Here is video example of that parasitic noise layer that appears starting from headphones volume 58 when use AKG k272HD headphones.
MixPre3II connected as sound card to computer. 40_Hz_-6dB.wav played back from computer.
Headphones connected to MixPre3II headphones out. I put headphones very close to smartphone microphone during recording to record that sound.
https://youtu.be/U2iDKTKM6dEUPDATE. I played back file from computer and also uploaded file to MixPre3II file system and play it back from there. USB powered or battery powered all exact the same. No any changes in noise sound.
I also found that in addition to headphones impedance, parasitic noise appears at different volume depending of different sound frequency:
40_Hz_-6dB.wav start to produce noise starting from headphones volume 58
100_Hz_-6dB.wav start to produce noise starting from headphones volume 62
Also today i had quick attempt to timecode jam MixPre3II with BMMCC. I still not sure if i do things right, but it seems it don't do nothing. I can not see same time indicated on the BMMCC screen. Should it show same time numbers as MixPre3II freerun when jammed?
I switched on timecode input in BMMCC settings, switched to BMMCC audio input to line in, volume set to 100%, switched both devices to same framerate. Have no idea what else i can do. Maybe timecode also somehow related to problem with headphones out? I don't know...
UPD. It appears that timecode is limitation of BMMCC itself. It can't Jam synk timecode. it can only record timecode to audio track when cable is connected.