the backpack "boom"box

So, you have seen the 80's boombox to the ear characters, always blasting music, looking like fools. Well, im not ok with the stranded lameness brought by holding a boombox to my ear. so instead, i made this... a backpack with 5 speakers, a 12 inch subwoofer, and a 1200 watt amp. Overkill? I THINK NOT! (ok, maybe a little, but its damn awesome)


So how exactly do you go about building one of these? to be honest, i cant give you exact directions, as if you want to throw your own together with spare speakers laying around, they will have different power ratings (in watts) and a different impedance. however ill explain how to make sure that everything is hooked up correctly later on. but first, ill go over the build procedures.

gathering the materials:
The first thing i needed was the box to put everything on. i had a small box already in my garage that fit a 12 inch sub wolfer. next, i needed a subwoofer that fit in the box. well, i had a 12 inch floor subwoofer that was self powered, the amp burned out but it was a nice box, which is what i have in my car. so the woofer itself was under my bed. its a single voice coil which makes it very easy to wire up.

now for the amp, i helped my mom with her car audio a while back, well she sold the car but not before i pulled the amp out of it. so i had that laying around as well!

my friend Vincent contributed 5 small 15 watt 4 ohm speakers and the car battery, so we had everything we needed!


building it!
well, the first thing was wiring the subwoofer to the inside of the box, i simply soldered two 12awg wires from the inside of the box to the woofer, then 8 screws to hold the sub in the box. mounting the amp was also easy, just 4 self tapping bolts anchored it to the top of the box.

the 'washers' we made.

the next thing we needed was shoulder straps. we didn't want to destroy anything expensive, so we found a towel that i bought at walmart a few weeks ago for around 3 dollars. we folded it down the middle and cut it (so we had two long straps). we then took each 'strap' and folded the outsides in towards the middle, so it was 1/3 the thickness. then we folded the ends over so that the bolt would go though 6 layers of fabric. we WOULD have used washers but we couldn't find any big enough, so instead i cut some small squares of sheet metal (and myself a few times). we put two self tapping bolts on each side of each strap, which holds the weight just fine!

the small speakers have wall mount holes on the back, so we tapped 5 nails into the box and hung the speakers from those.

next, we needed a way of securing the car battery that powers all of this... we put it inside of a basket and attached that to the bottom of the box with 4 hook screws, but later on we found that having all that weight on your back was bearable for all of about 2 minutes. we later switched to putting it in a backpack and having the backpack on backwards with the speaker on your back.


The wiring!
ok, so first off understand that the impedance of speakers is measured in ohms(Ω), now a 2Ω stable amp can not power a 1Ω speaker without damaging the amp or triggering an 'overload' protection.

•  when you wire speakers in series, you add the impedance together.
•  when you wire speakers in parallel, you take the impedance of one speaker, then divide it by the total number of speakers.

Since i have 5 speakers, each was 4 ohm, i had to come up with a more creative way of wiring them so they would function correctly. my amp can support 4Ω and 2Ω impedances. so one speaker could be wired on one channel. this leaves me with 4 speakers left. by wiring 2 speakers in series, you get 8Ω impedance. by wiring the 4 speakers into 2 groups of two speakers, i now have two 8Ω pairs of speakers. i can then wire the two 8Ω groups in parallel, giving me 4Ω of impedance, perfect for my amp!

 

now that my speakers are wired (one channel has 4 speakers, seen above. the other channel has a single speaker) i have two channels left over. the amp im using is bridgeable, which means you can wire one speaker to two channels so oyu get twice the power. i have a subwoofer that can handle 200 watts RMS and the amp is 100 watts rms per channel. so i used two channels for the subwoofer.

 

click for larger view

the only thing left to wire was the remote turn on. if you are not familiar with car amplifiers, there is no switch to turn it on. instead you have a remote turn on. because an amplifier will drain the battery, you wire the remote turn on to a fuse (usually the radio fuse) so that when the car is turned on, the remote power on gets power and turns the amplifier on. so think of the remote turn on as an 'electronic switch'. instead, what i did was wired the 12v+ to one side of a switch, then the other side of the switch to the remote turn on.
since i was working on this at 10 at night (22:00) the only store open was walmart, so i drove down there and found a light switch... i didn't want to JUST mount a light switch, so i also bought the face plate for it... it cost an extra 50 cents... well worth the added look.

yup, a flick of a light switch turns the back pack on.

the following pictures are of the backpack. we still had the battery mounted underneath the backpack, which we changed later on.

so far, i have 'tested' the backpack in public by showing it off to the local dunkn' donuts, and showing it off at school during both lunches.
the first lunch period, 3 administrators commented on how cool it was and let me play it inside the cafeteria... the students loved it and were all cheering.  during the second lunch however, the 'nazi teacher' that everyone dislikes told me to turn it off. after walking away and resuming the music, the teacher took me to the office. all the students outside (about 300-400) all started booing the teacher, and one student blocked the door with his body for about 2 minutes. i didn't get in trouble, the administrator she brought me to to write me up commented on how cool it was about 40 minutes before hand!