Choosing a headband

I attempted this exact project ~3 years ago and struggled with the ergonomics of the head band. I used an old V-Moda headband for initial prototypes, but did not enjoy the crustiness.

The AIAIAI headbands are readily available and very simple. Unfortunately the nature of their adjustment mechanism means this project requires PERMANENT modifications. It’s worth it though, I think they fit the aesthetic very well and I appreciate the inclusion of 3.5mm plugs for easy integration.

Interfacing with the User

Thanks to the simple construction of the AIAIAI headband I could use a consistent 8mm hole spacing to mount the “wishbone” earcups to any OEM setting, though I did have to drill additional holes higher up on the headband to get things to sit right on my head.

I recycled the 8mm hole spacing to create a mount for the DAC. This can be mounted to either side of the wishbone to support cable entry on the left or right ear.

There is a small interposer block between the wishbone and headband which gives ~10mm extra spacing.

The outer-most component contains a housing for the BMR breakout board & constrains both the BMR rotation mechanism & the headband. Looks clean too 🙂

I added some EVA foam to the wishbones for a little extra comfort.

The foam is affixed with standard super glue. Honestly it’s a bit too firm for my tastes, but it does help a bit with pressure points.

If you have any suggestions for better foam types please get in contact.

The DAC/Amp

This modified version of the features the following:

• Female straddle-mount USB-C connector
• RP2040 uC
• 2x MAX MAX98360A I2S Class-D Amplifiers
• 4 big ol solder pads

Originally the design was intended to have a male connector but I oopsied and selected the wrong one. WHOOPS! Works better for this project anyways.

I completely redid the layout to incorporate 4 layers because time is money & I am a sucker for good power distribution, even if it is unnecessary.

BMR Connection

I’ve seen a lot of janky ways to interface with the Index BMRs. If you look inside your index strap you’ll see a few copper pads that interface with the pogo pins on the speaker assembly. So I decided to mimic that!

These breakout boards are extremely simple, just a couple solder pads & corresponding pogo pads. The pogo pad shape is designed specifically to allow the solder pads to point in all 4 cardinal directions w.r.t. the arm of the speaker in neutral position. The reversible design allows you to swap polarity without having to re-solder, though admittedly that feature is unnecessary.

I gold plated these boards because it looks cool, but also something something good non-corroding surface finish or whatever.

Crossover Connection

The AIAIAI head straps include 3.5mm TRS plugs on either side of the headband.

Conveniently i had some 3.5mm jacks which just barely fit the solder pads on the breakout boards mentioned above.

In this configuration the tip is ‘hot’ and the sleeve is ‘cold’. This choice is arbitrary, but seemed to produce good results so I haven’t bothered swapping it back.

While I loved how slick the straddle-mount USB-C connector was, it just doesn’t make sense from a manufacturing cost standpoint. I have replaced it with a more standard USB-C connector 🙂

Previously to re-flash the RP2040 you had to precariously short two pads while simultaneously plugging the USB cable in. Didn’t work too well, so now there’s a convenient little switch to hold down!

To allow solder-less use I included a Molex Picoblade 1x4p connector. This is a SUPER common connector so you can buy off-the-shelf wiring assemblies or even pin your own out!

The back side has some features too!

I retained the big ol’ solder pads for the amplifier outputs for anyone who still wants to manually wire things in.

I also included VCC & GND plane connections for external power injection. The purpose of this is for power-limited applications such as use with the Bigscreen Beyond HMD.