Pop quiz. What was the fastest-adopted gadget of the last 50 years? The Color TV? The mobile phone? The DVD player? No, believe it or not, it was the boombox.
Yes, the boombox. Pretty much everyone has had one at some point in their lifetime. Regardless of whether it played CDs or cassettes or mp3s, the basic idea has always remained the same: play back stereo music, be portable, and be affordable. DIYers, however, have never delved very deeply into the world of boomboxes, though. Well, why should they, right? I mean, we all know that the best value about DIY speakers is the ability to achieve sound quality of speakers costing several times as much. But to only focus on home speakers misses out on the other important value of DIY: it's supposed to be fun. So let's have a DIY boombox that's fun to make, and fun to listen to. It can be user-friendly, too. Just plug in your ipod or smartphone, and crank up the volume!
The cabinet for the Sprite can be built with either 1/2" or 3/4" stock.
There is a dividing wall through the center of the enclosure, separating it into two separate chambers, about 4 Liters each. At the back of each chamber is a 1 3/8" diameter vent that is 5.75" long. This tunes the enclosure to 63 Hz, and will get an F3 of 51 Hz, but--perhaps more importantly--a nice 3 dB push between 70-90 Hz, which gives this box a supple amount of "boom." I didn't put any polyfil in there, or line the walls, but you're free to experiment (remember, the whole purpose of lining or fill within a vented enclosure is simply to absorb any midrange stuff that might be bouncing around inside. If it starts choking off the bass, then you've added too much damping)
While we're thinking about the enclosure, it's worth spending some time figuring out how to make this thing portable. For a long time, mine was just a rectangular box you carried with both hands. Then, in perhaps my most inspired moment of industrial design, I realized out that the best place for a handle would actually be on the side of the cabinet--not on the top, as is traditionally seen in boom-boxes (and guitar amps for that matter). With the handle on the side, picking the thing up and moving it was completely intuitive and comfortable, and easy to do one-handed. Now, if you make your own, you're free to put a handle on it wherever you want, but who knows, don't be surprised if the new line of Beats or Jambox starting putting their handles on the side!
Now, the whole design of the Sprite is based around a single Dayton ND90-8 (or Aura NS3, which is the exact same driver). In essence, the Sprite qualifies as a "single-driver" design. Unlike many single-driver speakers, however, this uses a passive filter to contour the response. When I first started this project, I did try listening to the ND90 run full-range, without a filter, but I found the sound to be a bit congested in the midrange, and the bass was lacking. So I worked on a filter that would add a bit of BSC (thus giving us some more sweet bass), and also to adjust the mids a bit to make the whole presentation more clear and pleasing. It's a night-and-day difference, to my ears anyway, going from a sort of "cheezy computer speaker" sound to something passably hi-fi.
Since this is meant to be a self-contained speaker, finding and installing some sort of amplifier is a must. There are tons of great options out there nowadays. Not a lot of wattage is needed, either. I think something like a T-amp (Lepai, Sonic Impact, Dayton, et al) is a great match. Parts Express happens to carry a "board-only" version of the T-amp, and this is what I used. Figuring out how to mount the amp and its parts was actually pretty challenging, so feel free to use my ideas.
A much simpler solution for the amplifier probably would have been to use a Lepai LP-2020 amp, but I've had this little DTA-2 amp board sitting on a shelf for years, and I'd kick myself if I never actually used it. And honestly, now that I've figured out how to mount all the little bits, it wouldn't be that much of a challenge if I used it in future projects.
Fun! Big sound! Loud noises!
But seriously, I really like this little guy, and I'm very excited to share it with the world. The high end from a pair of 3" full-rangers is more brittle than what you'd get from a decent tweeter, but it seems to work well with the over-compressed mixes coming out of most people's ipods and smartphones these days. As said before, the midrange is pleasing, and actually fits in with the kind of voicing I tend to like on my speakers--OK, maybe with a bit more of a "smiley-face EQ" than normal, but still, this is a boombox we're talking about. And the bass, well, it rocks on this thing. They may just be a pair of 3" drivers, but given ample space in a vented enclosure, they are ready to party!
While I'm at it, let me take a few minutes to talk about boomboxes and bass. You might be wondering why there are devices on retail shelves that are significantly smaller than the Sprite, yet claim to "bring the party" or somesuch verbiage. In my experience, the Sprite and its 4 liters per woofer, is the smallest enclosure I can design that still gives enjoyable bass. I've tried all sorts of ways to cheat it and go smaller; my garage is littered with test boxes smaller than 4 Liters, each one hoping I can achieve the same satisfying bass. But each time I got smaller than 4 Liters, the bass just sounded too punchy, and lost that glorious "depth" people like to feel and hear when they listen to music.
So how do the retail boxes do it? Mostly they use some sort of Digital Signal Processing (aka "DSP"), or built-in EQ, or a combination of both. Oftentimes, they will introduce harmonics into the bass to fake our ears into believing they are hearing deeper bass than what's really there. My Sprite doesn't use such trickery because, honestly, it's too expensive. My goal here was to design a project anyone could do, using off-the-shelf parts; in this case, a woofer, a coil, a resistor, and the small amp of your choosing. These amps are pretty much all very simple: usually just gain and a volume knob--occasionally you get tone controls, but certainly nothing as fancy as DSP or custom EQ curves. Maybe someday there will be such amps, and they will be as cheap as the little ones you see used in the Sprite, but why wait? This is fun to listen to as it is! And part of the appeal to a project like the Sprite is its simplicity, so let's keep it simple. :-)
And I think you'll be pleasantly surprised if you choose to build a Sprite that so few parts, when used with a properly designed enclosure, can create a sound that will easily rival those boomboxes and sound docks costing several times more.
Next is a slick implementation by [fixthedoor]. He had the smart idea to mount the Lepai amp in the front, and made a small access door on top. Build thread here
Is your boombox made from old hockey sticks? No? Guess you're not as cool as [adam215]. He made this for his hockey team to help get pumped up in the locker room. He called it the "Locker Rocker." More here.