I've been meaning to post this for a while but haven't because I don't go on this site very often.
I posted earlier that attaching a microphone to the camera (T3i) will remove the AGC. This is not correct (ML -for T3i-cannot either change the AGC yet).
But I did find this. I tried it and it works very well
(copied from the second post in the thread, from user jordy2324)
"If you're using an external recorder (such as a Rode Videomic Pro) make sure to send a hot signal (that will be something like a +db swith or a +20db switch) to your camera. Then, adjust the audio levels on your Canon DSLR almost all the way to the left, meaning turning them a few notches from being off.
By sending a hot (high) signal from your external mic to a low gain level on your DSLR, you're avoiding the aforementioned [junky] camera preamps as much as possible. In my experience with the 60D and Rode VideoMic Pro, you can actually get FANTASTIC sound this way. On par with the Zoom H4N."