Monday, June 18, 2012

Ammo box speakers

Last summer I decided I wanted to build a portable stereo speaker box so that I would have something to play music with at the beach. I settled on using a surplus metal ammo can as the enclosure, and picked up a 50 cal can at the local surplus store. I wasn't going to repaint it, but the can I purchased had been rather poorly resprayed at some point so I sanded it down and painted it with some tough paint. I bought a 15w Class D amplifier from Sure Electronics. These amplifiers are small, powerful, extremely efficient(~90%) and above all, cheap. I also purchased a set of 4" automotive speakers. I had some 3s LiPo battery packs on hand already, so I installed the appropriate battery connector on the amplifier. These batteries seem to work great with the amplifier since it operates well at 12 volts, and ceases working just before the batteries reach their minimum charge level(~9v), ensuring they don't become overdischarged. They also give a working time of around 8 hours with a 2200 mah pack, and there is certainly room for a larger battery if longer runtimes are desired. There is also room inside to permanently install a small balance charger if a complete system is desired, but I just take out the batteries and charge them outside. I put in an audio input jack, power switch and a power indicator I salvaged from an old control panel, and the final product is an attractive portable amplifier that has served me well for playing music on the go.

4 comments:

  1. Love this thing. Can you go into a more step by step process? I'd love to be able to craft my own, but not really sure how to start.

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    1. Well, it's a pretty simple setup, really. Almost all the parts are pictured there in the left picture. I ended up painting the box black, since at some point it had received a spray paint respray and I didn't like how beat up it looked. I think I used por15 black primer, but painting it is really unnecessary. I used the template that came with the speakers to mark the outline of the speaker cutouts, then used a jigsaw with a metal cutting blade to cut through the sheet metal. I installed the speakers and grilles, using the provided screws and some clips that go on the backside to receive the sheet metal screws. Small bolts would probably work as well. I made more holes in the can to hold the input jack, power indicator, and the power switch. The speakers were wired up to the outputs of the amplifier, and the input jack wired to the input. I used some double sided tape to hold the amplifier to the inside wall, but bolts would be sturdier. I connected the power indicator light parallel to the amplifier, so it lights when the amplifier is turned on, and connected the switch. The amp has a xt60 male connector, and the battery has an xt60 female connector so that the battery can be plugged in, and unplugged to charge it. The battery has Velcro on the side to keep it from bouncing around inside the can when in use.

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    2. While this amp has performed pretty well, and seen many days at the beach by now, I have been thinking about ways to improve it. The ds40 speakers are actually designed to work well without a perfectly sealed enclosure, but I think they would be substantially louder, bassier and cleaner sounding if I added a port to the can. That would make it less water/sand proof, but could improve sound quality substantially. Also since the speakers currently share the same enclosure volume, they probably also interfere with each other when each channel isn't playing the same thing, or if there is a phase difference between the channels. Either I should add a divider between them and give each half its own port, or mix the stereo channels down to mono so that they both always synchronized, and install one shared port. I am leaning toward the latter, since installing a divider that seals perfectly is difficult, and there isn't a whole lot of stereo separation between the channels anyway being that they are only a few inches apart. I am not really sure what the appropriate diameter and length of the port should be, but I will update the blog with the details when I figure it out.

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  2. Awesome, I'm definitely going to try this out. I've seen a number of ammo can conversions, but yours is definitely the best I've seen so far. Have you thought about doing one of the larger cans, like maybe the 3 foot mortar ones? Talk about some boom.

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