In an effort to reduce clutter around my house, I got rid of a bunch of old magazines I had been saving. That left me with a number of Knuff magazine files from Ikea. Staring at the empty files one day, I realized the bottom of the files looked to be just about the right size for a speaker box. I did some math to see if they were large enough for some small speakers I had on hand. They were so I got right to work.
This was a very quick and easy project, I completed it in one day. The total cost was around $70. That may sound like a lot but I already had most of the parts on hand. There are certainly cheaper amps, speakers, and Bluetooth units available. I selected high quality parts for optimal sound. I think the results were worth it. But of course you can design your speaker box around any small speakers and amp.
- Table Saw
- 2" Hole Saw
- Wood Glue
- Hot Glue
- Soldering Iron and Solder
- Palm Sander of Sand Paper
- IKEA Knuff Magazine File $5.00
- HiWave BMR2 Speakers $12.00
- Dayton DTA-2 Amplifier $19.90
- Sure Bluetooth 4.0 Receiver $24.93 (optional)
- 12v 3A Power Supply $8.99 (if not adding bluetooth, 2A will do)
- Stain (optional)
- Wood Putty
Step 1: Design
These are the calculations I made for the HiWave BMR2 speakers I used. Adjust the values accordingly for your components.
First measure the magazine file. Each one I have is slightly different. We are interested in the inside dimensions so measure from the inside of the finger joints or just subtract 8 mm from the outside dimensions. My file was 23.8 W x 8.5 H x 13.8 cm D (max). That gives a maximum volume of just under 2.8 liters.
Next I used an online calculator to determine the optimal sealed box size for my speakers. I came up with 1.11 liters (1,110 cm3) per speaker. That is the internal volume we need for each speaker chamber.
Then I measured the amplifier. It is 7.3 x 3.3 x 1.5 cm. So with a little wiggle room, I need about 3.5 cm across the width to fit the amplifier module.
Now we need to determine how deep the box needs to be. My file has an inside width of 23.8 cm. But I need to add two walls, one for each speaker. So we subtract 0.8 cm for the two walls leaving us with 23 cm. Subtract the 3.5 cm we need for the amp and we are left with 19.5 cm of usable width for the two speakers. Divide by 2 (for the two speakers) and each speaker chamber now has a width of 9.75 cm. The height is fixed at 8.5 cm.
Finally we calculate the depth. We know the height and width (8.5 and 9.75), and we know the volume needs to be 1,110 cm3. So we solve for X to determine the depth.
8.5(9.75)X = 1110 cm3.
82.875X = 1110 cm3
X = 13.39 cm
So the inside dimensions for each speaker chamber will be 8.5 H x 9.75 W x 13.4 cm D.
Step 2: Cut
Now we can cut the file. I used a table saw but you can use whatever you have. In the last step, we determined that the inside depth needs to be 13.4 cm. Add 0.8 cm for the front and back walls and we can set the table saw to 14.2 cm. Place the bottom of the file against the fence and cut off the top.
Next we need to cut pieces for the interior walls. Break down the top piece. I set the saw at 4 mm and cut the sides off of the back leaving me with three pieces. Then cut two pieces to a final dimension of 8.5 x 13.4 cm. Cut two pieces of scrap to glue over the 2.5 cm holes in the sides. I was just a little short on wood for the back piece so I used a piece of 4 mm MDF I salvaged from an old nok-hockey game and cut it to 8.5 x 23 cm.
If you want to make the entire speaker box from one file, you can place both speakers and the electronics into one chamber that is 2.22 liters in volume. Then you would use a depth of 11.4 cm. That should leave you enough wood to make the back panel although you may need to glue two pieces together to make a piece that is long enough. I prefer to keep my speakers isolated from each other.
Step 3: Assemble
Take the small scrap pieces and glue them over the holes in the sides to seal the box. Measure 9.75 cm from each inside edge and make a mark. Use the square to extend the line. Put glue on the edges of the inside walls. Insert the walls at an angle and twist them into position. The walls go on the outside of the pencil lines so you have a full 9.75 cm wide for each speaker chamber. You should be left with about a 3.5 cm wide space for the circuit boards. Clamp everything together and leave it to dry.
Step 4: Drill
Draw diagonal lines from the corners to find the center of each speaker chamber. Use the hole saw to drill a 2 inch hole for the speakers. The speakers are spec'd out at 2" diameter but I find they are just a little larger. You need to file some relief cuts in the holes where the wiring terminals insert. I used a sanding drum on my rotary tool to open the diameter just a bit until the speakers dropped in.
Find the center of the case and draw a vertical line. Drill a 5/16" hole on that line for the volume knob and a 1/8" hole for the power led. Drill a pair of 3/16" holes through the interior dividing walls for the speaker wires.
Be careful of tearout when you drill the holes. This is not high grade plywood and it will splinter and delaminate easily. Don't panic if you get any tearouts. I had two on the front but I just glued the chips back in. Once they were sanded the repairs disappeared.
Determine the placement of your amplifier and drill a 13/32" hole for the power plug. I drilled a 9/32" hole for the headphone jack you won't use that if you are adding bluetooth.
Step 5: Stain
There are probably some gaps around the finger joints. Use some wood putty to fill them. Let it dry and sand it smooth. Then erase any pencil lines and sand the case. I used 150 grit to round the corners slightly. Then I lightly sanded the case with 220 grit. Go easy, the plys of wood are thin. The case is already sanded so it doesn't take much.
Once everything is sanded smooth it's time to add finish to the case. If you like the light look you can just seal it with polyurethane or your favorite finish. I opted to stain first; Golden Pecan by Varathane following the directions on the can. I tested the stain on the inside first as plywood doesn't always stain well. Then I sealed it with three coats of semi-gloss polyurethane. I put on 2 coats with a foam brush then lightly sanded with 360 grit before applying the final coat. The stain really highlights the finger joints and makes the box look much higher quality than it really is.
Step 6: Wire
The amplifier is powered by simply plugging the 12V power supply into the power jack on the amplifier. Route the speaker wires through the interior walls and solder them to the speakers. At the same time, solder the capacitors included with the amplifier across the speaker terminals. Seal up the holes for the speaker wires with some hot glue. Plug the volume control and led into the amplifier board.
You can skip the bluetooth and just plug your device directly into the audio input jack on the amplifier if you like. But I wanted the convenience of bluetooth.
To power the bluetooth module, we can grab 12v from the amplifier board. Check the picture for the location of the terminals on both boards and solder wires between the two.
There are several options for the audio signals.
- The easiest way is to get a 3.5 mm to RCA audio cable. Plug the 3.5 mm plug into the audio input jack on the amplifier and the RCA plugs into the RCA jacks on the bluetooth module.
- You could cut the 3.5 mm plug and wire off an old pair of headphones, insert the plug into the amplifier jack and solder the wires to the bluetooth module. On typical headphone wiring, the bare metal wires are ground, the red wire is the right channel, and the white wire is the left channel.
- I didn't want to rely on mechanical connectors so I chose to hard wire the audio cables. NOTE: The audio input jack on the amplifier is normally closed. You cannot wire the audio cables in parallel with the jack because the jack is grounded. The left and right channels will be shorted to ground. Cut the audio input jack off of the amplifier board if you are soldering directly to it. Then solder the wires between the amplifier and bluetooth board as shown in the pictures. The downside to this method is you lose the ability to plug audio sources directly into the amplifier. But I couldn't see myself doing that when it has bluetooth so no big loss.
Step 7: Mount
Screw the speakers in place with some wood screws. Pre-drill the holes to prevent splitting the wood.
Insert the on/off/volume switch through the hole and secure with the included nut. For me, that left the knob sticking a little too far our. So I cut a piece of MDF as a spacer and placed it between the switch and the case then used hot glue to mount the switch. The LED was simply held in place with hot glue.
The amplifier is glued onto the back cover and the bluetooth module is secured to the dividing wall with hot glue.
Step 8: Glue
Test your circuits to make sure everything works. I tested the sound with and without different amounts of polyfill. I found the best sound with the amount of polyfill shown in the picture (for each speaker).
If it all works, seal it up. If you don't want to glue things together, you could always add strips inside the case and screw the back on. I didn't want to lose any volume inside the speaker chamber to the strips so I chose to glue the back on. Plus, it's much easier to glue the back on and you get a tighter seal for the speakers. So as long as everything works, it shouldn't be a problem. Smear some glue around the edges of the back, put it in place, and clamp it.
Step 9: Finish
Add some rubber feet to the bottom to protect the surface and improve sound. You can even add some decoration to the side holes and make it look like they belong there. The holes are just about the right size for a bottlecap. I glued in my company logo in laser cut acrylic.
Step 10: Enjoy!
Connect the speaker to your phone through bluetooth and start streaming some music. The sound is pretty good. More than enough volume for a large room, crisp highs and decent bass. This one has found a permanent home in my office. I've gotten many positive comments both on the sound and the looks. Now what to do with the ten other Knuff magazine files I have?