I do the same with my GH2, feeding a LINE level signal from my MixPre to the camera through a passive BeachTek adapter, but the limiting factor in this setup is the RMS self noise-floor of the GH2, which is -63 dB with the gain set to it's lowest level. At some point I will get my hands on a Canon 5D MK3 so I can test to see what it's RMS self noise-floor is. It would be great if it's as good as the GH2 or better.
