OK these are just beyond sick! Wireless Bluetooth version of my Resto-modded Koss DJ Headphones!
And they’re switchable from Bluetooth to wired connection! A removable cord is realized with a mini-DIN plug. The cord features a holder for the 1/4″ adapter, and a section of bungee-cord strain relief.
Before I gutted them, they were Koss K2+2 Quadrophonic headphones. They had two driver elements in each earcup, arranged vertically. I did not get to listen to them, they didn’t work, but from what I have read, the quad surround effect was underwhelming.
What they got going for my project is extra-large earcups! There’s plenty of room for installing Bluetooth electronics and a battery, and they have a large acoustic volume.
The stereo/quad switch just happens to be exactly what I need to switch between Bluetooth and wired operation!
I chose the Fiio BTR11 for the bluetooth headphone amplifier. It’s a cheap, but good headphone dongle. Importantly, it’s very simple, and has no display, and only three control buttons, which I will need to implement as mechanical switches on the outside of the Koss.
The board is freed from the enclosure and bodged into the earcups, along with some chrome-bezel switches and LEDs. The USB-C charging port sticks through an oval cutout. Soft silicone wires connect the switches and audio output to the board. Wires are silicone-schmooed to prevent vibration.
More detail on the battery and switches: I installed a much larger battery that should be good for about 40 hours play time on the highest quality setting!
And because I don’t like fire in my headphones, I bought a really nice name-brand LiPo battery. I’m not too worried about runaway heat, despite the closed environment. The circuit board does not get even slightly warm in use, and the battery charges slowly. Nothing gets even close to warm. I suppose the battery could die one day and become a problem but that’s true for every Lithium battery device.
Air gaps around the switch are sealed with silicone tape and caulk.
The electronics takes up volume inside the enclosure, so the same volume must be subtracted from the right earcup or it will unevenly affect the sound.
How to find the volume of such an odd shape? Millet and the Archimedes’ bathtub method! The millet is round and flows like water without being wet. With a little shaking, it fits into every crevice. (I didn’t actually need to weigh it.)
Now I have a volume of millet that is equivalent to the acoustic volume of the earcup. This volume affects the frequency response, especially in the bass region.
And then I need to install something in the other earcup to take up the same volume as the electronics. I stuffed the right earcup enclosure with layers of damping sheet material until the millet fills the cup perfectly. These two cups have the same acoustic volume.
Odd-shaped pieces of sound absorbing wool felt are tucked into every crevice of the electronics side earcup.
An equal mass of felt goes in the right cup. Half a gram of teased wool behind the driver.
The sides get a glitter-epoxy finish. Unfortunately I sanded down too far in one spot but I thought it looked kinda cool anyways, so I went with it.
USB charging port, rear buttons and DIN jack.
