We know many of you have waited for this post. This is a post about the boombox that has amused and annoyed people at several occasions. It was originally built to serve at Roskilde Festival 2010 but will surely come in handy in the future aswell.
Background
You won’t revolutionize the world by building a portable boombox. Nor is it very hard these day when cheap electronics are all around us. To make things even easier, there are lots of projects out there such as the Boominator that can inspire or even supply complete instructions and schematics on how to build the ultimate boombox. We’ve taken the mentioned Boominator concept a step further by adding lights that reacts to the music. It’s not rocket science, but even so it’s not very common either.
Design
The basic concepts for the boombox are derived from Saturnus “The Boominator” over at diyAudio forum. But we weren’t satisfied by just copying an existing design, so we decided to add some extra spice to it. Something that could show of our artistic and creative side (the sort of Achilles heal of an engineer 
). So in addition to making the original Boominator design a little more rugged by adding aluminum edges/corners, we thought: hey, let’s add a giant VU-meter on the backside!
Construction
Keeping in mind portability and durability, the material used had to be light yet strong. The choice fell on standard 12mm pine plywood, which is a good compromise plus that it’s fairly cheap and easy to work with. To make it even more lighter, there’s always the possibility to use a milling tool to remove some additional wood on the inside of the plywood faces. This might also affect the acoustics in a positive way since it probably will reduce the effects of standing waves in the speaker chambers.
To make the construction as rugged as possible we also added some 20mm aluminum L-profiles on the edges aswell as some heavy duty steel ball corners. Also, in order to protect the drivers in a effective yet good looking manner, a chrome protective grille was attached on the sides, covered by the aluminum profiles on the edges. In order to make room for this grille, the original Boominator design was slightly modified to create “edges” for it to attach to:
As for painting, the surface behind the grille was painted in a non-reflective black color to make the driver show as little as possible. As for the visible surface the choice fell on a metallic deep blue color. Of course it is absolutely necessary to use a good primer before applying the final paint. Finally, to make the surface as persistent to scratches and dirt as possible, a clear laquer was applied three times.
Amplifier
At the core of the box is a really cool AMP based on an almost legendary chip from Tripath: the TA2020, which is a 2x20W@4Ohm “Class-T” amplifier with an efficiency of about 90%. A Swedish guy called Jan has made a couple of excellent designs based on this chip and sells them over at 41hz.com. We’re using a kit called AMP6-basic which is a small footprint PCB (50x80mm) with mostly hole mount components for easy soldering.
We did however make one modification to the AMP6: in order to save some energy I replaced the input DC-blocking 3.3uF capacitors with 0.22uF ones. This will change the high-pass frequency to about 130Hz with a roll-off of 6dB/oct downwards. Because as some wise persons have pointed out, it’s a waste to play frequencies below 100Hz in an outdoor environment, at least for a system of this type.
Side note:
During a short period we experimented with a cheaper alternative to AMP6: a pre-built amplifier based on the same Tripath chip but with integrated volume + tone controls. However, it turned out that these were of really bad design, and they both popped while playing loud for a while. The only use we had for them was the neat little internal heat sink, which I removed and used with AMP6 instead =)
Speakers
As for the speakers. Saturnus over at diyAudio has done a great job finding the 10” P-audio HP-10W 8Ohm speakers which are high performance (200W), high efficiency (96dB/W) drivers. They are also fairly light and cheap, and they sound good! The boominator uses four HP-10W drivers in total. Each channel on the amp has two speakers connected in bipole (in phase) mode glued together magnet-to-magnet, giving a quite omni-directional sound.
In addition to the main speakers you’ll need some extra boost in the treble range since the woofers will cover frequencies up to only around 4-5kHz. Now, since two woofers are connected to each channel of the AMP (4ohm load per channel), adding real Hifi tweeter drivers would cause the impedance to drop further, potentially causing the amp to become unstable. So the only viable (and cheap) option was to add four piezo tweeters which doesn’t affect the total impedance to a significant extent. Sure, they don’t sound very crisp and detailed, but they do a decent job covering the missing frequencies.
Lights
Initially we had plans on putting a few LEDs on the side of the box. But when testing it out for the first time, we realized that by placing Malen in an upright position, the sound was spread better. And when we had Malen standing up on one of the ends someone cracked the idea: “Wouldn’t it be cool if it had a giant VU-meter jumping up and down to the music?”. Of course, everyone agreed and we went to work!
The VU-meter is built with twelve individual segments and four different colours: green, yellow, orange and red. The segments are constructed using 5mm sand papered plexi pieces with four 3mm LEDs drilled in from the side. Surrounding them is a 7mm plywood sheet with cutout holes for each segment. This will make the segments “sink in” a bit below the wood surface, preventing obvious scratches and damage when having it faced down.
Electronics
Of course, this VU-meter had to be controlled by a microcontroller. Since we’ve worked a lot with the super-easy-to-use Arduino platform (AVR ATMega328) before, we decided to use it for this application as well. However, one limitation with the platform is that it offers only 6 PWM outputs and we needed 12: one for each segment of the VU-meter. Fortunately there are nice little ICs for this and a widely used one is the TLC5940 from Texas Instruments. The circuit is a SPI-controlled “LED-driver” offering 16 channels with 12-bit resolution (compared to 10-bits on the ATMega328). Another nice thing with using a LED-driver instead of a standard current sink IO-pin is that you don’t need any external resistors as it automatically limits the current to a configured value (matching the LEDs forward voltage). So I soldered one of these onto a Arduino protoshield and we were good to go!
So basically, we sample the audio via a first order low-pass filter (@500Hz) through the analog inputs of the Arduino, do some software magic (please contact us if you want source code or details), and finally light up the LEDs according to the music.
gotcha:
Initially, we experienced some really bad audible noises due to the PWM signal controlling the LEDs. This was because the default PWM frequency of the TLC5940-library was set to around 1kHz. Since we are using the the same power supply for the lights and the amplifiers, some bad noises first appeared roughly following the music beat. There might be more academic and electro-theoretical fixes to this problem, but we solved it by simply increasing the PWM frequency to about 16kHz, ie. above the speaker range (and above the audible range for most people).
Energy
The heart of Malen is a 7.2Ah SLA (Sealed Lead Acid) 12V battery from Panasonic. The important thing to keep in mind when choosing battery type is that it can handle deep discharges without being damaged (as in losing capacity). Regular “car batteries” are designed for being constantly charged by the cars generator. In previous attempts we’ve ruined a couple of these bastards so we know by experience. We’ve also ruined a pair of pants and a really nice wool pullover by not having sealed batteries.
Renewable
At Roskilde festival energy is rare, so i order to last the whole week without paying for battery re-charges we bought a 20W solarpanel together with a simple regulator to charge the boombox while the sun was shining.
The end
Well, there you have it. To road to the final product has been a long and tedious one with alot of pitfalls. Please let us know if you have any additional thoughts or ideas on how to take this boombox to the next level.