Boombox, part 1
With my summer vacation now about a week away, I’ve already spent the better part of july planning and gathering some parts for a boombox project. Despite I have no solid design yet, the general idea is to end up with a portable audio system that outperforms the bog-standard crappy plastic speaker replicas. Something I can take with me to cater a decent-quality soundtrack for a sunny summer day outdoors doing absolutely nothing (besides maybe getting some tan on my moobs). The overall plan is (as usual), ” stick with what is available for free” and make it a fun project without stressing about appearance. Basically I’m fine with whatever, just as long as it can handle outdoor use. Shouldn’t prove too difficult with these specs ;)
The early stages of this project revolved around yours truly thinking what to do with a 11″ speaker and a 55Ah car battery. Spending a cabin weekend with a bunch of friends early june, we decided to hook up the car battery to a Harman/Kardon Go + Play boombox to save up on battery costs. The HK box uses the eight of the big D-type batteries (8*1V5 = 12V) and according to spec this gives you around 18 hours of playing time. Thus you could easily end up using three sets of batteries over a weekend, adding up to over 40€ plus a neat pile of problematic waste on the landfill. I knew for certain that the car battery should have no issues powering the boombox all weekend despite being drained round the clock.. And quite so it did! All this got me thinking about a boombox project of my own.
Now, using a car battery will allow for greatly extended playing time (or higher power amp solutions) but it’s also packs quite a bit of weight! At the cabin, there wasn’t really much need to drag the HK + battery combo around. But if I want to take something similar with me on foot, the weight instantly becomes an issue. Needless to say, the part about “carrying” got ditched right off the bat. Instead I turned to “haul”, namely in the form of a beer trolley. This allows for a heavier enclosure too. And what the heck, if it’s going to be a trolley thing the speaker elements could be bigger than on the average boombox.
In case you’re not familiar with the concept of a beer trolley, I’m talking about a cheapo metallic two-wheel shopping cart. This is something finnish people (stereotypically) use to haul excess amounts of beverage cases ashore after taking one of the 24-36 hour (beer) cruises in the Baltic Sea, usually all for the sake of saving some cents on the price of a beer can. Nevermind paying for the trolley or the cruise leveling costs.. Anyway, sad fact is that after this single use, most people hardly ever find any other use for the trolley and these perfectly usable items get binned. With this in mind, finding one in the dumpster was just a matter of time.. Soon enough, I had a car battery with wheels. It was time to start figuring out how this project turns out.. :)
For the amplifier, I was at first looking into some DIY power amps and various schematics. However after spending a while browsing all kinds of solutions, I stumbled across some relatively cheap chinese whatever-brand stereo amps on Ebay UK. There were multiple devices available around the 20€ mark (shipping included), all with output power around the region of 20W RMS per speaker. Surely something like this is enough for a boombox, so I figured maybe I’ll just cut corners here and go with something readymade. As I have very little expertise with power amps, building one myself would’ve surely meant spending time to study the topic, in turn “halting” the project for some time. If the amp proves underpowered, having it outside the enclosure as a separate module means upgrades will be easy later on.
Shortly evaluating the Ebay selection, I then placed an order for something called Lepai LP-168HA. Besides being able to drive the 2*20W RMS (2R to 8R load), it also had a separate 58W output stage for a subwoofer (adjustable crossover + volume) and a USB port for powering/charging compatible devices. Whereas subwoofer output sounds something like the 11″ speaker might like, being able to charge portable devices might come in handy while out and about.. Sure, for 20€ I’ll give this Lepai a spin :)
Whilst waiting for the amp to arrive, I figured maybe I’ll see if I could perform a bit of speaker analysis. The only thing I know about the 11″ speaker is its DC resistance (2,6 ohms). No brand markings or anything. So maybe, if I’m able to map out some parameters (known as Thiele/Small) for the speaker, I can also try properly calculating the volume of the enclosure. This is usually measured in liters (or cubic meters) and getting it “just right” is required to squeeze out the best the speaker has to offer.
So, with a bit of digging, I found Purebits’ Sample Champion. This program is a multipurpose tool for all kinds of audio analysis tasks and with a proper adapter, it can also calculate the Thiele/Small parameters. The analysis is done by feeding test signals through the speaker. The connection adapter for the task, no problem..
First, Sample Champion needs to be calibrated with the soundcard. A low value resistor (I used 7R6) is attached to the cable clamps and its value is input to the ‘calibration resistor’ -field (above the red box on lower right on the picture below). I forgot this about it at first and thus had to go through the whole setup/speaker measurement process twice. Here’s a analysis graph for the soundcard I used for measurements (M-Audio Delta66):
After this, clicking ‘Calibrate now’ will correct (zero) the curve and the actual speaker measurements can begin..
With these and a few additional values, the T/S parameters can be calculated automatically. Or well, they COULD be if I had had the full version os Sample Champion. Just so as it happens, the T/S calculation is disabled from the trial version. Not that I even thought of checking the feature list in detail prior to starting with the analysis. Full version 199€ and 24€ for the T/S plugin. Mmm, crap.
Sure, speaker parameters would’ve been a nice extra but I’m not going to splash out the +200€ (or so) just to get this single speaker analyzed. So, I’ll default to “whatever fits the trolley nice” then! If the boombox ends up sounding horrible, maybe I’ll just invest the money saved (from buying Sample Champion) into some beers and make it “sound better” this way. After all, that much money should get me about the same amount of beer as the granny in the pic has.. ;)
After this bit of meh, fast-forward to monday this week. LP-168HA in the mail!
The first impression sure was “spartan” to say the least, as the amp came with nothing else than cellophane wrapping inside a mailer bag. After unpacking, first things first (of course), open up the box just to have a look what’s inside. Given the price of this amp, checking the electronics for build defects is a very recommendable action. And what do you know..:
While fixing the heatsink mounting, I also replaced the thermal paste in-between. I don’t know what the stuff used in these cheapo devices is, but it sure doesn’t correspond at all with what my experiences of thermal paste are. What this amp had, resembled more something like skinned up silicone by touch, but which still came of clean with a single wipe. Whatever it was, I doubt the chips would’ve lasted much use had they been left as-is. After this small improvement, it was time to hook up the first test setup!
Without any enclosure, it’s impossible to evaluate the amount of bass. The driver sure moves like it’s doing a lot of work and some low frequencies can be heard. But, knowing how drastically the sound of even a small speaker changes once put into a enclosure, I’m not going to pass judgement on this 11″ just yet. Furthermore, with this setup the crossover dial of LP-168HA seems to work more as a combined volume + lowpass filter than crossover.
No idea how the 11″ can be even made to fit the trolley in any clever way. Looks like I just have to start sketching and building the actual enclosure.. :)