Zoom Gear & Home Recording Forum

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.

Enjoy...

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

Trev, a great contribution! Unforuntately I'm not savvy enough with electronics to make use of it (though let me know if you ever start selling kits )

I'd been looking into DIY preamps on the web, but saw that they were beyond my electronics skill level, so I've ordered an Rolls MX34 mixer to use with my AT-822 mic. However, the 822 has unbalanced stereo output while the Rolls has balanced inputs.

I'm planning to make a pin-to-pin adapter for each channel out of the 822, though I get the impression that there are adapters that will give better results than a straight pin to pin. You seem pretty knowledgeable about this stuff, do you have any info about his?

Thanks,

Nolo

No idea about the At-822, sorry. I have some AT MB-4000 but they are balanced output.

To save each of you PM'ing me here are some piccys...






The construction is 1980's breadboard style on copper shim. The top flap of shim folds over the top of the circuitry to shield the caboodle when the box is closed and sealed. Note the 7.2V of batteries in the AA compartment of the SERPAC M4 box. The switch allows me to turn off the power to the microphone sockets (in an emergency) if need be. It is not for use in normal operation, and so is hidden in the innards of the unit.. The yellow building-blocks are the 220uF surface mount capacitors. Inputs are at the far end from the camera.

There is no power switch, as the batteries last for better than 300 hours, so I just put them in when I set up for the recording, and take them out when I break down the eqpt.

What more can I say - it just works like a charm... I am very happy with my Z2 now...


-- Oh yes, I could say "Engineer(s) Zoom, why couldn't you have designed a decent low noise external microphone input in the first place?"

Indeed - that does remain a total mystery, given the perfectly quiet operation of the preamps for the internal mics. Perhaps at one point in the development of it they weren't going to have an external mic socket at all?

Heh, if they could simply have provided the means to connect into the same circuit as the built in mics - and provided TWO external mic sockets - now that would have been interesting! And on the face of it, it could have been cheaply done.

But as it is, as a standalone point-and-shoot, it's excellent anyway.

Thanks for posting your design, Trevor!

Peter,
If I had to guess I would say it is an error. Maybe a noisy component (eg a carbon film resistor) put on a critical part of the circuit board (the electret bias resistors), something which wasn't caught when Engineering checked out the current manufactured assembly. It does seem to me as though, since they use the same Low Noise op-amps in the internal and external circuits, their mic inputs should be working better.

Maybe somebody with a little spare time could refer the Zoom Technical Support people to this thread and have them report the issue to the factory...

Hopefully they will have either spotted it themselves by now, or noted that "noisy mic preamps" get mentioned on almost every forum where "Zoom H2" gets mentioned!

I don't think they have spotted it. I have certainly had no emails or PMs from anybody trying to find out more about my testing, etc... Which I would have thought should be their first step.

No, you folk need to start making it clear that this is a problem. Even if they won't fix your current units (and Japanese companies tend to be very responsible when correcting mistakes) at least future units will have it fixed.

Is all the noise because the preamps are actually noisy, or is it an artifact of the high impedance of the mic input, which as I understand it is way higher than most mic impedances. Guitarfx has posted some on this topic, I believe....

Trevor, sweet preamp! Let me know when you start making them to sell....

Dunno why - I only know that I've not found, nor can I imagine, a mic that works as well with the H2 ext mic input as the built-ins. The only setting at which the noise seems to disappear is at "H" and then the input sensitivity is close to line level (IMHO).

The ext mic input at L setting is only about 10dB more sensitive than Line-in, and reasonably well behaved (it doesn't overload with motor-boating like the M and H settings). The noise floor is pretty low on L, but my preamplifier is still below the noise floor there, just as it is with Line-in...

The ext mic input impedance is 2800 ohms, which should still be able to give much lower noise with 200 ohm mics than we are observing. That's what leads me to believe that there actually is a component error in the external mic input circuitry of the H2.

Hello everyone - I tried Trevor's preamplifier, but i wasn't very content with the result. Now i found at http://sound.westhost.com/project66.htm the "Low Noise Balanced Microphone Preamp" - this works for me pretty good with two 9Volt-Batteries - the noise floor is for a dynamic microphone and the LINE-In of the zoom h2 between -96dB and -102dB which is for my purpose - recording voices in the street for the radio- very good. If you have any questions (e.g. how do I connect the two 9Volt-Batteries) or want to hear an spoken example with a dynamic microphone - just write me: axonZ@gmx.de