I love my old Bose Quietcomfort 15 headphone, so much that I decide to add a Bluetooth receiver to it to keep it in service for a couple of more years. During the process, I needed to DIY a short audio cable to connect the headphone and the Bluetooth receiver. I couldn’t find any information about the pinout on the Internet, so I disassembled the original cable to find out the schematic.
After poking around, I drew the schematic below, it is pretty straightforward. The HI/LO selection is done by two pairs of resistors. I can also directly connect the L/R of the audio source to the R/L pin of the headphone side, and the audio still sounds fine. But I’m not sure if this will cause any harm in the long term, so I would recommend keeping the 57.8ohm resistors at least.
QC 15 audio cable schematic
Both ends of the cable are 3.5mm plugs. The one on the headphone side has 4 segments, while the end on the audio source side has 3 segments. You can also use a 4 segment plug on the audio source side and potentially add a microphone to the extra pin.