Piezo Vs. Magnetic Pickups

Posted on by Orpheo

I am always intrigued in how things developed. How did the guitar develop to what it is today? How do our views change on what constitutes good tone? Those kind of questions keep me occupied during the slow moments of a day, and one day I was asking myself the question: why do we, as electric guitar players, predominantly use sound systems based on electromagnetism and not on piezo-electrics? To answer that question I dove deep in history to see how the two fields, electromagnetism and piezo-electrics, developed to where they are today.

The end of the 19th century is when experimentation began with electricity, as scientists developed some feel on how solid properties behave in some conditions. Various scientists were exploring how magnetic fields, light, electricity, conductivity and many other physical phenomena behaved. The field got a major boost with the equations of Maxwell, which describe all possible behaviors of electromagnetism. His work was to be crucial to the study of physics, mathematics and later even quantum mechanics, and cannot be disconnected from what we as guitar players do on a very basic, scientific level. Electricity isn’t just scientifically monopolized by electromagnetism. Electricity plays a major role in practically every scientific field, for instance pyro-electrics and piezo-electrics.

A simplified explanation on how piezo works. The pyro electric effect is in essence the same, but in stead of pressure it utilizes heat.

A simplified explanation on how piezo works.

Pyro-electrics and piezo-electrics are connected in a sense that they are similar but with a different premise: pyro-electrics utilize a heat source on the base material to engage an electrical charge, whereas piezo-electrics use pressure. The first doesn’t really come in handy on a guitar. Maybe Jimi Hendrix would’ve liked a pyro-electric enducer on his guitar during his famed concert on the Isle of Wright, but most of us don’t carry lighter fluid in our back pockets! The latter does come in handy every once in a while. I mention pyro-electrics specifically because that was the get-go of this field of science. The Pierre Curie, the husband of Marie Curie (who became famous for getting two Nobel Prizes, one in physics and one in chemistry and later became infamous for severe radiation poisoning!) conjured a possible link between pyro-electrics and piezo-electrics and indeed, came to a conclusion that they are related.

Electromagnetism uses coils, currents and permanent magnets to create a voltage. There are several mechanisms at work in electromagnetism but it’s sufficient to know that a guitar pickup always has a magnet and a coil. The magnet permeates a permanent magnetic field. The vibrating metal string disturbs that field synchronous with the change of the pitch of the string. This change, in turn, forces the coil to create an induction current, which is also synchronous with the frequency (pitch) of the string. That’s how an electromagnetic pickup works, wether it’s a humbucker, single coil, P-90, dynastic etc etc. They all work by this same basic principle. It’s nothing more but a microphone but instead of a membrane moving with a coil attached to it and the magnetic field staying constant, the magnetic field changes.

A piezo-electric pickup uses pressure to create a current. The current is in close relation with the frequency of the source, of course. The physical reason as to why some materials exhibit this trait is a bit difficult to explain. Simply put, the material has internal charges, positive and negative, and they’re separate from one another. Through deforming the material these charges are released because the symmetry of the unity-cell is disturbed. You could regard it as a capacitor that releases a charge when deformed. This analogy isn’t perfect, but you get an idea on how it works.

A basic schematic for a preamp to work in conjunction with a piezo pickup.

A basic schematic for a preamp to work in conjunction with a piezo pickup.

The generated voltage of the piezo-electric system in a guitar is very small. So small that you need a preamp to boost the signal. That on itself isn’t something that would put guitar players off from using a piezo, so what could a reason be? A piezo system is very small and lightweight compared to electromagnetic pickups. Sure, you need a battery, but still. A reason could be that the rate of compression is different with a piezo pickup. Hit the strings twice as hard and a magnetic pickup will give a voltage twice as large. With piezo systems it works exponentially: twice as hard means four times as loud. Twice as soft means four times as weak! That dynamic range would make many players go crazy! I know I would! That’s why the preamps often have a compressor to soften the high and boost the low peaks of the voltage created by the piezo.

The compression is quite audible, too. Maybe a piezo is just too honest and too pure. There is no voicing of the piezo except via the preamp. A magnetic pickup voices the signal directly at the source. The coil geometry, wire gauge, magnet etc etc all tweak and shape the signal that’s being created, allowing you to refine that signal afterwards, but a piezo doesn’t have that capability. Frankly, you’re stuck with what you’ve got. I dare say it’s similar to the discussion of tubes versus solid-state. Tubes ‘sound’ warmer, more natural and organic than solid-state, and maybe that’s also the case with piezo pickups versus electromagnetic pickups.

At the end of the day, the final tonal verdict is all between your ears. Apparently our hearing is more pleased by the warm(er) tones of a copper wire around a magnet than a pulse generated by a compressed crystal when it comes to playing with an amp but we are more inclined in using a piezo in an acoustic guitar due to its pure, clean (dare I say) crystalline tone.

Written on January 5, 2014, by Orpheo

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  • David Morris

    Excellent article, Orpheo, as always.

  • Rick Rhodes

    intersting ! so it’s the compression in the pre-amp that causes the piezzo to breathe ! I get amazing sounds through my guitar rig with of my Gibson Chet Atkins, but it sounds to me like I should build a modified pre-amp… I’m thinking of trying steel strings on it instead of bronze acoustic strings… the Chet Atkins has so much sustain I have been thinking about putting a JB in the treble position, but maybe I don’t need to if I modify the pre-amp ?

  • Patrick Durham

    I like cranking my acoustic through both my old tube amps. But it is a wholly different sound. My question is, what cause piezo pickups to die? I’ve known some that kick the bucket same as regular p’ups can sometimes do.

  • Orpheo

    The chrystal may be broken, inside the saddle? The preamp can die on you, the wires are often very thin and more prone to breaking… a lot can go wrong and there’s not much you can do about that, unfortunately! :

    Rick Rhodes: I wouldn’t put a JB in the bridge of a hollowbody or semihollow! Much too hot! If you want the bite, cut and clarity of the JB, maybe you should consider the Pearly Gates (slightly sweeter highs) or the Jazz. Great pickup, too. highly underestimated. But I am also fond of the Seth Lover, but that one is very clean with little output.

  • Sircnawsnaws

    Nice article. Fun to read and informative.

  • Bronagh Mc Caffrey

    Hi,
    I am currently working on a school project- creating an ukulele amplifier. I am using a piezo preamp and I am using the circuit above as a guide. It says that C1 determines conversion gain, would you be able to tell me how i can calculate the gain in this circuit to optimize the gain for the speakers in my circuit?
    thank

  • SuperRonArt

    Great explanation. I think you meant to say “”The magnet permeates [its surroundings with] a permanent magnetic field”.

  • Alin Teodor Catană

    Thanks! This is useful info! Good job!

  • JFairweather

    You mean Monterrey Pop (June 1967), not Isle of Wight. Wight was his last major concert. He hadn’t set a guitar afire for several years by then.

  • Charlie P

    So I bought a Morgan Monroe MMS-4AE mandolin. It has a passive pickup with a tone and volume knobs. I’m trying to play it thru a Crate acoustic amp, but there is lots of hum and whine and produces low volume unless I crank the gains up, and then it REALLY hums and whines. I tried a Radial Stagebug SB-4 piezo active DI box with 48v phantom power. I drug out my Yamaha EMX 640 mixer to power the box. I plugged into one of the mic channels. The LED power indicator lit up for a second. I thought I was in business, but by the time I plugged into the Crate, the power light was off and no volume. Question: first of all, what kind of pickup do I have? Piezo? It is passive with no batteries, etc. I’m going to return the SB-4 and get something with a 9 volt battery. Should I get another DI box, or a preamp that can balance impedance? I’d really like to play this thing on over an amp soon. Thanks!