Piezo Pickup and SCREECH

[Zben3129]

Hi,

I recently made a piezo pickup for my guitar.

Goes as follows




Works great at low volumes. Anytime I try to put my volume on the amp past about 5 oclock, and insanely high pitched and LOUD screech comes out

Any ideas on how to correct this?

[Zben3129]

oh and also forgot to mention, would like to keep this passive if possible

[Pushtone]

I'm assuming the screech is NOT acoustic feedback.

Can you test the setup with another amp?
Where do you have the element attached? What part of the instrument?


From what I know about piezos...

They sound best feeding a very high impedance input, on the order of 10 or 20 megaOhms.
I wonder if driving  an 500k amp input is causing the screech.

Commercial piezo systems almost always use a preamp to get the high input impedance and to boost the signal.
Might be a good idea to get clever and build one that fits on the guitar strap.
The AMZ MOSFet boost has been mentioned in posts about piezos because of its high input impedance
The Tilman preamp has also been pressed into service for piezos.

I guess what I'm saying is that a passive piezo system may not be practical.
After all, few if any guitar makers offer a passive pickup system in their acoustic/electrics.

I would try inserting an active circuit between the guitar and the amp and testing for screech.

[PerroGrande]

My very first impression upon reading that was acoustic feedback -- to be followed by the advice to "move away from your amp and see if that changes anything".

[Zben3129]

yeah i used a 20ft cord at full length still feedback.What causes acous. feedback anyways

[PerroGrande]

Any device that converts motion to electrical signals can be subject to acoustic feedback. 

The most simple form, and easiest to understand, is the acoustic feedback that can be produced by a microphone.  The diaphragm of the microphone moves in response to pressure waves (sound) and induces a current in a coil of wire that is proportional to that movement.  If this signal is amplified substantially and fed into a speaker, the speaker converts these electrical signals back to pressure waves (sound) via the movement of the voice coil, cone, etc. 

If the sound waves produced by the speaker happen to cause the diaphragm of the microphone to move, a feedback loop can occur.  This loop is conceptually the same as oscillation in an electrical circuit.  *Positive* feedback causes a growing amplitude signal to occur until certain electrical limits are reached.  Acoustic feedback is the same, except the feedback occurs thanks to sound pressure waves rather than wires. 

It can manifest itself in other forms, too.  Turntables can suffer from acoustic feedback thanks to the sympathetic vibration of the various mechanical components -- ending with a vibration that is translated by the cartridge into electrical signals.  This can produce a nasty howling effect...

[Zben3129]

thanks, and I tried using the pickup through the headphone out of my amp, and same thing 

[PerroGrande]

okay -- that is good troubleshooting.  You've taken one variable out of the equation. 

So -- what we're left with is a piezo pickup and squealing when the gain on the amp is increased -- even through the headphone output.

Pushtone is spot-on when he mentions impedance issues.  Piezo pickups have a high output impedance and are poor current sources -- i.e. they don't "drive" a cable well at all.
I'd recommend an AMZ Mosfet booster or similar high-impedance buffer (something with 3 Meg+ impedance) right after the pickup -- prior to any tone controls, cables, etc.

[Zben3129]

thank you very much. Would it be possible to build the booster inside the guitar, with the 9v running through the ground of the cable with the battery external?

EDIT - Whoops...dur....not good to run +9v in the same line as ground. What was I thinking. Anywho, what if I had a 3 conductor cable and used the 3rd conductor to carry the +9v, would that work?

(9v battery in an external enclosure, velco on guitar, etc etc

[PerroGrande]

Well, sure it is possible to build a booster into a guitar.   But first things first...  Rig one up and see if that solves the screeching issue -- THEN worry about jamming it in a guitar.

[Zben3129]

yeah I'll breadboard a simple one in a little while, dinner time now though Turkey leftovers yay

[Zben3129]

RESULTS

Okey dokey

The preamp fixed the problem, but it didn't

It still screeches, but I am able to get more volume out of it before the screech starts. The screech now begins at 8 oclock, and is much louder. Due to the fact that its a higher volume level.

This just may be the limitation of my 30 cent pickup 

[Electric_Death]

I'm incredibly experienced with piezo, without a feedback loop, there would be no screeching.
In fact, adding a volume pot won't exactly eliminate the screeching, it will just allow you to reduce it. Piezos are just really really sensitive generally.               

How do you fix it?
You need to add some dampening material to limit the piezo's vibration.
A layer of rubber, some thick layers of fabric, foam rubber, cotton balls..about a thousand different things will work.
Glue it onto the front AND back of the piezo, glue the piezo in place and of course, don't forget to incorporate a volume pot.
Depending on the material, you will lose some volume but not necessarily.

I just built a piezo microphone for recording vocals. Talk about incredibly high quality. I used components from the medicine cabinet and this sucker will give condensers costing in the hundreds a run for their money. I'll add a Smash Drive with EQ to it, find something to use for an enclosure and presto, instant high grade studio mic I built and designed myself.

[Zben3129]

thanks, but I have 1/8 inch craft foam on either side, hot glued on. I used 20 lbs weight on the "sandwich" to make sure it was a perfect adhesion. I am stumped with this thing. I have done everything I can think of, and I can't come up with a resonable explanation of what is happening


Strange

[Aren]

Electric death -
Want to elaborate on that piezo mic? (presuming, of course, you weren't kidding...  )

[Zben3129]

+1

[tranceracer]

I tried using the pickup through the headphone out of my amp, and same thing 

Try using another amp or plug into a mixer.  If you still have screeching it may be that the pickup may need a different buffer.

-tR

[IGR]

Don´t forget than piezo has own capacitance in order nanofarads, with input impedance of preamp, say about resistive character, it forms RC network. At harmonic overtones some few nearest octaves above of its ground frequency could certain preamp becomes oscillator. If you´ll shift input impedance 10x higher, ground frequency will be 10x lower, in order of 0,1 Hz. Though a high end pramp could oscillate  at this frequency as well, making stuttering tremolo. It depends at quality of input cap too, if its present. Ceramic, I know, I profane, is better than polypropylene/foil.

[Zben3129]

would adding a high pass filter help?

[Paul Marossy]

What preamp circuit did you use? This one works good: http://www.diyguitarist.com/Guitars/PiezoBuffer.htm

I think your problem is that the piezo element you are using is really not intended to be used in the manner in which you are using it. You can use the same sort of arrangement to induce feedback on a magnetic pickup. See this page: http://www.projectguitar.com/tut/sustainer.htm

I think you need to try a different approach... (like a piezo element under the strings, or if it's an acoustic, a Martin Thinline type of pickup)