Hi folks.
I was in the same situation and I purchase the Zoom U-23, not expensive, around 85$ here in Switzerland.
For my tests I got my good old BMCC, the brand new preamp Zoom U-23 and a Beyerdynamic MCE72 stereo microphone. Connected with XLR - XLR (mike > preamp) and TRS (preamp > cam).
After just a few tests,
it works really well, level-wise. I didn't test it to preamp another device, for the sake of comparison.
Because the BMCC is generating a base hiss sound, we have to turn down to 20% input sound level on it. At 40% the hiss starts to be a little too present, still fixable in post but if you stay around 20%, you're good. At 90% level, the hiss sound reads -79 dB. Fixable too but the "fix-it in post" is our enemy right?
The zoom can bring up the level and the gain high enough to counterbalance the low input signal on the cam without creating ground noise.
- So for now and before more tests: I recommend
BMCC > MIC level, input level 20%
Zoom U-23 > volume and gain 8
Important: the sound quality (beside audio level or hiss sound) inside the BMCC is not great anyway. It brings up some mids and highs, the lows are a little weak. To test that, try to record with the BMCC with a preamp, a song you know well and then you realize how much it's not really good.
The good news is that you can fix in post (sometimes we have to do it) with a dynamic EQ or Fast Fourier filter.
Another trick: we can power the Zoom U-23 with the cam battery. I use a Globalmediapro Li95 and this adapter:
I put the Zoom, the battery inside a little wraist bag, the BMCC on a monopod (or any single tube) in an holster on the waist belt and and so I can shoot freely. Not that much shaky cam, looks good for a 1-man crew.