The most painful part of listening to music everywhere you go is the inability to share it properly with others. We've tried earbuds, portable speakers, large portable speakers, cars, and beatboxing. Each has an ideal situation where it really shines, but no one solution transitions from the individual listening experience to the group very well. So just like a belly button, we need something that can go from outtie to innie on command to meet the necessities of the situation. Innie/outie headphones.

This is the main Instructable for the outie/reversible headphones project. A finished sub-Instructable on the parametric CAD bent wood lamination part can be found here.

Step 1: Cannibalize Ze Speakers (and Electrical Planning)

Take the speakers apart. The 4x screws that hold them together can only be accessed once the speakers are expanded. Remove the innards, being careful to not break any of the small wires. Measure and model the interfaces needed later for CAD.

The X-Minis have several inputs/outputs for music:

external 1/8" male audio input (white cable)
mini-USB female jack input (for charging and audio through the provided cable)
1/8" female audio output (for daisy chaining the output of one speaker to the input of the next)

Each of the X-Minis normally act as their own speaker, with their own power switch, volume, and audio input. As headphones, it would be ideal at all times for each to share the same volume, audio input, and power switch. The cable that comes with the X-Minis connects a USB Type A and a 1/8" audio input to the X-Minis with a 2x USB-Mini male connectors through a volume knob (see the picture with the black X-Minis).

This configuration works well for a pair of headphones: a master volume switch, shared audio source. The downfalls are no shared power switch (I decided to just let that be) and the USB-Mini inputs for audio are a little bulky and unsightly. To fix this, I decided to eliminate the USB-Mini male plugs from the cable, and solder the internal wires directly to the inside of the speaker. Additionally, decided to remove the audio output jack to save space, and to remove the volume dial on the speakers themselves so only the volume knob on the cable is used.

Step 2: Replace the Volume Potentiometer, Remove Audio Jack

Each speaker has a variable potentiometer with 3 leads to control volume. As you rotate the dial, the resistance between A-B and B-C change (but A-C remains the same at 20 kOhms). Since we want to vary the volume on the cable instead of the individual speakers, we want to replace the variable potentiometer with 2 resistors, that to the speaker look like a volume knob always at max volume.

Remove the circular volume knob (I used a solder removal tool).

Then solder 2.2 Ohm and 20 kOhm resistors in the proper positions. Try playing music through the speakers to make sure you wired things correctly (if nothing plays you probably reversed the resistors and it thinks it's at minimum volume).

Remove the audio output jack using a solder removal tool as well.

Step 3: Splice in the Audio Cable

Now to eliminate the bulky USB-Mini cable head from the system.

Cut the USB-Mini plugs from the cable. I tested continuity between each of the wires (gold, red, and green) and the USB pins to identify which cable goes to which pin (as show in the sketch).

Very carefully solder each of the wires from the cable harness to the appropriate pins on the USB-Mini female connector on the speaker board. Once done, test your work by playing music through the speakers. If it doesn't work, check continuity between the connections you made. The pins are so close that if you weren't careful enough, it's likely you inadvertently soldered 2 together.

NOTE: I actually had to undo this step and thread the cable through my 3D printed pieces, then resolder for the final headphone. Allowed me to have a cleaner cable routing scheme.

Step 4: Headphone CAD Creation

Using the measurements from the speaker disassembly, I designed new 3D-printed backplates for the speakers so I could connect the headband with the X-Minis nicely. Considerations and features for the CAD:

Use the original screws and mounting holes for fixing the speaker to the 3D printed piece
Make sure the screw pattern and headband connection location results in the proper speaker orientation
Create an outlet from the speaker internals for the cable harness
Rotating joint between headband and headphones to allow speakers to rotate from innie to outie
Mechanical stops so the headphones rotate only to the proper positions
Connection channel for the headphones (decided using hot glue in the end would be fairly robust and let me disassemble if necessary)
Attempted to match the style of the speakers and headband

The headband CAD will be addressed in the next step.

Step 5: From CAD to Existence

I used an Objet Connex 500 to print both headphones from Vera White. If you happen to have access to one of these beautiful machines but don't have any idea how to use it, here's a great guide.

The headband is enough of a project in itself I decided to make another Instructable for it. Made it by creating a bent wood lamination using CAD, a CNC router, and a laser cutter.

Headband parametric CAD bent wood lamination

Step 6: Assembly

Couple quick pictures of final speaker assembly. Close attention to detail in modeling the interface between the new backplates and the existing speakers really paid off. Everything fit juuuuuuuust right.

Hot glued the headband to the headphones and wiped away excess glue.