Introduction: Ammo Can Speaker Insanity: the MOAB
Warning!!!! This is not your everyday bluetooth speaker build...
We've been perfecting the art of portable music systems here at the Check 6 Labs for several years (home of AmmoAudio.com and our popular Ammo Can X, an ammo can speaker made out of a 50 caliber ammo can). We recently put together a build that's so over-the-top we call it the MOAB - "The Mother of All Bluetooths." Crystal-clear audio, big-time bass and a bluetooth range longer than a football field! We wrapped all of this into a 120mm mortar can designed with exquisite sound and maximum portability in mind. The ultimate bluetooth tailgate speaker has arrived!
Step 1: Design Considerations
We've already mastered the smaller form-factor ammocan builds and thought it'd be fun to put together a large, portable hi-fi bluetooth system as a challenge and to practice our craft a little more. Why ammocan speakers? Watch this and you'll understand:
Ammocans come in many sizes, and we chose what we felt would be the largest possible size that we could make portable. Ammocans are not exactly built with acoustics in mind, so they'll need an inner shell made of MDF to minimize vibration and provide a decent sound chamber. Unfortunately, none of them are big enough to present an optimal sound chamber for the drivers we wanted to use, but don't get too wrapped up in that - I've seen open baffle speakers that defy the laws of audio engineering ;-)
We thought we'd try to build these with parts we had in stock, mostly stuff we've been collecting for builds like this. We chose to mount the drivers back-to-back on separate channels to take advantage of the 2.1 config of the amp. This is neither bipole nor dipole configuration, which would have opposing speakers for each channel, the former wired in phase and the latter out of phase. While certainly unconventional by audio design standards, it was the only way we could stuff two of these awesome Eminence Alpha neo's into the can! The added bonus is that this pseudo- bipole config will give it 360° sound - not exactly stereo but very nice as a party box.
We needed a decent tweeter to compliment the Alphas and chose the Swan Audio (HiVi) SD1.1-A, a textile-dome speaker that is close in sensitivity to the Alphas. These tweeters are huge and heavy! So sticking with our pseudo-bipole design, the front and back gets one of each. But hold on there...this wouldn't be an epic build without a subwoofer, right?
We had a bunch of 2Ω, 4" Tang Band subwoofers. If we put them in series they'll be a 4Ω load on the amp, so two it is, each mounted on the opposite sides of the can.
We since we're powering this with two 12v SLAs in series, we needed a way to charge the batteries when the MOAB was not being used. SLA batteries like to exist in a charged state and running them down too much will cut their lifespan. We also needed an external antenna for the bluetooth as it's mounting location inside the sound chamber would not give it much range. Like our other designs at AmmoAudio, we included USB charging capability so we can use the batteries to charge our phones.
We also wanted access to the battery compartment which meant we'd need two chambers in the MOAB - one for the batts and one for the speakers.
Step 2: Parts
There's a million ways to skin a cat in a build like this, but the following gives you an idea of what parts we used for our version of the MOAB.
The choice is yours, depends on how big you want your build to be! We opted for anPA154 120MM Mortar Can, 32" Tall x 12" Wide x 6 1/4" Deep. You can usually pick these up at any military surplus store, gun shows or online.
For this ammocan, a 6" mid-bass is about the largest we could fit back-to-back with the inner lining. Might as well go with the best - Eminence Alpha Neo's rock! Great range and very efficient.
We had a bunch of these Swan Audio 1" dome tweeters, and it turns out they are a great fit with the Eminence Alpha's.
It's cheaper to build your own, but the ready made ones are not much more and save a bunch of time. We chose to cross these over at 3500Hz
Subwoofers: We used this one because we had a bunch in stock -- unfortunately it's no longer available. A sub is not really necessary and audiophiles will laugh at one this small. We're not audiophiles -- we just make really cool stuff -- but if you want added punch, add a couple of small subs.
There are a ton of Class D amps out there that would fit this build nicely. Because we opted to add subs, we chose this one with theSTA508+TC2000 onboard chips. This amp is very efficient and is built in the 2.1 configuration, which will give plenty of juice to the subs. It's rated for 18-34v, which is a good fit for our in-series 12v SLA's.
Again we went with what we had in stock. This board is a great little bluetooth receiver and with our hacked-in external antenna, we expect to have no problems with bluetooth range.
For the external antenna, we used a TP-LINK 5dBi Omni-Directional Antenna:
We used stock ones from our AmmoCan X builds. Any 12-24v USB port will work if you want to include USB charging in your build, but this is not a requirement.
1/4" and 1/2" MDF for the sound chamber
6061 Aluminum: .063" for the control and battery access panels, 0.625" for the wheel mounts.
Suitable wheels - we used 96mm All Terrain Wheels with bearings
3.5mm stereo jack
Suitable switches (we used an off the shelf missile switch for PWR and a toggle for the USB)
RP-SMA connectors and cable for antenna
Voltmeter - something like this: http://www.ebay.com/itm/Durable-DC-0-30V-Blue-LED-...
Step 3: Control Panel
A cool control panel is a must for a build like this, as there's a lot to control! We CAD'd up the design and cut it on the CNC. The back is lined with acoustic matting like the stuff used in car stereo installments. This dampens any vibrations from the switches and helps with the acoustics of the MDF box. If you don't have access to a CNC you could use a piece of ABS, MDF or anything that you can use to mount your switches and use to cover the inner sound chamber. From left to right there is the amplifier controls, USB charge port, Aux in port, fuse, bluetooth antenna jack and the volmeter and its corresponding momentary switch. Engraved on the panel are our recommendations for SLA battery charging.
Step 4: Power
The amp we used was rated at 18-34v. The manufacturer claims output of 80w each channel + 160W for the subwoofer @34v - yeah right! I learned long ago to take these specs with a grain of salt, but this is a powerful little beast that will fit this build nicely. We opted to use two 12v/10ah SLA batteries in series, which when fully charged get up to as high as 13.5v each. The amp will have up to 27v to power it, not optimum but certainly plenty of voltage to allow it to put out a great sound.
We considered using three 11.1v lithium batteries packs for this, but the only really affordable ones are those wrapped-in-blue ebay specials which in our experience do not live up to their advertised amp capacity. We've also had lithiums combust in the lab on two occasions and figured the safest way to go would be sealed lead acids. LiFePO4s would be a great option, but while they are extremely safe, they are very expensive - two 12v LiFePO4's would have set us back about 200 clams and defeated our goal of using parts we had in inventory to build this thing. In our experience, SLAs have the best bang for the buck and will give the MOAB hours of play time.
While the amp is powered by the 24v+, the Bluetooth board and the USB charge port are both 12v. A voltage stepdown module would have worked nicely, but we believe in keeping our designs as simple as possible. The solution? Route the amp to the two 12v's in series and run the bluetooth and USB off of one of the 12v SLA's. We fused the amp with a 10a fuse and the Bluetooth and USB with a 5a inline fuse. There are basically two wiring harnesses in the MOAB, a 24v and a 12v.
We opted to put the batteries in the bottom of the can in a separate compartment with an access panel to keep the weight distribution down low and allow us to service the batteries if required. This of course required a badass cover for the panel, which we mounted with a piano hinge and a couple of latches to keep it closed.
Step 5: Fabrication
Once we figured out the driver mounting configuration, it was time to start making chips fly. The tweeters and minisub holes were cut with a bimetal hole saw and the woofers with a Jasper router circle template. Unfortunately we didn't have a good way to clamp the ammocan down to the CNC table, so we opted for hand power tools. Ammocans are made from heavy galvanized steel, which makes them much tougher to machine than softer metals like aluminum and brass. A good bimetal hole saw will cut it, but you're in for a fight - go slow and go at a slow speed with your drill.
The 6 1/2" holes for the woofers were a bit of a challenge. We used a Bosch router and a two-flute, 1/4" carbide mill bit at the slowest speed the router would allow, about 8000rpm. Way too fast but when you're building something epic you just need to suck it up and press on. I'll throw in a disclaimer here:
Cutting steel with improper tools and improper knowledge can lead to serious injury
Make sure you take proper safety precautions and use heavy gloves, eye protection and long sleeves! Those little chips that spin off a router at those speeds are hot and hurt like hell. Ask me how I know :-)
Step 6: Sound Chamber
Now that we had the ammocan fabricated, we can start fabrication of the sound chamber. We built our box with 1/4" and 1/2" MDF and routed the edges so it would slip into the mortar can with ease. Box building is a great way to bone up on your woodworking skills but it's beyond the scope of this Instructable. I'll leave details of doing it up to the woodworkers out there, and there's a ton of forums and Instructables that deal with this art. You need to have a fair amount of precision in this step along with the proper clamps to bind the sides together properly and get a tight fit. Just make sure you drill some vias for whatever wiring system you end up going with!
Once finished, slip the MDF box into the ammocan and check for fit. There is no need to glue it in, the speaker mount bolts will pull the box tight against the sides of the can. Once fitted, trace the circles we cut in the previous step so you have a reference for cutting the holes in the exact spot in order to fit the speakers. We used the same hole saws and circle templates we cut the ammocan with to cut the holes in the MDF.
Step 7: Mount the Speakers
With the MDF sound chamber clamped in place inside the ammocan, mark out the holes for the speakers and drill them through both layers of the MOAB. Get some mounting bolts that will fit through the can and the mdf with room to put on a lock nut. We CNC'd some speaker grills matching our logo to offer a bit of protection for the speakers and because, well, crosshairs on ammocan bluetooth speakers look badass!
Step 8: Wire It Up
Take your time and map out how you're going to wire this thing, and cut your wires much longer than you think you'll need. You'll want enough slack to allow you to remove the control panel if need be and enough to reach the battery bay, plus some!
For the MOAB we need hot wires from the 10a fuse and amplifier going to 24v in the power compartment, and hots from the 12v bluetooth board and USB going to a single 12v battery via an inline fuse. We also need wires going from the charger port to the 24v in-series batts, as well as wires to the speakers from the amp. We'll keep the 24v and 12v grounds seperate. Pretty simple really!
For the charger we used a 3-pin XLR male and female. You only need two of the pins for this, one + and one -. The charger is an 24v/2a XLR, easily found on eBay unser "scooter chargers." This should charge up the SLA array in about 2-3hrs., assuming we don't let the battery voltage to drop below our own voltage recommendations on the control panel :-)
Step 9: Add the Wheels
To maximize portablity, we added a couple of all-terrain rollerblade wheels with bearings. We machined a couple of wheel mounts that we secured on the back side at the proper level to insure free movement.
Step 10: Test It Out!
First thing to check is that you have voltage. 24v + on the voltmeter shows all is well. Next check 12v into the USB port and Bluetooth switch. All LED's are lit so this project's a GO!
We made a few videos of the MOAB in action. a couple are fairly long as we were testing the bluetooth range. The song is "Turn Down the What" by Lil Jon. It's heavy on the bass so we thought it'd be a good test of the Midbass and subwoofers. These youtube vids don't do the MOAB justice - this thing sounds phenomenal, very clear highs and whopping bass, very high-end!
1. First test
2. Bluetooth range. 300+ feet of awesomeness!
You can see it starts cutting out a bit past about 100yds.
3. Wheel test while playing:
Step 11: End Game
We hope you enjoyed our little instructable, and hopefully we inspired you to try to build an epic masterpiece like the MOAB. This thing will be a big hit at indoor/outdoor parties, the beach, tailgates or campsites, and is portable and rugged enough to take on the road. The bluetooth range with a simple external 2.4ghZ omnidirectional antenna is huge, the longest we've seen here at the Check 6 labs! And the sound? We'd put this up against anything out there, even our popular AmmoCan XLT! I will warn you this is not a cheap build, and the labor that went into this was incredible. Fortunately we have a group of very capable guys here at the Labs that love music and love building over-the-top stuff like the MOAB!
Next up, after we celebrate the completion of this project - portable ammocan guitar amps for our version of Johnny Ramone's guitar! Visit us over at AmmoAudio.com for updates!
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