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Piezo characteristics and problems

I'm not sure where this should go So I'm starting it here. If y'all think it belongs under another thread please point the way.

I was going to keep this to myself and try to market a board to solve some of the problems but decided I'd just as soon make it public and if there is enough demand maybe proceed with a design. I've already built one board to test with and initial testing looks like I'm on the right track.

From what I've read, people are unhappy with two characteristics of Piezo pickups.

(1) Feedback and runaway squeal?

(2) really nasty sound when overdriven.

I'm going to address issue 2 first as I feel like i have a better hand on this. Although, I think issue 1 is related to issue 2.

I picked up a Radio Shack 273-0073 to experiment with. First test I hooked it up to an oscilloscope (instrument to visually display the relationship between Voltage (and possibly current) with respect to time. I removed the piezo element from the plastic housing and laid it flat on a table top. I connected the wires to the oscilloscope and pressed the piezo element down on the table top, then lightly tapped the table with a spoon.

Ding! 3V peak output. 3V is equivalent to a Humbucker output. Most inductive pickups are much lower, typically around 0.5V.

So the Piezo has a high output voltage drive (with little current as it has a high impedance).

Next I struck the table a smart rap with the spoon.

Output went right off the limits! I re-adjusted the scope and tried again. 20Volts peak! Damn! These things can overdirve the hell out of amp inputs.

In fact, some Piezo elements can produce several thousand volts when properly configured and are used to ignite propane grills.

So, what to do?

Most people are dampening the response of the element by mounting it with softer adhesives, and carefully placing them near the bridge and away from the more resonant back panel (I believe this to be the current best case recommendation, yes? no?)

Limit the output when heavily driven to (1) protect the amp inputs, (2) limit the overdirve to produce more pleasant sound.

How do you limit the output? Clamps or compressors.

The simplest clamp is a pair of diodes in parallel with the piezo output.

A better clamp would be to add a resistor in series with the diodes to limit the current.

Even better would be to buffer the output, them add the clamps, follow it with a buffer.

Finally, a compressor that detects the output level and gradually decreases the gain of the amplifier to control amount of compression.

Right now I have a TL082 with the first op amp configured as a gain of 0.5 (yes,negative gain since the output of the piezo element is so strong). The output of the first stage drives a 10K resistor, then a 10K resistor to two switching diodes (1N914) to the ref level. The connection of the two 10K resistors is the output and goes to a 1K resistor to two 2.6V zener diodes in parallel but cathode to anode and anode to cathode. These also tie to the ref level for the op amps. FInally the second amp (TL082 has two op amps in one 8 pin package) is used to buffer the output.

My initial test showed that this does indeed provide some soft clipping.

Next up is to build a LM380 power amp and mount the piezo element under the bridge of my two string slide I built last night.

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Permalink Reply by toasterburn on July 6, 2009 at 2:38am

on my last build I tried leaving it in the case, screwed to the wood inside the tin. sounded fine to me. I experimented with putting it different places inside the box and got better results screwed to the wood near the bridge. wish I had taken a video of it before I sold it.

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Permalink Reply by Sven Pfitt on July 13, 2009 at 5:10pm

Yup, day late and a dollar short.

I suspect the issues have been addressed by mechanical mounting techniques.

I posted my start of this thread after reading a lot if intro stuff (2002-2004) , but before finding a lot of late stuff (2007-2009). As an engineer I would like to understand the mechanics behind the resonance issues. As an EE I tend to look for and electrical/electronic solution rather than a mechanical one.

Right now I"m off on a tube design tangent. I've built two CBGs and have the second tuned Drop D (DADG). So I'm trying to learn to play it.

Sven

Michael Puckett said:

I am building my first CBG myself and plan to use a piezo in it as well. The site contains a lot of piezo information which I have been reading up on but have to admit that I am a bit confused by what seems to be conflicting information though it is probably just me.

What I find confusing is the seeming conflict between hard or soft surrounds and attachment methods. Some seem to prefer a hard attachment/surround such as hot glue or gorilla glue. Some seem to prefer a soft attachment/surround like silicone caulk or the rubberized tool dip. The Tesla by Ted Crocker seems to use a hardwood surround coupled with a soft, spongy sticky tape attachment.

So my first question would be, is the attachment method solving one problem and the surround method solving another or are they both addressing the same? For example does a soft attachment reduce the "tinny" sound and a hard surround reduce say the peak output?

Sven, it seems that you are proposing to solve the same problems electronically that are being solved mechanically by one or both of these methods above.

I guess I have some characterizations to do with my own piezo pickup.

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Permalink Reply by Brian Lemin on July 14, 2009 at 8:56pm

I have discovered that this web site: http://www.windworld.com/products/catalog.htm#smallelec Is selling Piezo in various sized strip forms.

I wondered if anyone has tried this as I thought that sticking a strip on a bridge might be quite effective?
Any one tried? Anyone like to offer an opinion?

Thanks

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Permalink Reply by Randy S. Bretz on March 26, 2012 at 1:17am

This is a film piezo on my whiskey box bowed dulci, it works by pressure on it by the bridge. The sound produced from it awesome. Very crisp and clean. Only problem that i don`t like is it can`t be cut to make it shorter like a rod piezo can be, but i`m very happy with the overall sound.

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Permalink Reply by Glenn Reither on March 26, 2012 at 1:30am

Yeah, the films need LOTS of pressure. I can see how sitting a bridge directly on them would achieve that. So on a lid they need to be sandwiched on really tight. I have a couple I haven't been able to use because I can't get enough out of them but will try putting them between bridge and lid. Thanks Randy!

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Permalink Reply by Tom Manno on November 13, 2009 at 7:36am

#2 - "sounds nasty when overdriven." How did you do that? That's how I like it, nice and swampy :)

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Permalink Reply by Glenn Reither on March 25, 2012 at 6:04am

I use the Tillman preamp design with some extra stuff on the input stage to even out the variance in FETs and a trim pot before the output to bring it down a little if necessary. I use 25k pots for tone and volume, 0.047 cap for tone. So far as mounting piezos goes, I've tried a few things, but now I just cut out a circle of ply slightly bigger than the disc, supa-glue the piezo to the underside of the lid in the bridge position (maybe just slightly forward and slightly toward the bass strings), run hot blue around the edge of it and sit the ply circle on top, then run hot glue around the ply edge so the disc is completely sealed off. Quick, easy, sounds good, no feedback.

Using two piezos in parallel will halve your impedance, which can't be a bad thing. Just splice red to red, and black to black.

I prefer the larger size discs because you get a more even response across all strings, but I guess if you use two discs parallel the small ones are fine. Mount them side by side and they cover the whole bridge width.