Blaster in its natural environment.

We sat out in the sun during the first summer days of 2011 and listened to music, using the iPhone's built-in speaker. Not exactly great sound quality or volume. So we got an idea: why don't we build ourselves a portable battery-powered ghettoblaster?

Neither of us had any experience in building audio systems, very limited experience in dealing with DIY electronics and we're not exactly carpenters either. Luckily, we're both software engineers, used to swimming in the deep waters of the Internet, so - how hard can it be?

We started the project mostly because we wanted to get ourselves a portable boombox, but discovered that the building process itself is actually quite fun. To be honest, there were also times of frustration, but overall the experience was definitely enjoyable.

If you're interested in building a boombox yourself, we've created a separate listing of various helpful resources, that will be referenced throughout the text.

What we imagined it would be like

A collection of traditional ghettoblasters. We though ours would be quite large.
Image © Tom Meyer

A heavy duty plywood or aluminium box was the first thing that popped into our minds. Fitted with a couple of of 8" car audio speakers and a car battery, it would be fairly loud and play for around 8 hours or so.

Weight in the ballpark of 20kg, still something that can be carried by two people. And if that would get too hard, we could fit in all-terrain inflatable wheels so it can be dragged. A box like that would also certainly be sturdy enough to withstand some hits and to be sitted on.

Bootstrapping the project in a couple of days

We used a lot of time to look into online resources. 41Hz forums were extremely helpful.

We talked to some audiophile friends and searched the web. A lot. One of the best online resources turned out to be 41Hz forums. 41Hz is a company selling DIY amplifier kits and they have a very active community.

After doing the research and thinking it through, it turned out we could do with a far more convenient solution than we imagined. It just isn't necessary to drag over 20 kilograms just to enjoy some portable boomblasting goodness. Our final blaster ended up weighting about 10 kg. And by the way, standard car batteries hold an enormous charge for a well optimized boomblaster. Really, enormous.

At this point we would like to thank Audiopoli people for helping us out. Special thanks to Tuomas Luttinen who helped in the design, soldered us an amplifier kit, and shipped speaker elements from Germany with a very competitive price.

List of useful internet sources.

Requirements for the box at this stage

We wanted to keep the thing portable, at least by dragging, and decided to strive for lighter solutions where possible.

We had the blaster on board in an offshore sailing competetition. It did well.

We also wanted to keep the box robust in outdoor conditions, so we need to resist some water, dust and funnily enough, bugs. As software engineers we are used to seeing bugs in software, but in this case there are real bugs that are trying to get into our system in the woods. Get out!

Neither of us has golden ears, so we believed sound quality not to be too big of a concern, as long as it's loud enough. In the end, the quality turned out to be pretty good, though. Our boombox can't stand comparison to high-quality indoor speakers, but when compared to other portable devices, it kicks ass.

We didn't have a good idea how well the batteries would last, but to keep on rocking long enough, we set the minimum battery life expectation to 10 hours with reasonable volume level. In the end, that wasn't too hard a goal: our actual playing time is somewhere around 50 hours.

We went with only a standard 3.5mm stereo input plug, no embedded audio sources.

In order to attach any smartphone or MP3 player, we wanted to install a standard 3.5mm stereo input plug. We could've also embedded an audio source, like an MP3 player to the blaster, but decided not to. It would complicate things and we're carrying perfectly good audio sources with us in the form of smartphones and MP3 players anyway, so why bother.

Our initial price cap was at 300€. We kind of lost track of the exact cost somewhere along the road, but calculations afterwards point to somewhere around 420€. Oh well.

A list of electronic parts we used.

Selecting and assembling the electronics

It all started with the electronics. We couldn't really build or even design an enclosure, before we knew what's going to go inside, because the electronics define all the constraints that have to be fulfilled. So we started to take a deeper look into the electronics that are going into the box.

Amplifier

Because portable devices have a limited battery capacity, the most important selection criteria for an amplifier is efficiency. In other words, how much power will it waste when amplifying the sound. Car amplifiers turn out to be not that good in this respect, even though they can have good quality sound.

AMP6 basic amplifier with some power resistors and capasitors to test high pass filter.

A very popular choice among boomblaster makers seems to be AMP6 from 41Hz. It's priced at 35€ for the DIY kit and 55€ for a pre-assembled one. AMP6 provides good sound quality and good efficiency with maximum of 2x25W average power (RMS) based on the spec. The maximum power is 2x14W in practice, we were told. After that it will start to sound bad.

This seemed like a low number at first, but even though various market speakers announce huge power numbers (2x100W or something similar), it's actually a surprising low amount of power that's required to power up efficient speakers. 2x2W average power with good speakers can produce loud enough sounds for many situations. This means that AMP6 is a really good fit for a portable battery-powered device.

Speakers

All speakers connected to the amplifier. The speakers won't sound that good in open space.

Speakers were the hardest to pick. There are a ton of things that affects the quality and suitability of the speakers, and we can't really understand even half of them. We read quite a bit about these things, and then went with a reasonably sounding recommendation based on other already built boomboxes.

As with the amplifier, efficiency is also critical for the speakers. Speaker efficiency is measured as the volume the speaker can produce to 1 meter distance, using 1W of power. 90dB is a good number, 85dB is not. The bigger the number, the louder it is. This is simple.

But then we get into the impedance math. I can't really say I understand the underlying phenomena, so I'm not even trying to explain why, but here's the deal: the larger the impedance of the speakers, the more power they require. 4 ohms is a good fit to be used with AMP6 amplifier. Selecting 8 ohm speakers would mean less output volume with the same power. So 4 ohms it is.

Other selection criteria include linearity of the frequency response and some other magic stuff that only audio wizards can understand. Our first intuition was to go with car audio parts, but turns out they're often not optimal in terms of efficiency, price and the mentioned other magic criteria, when compared to PA elements. Car speakers have good resistance for water, dust and other crap, so that's a plus, though.

In the end we decided to leave the selection process to the experts and decided to use Monacor SP60/4 bass-midrange PA speakers, that were highly praised in several forum posts. They have good efficiency (90dB), are pretty cheap (around 16€ per speaker) and the wizard-stuff is good, they say. SP60s are also pretty well sealed, so water and dust shouldn't be that big of an issue.

We tested how much a cardboard enclosure would boost the SP60 sound. The answer: a lot.

SP60s are not full-rangers as they can only play bass-to-mid-range level frequencies up to 8 kHz. So we needed a separate pair of tweeters to play the high-pitched parts of our hipster techno songs. The selection process was again reading posts and Monacor DT-99 was the one selected. DT-99 are not as cheap as the tweeters could be, around 20€ per speaker. We could have gotten a pair of cheaper tweeters, but at this point we though what the hell, it will not impact the total budget much anyway and these are supposed to be real good. We have no idea how it would have sounded with cheaper tweeters, but DT99s are certainly good enough.

Battery and the charger

A few words of caution: a big concern with batteries, of all kinds, is the danger of explosion. We're not experts on this subject, so please, check up these things on some reliable source before doing your own work with batteries.

The battery, LiPo vs VRLA

We ended up buying a 7,2Ah battery. Turned out it's quite a lot of juice.

Batteries were a bit of a scary subject for us since we knew they could explode if handled inappropriately. That is why we decided to do a fair amount of research on this subject.

For a boombox, there are at least two realistic alternatives: Lithium-Polymer (LiPo) or Valve-Regulated Lead-Acid (VRLA) batteries. The former are more compact and lighter, whereas the latter are cheaper. We weren't that picky about the weight, so we discarded LiPos pretty early on.

Lead-Acid batteries have the problem of leaking explosive gases, mainly hydrogen. Luckily, the worst can be avoided by selecting a suitable LA battery type. Standard lead-acid batteries will leak gases almost every time they are charged, so they are not suitable to be used in a sealed container like a boombox. VRLA batteries, however, are built in such a way that the gases will not leak the sealed battery unless something goes badly wrong (the battery is overcharged, for example).

VRLA batteries seem to be quite widely used in boombox projects, so we figured out it's probably safe enough for us too. We picked one that had suitable capacity, dimensions and price, without being too concerned about its datasheet. We reasoned that we'll be drawing quite small currents from the battery anyway, so all batteries of suitable size are able to deliver what we need.

Battery size

Bigger is better, but when capacity goes up, so does dimensions and weight. Using our setup we had an estimate of 300 mA current on the maximum power. This means that for example a 7Ah battery would last around 7Ah / 0.3A = 23.3 hours. We were sceptical about what it would be in reality, but thought we'd still exceed our original 10 hours requirement.

We ended up buying a 7.2Ah VRLA battery. We kind of delayed the actual buying of the battery until the case was very close to being finished, and at the time we went to the shop the 5Ah battery that we had decided to buy, was sold out. We really couldn't wait a few weeks, because the traditional Finnish midsummer party was only a few days away and we needed the blaster there. 7.2Ah it was, then.

The longest test run on one charge has been around 30 hours. Charging the battery fully after this took surprisingly little amount of time, so we are expecting the battery to actually last a lot longer than 30 hours with medium voice level.

Because the thing just keeps on rocking (we really haven't been even close to a dead battery), and the smaller battery would've weighted one kilogram less, in hindsight it seems a smaller battery could have been a more ergonomic investment.

The charger

Several chargers worked with our battery but investing a bit more, we could get one that is much smaller and has useful features.

Most of our parts were selected based on recommendations and information we found online. However, charger availability seemed to vary a lot depending on the store, and ordering these parts from the Internet didn't seem very cost-effective (they weigh relatively much), so we bought the battery and the charger from the local electronics store, based what was available, with the assistance of their helpful staff. Props to both Yleiselektroniikka and Partco for good customer service.

Optimum charger is one that can match the maximum charging current of the battery without exceeding it. We ended up buying a charger that cost around 40€. It's not the cheapest one but it is considerably smaller, lighter and also faster than the cheapest option. Additional benefits include a status indicator led that was missing from the cheapest chargers. Overcharge protection was also a very nice feature, as we can now leave the device plugged to the wall without constantly monitoring it. Overcharging is one of those things that can blow the battery.

Wiring it up for a test run

We bought large-ish standard connectors, a bit excessive in size. Smaller ones might've been trickier to solder... :)

Rest of the electronics were mostly just about wiring the audio components to the amplifier and the amplifier to the power source. We put a power switch between the battery and the amplifier. For the audio, we put a stereo socket to the amplifier and then used a stereo cable with jacks at both ends to connect it to the audio source. As we needed to assemble and disassemble the electronics many times, we used screw terminals to connect the other components.

Before continuing on to building the casing, we did some test runs with the electronics only, to verify everything is working as expected. If was an extremely satisfying moment to see and hear the speakers dance with the amplifier. We also build some simple boxes out of cardboard to test how the sound would improve. It turned out to improve a lot with just the cardboard, so we were thrilled to see how our speakers would do with a real wooden box.

A word of warning about the screw terminals: we managed to short circuit the battery once, luckily just for a fraction of a second as sparks started flying, because one screw terminal was not tight enough. Luckily the components didn't break. After this incident we started double checking that the screws were holding and applied a bit electric tape so that the same thing wouldn't happen again with worse consequences. We also started double checking that the power switch, as it protects the amplifier, was always turned off unless we really needed the power.

Designing the enclosure

Pen and paper - unbeatable when sketching.

Now that the soldering was done, we moved on to the enclosure. The design came mostly mostly from "form follows function" and "keep it simple", because neither of us had experience with this kind of work. Our box design emerged from these the requirements, each mainly meant to enhance the lower frequencies and keep it maintenance and bug free (literally).

Prototype made of cardboard gave us quite a good idea of the size of the final box.

Before ordering the wooden parts, we assembled a 1:1 cardboard prototype of the box to help us imagine how it will be in practice. This was a good idea, because it gave us confidence we're doing the right thing, and building it didn't take much time.

We made only one major change to the casing while building it. Initially we planned to include the charger into the box, so it wouldn't need to be carried separately. However, after realizing that we would still need the electric cord, which can be forgotten the same way as the charger, and that one charge lasts for tens of hours, we decided to leave the charger out. This allowed us to make the box design simpler and more lightweight. That decision has proven to be a good one.

Preparing for the assembly

The locking mechanism of the back panel was made by gluing big bolts and washers to the L-pieces screwed into the top and bottom panels.

We didn't really know what's needed, so we ended up going back and forth the stores several times. It got annoying rather quickly, which is why we have set up a list of things we needed. Hopefully this can save someone else a trip or two.

Tools

We tried to minimize the carpenting work as we didn't have many tools. The enclosure design made sure we needed just a few basic tools.

We didn't have many tools available beforehand and wanted to minimize the cost. Good tools cost a lot, so we tried to borrow from friends when possible, checked out for local workshops, and as a last resort, bought them. Local workshops turned out to be a little too expensive for our taste, but that's probably not the case everywhere.

For tools that were only needed for this project, we tried to get away with the cheap alternative. For example, we bought a hole cutter for under 10€. It was clumsy, started to get blunt, and honestly was a little scary when spinning fast. We kept a piece of plywood between us and the cutter, just in case it would smash into pieces.

While our cheap cutter had its flaws, a high quality cutter would've cost 40 €. In addition to the cutter, we also needed a saw and a clamp, among others. We're not likely to need these tools again any time soon, and savings of 10 to 30 € per piece pile up pretty fast. We chose to buy the cheaper ones and live with the frustration that comes along.

Listing: tools we needed.

Assembling the case

Front panel being attached.

Building was fairly simple, once we just got our hands dirty and started working. There were surprises for sure, but we managed to solve each and every problem we encountered, just by using common sense.

We weren't experienced with the tools and didn't have knowledge of the properties of the materials we used, so we took extra precautions for each step.

For example, we tried out on a piece of excess plywood if it would chip when drilling with different sized drill bits. And it did. We checked if it would fracture when driving the screws from different sides. And it did. It was very helpful to test these, so we had confidence when handling the proper pieces. We wanted to avoid reordering the pre-cut pieces, and succeeded.

An important lesson from the building is that you should always think through what you are doing. When some parts are screwed and glued, it might be extremely difficult to access some parts of the box, so building order matters a lot. We were lucky enough to avoid all catastrophes, but did some things in a more difficult way than was necessary.

There isn't actually that much going on inside the box, everything fits to rather small space.

We begun the construction by first building the center pillar. It would house the amplifier, battery and the input device. After that was completed, we attached the L-pieces to the top and bottom blanks. Those would be used to lock the back panel once the build would be finished. The top, bottom and side panels were then attached to the central pillar. Finally the front panel, which already had the speaker holes and mount plates done, was attached.

For carrying the box we used a shoulder strap from an old laptop bag. It's conveniently padded and has quick release clips at both ends. So far the clips have endured but they are still a slight concern as the blaster weights considerably more than your average large laptop. We'll have to monitor this and probably make a new shoulder strap at some point.

Part list for the enclosure.

Painting

The blaster assembled prior to painting, also the grilles and carrying hooks are still missing.

Our aim was to make the blaster smooth black matte color with neon color details to make the features stand out. We were recommended to use spray paint, as it would be easy to apply (no brushes and no thinner), and it would be moisture proof. For the details, we picked up some neon yellow furniture paint that can be applied with a small brush.

Black paint, grills and speaker holes waiting for the decals.

We didn't have much knowledge about painting plywood, so we just sanded the surface smooth with a finely grained sandpaper and applied the first coat of spray paint. The result was not quite what we expected. When plywood gets wet, of water or paint, the strings of the wood will rise considerably, making the texture of the plywood very visible in direct lighting conditions.

The twice painted box masked and waiting for effects to dry.

After investigating this issue, we found out that you could try to apply a bit of water and then sand it before the first layer of paint, thus saving some time and paint on the first round. Maybe we'll try that next time.

In any case, in order to get the smoothest surface, it is necessary to sand each layer of paint before applying the next one. Sanding the first layer revealed the plywood underneath, so a thicker, non-spray paint might work better as a base coat.

The bolts were painted while attached to the box, rest were painted just over a paper sheet, grilles on both sides.

It took most of the can to coat the outside of the box once, so several cans were needed for a smooth result.

While painting, we noted that spraying multiple layers fast covers far less than spraying slowly. Moving slow but steady, rather than passing each surface fast many times, produced a better result.

Conclusions

You can still see the texture after sanding, painting, sanding and painting. Really smooth surface needs thicker coating to cover the wood texture.

There are several things to consider when building a boombox: sound quality, robustness, loudness, weight, price, complexity of construction and so on. Improvement on each sector will usually make it worse in some other aspect. We're pretty pleased with our compromise, especially given that we had no background in building these kind of things.

The blaster sounds nice and with the weight of 10kg, it's not too heavy for carrying on most occasions. An even lighter version could be beneficial at times, though, so we'll have to think about that next summer...

One of the hardest battle tests so far has been a bungee jump from 150 meters. No, we didn't jump with the blaster, but it was at the ground level, and you could just about hear it booming even at the top at 150 meters. That was neat!

Blaster was also on board at an overnight offshore sailing competition. Wagner's Ride of the Valkyries has now been blasted on a sailing boat in the middle of the ocean in the darkness of a Finnish autumn night. And Danger Zone from Top Gun, too.

Feel free to contact us or check out the related 41Hz forums thread if you have any comments. We'd be pleased to hear about other similar projects!