I found a discussion about 5909’s “passive” mode. They are apparently repurposing the USB-C connector in this mode to pass the analog signal straight into the headphones.
There’s more information about “passive” mode on pages 2 and 3 of the downloadable FAQ document.
Including that the headphones power-off when passive mode is initially detected. You can use the headphones like that, powered off. Or you can turn them back on and engage ANC.
HOW DO I LISTEN IN TRUE PASSIVE MODE (WIRED)?
For airplanes or analog listening, plug 3.5mm audio plug into headphone audio input to engage passive (wired)
audio. Headphones power off when audio cable is plugged in. Passive audio listening will work even if battery is
depleted. Microphone and buttons are disabled during passive mode.
NOTE: The No 5909 is only compatible with the Mark Levinson proprietary passive audio cables included.
DOES THE ANC FEATURES WORK WHEN USING THE ANALOG CABLES INSTEAD OF BLUETOOTH
WIRELESS?
Yes, if your battery is not depleted in your No 5909, the ANC settings that you selected as your default settings in
the app can be accessed when connected in a true passive mode (wired).
With your No 5909 powered ON, connect either the 1.25m or 4m, USB-C to 3.5mm proprietary audio cable
between your source device and your No 5909. Once the cable is connected, the power LED on the left ear cup will
turn off indicating that Bluetooth is no longer engaged and by pressing the ANC/AWARENESS button on the left ear
cup, you will cycle you through the default modes associated in the app.
DOES THE No 5909 HAVE A PASSIVE MODE THAT BYPASSES THE ANC AND DSP??
Yes, with your No 5909 turned OFF, connect the headphone to your source device’s headphone connection using
either the 1.25m or 4m USB-C to 3.5mm proprietary audio cable (included).
In this situation, all electronics; Bluetooth, internal amplification, and DSP are turned off and you