DIY Musical Instruments: Experimenting With Piezo Contact Mics

I thought I’d share some awesome thoughts on piezo mics, compliments of my super-talented friend Brian. I recently asked him for a coles notes version of how contact mics work so I could better understand why my experiments were working well or not working at all. He uses them to amplify his double bass, and here’s what he said:

“I have used piezo mics these ways:

– in a copper sheet stuffed under the bridge foot

– in four little steel (or aluminum?) cylinders each set in a hole drilled in the bridge behind each string

– In a plastic coating wedged in the wing of the bridge

– In a plastic cube glued on to the bottom of the bridge

– In a stick, with a spike on one end, wedged between the feet of the bridge

What all this tells you is that piezos can detect through a variety of surfaces, though each will sound different.  There’s no way to know what will work without trying it.  All of those bass piezos sound different on any given bass, so there’s no way to say one of your ideas is good or bad or better than another without trial and error, unfortunately.

Another important consideration is impedance.  If you are thinking of a guitar amp as the primary way to amplify your stomp boxes, you are likely to have an impedance mismatch between your piezo and the guitar amp.  Piezos are very high impedance and magnetic guitar pickups (which guitar amps are designed for) are fairly low, comparatively.  It will still work, but some of the low end of your piezo will disappear and the equalization curve will be fucked in some way.  That’s why acoustic guitars and basses use acoustic specific amps (sometimes) or (more often) put an impedance adjusting preamp in between the piezo pickup and the guitar amp.

I use this box to match impedance. It also has a phase reversal switch and a high pass filter (which is an eq curve that rolls off lows below a certain point at a pre-defined slope).  You should try this with your stomps when they’re done, and see if it improves the sound.  Or, build a similar circuit right into your stomps.”

I was really confused reading the overwhelming amount of info on the web and found his simplified version to be very helpful. Hope it helps you as well.

Thanks Brian!!!

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~ by hooksandladders on June 1, 2009.

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