This is a super simple little headphone amplifier circuit that sounds good and can get very loud.
It's a design that I first learned about from another youtuber, Dustin Watts, but his version is based off an older one called the CMoy amplifier.
Dustin's channel: https://www.youtube.com/channel/UCYPmdetwhvFeRkgXhIWq_eg
The wikipedia page about the amp:
https://en.wikipedia.org/wiki/CMoy
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This isn't the only way to do it.
It's probably not the best way to do it.
But it's how I do it.