Somebody asked me about a low noise preamp design for the Zoom H2. Here is what I have built for my own use. I fitted it into a SERPAC M4 case with two SAFT lithium 14250 CR2 cells (for 7.2 volts nominal). You can use standard 9V batteries, if you wish.
First, building this preamp is not for somebody who has not built lots of electronic thingys before. I constructed mine upside-down breadboard-style on thin copper shim which I wrapped around the electronics after stuffing everything into the M4. Maybe somebody will do a PCB, I didn't have the time spare (it took longer than I thought to get it designed so that it would cover the entire dynamic range of the H2 line input).
The schematic is at URL http://TrevorMarshall.com/ondio/H2_LoNo ... mp_TGM.pdf
This preamplifier was designed to feed the H2 Line-in socket, and it has a noise level below that of the H2.
When you plug in a low-output microphone you can hear the noise from the microphone preamp (if it is a condenser) when your H2 headphone volume is at maximum, and you are listening intently. Most microphones will need a fairly high level to overdrive the H2, but I recommend using a power-though attenuator, like the M-Audio In-Line 10dB PAD, if you are going to be in sound pressure levels greater than 110dBA with a 200 ohm -45 dB re 1V/Pa microphone, or 100dBA with a 200 ohm -35dB re 1V/Pa mic. A 600 ohm mic will handle about 4 DBA higher SPL as the preamp's gain is a little lower from a 600 ohm source. At these SPL the microphones will be driving the H2 to its full available dynamic range (assuming they are low-noise microphones).
Those of you who want to check my PSpice simulations before constructing your own unit, my ORCAD project files are at URL http://TrevorMarshall.com/ondio/H2_project.zip
For the 22u input capacitor I used two 47u low-leakage surface mount tantalum electrolytics back-to-back with a 0.1 ceramic across them to deal with any electro noise. The output capacitors can be of your own choice, 10u is plenty big enough. All resistors MUST be metal film types.
When you plug in the preamp it does not add to the H2's noise floor in any way, which can sometimes cause low level whistlies. Once a mic is plugged into the preamp the A-D converter settles down, however. There is an additional filter on the power to the microphones. I used a resistor to keep the voltage applied to the microphones below 5V, as some can't take more than this. Check the specs of the microphones you will be using, they may take 9V quite happily. Nearly all distortion is 2nd harmonic (supposedly desirable tube-style 'warmth').
Due to the way I designed the input-impedance-setting feedback circuitry, this preamp will work fine with cheaper Electret condenser microphones rated at "typical 2K ohms impedance". It will also work well with 50 ohm microphones, as the equivalent input-referred noise voltage has dropped to 1.7 nV/sqrt.Hz at the 50 ohm source impedance.
The 3dB point is below 30Hz, and you can increase C4 to a 330pf if you are more worried about RF than an upper 3dB point of 100KHz.
ps: since the auto-magic forum link-maker provided an incorrect link, a proper link to the attenuator is URL http://www.m-audio.com/products/en_us/M ... -main.html