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Another Stab at Contact Mics

Posted by Tom Benedict on 07/05/2017

Several months ago I ran a bunch of tests on a contact mic to try to figure out the best (ok, best of several options) way to interface them to my recorder. It wasn’t the most exhaustive test, but it at least pointed me in a good direction.

Specifically I wanted to find which configuration had the best low frequency response. To that end the mic under test was a 35mm piezo element stuck to a subwoofer, and the tests consisted of running a frequency sweep through the subwoofer while I recorded the output of the contact mic.

I tried four configurations: 1 – direct TS plug-in, with all the high pass filtering that leads to. 2 – HOSA MIT-129 impedance-matching transformer. 3 – Alex Rice impedance matching preamp. 4 – Stompville impedance matching preamp.

It wasn’t the best subwoofer in the world so the data was lumpy, but a couple of things came through pretty clearly: Not surprisingly, the direct-plugin lost some of the low-end. The Alex Rice preamp is known to have noise issues, which also came out in the tests. As far as signal to noise went, the HOSA MIT-129 and the Stompville preamp were very close across the range of the test (10Hz-1kHz), but the Stompville had higher overall signal and required less gain out of the recorder.

My point in running these tests wasn’t to prove once and for all which setup was the best. It was just to get some direction for how to build my next round of contact mics. I decided to go with the preamp from Stompville and run with it.

Before going into just how I ran with it, here’s some background on the Stompville preamp: SmudgerD started the same place I did, with the Alex Rice preamp, but instead of just being frustrated with the noise floor, decided to redesign it, and then further refine it. The result is a very well thought out design. Even better, SmudgerD packed the whole thing onto a tiny PCB that fits on the back of a Neutrik XLR panel jack.

I bought two.

(But I wish I’d bought four.)

Then I stuck them in a box.

Contact Mic Preamp Box

The bottom of the box is tapped 1/4″-20 so I can attach it to the top of my DR-70D using the camera mount thumbscrew. Two of the Stompville preamps face out one side, and as soon as I buy another two they will face out the other side. The third face has a row of mini-XLR connectors for plugging in the contact mics themselves.

I was turned on to the mini-XLR connectors by Mo, who shared them in a comment on my EM-172 and XLR Plug article. I have to say these things are the cat’s pajamas. They’re small, they’re inexpensive, they have a positive lock, and they have the neat feature of having pin 1 make contact before the other pins. This ensures ground makes contact before the signal pins. I love ’em.

Unfortunately, right after I finished the box I was drawn into other things. So I only have two contact mics built so far. The first is the one that’s still attached to the subwoofer. The other is a prototype I built to test the preamp box in the field.

Prototype Contact Mic

It’s ugly, but it works remarkably well as long as you don’t apply any pressure on the center of the piezo. (This introduces a large DC offset.) The next rev, of which I’m building four, will be fully enclosed.

Just for grins I took the whole mess in to work to play.


– Tom

One Response to “Another Stab at Contact Mics”

  1. […] finally finished installing the other two channels in the impedance-matching preamp box I built back in July for plugging contact mics into my recorder. The amplifiers I used came from […]

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