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Thread: Piezo pickups

  1. #1
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    Piezo pickups

    Hello

    I have a little question about the piezo pickups. A friend of mine asked me to help him ''electrify'' his xylophone with wooden bars. So far so good, but he wants to be able to send a stereo signal to the mixing board directly (instead of dozens of mono signals from each bar). So the question I have is: do I have to build an active preamp for each piezo pickup or I can simple build a passive attenuator/mixer that regroups piezo inputs and sums it passively to a stereo signal?

    Thank you very much in advance!
    ''I'm a quick learner, you just need to explain it to me very slowly''

  2. #2
    Senior Member Enzo's Avatar
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    The problem is the Xylo does not have a resonant frame or sound board, so there is not one source of signal for a piezo to pick up. COntrast that to a guitar, wher the bridge and/or top face of the guitar body resonates.

    If he wants a stereo signal for recording or for a PA feed, I really think a pair of suitable mics might be a better choice.


    However in the spirit of the OP, you don't have to send dozens of signal to the mixer, but you can have dozens of pickups and a small pre-mix box for them. Think of a Fender Rhodes where each tine has a pickup. The pickups are not unlike guitar pickups. Those several octaves of pickups are all wired together, and that sum is sent on to a preamp. But any piezo on a tone bar, it would seem would interfere with its free motion and vibration, and thus affect tone and play.

    I don't think I would send a raw piezo signal to my mixer anyway. I would want some sort of preamp or similar processor to take the picked up signal and make it more friendly to a mix board. Unless your mixer is sitting nearby, you'd probably want a lowZ balanced feed to the mixer.
    Education is what you're left with after you have forgotten what you have learned.

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    Thank you Enzo for a detailed response!
    I was thinking of using the little round piezos with a double-sided tape to mount them underneath the bars. It will dampen the resonance a bit but I think that my friend could live with that
    Your suggestion of using a premix (passive?) box to sum let's say on octave worth of pickups and to send them then to the preamp (again one per octave) is brilliant! My only worry is to overload the piezo signal by sticking it directly underneath the bar... I guess I will have to buy a couple and to test it out
    Thank you again!
    ''I'm a quick learner, you just need to explain it to me very slowly''

  4. #4
    Senior Member Enzo's Avatar
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    Piezos are robust, that is what they glue to a sheet of wood inside electronic drum pads.


    By all means try a few "keys" and see what results. I still like a pair of microphones.


    Piezos like really high input impedances on an amp, just one more reason for some sort of active box to gather signal and then spit it out in a manner suited to the PA mixer.
    Education is what you're left with after you have forgotten what you have learned.

  5. #5
    Old Timer olddawg's Avatar
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    Honestly... sounds like a big cf. I would just use a pair of Shure SM57s or 58s and pay attention to the placement with decent articulating mic stands. If cost is an issue, Behringer XM5800s would do fine. You can get 3 with cases and clips for $50.
    rjb likes this.

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    Thank you all again for suggestions!
    The thing is that my friend wants to put those pickups through different effect pedals (let's say one octave through reverb+delay, another octave through an overdrive, etc.) that's why I was thinking about piezos instead of microphones (because of the bleed, potential feedback, etc.)
    I will mount a few piezos on a couple of bars and report back!
    ''I'm a quick learner, you just need to explain it to me very slowly''

  7. #7
    Old Timer J M Fahey's Avatar
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    Your friend dreams and has no clue about the Technical problems he´s creating. Sorry.

    Just for starters: "Stereo" is impossible for contact mikes, since each pickup detects just *one* bar, without space information.
    Think about it.

    True Stereo is picked by two "air" pickups, placed respectively near each end at at some convenient distance away, where each of them picks all bars, just with different volume and distance.

    That varying volume and phase/delay information for each of them conveys the "Stereo" information, so you know the project is flawed from the beginning.

    You may simulate Stereo, the way most Studio recordings are made, by individually picking sound sources (microphones or pickups) and panning each of them left and right as needed ... you´ll need one preamp each (even if a single Op Amp) and you might mix them into left-right inverting Op Amps, using a net of varying value resistors, say 10k to 100k so going left to right you hear them gradually move in the recording (or live sound feed) and viceversa.

    So it is Technically possible, just complicated.

    I have made pickup systems for Les Luthiers, an Argentine group famous for their "unconventional instruments" , including the Cocófono, a xylophone made out of tuned coconut shells, the Dactilófono, where a converted typewriter hammers hit glass test tubes, the Latín , a violin using a Danish Ham can, the Viola da Lata, a cello using a 20 liter (5 gallon) oil can, and so on:






    here´s a free sample of the Pelotófono(Ballsphon):


    In MOST cases I used small Electret capsules practically touching the vibrating surface (like in Crown PZM microphones), sometimes 2 or 3 capsules picking "air" stereo sound as I mentioned above and only a few times used Piezos ... sometimes as noise gate triggers to enable Electrets (the real signal pickups) do their thing but self mute when not actually playing to minimize ambient noise pickup, which was considerable otherwise.

    Sometimes there were almost 100 microphones on stage , depending on instruments chosen.
    rjb and Arhythmic like this.
    Juan Manuel Fahey

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    Piezo pickups need to see a high impedance - 10M Ohm is the usual value, but depends on the pickup type and design. There are a few guitar systems that I've recently fitted that work fine directly into a 1M Ohm amp input. These are best glued directly to a resonant surface. The commonly available disk types can be highly variable and you'll need to experiment with a few to get the best sound.

    Bear in mind that the relative signal produced from high to low in a (say) 3 octave instrument means you need to be able to trim the individual output levels. A passive mixer is not a good choice as the resistance values are high which results in a low-output, noisy configuration that has limited frequency response and a hugely attenuated low-end.

    If I was to do this with cheap piezos, I'd build a buffer that would handle each pickup. So you may have 37 of these. Anyone who builds keyboards will tell you that this type of 'one-circuit-per-key' approach is not unusual. Then you have the flexibility to mix these down, equalize levels and assign them to different channels. The components are pretty cheap and it's easy to put something together on a single piece of Veroboard. A little repetitive, though and lots of wires.

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