The FiiO E5 headphone amplifier is a tiny device, nearly a 1 1/2"-inch square, that makes a big difference in listening.
It serves multiple purposes. It can amplify signals from music players that sometimes have trouble driving better-quality headphones. It can serve as a volume control to tame devices that put out too much volume, such as some Tecsun radios. And it has a switch to turn on a bass boost. The effect is subtle. You won't get boom-boom-boom. But you will get a richer sound that seems to have more depth.It charges up via a mini USB port. A blue light indicates it's on. An orange light indicates it's charging.
And that's it. The battery is sealed, so when the battery gives out, the E5 will be unusable. There's an E3 model that uses a AAA battery, but I don't know if that has the bass-boost feature. I have two E5s, both bought through Amazon for about $25 each. I think they're well worth it.