I recently finished a little portable mini-amp project. I wanted to have something powered by a battery that would have a decent amount of power and would be easy to plug other portable things into - iPod, etc, and also use it as a testing speaker for other projects. There are lots of portable speakers you can buy, but this allows for some customization and can end up costing less (if you can put up with some headaches along the way).
I started out looking for the highest wattage chip that could take a battery operated supply. I found the LM384 which is a 5 Watt mono chip amp and can take a 12 v - 26 volt supply. As I'm writing this I am realizing that I made a mistake and I'm actually underpowering it with only a 9 volt and it still works (?!), although that might explain why it is distorting pretty quickly at louder volumes. Two 9-volts would probably be a safer bet...although the chip might need a heatsink then since it gets hot. I should've probably gotten a lower wattage chip, but then that wouldn't have been as fun.
I built the example circuit included in the LM384 datasheet but it sounded TERRIBLE and crackly. I bypassed prototyping on a breadboard and just used a perfboard so there is a good chance I could have messed up soldering something. But rather than have a test circuit on a breadboard and never finish it, I wanted to just go for it. I discovered that when I touched the back of the circuit, the sound would clear up for a second. When I let go it would slowly start sounding crappy again (suggesting some weird capacitive business). Employing some circuit bending techniques, I started connecting random leads together using first a resistor and then a capacitor. I discovered if I put a .01 uF capacitor between the pot ground terminal and ground it cleared up and worked. Not really sure why...
I used a 4 Ohm, 6 Watt satellite speaker from my old 5.1 Creative speaker system. I only ever used 2 speakers, so I just opened up an extra one and took the speaker out. I could've used an 8 ohm speaker and used less power I think. Again not as fun...
Found a wooden box at Jo-Ann for $4 bucks. I almost wanted to go with a smaller enclosure but glad I didn't because it ended up a very tight fit. I was first just gonna cut a regular round speaker hole on the front but figured it was a chance to customize a little bit. Using an exacto knife, a jigsaw blade, and a soldering iron (burning holes) I finally managed to punch out the design that you see of the guy wearing headphones. This was a painstaking process, but kinda worth it in the end. I used the original speaker's grill cloth to cover the back.
On the front I put a 1/8" stereo jack with two resistors, so it sums the stereo into mono. Interesting to hear how certain out-of-phase stereo things cancel out in mono. I hooked up a huge volume knob, and put a power switch on top w/ power LED to finish it up. The speaker inside still needs to be mounted properly. I also need to add some absorbent material on the inside to reduce the "boxy" resonant sound from having it in...a box. The cutout design is a little small and does seem to block some highs though - I could probably cutout some more holes or slats to open it up more. But this speaker is all mids anyway so I'm not gonna pretend like I'm gonna get a full-range sound out of it. There is also a quiet noise hum on certain input configurations that goes away when I touch certain wires, which I haven't figured out yet.
Anyway glad I got it done - it was a really simple and small project but a good place to start. And useful to me for future projects. Designing this stuff, I feel like I have to re-learn a lot of things I know and go with more intuition, trust the trial and error process. Just try stuff until it breaks and if it works...don't try to break it (or myself) trying to understand why. ;)