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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 Slug on June 25, 2009 at 5:04pm

Ya lost me about halfway in.

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Permalink Reply by Sven Pfitt on June 25, 2009 at 5:09pm

Bottom line is the Piezo drive is too strong. I'm trying to limit it in a graceful manner.

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Permalink Reply by Slug on June 25, 2009 at 5:14pm

I don't know about others, but when it comes to electrical resistors and capacitors and things of that manner, I just get completely lost.

I need a visual representation. Like a diagram or a picture of the actual wiring of the piezo. Etc.

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Permalink Reply by Michael Puckett on June 25, 2009 at 6:22pm

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 June 25, 2009 at 8:22pm

Quote:
"By simply using a moderate amount of hot glue on each side, all the negative properties disapear and you get a nice, natural acoustic sound if properly placed."

"Hot glue" on the actual disk or on the surrounds? A spread or a blob of glue? Do you use the hot glue to attach it to the sound board?

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Permalink Reply by Michael Puckett on June 25, 2009 at 9:04pm

Ted,
Have you experimented with a softwood surround? Does it give you the isolation needed for clarity? I am thinking something really soft like balsa wood or did you find that a hardwood was necessary for the surround?

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Permalink Reply by Doc Oakroot on June 26, 2009 at 12:17am

Since a piezo has high voltage and high impedance (=low current), wouldn't it benefit from a transformer? Could you solve the impedance matching with that?

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Permalink Reply by toasterburn on June 26, 2009 at 4:01am

I've taken a few electronics courses and I have experience working with an oscilloscope, so I followed most of what you were talking about. Good luck with it, I'll be interested in seeing how it turns out.

I can say what works for me, since I make my guitars with tin bodies I do things slightly differently. I use a silicone glue to mount the piezo to the wood, directly under where the bridge will be but on the inside of the tin. Then I put a layer of electrical tape on top of the piezo so that it is shielded from the tin body. Then I use glue on top of the tape to adhere the whole thing to the body.

Whatever the reason, I get a great clean sound from it and haven't noticed any feedback. And overdrive sounds good too, in my opinion.

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Permalink Reply by Sven Pfitt on June 26, 2009 at 2:12pm

If you could find one with a 20:1 turns ratio (1Meg:47K, 47K being the standard for input impedance) it might work, however you will be reducing the amplitude by a factor of 20 at the same time. I probably have some old interstage audio transformers somewhere. If I can find them I'll try it.

If most piezo elements are driving amps hard and the volume has to be kept turned down this might be a good thing.

I probably won't get a piezo mounted and amp running till next week (promised to take the wife to see Aron Tipton tomorrow and we normally go to visit her dad at the nursing home on Sunday so the weekend looks shot).

Sven
Doc Oakroot said:

Since a piezo has high voltage and high impedance (=low current), wouldn't it benefit from a transformer? Could you solve the impedance matching with that?

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Permalink Reply by Sven Pfitt on July 5, 2009 at 4:06pm

OK, the saga continues.

I finally finished my 4 string with piezo pickup. (The 'kids' had to check it out).

I built a pre-amplifier to buffer the signal from the Piezo element, and finally got it working (forgot a ground and spent half a day chasing a 60hz hum problem).

It appears that a good strum on the top E string produces a nice 1V peak signal that rapidly decays. 1V is nominal for the input to most amps and should not overdrive the input. So with the piezo element mounted under the bridge the piezo element produces a good nominal amplitude signal. This seems to be the recommended location now days for the piezo element. (I mounted it with hot glue per the nom suggestion.

I'll mount another piezo element on the back of the cigar box to see if it produces a greater signal strength, as some have reported what I interpreted to be excessive drive and induced clipping at this location. I'll check the amplitude at this location and report what I find later in the week.

I hope to get the power amp going this week as well, but I need to pick green beans and can them so I may be a bit off on my expectations.

Attachments:

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First_4-String_1.JPG, 85 KB

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Permalink Reply by William Bannier on July 5, 2009 at 5:38pm

Riddle me this, please- Everything I've read about using a Piezo transducer says to take it out of it's casing and then glue it into the guitar. Now I'm reading that a lot of you encase it in wood, tape, glue and even cowhide. What's the point of taking it out of one casing and putting it in another? Couldn't it just be left in the original casing, glued in and wired to the jack?

Also, where's the "best" place for putting the transducer?

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Permalink Reply by Sven Pfitt on July 6, 2009 at 1:55am

The piezo element is designed as a speaker, not as a microphone. As such it is mounted in the plastic case with the rim supported to allow the disk to move when driven.

Since it is being used as a microphone, this is not nearly an optimal configuration. Experience has shown that removing the piezo element from the plastic case and mounting to wood in the guitar is a better solution. I have mine mounted under the bridge, and glued to the neck that runs through the guitar. It logically makes sense to mount it in this location since the bridge will conduct the sound/mechanical vibrations directly to the piezo element at this location.

William Bannier said:

Riddle me this, please- Everything I've read about using a Piezo transducer says to take it out of it's casing and then glue it into the guitar. Now I'm reading that a lot of you encase it in wood, tape, glue and even cowhide. What's the point of taking it out of one casing and putting it in another? Couldn't it just be left in the original casing, glued in and wired to the jack?

Also, where's the "best" place for putting the transducer?