What is the project about? It's a class D stereo amplifier. It outputs around 4W/Channel when supplied with 15V DC . Can be supplied with 28V, the output power delivered will be higher. You can use it in your car, build your own portable speaker, BT speaker, etc., You'll need to make cable with a DC-DC Boost Converter (something like this) and you'll be able to power it with your Laptop's USB Port, Phone Charger, etc.,

Who made this? I made this using the reference design from TPA3122D2-EVM

Why do it? I needed an amplifier for an old car of ours and I needed to learn eagle CAD to design an OSHW HAT for RPi.

How do you make one? Use the files I've uploaded, send the designs to a fab house, purchase the parts and get it assembled. Plug and Play (well not exactly), You are good to go...

How much does it cost? It costed me around <1000(INR) that's about $15

What's next? I made a very simple clock. Same reasons, I needed to get to know eagle.
My RPi HAT is ready. Kernel drivers are done. Will hit Hackaday in a couple of months.