Work? Yes. Work well? No.
Piezos like to see really high impedance inputs on an amp.
Look up input stage circuits on so-called Acoustic amps from various manufacturers.
Piezos like to see really high impedance inputs on an amp.
Look up input stage circuits on so-called Acoustic amps from various manufacturers.
There seems to be 2 (op-amp) versions for piezo.
VERY different (seems to me)
#1 .... Non-inverting, high input R (1M to 10M), same basically as most magnetic.
#2 .... Charge-pump, piezo directly to inverting input, capacitor feedback (integrator)
VERY different (seems to me)
#1 .... Non-inverting, high input R (1M to 10M), same basically as most magnetic.
#2 .... Charge-pump, piezo directly to inverting input, capacitor feedback (integrator)
#2 doesn't make sense to me. Example?
A good piezo preamp would use a discrete 1st stage, not opamp.
A good piezo preamp would use a discrete 1st stage, not opamp.
Yes, but AFAIK you need to run a TL07x at a more modest input impedance to get best noise performance. With a discrete FET the sky is the limit.
Just because it was there.
Piezo Film Buffer and Amp Combo for Guitar Electronics
Another idea
8 Application and Implementation
OPA1679 datasheet Low Distortion, Low Noise, General Purpose Audio Op Amp | TI.com
Piezo Film Buffer and Amp Combo for Guitar Electronics
Another idea
8 Application and Implementation
OPA1679 datasheet Low Distortion, Low Noise, General Purpose Audio Op Amp | TI.com
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The TL07x amps are excellent to extremely high source impedance. They work fine on condenser mike capsule of nominal hundreds of Megs. Yes, a just-right JFET can be a thin-hair better.
Very true, they are based on two different properties of piezos.
This is based on pretending the piezo is an AC voltage source with a small capacitor in series. The capacitor may be only a few hundred picofarads in some cases (undersaddle piezo, for example), or may be as much as a few tens of thousands of pF (larger circular piezo elements).
The capacitance acts as a high-pass filter with the input resistance of the preamp. As long as the input resistance is big enough to let eighty hertz signals through without much loss, this works fine for guitar.
The downside of this approach is the very high input resistance (particularly necessary with those small-capacitance undersaddle piezos.) This makes the circuit sensitive to hum and noise pick up. Careful shielding and short lengths of piezo-to-preamp cable may be required to keep things quiet.
This is based on a more realistic model of a piezo: an insulator that generates an electrical charge across its two surfaces if it is bent. Those two surfaces are metalized so the charge can be drawn off.
Since current is the rate of change of electrical charge, if we were to short out the piezo, the current it generates is proportional to the frequency at which the piezo is being bent back and forth. (A higher frequency equals a higher rate of change.)
This means the shorted piezo generates an AC signal current proportional to both the frequency, and the amplitude, of the strain on the piezo. In other words, the current from the piezo rises at +6 dB/octave.
Feeding the piezo directly to the inverting input effectively short-circuits the piezo, and so sends its entire output current through the feedback element from op-amp output to inverting input. If that element is a capacitor, the preamp gain falls with frequency (-6 dB/octave), which exactly compensates for the rising-with-frequency characteristic of the shorted piezo itself.
(What if we don't short the piezo, but leave it open-circuit? The piezo is two metalized surfaces separated by an insulator, so it acts as a capacitor itself. The electrical charge generated by bending the piezo is therefore used to charge and discharge this internal, parallel, capacitance. Once again, the rising +6 dB/octave current is compensated by the falling response of the internal capacitance, and the output of the bare piezo is therefore a voltage that is flat with frequency. Connect it to a high enough input resistance, and you get a flat frequency response...exactly what we saw in hitsware's approach #1.)
In my experience, the second approach - feeding piezo short-circuit current to an integrator (sometimes called a charge amplifier or charge mode amplifier) is quieter, less prone to noise and hum pickup, and might also cope better with sudden huge spikes (transients), which piezo pickups have a nasty tendency of putting out.
The charge amplifier approach is widely used with piezo transducers in some areas of electronics (such as ultrasonic receivers). But, probably for historical reasons, it seems to be rather rarely seen in guitars with piezo elements.
As an aside, piezo's can generate huge voltage transients. My mother used to light her gas stove with a piezoelectric igniter: it generated, quite literally, thousands of volts in response to a spring-loaded impact, enough voltage to spark across an air-gap at the tip, and light the stove.
In the same way, guitar piezos, which normally generate only millivolts, can produce hundreds of times their normal output during brief initial transients if someone is very aggressive with their playing technique ("pick attack" seems to be the term in vogue lately.)
I suspect that one reason why piezo pickups on guitars sometimes sound harsh is that these short but huge transients briefly overload and clip preamp inputs.
-Gnobuddy
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