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You may be getting just a little bit of drive from the A/V Output connection. The 5D Mark ii has no headphone connector. All it has is a TRRS (tip, ring, ring, sleeve) connector that happens to be the same size as a headphone plug. It is designed to feed an analog video feed out of the camera and in to a TV.
While there would be a drastic impedance ...
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In general, I'd say don't bother. Just do a bit of pre-recording testing to make sure your input levels are good. You're not going to want to tweak the input levels a bunch while you're recording, anyways. That would just be distracting to you and the other people that are recording.
But if you really want to, and you're on a Mac, check out "Line In" from ...
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