5DII : monitoring sound input

I have a 5DII, but have never seriously tried using the video capability. This morning, I connected my Sennheiser MKE 400 external mic, then my trusty old Sennheiser HD 25-1 headphones to the A/V out socket, thinking to monitor sound input.

As soon as I pushed in the headphone jack, the LV disappeared from the screen and I heard a loud, low frequency buzz through the cans. The buzz persists even if LV is disengaged and I try to replay a pre-recorded sequence with the mic disconnected.

Am I misunderstanding Audio out, or is the headphone 70 Ω impedence to blame? Is there some sort of work-around? Dumb questions, maybe, but a total newbie at this . . .

An A/V jack is not a headphone jack, and the 5DmkII has no headphone monitoring capability. You will need a separate pre-amp device like this JuicedLink:

http://www.bhphotovide​o.com …_Channel_DSLR_C​amera.html

Ah, gotcha. Thanks for the quick response.
I have a good friend on Oz who's a sound engineer (but asleep at mom); think I'll hassle him for a cast-off 4>2 mini-mixer.

AV outs typically have 3 connections + ground, and I believe it will only work for playback, so no real-time monitoring without some hacks or an external preamp / recorder in the signal chain.