What we have here is a bass booster device for headphones. It amplifies a range of signals that you can choose with the potentiometer on top. Although you can also boost high frequencies with this, it is intended to amplify low ones which are which mobile phones and MP4 players lacks.
This is the thing out of the box. Each PCB deal with one channel.
The diagram is from somewhere in Makezine, but I modified it to suit my preferences and for improving it a little bit. Mainly, as I didn't want it to be portable, and I hate batteries, I fed it with a wall adapter. I don't want to go too deep into electronic matters, but for this you have to get some adapter rated with more voltage than what you need (I chose 9V), then regulate and filter at the same time with a LM7805 to avoid noises. I made some other changes too.
The thing is based on a LM386. The green wired PCB is right channel.
The result is great. I was worried because when you work with sound stuff you get annoying noises very easily, an as I'm sort of an audiophile, I can't listen to music with a noisy machine. It would have gone straight to the bin if that happened. But fortunately I get absolutely no noise from it!, just pure sound!
Besides it boosts a selected range of frequencies, it also amplifies noticeably the sound in general. Nevertheless, as it's not its purpose, I haven't measured what amount.
I added a nice yellow LED and fitted it into a box with a transparent cover.
I'm very happy with this little thing. I reckon it'll be very useful. For me, it's not about simply using this and there it is. I think the greatness comes when you set up properly an equalizer to manage all the frequencies, then with this extra boost in low ones you'll get a very nice sound.
I want to show it to you the best I can, so I've made a video where I play with the potentiometer. The audio (courtesy of Evanescence) is recorded through the computer so you can listen better. I promise it's not altered in any way, I've turned off all the equalizers.