Introduction: How to Make a Loud Bluetooth Enabled Mobile Speaker With Subwoofer

My sister asked if I could build her some kind of mobile "boombox". As I always build my speakers myself I thought, well, how hard could it be?

I´m aware that you can buy many bt speakers with reasonable sound quality but all share the same drawbacks: they are expensive, small and can not be used for a bigger audience in free air situations. Or at least I could not find any.

The requirements were:

  • Loud
  • Good sound quality
  • mobile with at least 5 h play time
  • bluetooth interface
  • pink
  • 300 € price tag (approx 322 US$) (+/-)

Sure, short list, should be done easily.

First up I started to think of a concept. Most people I found online were using some cheap car hifi speakers but that wasn´t enough for me. I already built 2 speakers in the past which I knew sounded great and were reasonable priced so I chose between them. Additionally I wanted to have a little more bass than you could put into a small cabinet so a subwoofer was a must - and a bigger cabinet. Bass frequencies and small enclosures don´t fit together. And I did not want to blow up the mid bass to make it "sound" like a bigger speaker, it should be have a straight frequency response characteristic over the whole spectrum.

It´s not easy to find a cheap subwoofer chassis without the need for a big box.

On ebay I found a digital 3-channel amplifier where the variable subwoofer cutoff frequency and volume could be set to fit the characteristics to the main speakers. Unfortunately it did not run on 12V. I tried but it did not turn on at all. Since I planned to run everything on ONE affordable 12V deep-cycle lead-acid battery (and not two, which would add to the weight of the speaker with another 7kg) I needed to add a so called step-up-converter I also found on ebay.

As the speaker should be some kind of "idiot safe" (means it should not be possible to kill the battery with deep discharge and I wanted to have a display where you can read the capacity that´s left on the battery I found two circuits that were great to add to this build.

The complete list of materials I chose:

  1. Battery level meter with LCD display, 6,96 €

    That will display the percentage of battery capacity that´s left before recharging is necessary.

  2. 3-channel digital amplifier, 17,52 €

  3. Bluetooth module, 8,98 €

    In the final build I chose a different one because the one I ordered first did not work on my 3.3v step-down-converter (maybe needed 4V). So I took one with an included 3.3V voltage regulator that worked perfect.

  4. Car battery charger, 24,32 €

    Of course you can use any charger but I wanted it to charge in a reasonable time (4A charging current) and not 24h or so so this was the cheapest I found with AGM capability.

  5. Step-Up-converter (12 to 24V), 6,99 €

    This is the regulator I found with the highest output current (10A). I don´t need 10A, but it´s better to have some not used power left.

  6. Deep-charge protection module, 7,95 €

    There are several protection modules on ebay, too. But I wanted to solder some DIY kit so I used this one. It works fine. But at one small disadvantage. It has a reset switch. So whenever the module turns off the power you need to push the reset switch. AND if you turn it off and on again. So double "turn on" action.

  7. Battery 12V/11 Ah, 29,90 + 4,90 €

  8. Subwoofer chassis, 33,00 €

  9. Speaker, 55,55 € x 2

    The speaker pair I chose was a "broadband" speaker kit I had used in a speaker called "Cyburg´s sticks". They sound awesome and can play really loud. Sure, since it is not 2-way (or more) the heights can be a little harsh but they are the best bang for the buck I know.
    If you don´t like the sound of broadband speakers you could use a 2-way speaker instead. But also difficult to find good sounding ones for a reasonable price. I would take these:
    It´s sold from a small hifi shop and I´m blown away from their performance. It´s easy to change the build, just put it into an closed box and everything else doesn´t need to change.

  10. Wheels, 13,88 + 4,90 €
    Of course you don´t need wheels. I assume it is not good for the electronics anyway to roll the speaker around on not so smooth surfaces. But it may be convenient to place it within a room without lifting it. At last the speaker will weigh almost a ton.

  11. Wood, MDF 16mm around 20 €, and some scraps and screws

So that´s it. Hope you enjoy the build and take some use out of it even if you don´t build it yourself or modify it. That´s how it´s meant to be. Cheers!

Step 1: Build the Enclosure

I chose to build the speaker into some more interesting shape than a simple box. But that´s no problem at all if you decide to make it for example square.

Unfortunately I lost some of the pictures and only have the sketchup draft and some final pictures left. I´m sorry. This may happen only two or three times, I promise. But you already know how to build square boxes from MDF I assume, so give it a try.

I will describe it anyway with help of the sketchup model.

Step 2: Cut the Parts

You will need the following parts (if not mentioned otherwise in MDF 16 mm):

  1. Top & bottom 2x 426,6 x 359 mm (45 degrees bevel on both short sides)
  2. Sides 2x 176,3 x 359 mm (45 degrees bevel on both short sides)
  3. Diagonal sides 4x 125,7 x 359 mm (no bevel)
  4. Front 1x 574 x 322 mm (speaker and sub woofer reflex port are cut ot from this)
  5. Back 1x 424 x 322 mm
  6. Subwoofer sides 2x 312 x 322 mm
  7. Subwoofer vent top 1x 274 x 262 mm
  8. Subwoofer vent bottom 1x 312 x 262 mm
  9. Speaker back side (inside the box) 2x 140 x 322 mm
  10. Backpanel for electronics 1x 138 x 320 mm (Plywood, aluminium, cardboard 3-4 mm max)

  11. Strip of 15x15 mm (more or less) hardwood for mounting the electronics panel
    cut to 2x 66,1 mm (45 degree bevel on one side), 1x 174,3 mm (45 degree bevel on both sides)
  12. Strip of waste material for backing up the battery (approx 100x160+ some 15x15 strips, depends on your battery and size of it
  13. Screws if you like it sturdy (and did not cut exactly to size :)
    Be warned: screwing into MDF can´t be made invisible later. If you put varnish on even with most careful preparation of the screw holes they WILL stick out. So better work carefully and don´t screw up :)

Step 3: Front

First you cut out the holes for the speakers. If you like (but it does not make a difference) you can recess the speakers into the MDF to get a flush surface. You will need a router with a dado bit and a circle cutting jig then.

Otherwise you can drill the holes to fit the speakers you chose. I recommend to cut the bass reflex opening later with a flush trim bit to make it perfectly straight rectangular. If you don´t have one you need to be super careful to cut it out with a scroll saw or a jig saw. Mark the opening exactly in the middle of the front and cut it out.

Don´t mount the speakers yet - but you can dry fit to be sure everything is fine and drill the holes for the screws to mount the speakers later. On the DIY kit there were plenty black screws to use, but you can use any screw you have that will hold the speakers in place.

Step 4: Subwoofer Enclosure

Then you glue the subwoofer cabinet. Get one part no. 6
and lay it flat on your workspace. Mark where the vent top and bottom go (parts 7 & 8) and glue them flat on. Then put on the other side (no. 6). Align if necessary and screw the sides (don´t forget to counter sink the screw holes and pre-drill the holes for the screws). MDF does not forgive and will split if you don´t pre-drill the holes. For example if you use 4x50 mm screws, use a 3 mm drill bit.

Step 5: Align the Front Panel

Glue on the front panel. Align the sides of the subwoofer enclosure so it is as square as possible. Fix with clamps and let it set. If you like to screw this on make sure the screw holes get pre-drilled as well and countersunk to have a tight fit.

Let it dry over night. If you used screws this is not necessary.
At last use a flush trim bit (drill a hole big enough in the middle of the vent port to take the router bit and bearing) and cut out the vent port. Round over the edges from the outside.

Step 6: Speaker Back

Put on Parts No.9 on both sides aligned 126 mm apart from the front panel.

Drill some holes for the speaker and subwoofer cables with enough room to fill in hot glue for sealing. Make the holes big enough to feed the cables through without using force. You may put the cables in now. I chose to drill the holes near the top of the cabinet so the cable will not "pull" on the connectors causing trouble later.

Step 7: Bottom

Now put on the bottom of the cabinet (part no 1). I used screws to fix them into place because you can´t see them later anyway and I wanted continue to put it together as it dried. But as said, glueing the rest of the speaker will result in a better look, depending on the finish you want to apply.

Step 8: Back Panel

Add the back (right side of it), part no. 5.

Step 9: Battery Holder

Depending on what type of battery you use you cut some scraps into size to fit the bottom and sides/top of the battery.

To hold it in place I used two strips with two holes each and screwed them tight so that the battery can not move at all even if you put the speaker upside down.

If you want to reduce the impact on the battery you can put some rubber spacer pads below and between the strips and screw them on now.

Since my design included some vents into the electronics panel I needed to cut a beveled edge on one side of the battery plate.

Step 10: Sides and Top

Continue with the diagonal parts no. 3 and glue them to the beveled edges of the bottom part.
It will stick a little bit out which is no problem for now, it will be sanded or cut flat later. Just in case you wondered what the red triangles are. If you are a ninja class woodworker and have a first class table saw you can saw them with the double bevel before glue up. Then you do not need to sand them flush (destroying the surface of the MDF) - this will result in a smoother finish without flat spots. MDF is really smooth on the flat sides but it will suffer if you try to sand it. Put on the sides and other diagonal parts, finishing with the top. String everything together with some ratchet spanners to make the glue joints sturdy and squeeze out excess glue. Clean up now with water or after a short time with a chisel. Do it because you will need to sand it away later ruining the surface of the MDF!

If everything fits perfectly let it dry over night and tadaa, the cabinet is finished.

Step 11: Fit the Electronics Panel

Now glue on the last two strips cut to size so it will sit approximately 16 mm away from the back edge of the cabinet.

Cut the electronics panel a little bit undersize (I chose 1 mm from each edge/corner) and drill every hole to mount it to the strips later. Pre-drill everything below.

I used some M5 screws and since I used hardwood strips (oak) I pre-drilled with a 3.8 mm drill and tapped with M5 directly into the wood.

Step 12: Lay Out & Build the Electronics Panel

Depending on what electronic parts you want to use or have at hand you need to design your own panel. I´ve included the shape and layout of mine but I assume depending where you live it may be impossible to get the exact items I used. So have this in mind and start building the panel after you bought all parts you want to use.

If you want to build cheap without additional fail safe functionality you even can start with an amplifier and on-off-switch without any other parts. Will work, too.

Add Bluetooth if you like, but you also can use a 3.5mm headphone jack to connect your mp3 player or phone.

Old school baby!

After I cut all holes I painted the panel, too. I used a brush, that´s why it looks so awful, and several coats of paint and clearcoat to make it look a little bit better. Before I put on the second last coat of clearcoat I put on the laser printed labels I made and "glued" them on with a coat of clearcoat, sealing them off with two final coats of clearcoat. Remember not to sand in between layers after the labels are glued on.

After that I fit all electronic components and did the wiring.

Step 13: Mount All Electronic Parts

I made several connections to the modules and the battery. I tried to solder only the cables that don´t need to be changed later.

To the cables that lead to the battery terminals, the the power switch and the fuse holder I crimped some flat connectors. I already owned a cheap crimping tool which works better as I excpected. After crimping just to make sure I put some solder on the connector to prevent the cable from slipping out.

All other cables got soldered onto the PCBs or tinned to connect them with the screw terminals on the modules.

I tried to use red wires for + connections, black for - (ground) and other colors for switches. Audio in/out/ground got red/white/black. LED connections get red/black, too.

First I connected the battery + to the fuse holder and inserted a simple 10A car fuse. The output goes to the power switch and from there to the step-up-converter module (IN +) and the battery level meter (IN+).

The bluetooth module (AUDIO OUT) connects to the amplifier CINCH-Input-Connectors. I used some cheap cinch connectors to solder the cables to and plugged them into the back of the amp.

I used a 3.5mm headphone jack (stereo) to connect to the bluetooth (AUDIO IN) so it will be possible to connect any device with a headphone out port (no bluetooth) to the speaker box.

All ground connections are connected directly to the GND-pole of the battery, except the audio signal connections to and from the bluetooth module, they are connected directly to the module GND where necessary.

Step 14: Fine Adjustments

I tested everything out and made sure the battery protection works. To do that you need to set the "cut off" voltate to approx. 10.5 volt. This means that if the battery capacity drops below 10.5V the protection circuit shuts off the power through a relay switch.

After that you need to recharge the battery and push the reset button. I checked that every light and switch worked and put the panel into place.

Step 15: Build the Speaker Cover

To protect the speakers from damage (kids like the speaker cones) and maybe from idiots hitting the speaker with their foots :) I made a protective cover from baltic birch I had left from an old table. I cut out with some space (2 mm on each side smaller than the cabinet inside) and painted it black.

I used four plugs I don´t know how they are named, you can get them at any speaker trader or wherever you buy your hifi stuff.

The fabric I used is not any "hifi" approved fabric but I had it laying around and didnt want to spend another 15 or so € for special audio fabric that may not alter the frequency response if you know what I mean. But the acoustic fabric looks way better and is available in several other colors. I chose black for the obvious reason: it was free.

And it does not look this bad.

Cut it to size, wrap it around tight and clamp it with panel nails. Cut off the recess in the back and put it in.

Step 16: Sound Check

Finished. Connect your phone through bluetooth or audio jack input with a cable and get the party started.

Hope you enjoyed the build. Sorry again that I build the speaker without taking pictures ... will not do it again, I promise.