Introduction: Mobile 12V Sound System

About: Owner of a small local bicycle shop in sunny St. Leonards on the East Sussex coast in England. Apart from the bicycles I really enjoy metal working. Which is handy as I really enjoy making crazy mash up bikes…

The aim of this project was to build a fairly moveable 12V sound system to accompany our "Pier to Eternity" illuminated bicycle rides.

I've built a few things similar in the past and some of this project already existed as part of our previous sound system.

I should point out that I'm very much a 'bodger' when it comes to audio (and electrics). I'm sure there's a million ways to make this thing more efficient. But as long as it's quick to make, loud and plays music for a few hours. I'm happy.

Tools needed:

  • Basic electrical tools; side snips, wire strippers, crimpers, good set of screwdrivers all come in handy.
  • Basic wood working tools.
  • If you're getting fancy... Soldering iron and the needed accessories.

Materials needed:

  • 12V amps - Car kit is 12V and it's pretty easy to pick up cheap car audio kit.
  • Fairly big 12V battery.
  • Speakers - As many and as varied the better. As you'll probably be using car amps, car speakers would be best.
  • A bunch of wire, from fat to thin.
  • Electrical connectors - Chocolate blocks, crimp connectors and so on.
  • Audio connectors - Depending on the amp setup you will invariably need various RCA / jack connectors.
  • Off cuts of ply wood, bunch of screws.

Step 1: Transport?

How are you going to move this thing around? It'll end up weighing quite a bit so it's worth thinking about it now.

We're going to use a bicycle trailer and pull the thing around by bike. The trailer is just a flat platform so we making sure everything fits and we can strap it down easily.

We're also going to put handles on the system to help move it about. Our battery is totally separate from the main system and easily detached, so the battery and system can be moved separately, both weigh about the same. Quite a lot.

Step 2: Find One or Two 12V Car Amps

When looking around for car amps pay attention to...

  • If it looks like it's made of plastic and might fall apart, it probably will.
  • Is it bridgeable? This would be marked around the outputs for the amp. If it's bridgeable it means you can use the amp with mono output, which is useful for mobile sound systems.
  • I look for something that looks well made and has nice big well made connectors for power and audio out.
  • Another good thing to look for... Does it have it's own easily accessible standard car blade fuse.

Amp1 in our system is an 80W bridgeable amp, with outputs for two speakers.

Amp2 in our system is an unknown W non-bridgeable amp with outputs for two speakers.

Step 3: Find Some Speakers

As we're using car amps we scrounged for car speakers.

Luckily we had a good start as we already had a bass speaker and two mid ranges speakers, already in a box, from our last sound system.

I knew from our last rides that Amp1 could just about drive all these speakers, if you didn't turn it up to 11.


Low pass filters:

Amp1 also has a low pass filter built into it. By turning this on you're basically saying 'just do the low notes for bass'. If we turn this on for Amp1 we can turn the volume up to eleven. It's loud. But it's just bass and doesn't make best use of the two mid ranges speakers in this box.


Buzzing speakers?

If you're speaker is making a nasty buzzing sound, like our bass speaker was, then there's probably a tear in the speaker somewhere. Look for any holes and just tape over them with electrical tape or whatever. In our case there was a weird seem in the bass speaker and when we taped over the whole seem it fixed the buzzing problem.

Step 4: Find More Speakers

So we were lucky enough to have some more speakers lying around. A couple of car speakers from the doors of an old Ford, a single tweeter and a horn speaker.

I wired all these up to Amp2 and checked that they made a nice loud noise. The horn speaker was from some public address system and had a funny dial on the back that allowed you to change the resistance. But the lowest setting was a 1,000Ohms! Still pretty high and when I tested it you could hardly hear it.

So we swapped it out for another horn speaker, this one being 8Ohms, which is much closer to what the amp is expecting and made a good loud sound.

Once I was happy that Amp2 could drive these speakers I built an extra panel on top of existing bass box and cut holes and mounted these speakers.

Step 5: 12V Power Source

We need some 12V power to drive our amps.

You can use car batteries. But these are open/wet (which means you've got to be careful otherwise you might start spilling acid everywhere) and designed to give massive amounts of power in short bursts.

It's better if you can get your hands on mobility scooter / golf cart / electric bicycle / caravan batteries. These are usually sealed and designed to 'deep discharge'. Which basically means if you fall asleep and leave the sound system on all night you're not going to kill the battery.

I bought a 20Ah 12V sealed deep discharge battery.

This battery is a bit small for my rig but is fine for build and testing.

I then picked up a big 80Ah 12V battery, again sealed and deep discharge. This should give us a fair few hours of music.

Step 6: Strenghten the System

I added a single side panel that held the new top panel with the fairly sturdy bass box below, making the whole thing less wobbly.

I had a 12V caravan power cut off switch and a sexy 12V LED Voltmeter lying around. So I mounted them into this side panel.

I also bolted two plastic handles into the bass box. I'm starting to regret using plastic handles, I'm not sure they're up to the job. Use metal.


Update: As suspected the plastic handles broke when I was strapping the system to the trailer. So I bought a couple of metal handles instead. Should have done that in the first place.

Step 7: Wire Up the Power

So everything is 12V and the battery is 12V. So you just need to wire everything up in parallel. Join all the red wires together, join all the black.

My system is a bit fancy. I've got a voltmeter, some USB outputs for charging stuff on the move, the big cut off switch, a FM transmitter and two car amps.

Your system might just be one car amp directly wired to a car battery. It will still work and still make noise.

Because my system is pretty silly I've added a positive bus bar and a negative earth block. Basically making it easy to connect all the red wires and all the black wires.

You'll notice in the photo that I've used fairly fat wire for the connections to the amps. They draw a lot of power and you want to make it as easy as possible, so as fat as possible. Power to anything else can be whatever makes sense.

Sometimes you'll need to make a wire longer to reach it's goal. I soldered all my joins, as I'm trying to get better at soldering. But you could use chocolate blocks or whatever. Try and cover any bare wire in electrical tape. Stripping long lengths of copper, twisting the wires together and then taping can work well.

If you short a car battery it can cause quite a bang and if you're sound system is for use outdoors you might get wet! So I put my positive bus bar on the top of the box, over to the left hand corner. I put my negative earth block on the back of the box, over to the right. This makes it pretty difficult for water or a dropped spanner to short the system. That said, I'm thinking about boxing in the earth block.

Your amps will probably have a ground / earth connection, a positive / power connection and also a REM or RMT connection. The REM or RMT stands for 'remote' and all you need to do is cut a bit of wire and connect the REM/RMT to the positive connection to make the amp work, basically trick it into thinking that the remote says yes, the power is on.

Step 8: Optional Extra: FM Transmitter

When we've had really big rides the people at the back can't hear the music. So rather than making a bigger, heavier, louder sound system it's better to distribute the sound. So we're going to broadcast FM whilst riding.

With a bit of research we found this transmitter that could run on 12V. It has two settings; 2W or 0.5W. Apparently 1W = 1 mile. So we'll probably only need the 0.5W setting.

There's lots of laws in broadcasting radio, so if you do choose this optional extra then best look into them. Our transmitter is very low powered, which is probably a good thing. We're also going to broadcast on a frequency that's used once a year for a local community radio station. So we know that it's open and not going to mess with anything important.

We strapped our transmitter to the system with a combination of staples and re-usable cable ties, so we could remove it easily enough.

Step 9: Fuses?

So fuses are good. They blow and your system doesn't.

Both the amps had their own car blade fuses, which is a good thing to look for in a car amp.

I wasn't too worried about the voltmeter and USB ports. But I wasn't sure if the FM transmitter had an internal fuse so I soldered a car blade fuse holder on the positive lead.

I also fixed in a 100A truck fuse directly on the isolator switch. This is a pretty big fuse and you've got to hope it never blows. But it's a fail safe, and not a bad idea.

Another little safety thing was where the bolt for the truck fuse passed through to the outside of the box I covered it with a lump of Sugru, which should make it harder for someone to short the system by mistake.

Step 10: Connect Your Audio Cables

You should already know how to connect your speakers to your amps from the testing phase earlier in the build.

But you need to connect some kind of player to the amp(s) and FM transmitter, if you're using one.

Most decent amps will have two female RCA jacks for the input. Most players will have a female 3.5mm jack. So if you've just got one amp you need a male 3.5mm jack to two male (left & right) RCA lead.

In our case we've got two amps and the FM transmitter to get audio to. So we need to split our audio three ways.

After a little hunting on ebay I found some RCA splitters, cables which take one RCA in and give you two RCA outs. With a combination of these I managed to split our audio in three ways.


Stereo / mono... Bridgeable / Non-bridgeable

As all our speakers are in one big box and we've got an odd assortment of speakers, stereo doesn't make a lot of sense, it's better to send the same signal to all our speakers. Amp1 is bridgeable, which means that it can act as a mono amp. Amp2 isn't bridgeable, so we bought a few audio connectors which basically turned a stereo input into a mono output, bit of a hack, but it seems to work.

Step 11: Optional Extra: Crazy 'stereo in a Field' Box!

As well as broadcasting FM I've always wanted to give this a go... But this is EXPERIMENTAL and also I got help making this part... So you might have to work it out for yourself as I'm not entirely sure how it works!

The basic idea is that when you're on the move with your sound system and there's other sound systems picking up your FM broadcast you want everything to be in mono, as you've no idea where the various sound systems will be.

But when you stop for a drink in a field, it would be great if the main sound system played the left channel and the other sound systems picking up the FM play the right channel, which lets you set up a stereo sound system in a field!

So this magic box allows us to switch between the two. The magic box basically takes the place of two RCA splitters, which I mentioned on the previous step.

The box has a left and right RCA audio in and two left and right RCA audio outs. And a big switch.

Switch position 1 > Sends in left to out #1 & #2 left, in right to out #1 & #2 right (straight forward splitter).

Switch position 2 > Sends in left to both of the #1 outs, in right to both of the #2 outs (stereo, the amps see the left channel, the FM transmitter sees the right channel).

Right... hope that makes sense?

Step 12: Paint Job

To cover up our bad carpentry we used up some putty that was about to go off and tried to fill as many holes as possible, smooth off bits and bobs. Then white undercoat and some blue paint we had lying around.

Some aluminum tape, sticky vinyl and a pair of scissors later and it's starting to look proper nuts.

Step 13: Make Some Noise!

We ratchet strapped the system and the battery down onto the trailer and wired up the battery.

A few tests playing around with the volume of each of the amps and the volume on the player and we found a nice point where it's nice and loud but not distorting (too much).

It's pretty heavy and bumps around a fair bit, so had to make sure all connections were nice and tight.

We've had a couple of rides with the system so far and it's been great. The red horn speaker had a loose connection somewhere, so kept cutting out on the bumps. So we replaced it for an even sillier horn speaker.

Our rides last a few hours and so far the battery has not dropped below 12.4V, so the battery is plenty big enough.

The only thing I might change down the line is some kind of lightweight (plastic sheeting?) cover for the top, just in case we get caught out in the rain.