www.steeman.be

Ever since I had bought an mp3 player, I had felt the need to have a nice portable amplified boombox with integrated batteries.

We used to go skating in the park near my place, and skating is always more fun with some skatepunk playing. Nowadays I take it with me when I’m are hanging out with friends in the park or the city.

I didn’t like any of the commercial versions I saw, and decided to build my own.

This box features:

• 2 car audio speakers
• audio amplifier
• 12V 3Ah sealed lead battery
• Power converter for MP3 player
• Velcro-mounted Creative Zen Xtra 30GB mp3 player

The audio is excellent, no distortion, too much volume, and great bass response!

Originally I was having problems with a ground loop hum in the audio. This was solved by isolating both audio channels coming from the MP3 player using two 600:600 audio transformers.

The casing was constructed from a steel chimney tube (diameter 131mm), with a speaker mounted into each end. The lead-acid battery is jammed between the two speakers, with some foam disks behind every speaker to dampen mechanical vibrations and to fill up the excess space between the speakers and the battery. The audio amplifier and circuit board are tucked away somewhere behind the battery.

The box has a switch that selects between internal battery (”on”) or external battery (”off” when not connected). There is a large 10mm LED for either power setting. I have 2 cinch connectors, one for charging the battery (connected directly to the battery terminals), and one for connecting an external battery (connected to the selector switch).

The electronics are quite straightforward. To the left of the board there are two 600:600 Ohms audio insulation transformers. These isolate the ground return path for the audio signal, preventing that annoying ground loop hum which I had at first.

To the right of the board is a sweet 12V-to-5V convertor. I had a lot of trouble building this, since my mp3 player consumes 1,5A peak, so the convertor has to dissipate 10,5W. A single 78T05 didn’t cut it (hot hot hot!), so I tried to put a few 1N4005’s in front of it to create a bit of voltage drop. Now it was the diodes that got hot. I finally resorted to using three 1N4005’s in series, then a 7809 to drop the power to 9V, then another 1N4005, and then a 78T05. I know I should’ve used an LM7628 type of switching convertor, but for some reason these seem to be hard to find here in Belgium. Hey - if nobody’s using them, why should I? It wastes a little power, so what?

Oh, I didn’t feel like drawing the schematic diagram in CAD, so a quick scetch is what you get. If you fancy more info, feel free to mail me.

The amplifier I use is an off-the-shelf Altai car amplifier. It supports rear speakers as well, but I haven’t found a chimney tube with 4 open ends yet… I stumbled across it in the hardware shop one day, and decided that I couldn’t homebrew it myself for that price. Saves me the hassle of trying to lose the heat from a couple of TDA’s… My amp does not have a volume control. I simply use the volume on my mp3 player, and this works perfectly. I figure the car amp was intended to be installed behind an older car stereo, which is essentially the same as what I’m doing.

Old version

The first version I built featured a 12Ah lead-acid battery and an integrated 230V charger with transformer. It took me a while until I figured out that I shouldn’t be dragging all this heavy stuff around. I estimate that the boxes’ weight was close to 10kg… Take it to the thrash!