Sennheiser HD400 Instructions For Use Manual page 50

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Headset HMEC 302:
Connect the 1 / " (6.35 mm) stereo jack plug for the headphones and the PJ-068 jack
4
plug for the microphone to the corresponding jack sockets of your intercom.
HMEC 305 headset:
Connect the XLR-5 plug to the aircraft's XLR-5 socket.
Headset HMEC 305-C:
Connect the XLR-5 plug for the headphones and the microphone to the aircraft's
XLR-5 socket.
Connect the XLR-3 plug to the XLR-3 socket.
HMEC 306 headset:
Connect the 6-pin Redel plug to the corresponding socket in the aircraft.
HMEC 322 and HMDC 322 headsets:
Connect the U-174/U jack plug for the headphones and the microphone to the
U-174/U socket of your intercom.
Connect the XLR-3 plug to the XLR-3 socket.