My first DIY headphone amplifier, which I put together after learning a lot about electronics and audio. It probably has some flaws that I would address in a future build.
Here is the amp in the early stages of my build. You can see the basic power supply and op-amp along with coupling capacitors, but not much else.
The amp is based on this circuit which is really just a basic op-amp application. I used the OPA2132 as the op-amp, which is a really easy to work with op-amp, and has dual channels built in, so I save quite a bit of space on the board. The main issue with this amp is the power supply, which is just a simple voltage divider circuit with some electrolytic capacitors. It's prone to being very noisy, and in order to get rid of the noise you have to run the amp on a linear regulated power supply, which is properly isolated from mains with a transformer. Given that most power supplies are either poorly isolated, or switch-mode, this makes it a bit difficult to use. I fixed this issue in my next headphone amplifier.