I find the Sennheiser MKE 400 signal at the "+" setting just a little too low to be used with the 1, 2 or 3 rec level of the camera. I didn't want to add a third battery to the system therefore I had the idea to boost the signal using an audio transformer. My solution boosts only one channel following an old sound recording trick. If you set one channel about 6 dB lower than the other you minimize clipped audio because when one channel clips you will most probably be able to use the -6 db one. Since the 5D Mark II doesn't show audio level meters during video recording such a safety net is very welcome.
I have no experience with audio transformers so I virtually tried my luck and bought a specs wise good looking shielded 1:2 audio transformer from RS-Components. The part number is 667-6044.
The shielded Z21805C 1:2 (6 dB) audio transformer is made by Oxford Electrical Products. Here's the data sheet. It has a very wide frequency range and low distortion and it doesn't add any noise. I think any quality transformer with similar specs will do.
I also bought a ready made stereo 3.5mm plug to socket cable and used a short piece of both ends.
To my surprise this simple solution works very well. Soundtrack Pro reports a 5 dB difference between the channels. The noise floor of a recording in an absolutely quiet room is at -65 dB at both channels (using rec. level 1 of the 5DMKII) and is practically not audible. It's a huge difference to the annoying hiss in the former AGC only recordings. I have no means to measure the audio quality but I can't hear any difference between the two channels other than the volume. For many applications this quality will be good enough and double sound is no longer mandatory.
