Boom Bag

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Introduction: Boom Bag

I was upset. Everywhere I turned was an ipod. On display, in someone's hand, strapped to an arm, hidden in a pocket (the head phones are a give-away), in the train, on the bus, walking down the street. I kept thinking to myself, "What happened to that guy from the 80's with the HUGE book box on his shoulder?" Well, no more! We've been isolating each other from our musical tastes for too long, and have been acting far too respectable. Enter the Boom Bag.

Sure you can buy these. For $150.00. Which is insane. So here's how to make one. It cost me five bucks, but then I had the back pack and speaker already.

Step 1: Materials

- A back pack (I used my fourth grade back back for that real 80's look)
- An 8 ohm speaker that will fit in an outside pocket on the back back (I used a Marshal guitar monitor)
- A battery holder (Mine's from a toy)
- Some electronics including:
-- LM386 IC
-- 100K pot
-- 10 ohm resistor
-- 0.05uf cap
-- 250uf cap
-- 10uf cap
-- 0.1uf cap
-- A headphone plug (I got mine from some crappy airplane headphones)
- Wire
- Soldering iron
-1/4" MDF or plywood
- Some polyfil
- A stapler
- Some screen or mesh

Step 2: Breadboard

Test that circuit out! I'll be honest, I didn't come up with the amp myself, I used this guy's. But you'll notice my parts list is a little different. I left out c2, the cap that connects pins 1 and 8. This is the gain pot, you can increase the cap for more gain. I didn't like any gain, so I left it out. The pot controls the volume, and the last cap, c5, can be increased a bit for some bass boost. Try stuff out and see what you like.

Step 3: Circuit Board

Next, hard wire those bits up. As you can see I etched my own circuit board, I'm not going to get into how, there's many good instructables on it already. There's so few parts, you could probably solder it up without a board at all. I'd give you the image for the board, but my computer ate it.

Step 4: Cut It Up

Cut a piece of MDF to fit in the pocket of the back pack, then cut a circle out for the speaker. Speakers usually mount on the back of the board so you.ve got to leave a bit of a lip for the speaker to screw into.

Step 5: Wire It Up

Wire everything together, make sure it works.

Step 6: Mount It Up

Screw down all the pieces. Try to locate the battery pack where you'll be able to access it. Or maybe you could solarize yours! When your done here, cut a piece of screen or mesh and staple it to the front side of the mdf to protect the speaker.

Step 7: Stuff It Up

Shove your workings into your back pack pocket, and stuff the gaps with polyfil. Trace the speaker circle with chalk (you should be able to feel the MDF cut out) and carefully cut the pocket. You might want to run some seem sealer around the fresh pocket edge, or seem sew it.

Step 8: Finish It Up

Almost done, just cut a hole inside the pocket to thread the headphone jack through, or send it through the outside, what ever works for you. This old back pack had another pocket inside the main one that I could keep my phone (and mp3 player) in. Another option is to get a usb headphone connector and leave it wired in the speaker pocket. Good luck!

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    37 Discussions

    http://web.mit.edu/6.s28/www/schematics/lm386.htm

    sound quality is? I'm a bit of an audiophile or at least appreciative, so, just curious.

    1 reply

    This is definitely not super fantastic sound quality, but if you want to pay more money for a better amp, that's easily doable.

    i made one a while back with a battery from them kids electric cars and a car amp, it's epic-ly loud!

    DSC00743.JPGDSC00741.JPGDSC00744.JPG

    Capacitors are generally used to block DC while passing AC. where they're used determines what that function is called. On the input and output leads, it's DC isolation (also called high-pass filter), on pin 7, which internally is connected between the input transistors and the power supply, it's used to decouple or filter any AC by shunting it to ground (low-pass filter). Another could be added from pin 6 to ground for additional power filtering. From pin 1 to 8, it's giving feedback for adjusting the gain, different values give different gains. I've also seen circuits using a resistor instead of a cap there.

    here's the basic LM386 amplifier Little Gem. The cap on pin 2 decouples (filters) DC from the input, on pin 5 it decouples from the output, and on pin 6 it filters any ripple on the power supply. In this case (Little Gem) they use a potentiometer instead of a cap to give variable gain feedback.

    I had this same idea(though slightly different) before seeing this instructable. I really like how this turned out. I'll post pictures if/when I make mine.

    mbmosher question. how long does it take to make one of these. I nevermind one before where can i find part if i don't have it. would radio shack work? also how loud does it get with one speaker and how long will it last with 4 aa batteries? i want to try to make it this weekend.

    2 replies

    A single charge will last for about three weeks of consistent use, and the single speaker (8" in this case) can be VERY loud. I think you could get all the parts (besides the bag and speaker) from radio shack. I took be the better part of a day to make - laying out the board and cutting the mdf took the longest.

    Thanks for answering my questions. but i do have a couple more. now you used a Marshal guitar monitor(speaker) right. Can i use a car speaker or speaker from a old boom box. If i decide to use two speakers, do i need to have two of everything. or can i use one curcuit board. also in part step 2 the bread board. that is just to test the power.

    I'm a little confused. I cant't seem to get mine to work and you only list 4 capacitors in your materials list yet I see five in your pictures. I'm getting power to the speaker but nothing is coming through. Maybe you can clarify these things for me.

    1 reply

    You know, I've been staring at that breadboard photo for about ten minutes and for the life of me I can't figure out what that 5th cap is for - I don't think it made it into the final circuit...

    I've done this myself with a pair of cheap speakers and a woofer I saw on sale (from 50 to about 15, had a random gift card as well). Attached two of the big 6volt flashlight batteries. Sounds great, it's quite loud, and easy to take in/out of my bag when I don't have the space for it. However, this is great if you've got a bunch of spare parts and/or you're looking for a cheaper alternative, +1! Maybe add a step to show where you'd attach some LEDs for people who are curious about that, it could be a neat addition to the project.