This started as a basic crystal set, as you can guess from the earpiece used. However, when you're an 11 year electronics geek (and have a father that's one too) you quickly start improving it. Our 'improvement' was to build an amplifier circuit.
It's a basic transistor circuit, available from most books, but built on a tag strip to use the least amount of space.
AFAIK, it still works :)