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!

Enclosure Design

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!

The Filter

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. 

 Frequency Response, with and without filter.
This is how I mounted the filter to a piece of pegboard.  No soldering required.  

There are two versions of the Sprite.  Originally, I only did a filter for the 8 Ohm version of the ND90 (because that's what I had on hand).  But after lots of requests (and people sending me the driver), I finally worked up a filter for the 4 Ohm version of the driver.  How do they compare?  Well, the 8 Ohm woofer only needs 2 parts, where the 4 Ohm needs 3.  Still, the cost difference is probably pocket change.  

On the other hand, the 4 Ohm woofer is more efficient--almost to a fault.  It goes LOUD, like "I can't hear myself think" loud; but past about halfway on the dial, and you become intimately aware of what Intermodulation Distortion is.  But what do you expect when you are asking a driver to not only play bass down into the 40 Hz range (with 4 mm of xmax!), and simultaneously play treble up past 16,000 Hz?  So horses for courses, I guess.  For most practical applications, especially in smaller rooms, I'd use the 8 Ohm version.  But if your application involves larger rooms or crowds of people and you want them to shut the hell up and listen to the music, then the 4 Ohm would do you right.  



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.

 Dayton DTA-2, mounted to pegboard.  I used a set of spring terminals to make it possible to quickly disconnect from the speakers, if need be.
 The business end of the Sprite.  I used a standard household wall plate to mount the input and power jacks (not included; had to buy from Radio Shack), and the power/volume knob (included with amp board). 
 This is the cabinet with its insides showing.  From here you can kind of see how I wired it up, and also how I was able to free-hand the receses for the rear-mounted drivers.  Notice the custom paint job, courtesy of my 2 year-old daughter.

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.

Listening Impressions and a few words on boomboxes

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.

Sprite Gallery

One of my main hopes with this project was that it would inspire lots of creativity and variations on the design.  So far, we're off to a good start!

First is a nice, pretty veneer job by [kevin007].  Nothing says "executive tabletop radio" like Bubinga.  Details on his build are in this thread.

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

Now, just because I made the sprite into a single box doesn't mean it has to be that way.  A few builders have gotten wise to this, and have built some petite bookshelf "cube" speakers using the Sprite filter and a 4 Liter box, such as [generic's] build here.  Plenty of boom and understated looks.

The Lola Redux, by [asmd] was a Sprite variation that took on many iterations, ultimately incorporating VU meters and a stunning Candy Apple red finish, amother other nice touches.  Additional build details here.

So there seems to be this new trend of people buying old luggage at thrift stores, slapping some drivers in there, and re-selling it for huge money as a boombox.  Problem with these is that there is zero attention paid to actual sound quality.  That's where [Don Radick] got smart, and incorporated the Sprite filter (and a bit of wall reinforcement) into his "boomcase."  Now that's how it's done right!

Taking things to a degree of precision that most of us will only dream of is [mountain] who used CNC machining to create a curved Sprite.  He made them for Christmas gifts for his children, those lucky kids!  The build thread is definitely worth a look, to see his mastery of machining aluminum and curving plywood, among other things.

I love how [dstmbgh] introduced his retro-inspired, "throwback" Sprite: This past Christmas my youngest daughter was home from college and informed me that one of her roommates had moved out, and now, when she and her other roommate wanted to listen to music on something "better" than their laptop speakers, they had to plug into the TV. Any self-respecting speaker-builder-hobbyist father can't have that, now can we?  Read on 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.

[bpmoose] built his as separates.  They use poplar plans stained in the traditional steel wool + vinegar solution.  His incorporation of the amp and volume controls is so classy.  More details here

[Carbon13] got his first taste of box joints with this Sprite he built for his wife in Cherry.  Beautifully done.  More info here.  

Going for "highest Spouse Approval Factor is [JDCrae].  These use a gorgeous Quartersawn Oak veneer with aniline dye to make the figure and grain pop.  You can see how he constructed them here.