DIY contact microphone

Question

Hi!

Using a contact microphone for field recordings seems to be popular and there's loads of tutorials for building one on the web.

I bought the parts and am thinking of the adapter and amplification I want to use. I've got a Zoom H2 recorder that has a line-level and mic input.

Question:

• What kind of amplification does a contact microphone need to be able to connect to a line-level input? Or could a simple 3,5mm jack mic input provide enough gain?

• I also understood that piezo's have a unnatural frequency response because of their high impedance. Will a line level preamplifier be the easiest way to amplify the signal with a more natural frequency response?

Thanks.

Answer 1

Here is a nifty preamp circuit that provides a FET buffer and balances the signal, which deals with all the issues you are discussing. The circuit is based on the work of Alex Rice.) You attach the piezo to the circuit with a length of shielded mic cable, then run a mic cable from the circuit to a male XLR. This plugs into your recorder which supplies phantom power to the circuit. (It's shielding, not grounding per se, that controls noise, so the circuit needs to be shielded too.) It's super-quiet and can drive a signal over a very long (100'+) cable, which can be helpful in some field situations. If you are not confident with reading circuit diagrams, there's a kit you can get here (disclosure: I make this kit). There's a super-detailed instruction manual on the same page that you can download that will show you exactly how to assemble the circuit; if you are handy at "bread-boarding", you can make this from your own parts and skip the kit.

At R8 we usually use a 680 Ohm resistor, but this may vary depending on your particular FETs. Try resistances from about 400 - 1200 Ohm to get the perfromance you want.

Answer 2

It's not a matter of gain, it's a matter of impedance. Piezos have a very high output impedance (actually a capacitive impedance), so they need to see an even higher impedance at the input of your preamp, or they'll be prone to picking up interference and having low frequencies rolled-off.

(A 500 pF pickup, for instance, has a 6 MΩ output impedance at 50 Hz, so even if you plug it into a "high-Z" 1 MΩ guitar input jack, it will be attenuated by -17 dB at 50 Hz. At 1 kHz, however, its impedance is 300 kΩ, which is only a -2 dB attenuation. That's why the low frequencies disappear and they sound tinny if you don't use a very high input impedance.)

Just an FET-input buffer will do (input impedance ≈ 1,000,000 MΩ). It shouldn't need much gain. Piezos can produce very high voltages on their own.

Answer 3

assuming you've bought a piezo element and are looking to connect it to your portable recorder:

• just wire the element to a jack and treat it as a dynamic microphone - i.e. no phantom or plug-in power is required, but you need to stick it into a microphone input and will need to turn up the gain quite a bit.
• you will run into response curve problems more due to the way you're attaching the element to the surface you want to record, so i'd say it's safe to postpone this worry for later :)

one thing you might want to try is to wire one end of the element to both tip and ring (left+right), then after recording sum the channels to mono - this may yield better results in terms of noise.

p.s. have fun destroying the mic =) with some care when re-soldering naked piezo elements have more than one life.

Answer 4

Great answers so far! One thing you need to be VERY careful of is the potentially high output of the piezo! Under normal circumstances, a piezo element will output a decently normal level when connected to a mic pre. However, if you drop it, flick it, or hit it hard in some other way, many piezos are often capable of outputting over 30v! That can blow up a pre if you're not careful!

I've had a lot of luck improving the overall frequency response of my contact mics by terminating them 1/4" and plugging them into an inline impedance matcher, then straight into a 744T. Might also be interesting in playing with plugging one into a DI box. However, I'd be VERY interested in hearing the results of connecting one into a variable impedance mic pre (I used to own one of the ART preamps that had this function, but I since upgraded to my Chandler Germanium...)

Another possibility (which I've thought about doing but never done): if you have Logic Studio, there is an EQ plugin called "Match EQ". Quite possibly, if you could find a surface that resonates at a fairly flat frequency (maybe an NS-10?), you could shoot pink noise through it, record it's response, and then match that to a direct measurement of the pink noise with the EQ (internally in the software). This would account for the abnormal frequency response. You could then save your new EQ setting and apply it in post to every recording on the mic, which would essentially "decode" your mic. Of course, all this is assuming you can find a surface with a fairly flat response.

I have other ideas up my sleeve about how to accomplish this with Match EQ if anyone is interested... let me know.

Hope this helps!!!

Answer 5

Thanks, the software EQ "decoding" seems like an interesting idea. I'd like to hear more ideas on how to record a good flat reference EQ impulse. I don't own the NS-10's and my Genelec monitors' body seem too sturdy to vibrate well. Or do you think I could get a good "natural" frequency response by laying the piezo on the Genelec's wire mesh and recording the noise?

Hardware-wise, I was alright with soldering a piezo and a plug, but I'm not sure I'll understand the schematics for building a FET buffer.

A second question concerning voltage is about Phantom power and piezos. I was going to try and amplify a piezo's signal with my MOTU Ultralite's 48V phantom mic pre, but somehow that didn't seem like a good idea, just by my instincts. The piezo plate and wires I made is completely open so I guess my instincts said 48V isn't a good combination with this.

How about it? Any more scientific explanations concerning the use of piezos with phantom power to amplify the signal? Naturally phantom power gives additional volts for a mic and it won't change the frequency response, or will it?

Answer 6

im diggin with mine contact mic now...

can anyone say what can i do with mic noise? grounding?

btw i tried phantom power too and it worked good through XLR. it gave needed amplification, but noises...

Answer 7

This is not actually related to your contact microphone project, but the external input circuitry on the Zoom H2 is noisy to a degree where the dynamic range achievable on it is smaller than from the internal microphones before you even connect anything. So you might want to consider recording options with better input circuitry.