Step 3: Get an Amplifier and Battery (and Charger)

You will need an Amplifier. The amp discussed here is a standard ordinary car audio amplifier (except the switch we added). Amps like this have one input - a "Line-Level" stereo input. This signal can come from an IPod or other headphones-ready device, OR a mixing board with several inputs (including Microphone jacks). Amplifier wattage numbers are very subjective, so don't worry about whether it says it's 200 watts or 1000 watts, just see if it WORKS and sounds good. It will probably supply more wattage than your speakers can handle either way though, so be careful.
(When testing a suspicious used amplifier, don't connect it to a speaker you don't want to fry!)

Another thing about a Car Amplifier like this: it runs on 12 volts, like a Car Battery supplies. Using your home stereo amplifier won't work unless you have a very long extension cord. You may be thinking "but I can just use an inverter, and power my favorite plug-in amplifier!". DON'T DO THAT. You will be wasting a third of your battery life, carting around extra equipment, AND inverters make a horrible 120Hz "buzz" that gets into all your sound. Plus people who know better will scoff at your lack of efficiency.

You will need a Car Battery to power your soundsystem. It can be a used one, a small one, a largish one, or a Boat battery, or other 12 volt battery of a decent size. Deep Cycle batteries are better, but more expensive - basically, they can be drained further without lasting damage. Ordinary car batteries are not supposed to be drained more than halfway during normal use. But if a Car battery is available to you, use it. You probably won't drain it more than halfway anyway before the Cops take it away. (Try to get them to let you disconnect the battery so it's still good when you get it back!)

And the Charger: batteries like this must be charged back up after being used - allowing them to sit discharged will ruin them, besides not being ready next time you want to party. I recommend strapping the charger into the shopping cart and wiring it in - so there is no reason not to plug it in when you get the chance, even if the cart doesn't go "home" to the same place every time. You can't skip this step; charging is essential to the preservation of your battery and your whole project.

Read up on car battery chargers to understand how long to leave it plugged in, and what Charge Rate to set your charger to. If you have an automatic charger with a two-amp setting, overnight will charge your battery back to full, and you can leave it plugged in up to a day or two without danger.
It would be a lot cooler if you built some speaker boxes with car speakers, but standard hifi speakers like the ones you used are readily available. I see really nice ones at the goodwill store all the time. In fact, every piece of stereo equipment in my room came from goodwill, from the 200 watt sony tower speakers, to the 25 disc cd changer, to the technics turntable. But you could still get lots of new parts for this cheaply on a website like parts-express.com. That way you could get like a 4 channel amp and hook up subwoofers and everything. Also, like someone else mentioned, an amp that would work with 2 ohm loads would be a lot more efficient and you wouldnt drain your battery as fast. Also, if you bought car speakers and assembled the boxes, you could get coaxial speakers that have a woofer and a tweeter built into one driver. That way it would be a lot smaller and lighter and you could improve your score in the running-from-the-cops-over-terrain category.
looks great, I'm up for a shoppying trolley sound battle....<br /> <br /> Enter &quot;The Hobo Hifi&quot;<br /> <br /> All our photos are on our facebook page, <a href="http://www.facebook.com/Hobo.HiFi" rel="nofollow">http://www.facebook.com/Hobo.HiFi</a><br /> <br /> <br /> We have blown nearly 10 speakers making ours, its one of those, its not loud enough, get a bigger amp, oh our speakers are blowing, buy better speakers, oh its not loud enough, buy a bigger amp..............<br /> <br /> we have kept it pretty budget, are are slowing creating a hobo franchise, with the Hobo Hifi, the Hobo BBQ, the Hobo Brew. ( music food ( spit roast )&nbsp;beer ( home brew kegs ) ) and about to make Hobo Tendo so we have have summer mario kart tournaments in the park.... that will just be another battery inverter snes and a tv, that should keep us entertained while we plan the 12v Projector for movie nights.<br /> <br /> yay for portable tunes!<br /> <br /> <br /> <br /> <br /> <br />
Nice work.<br/><br/>See my efforts here: <a rel="nofollow" href="http://www.wheelbarrowsoundsystem.co.uk/">http://www.wheelbarrowsoundsystem.co.uk/</a> for making a portable sound system.<br/><br/>If you want to make a system that can be run at high spl (loud enough to party to) you need to achieve efficiency.<br/><br/>Do not use AB class traditional car amplifiers. seek out more efficient class D amplifiers.<br/><br/>working from a low voltage, high current source (e.g a battery) it is worthwhile seeking an amplifier that is capable driving low impedance loads - down to as low as 2 ohms.<br/><br/>I'd recommend using a greater number of smaller drivers to increase the radiating area - for example:<br/><br/>using 4 x 8&quot; drivers on bass will give you a larger radiating area over a single 15&quot; driver will give you 13% greater radiating area. it will also be possible to wire the drivers in parallel to present your amplifier with a low impedance load to make the best advantage of the high current source that is a battery supply.<br/><br/>when using multiple drivers in the mid range and above ensure the radiating centres (the middle of the drivers are within 1/4 of the wavelength of the crossover to any HF (tweeter etc) device.<br/>
The gauntlet has been thrown down! I challenge you to a sound-off! Wheelbarrow versus shopping cart - who will prevail? The contest will be judged on the following merits: Cost Theoretical electrical efficiency Actual operating time Funkiness Time to assemble Time to assemble by a random Instructables reader Time to acquire materials Roundness of speakers Correctness of phasing over the audible range Beer carrying capacity And for bonus points: running-from-the-cops-over-terrain contest! much love
If I'm honest I reckon I score out of 10:<br/><br/>Cost - &pound;Lots, 0/10<br/><br/>Theoretical electrical - 5/10<br/><br/>Actual operating time - 8hrs 8/10<br/><br/>Funkiness 10/10<br/><br/>Time to assemble - 2 years 1/10 <br/><br/>Time to assemble by a random Instructables reader - INFINATE 0/10<br/><br/>Time to acquire materials - none 8/10<br/><br/>Roundness of speakers - (ERM mine are round) 10/10 If you are referring to 'the sound' then that is a very subjective thing. to my ears its good. The frequency response is not perfectly flat, but then that was not really my objective.<br/><br/>Correctness of phasing over the audible range - I'm not sure you know what you are talking about here. If you are asking if the speakers are in phase with each other at the cross over points, then yes. (or as close as they realistically can be without some DSP). the speakers in the different bands have their own individual transfer functions, as long as the outputs are in phase at the cross over points - bass - mid and mid - high then all will be cool.<br/><br/>Beer carrying capacity - 1 pint - 4/10<br/><br/>And for bonus points: running-from-the-cops-over-terrain contest! - 4/10. it may be a wheelbarrow, but its not fast!<br/><br/>really like what you have done with your system. Mine will never be finished.. I've always got new ideas and things I want to improve/change.<br/><br/>If you'd like to be kept abreast of the developments join the wheelbarrow sound system facebook group! - <a rel="nofollow" href="http://www.facebook.com/group.php?gid=10337113819">http://www.facebook.com/group.php?gid=10337113819</a><br/><br/>I am planning an event over here in the UK that will be the wheelbarrow sound system VS (an as yet unnamed opponent)<br/>
Looks like a fun setup! Only issue I see is that you're driving 8 Ohm house speakers with a car amp intended for 4 Ohm speakers. Any idea of the longevity of this setup?
aha! Very perceptive, except that you didn't read STEP 6, where I mention this. While it is true that hooking 4-ohm speakers to an amplifier designed for 8-ohm speakers could cause trouble, doing the opposite as we are doing here will not. The effect is that the amplifier will only be asked to supply half as much current as it would otherwise supply. I point out that it is possible to put four such speakers, doubling them up - because two 8-ohm speakers in parallel is like a 4-ohm speaker. But the magic thing about this instructable is that it is made with readily available parts such as household speakers, and works well with a little room to spare in the amp department. People know what happens if they turn up their system too loud for the speakers - they can burn their speakers out. Then the people might choose fancier speakers to replace them. This way, they don't have to worry about messing up their amplifier. Thanks for your comment!
Ah, I read it incorrectly the first time around, as I thought that you were recommending that people hook their two speakers up in SERIES instead of parallel. Nice setup though.

About This Instructable


28 favorites


Bio: whats an engineer to do?
More by jerkey: Brain-Controlled Wheelchair Easy rechargable portable ipod/USB recharger How to make a shopping-cart sound-system for street parties
Add instructable to: