from the 3.5mm Stereo Receptacles and goes through 4 precision (multi-turn) trimmers to a pair of stereo potentiometers. From there the signals are paired into right and left and amplified using two LM386 chips.
The second stage of amplification is done by a dual audio amplifier, the TDA7050. The reason for using two stages was to provide simple and precise audio balancing between the two inputs.
Check the Youtube video of this project for more details about this.
The TDA amp can also drive two small 8 Ohm speakers. Make sure you carefully read the datasheet of this chip before connecting anything else than headphones on it's output.
If you decide to use cable audio connectors (for the audio inputs and/or headphones output) make sure you solder the filtering caps (C30, C31, C32, C33, C28, C29) as close as possible to the connector.
The board is supplied with 5V coming from the USB receptacle. A 400mA fuse (self resettable) is used to protect your USB port.
Please watch the youtube video to see how I calibrate the whole thing before using it and for other useful details.