Piezo pickups: Placement, output voltage and resonant frequency

[saigon-condom]

Hello forum,
piezo pickups have completely different properties than magnetic pickups, because you can place them more or less wherever you want, for example under the bridge or somewhere on the body. However, piezos often sound very harsh and sterile and they have a low output voltage, that's why they are often used in combination with a preamp and EQ. Would it be possible to get a good piezo sound without editing the signal via preamp and EQ? What does the piezo resonant frequency say about the tonal character of the piezo? For example, does high resonant frequency mean harsh sound? Would it make sense to connect multiple piezos in series in order to get a higher output voltage or could this cause unwanted phase issues?

[Fluoroscope 5000]

My Parker NiteFly V1 came with Piezos on a volume pot that could be mixed with the magnetic pickups without a preamp. I didn't like the sound because of the very hard attack from them being mounted under the bridge saddles. Mixing with magnetic pickups will have the same issue as mixing one magnetic with another. It can be done, but one pickup will load the other. I guess you'd want Piezo's of similar inductance to your magnetic pickups for efficiency. There's no way to predict how multiple Piezo's placed around the body will sound. Yes, there will likely be phase cancelations depending on how the wood is vibrating where they are placed. I've heard that placing one in the neck pocket can work well. It may be more or less in phase with the magnetic pickups depending on the wiring, but some freqs might not be in-phase anyway. You'd just have to try it.

[Helmholtz]

Piezo PU are essentially capacitors. They don't have inductance.

If they show a resonance, that would be a mechanical one.

Piezos cannot directly sense string vibration like a magnetic PU. Rather the output voltage is caused by varying pressure/compression.

If the load resistance = amp input impedance is lower than around 1M, bass response will drop.

Paralleling with a magnetic PU would mean severe loading and bass loss.
Also the performance of the magnetic PU would be impaired by the high capacitance of the piezo.

[saigon-condom]

Thanks. That means that I have to wire the piezos in parallel in order to get a higher output signal: Cn = C1 + C2 + ....

Would it be beneficial to install the piezos in a hollow or semi-hollow body guitar because a solid body guitar has more inert mass and is less willing to vibrate?

[Helmholtz]

Thanks. That means that I have to wire the piezos in parallel in order to get a higher output signal: Cn = C1 + C2 + ....

Sorry, no.
Paralleling will cause mutual loading, resulting in lower output.
To add signal voltages you would need to wire the piezos in series, but that is likely to introduce noise as only one of the PUs could be grounded.

Would it be beneficial to install the piezos in a hollow or semi-hollow body guitar because a solid body guitar has more inert mass and is less willing to vibrate?

Can't say.
Remember the piezos are pressure sensitive. Vibrating the piezo as a whole will not produce a significant output.
That's why piezos are best placed under the bridge, where they can sense pressure vibration.

[J M Fahey]

IF you have many, as in,say, one per string or at least one under each end of the bridge, by all means wire them in parallel, if anything for shielding reasons, feed them into a proper impedance buffer/amplifier, and IF needed, give that preamp some gain.

Under NO circumstances must they be used mixed with Magnetics, as mentioned above.

Most "passive" you can get is using one or two large "alarm" or "tweeter" disks under the bridge, signal is strong because that is a maximum vibrating pressure point and large disks typically have between .02 and .06uF , so they can drive any typical Guitar amp input.

They are capacitors and follow standard RC equations, so worst case .02uF and 220k input (lowest you will find in a Guitar amp) still give you -3dB at 36 Hz.