Portable, loud, and fashionable. Take a vintage suitcase and install a set of speakers, passive crossovers, an amplifier, and a couple of rechargeable batteries. Now, you have a portable, albeit heavy, stereo that will make your ears bleed.
I got the idea from a handful of Internet vendors that make custom, build-to-order jamboxes. They sell them for stupefying amounts of money, often around $800+ (U.S.). In some extreme cases the product costs over $2,000. I liked the idea and the sense of profit! I also had the know-how and decided to build one for half the cost.
The Critical Case
It was clear from the outset that the look of the suitcase was almost as important as how it sounded. After striking out at several thrift shops, I found that the best place to shop for vintage and unusual suitcases was online. I purchased a striped tweed suitcase from the 1940s that was to my liking and reasonably priced.
As an application engineer, I have access to the full catalog of Maxim Integrated parts and evaluation (EV) kits. I picked the highest wattage Class D amp, the MAX98400A. This part was appealing because it could be configured to work as a stereo amp with 20W per channel, or with minimal tweaking, it could be configured to work as a mono 40W amp. I really liked that the EV kit (Figure 1) had two amps on it with shared power rails.
This EV kit demonstrates that filtering the output of a Class D amplifier is not necessary because the speaker that it drives acts as a filter. So one amp on the EV kit is specified for filterless evaluation and the other has an on-board passive Class D filter. At first I thought that I would build a 2.1 system with one amp outputting stereo and one amp driving all of its power into a single woofer. After careful consideration I decided to go with a bi-amped design driving eight speakers. One stereo amp powered the two woofers. The other stereo amp went to a crossover and then to a midrange driver with two tweeters per channel (Figure 2).
The Audio Setup
Having rebuilt home audio speakers before, I selected drivers with two design considerations: size/geometry and look. I made paper templates to ensure that they would fit in my desired layout. Once the speakers were installed, I noticed that the walls of the suitcase could not support the weight of all the drivers. I then cut out the lining and glued in plywood to improve rigidity and support for the speakers. Next, I selected the passive crossover that made sense with the speakers’ frequency response.
I originally planned to lowpass filter the woofers as well, but listening tests indicated that filtering for the woofers was not really needed. I later confirmed this decision by the improved loudness of the unfiltered design. While a filtered scheme would have produced more balanced audio, I was not really targeting a “balanced” sound. I wanted loudness!
During assembly, I used 12-gauge speaker wire and banana jacks to connect the speakers and crossovers to the EV kit. Then I glued the crossovers to the suitcase walls (Figure 3). I soldered wires to the pins of the headphone jack connector, then heat shrunk the solder joint, and did the same to the terminals of my gigantic power switch. Don’t skimp on build quality!
The Power System
Each amp has an absolute maximum supply voltage of 28V, and 24V is within the operating range. I picked two UPS, lead-acid batteries for their high-energy density. Yes, they are heavy, but they are contained in a suitcase built to carry a modest load — weight is not a problem. With this much power the boombox will run at maximum volume for about five hours and much longer at more reasonable volumes.
The battery holster is made from plywood and pipe strap (Figure 4A). To manage the weight distribution, I placed the holster at the bottom and back of the case to counter balance the weight of the speakers. I cut out some suitcase lining to glue in the battery holster (Figure 4B). I also used an XLR port on the side to connect to a 24V battery charger and an on/off switch with indicator LED. Finally, I stuffed the case with polyfill which helps deepen the bass and reduce rattling (Figure 5).
Enjoying the Sound
From the beginning I had decided that the audio source from this boombox would be a 3.5mm jack. This way it could play music from smartphones and other portable devices. Then I ran into an interesting problem when using my phone which uses a jack-detect circuit. The first time I plugged my phone into the boombox, it was barely audible. I added a 32Ω resistor to ground on the amplifier’s input to the amplifier (Figure 2) to trick the jack detection circuit. Now when the smart phone checks the impedance during its jack detection, it thinks that it is powering headphones and puts out a larger signal level.
The boombox is loud and was really fun to build. And it sounds as good as it looks (Figure 6). But, I am not done. I want to try to create a better seal between the suitcase and the lid. Currently if you play “bassy” music at loud volumes you can hear what sounds like port noise. For now, the plan is to use something like thin foam strips, similar to weather stripping, and line where the two parts of the case meet. I also plan to add Bluetooth connectivity. The boombox already has style for miles, but ditching the headphone wire would really put it over the top.
|MAX98400AEVKIT||1||Evaluation kit with board for the MAX98400A Class D amplifier|
|WKA12-5F2||2||Werker 12V UPS batteries|
|260-198||2||2-way 8Ω 5,000Hz, 150W crossover, Partsexpress.com|
|264-809||2||3in full-range driver, Tang Band|
|264-805||4||2in tweeters ,Tang Band|
|295-332||2||8in aluminum cone woofer, Dayton Audio|
|2404SX||1||24V, 2A battery charger, Soneil|
|F3FSTF||1||Female XLR connector, Switchcraft|
|GRB292A201BBNA2||1||Rocker switch, CW Industries|
|30-711||1||.5mm female jack receptacle, Markertek.com|
|Builder’s choice||1||“Sweet-looking case”|
The Bluetooth word mark and logos are registered trademarks owned by Bluetooth SIG, Inc. and any use of such marks by Maxim is under license.
I’d like to thank Dominic Odbert for inspiring this design. To check out some of his great work, click here.
This piece was originally published on Imgur.
About the Author:
Patrick Gallagher received a B.S. in Electrical Engineering from Texas A&M University and joined the audio division of Maxim Integrated immediately after graduation, as a member of the technical staff. His work includes precision amplifiers, video amplifiers, audio amplifiers, codecs, and gamma references.