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Balancing outputs from humbuckers with piezo pickups in a semi acoustic guitar.

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  • Balancing outputs from humbuckers with piezo pickups in a semi acoustic guitar.

    Ok. *Help needed! *I will try to explain.

    I have a semi acoustic guitar that I have built from scratch. *I really struggle with the tech side. *Are there some clever people that fancy exploring this project?:

    I have two humbuckers like a Les Paul (let's call them H1 & H2) and two Piezos (let's call them P1 and P2).
    The circuit allows you to 'swap' (with two simple switches) H1 for P1 (bridge) and H2 for P2 (neck).
    I thought this would be good for 'noise-outs', chopping up and feedback. * One piezo is in the guitar neck and the other is under the bridge. *With the three way switch I can choose the following combinations:

    3-way down:
    H1 (Bridge) or
    P1 (Bridge).

    3-way middle
    H1 and H2 or
    H1 and P2 or
    P1 and H2 or
    P1 and P2.

    3-way up:
    H2 (neck) or
    P2 (in neck - 3rd fret)

    Here is the problem:
    Both the Piezos are really quiet.
    Both the humbuckers are really loud.
    I just cannot mix H1 with P2 or H2 with P1 (3-way mid pos.) because the Piezos are too quiet.
    Even when ive tried to put the piezo though my acoustic guitar preamp the signal is still very quiet.

    How can I achieve what I am trying to achieve?

    Just for clarity I would like to achieve the same volume output for each pickup H1, P1, H2 and P2 so I can switch between them without volume drops. *H1 and H2 are about 16k.

    Please please help me find a way.
    I have looked into a pair of FET or Op-amp preamp circuits but a) I can't find a circuit that I'm sure can give me a good volume boost and b) I'm in England and most of the parts are listed in American shops and I can't find equivalents.

    Ps I have plenty of room for battery storage in the guitar body. *This is not a problem! *I'm not looking to create an authentic acoustic tone with the Piezos, just for feedback and noise-out.

    Your thoughts: *what do you think?


  • #2
    The two things that are working against you are the relatively low output from the piezo elements and the fact that the piezo have a very high output impedance. The only way to balance them will be with some sort of active preamp buffer.

    Based on the way that you describe the switching system, you will need to have a buffer preamp for each piezo pickup.

    Most single fet buffers will probably work and do not require too much to build them, just a handful of basic parts.


    • #3
      Being in England isn't really an excuse. RS, Farnell and Maplin should have everything you need between them. I've had trouble finding some JFETs called for in American schematics, but the BF245A usually works as a replacement.

      No need for FETs really, an op-amp circuit based on the TL071 or TL072 should do the job.
      "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"


      • #4
        Thanks guys, a good start but as a beginner (regardless of enthusiasm) I'm still swimming in a sea of information I find hard to grasp.
        I know I am going to build two buffer preamps and they are not too hard to put together (I'm told!). *I assume I will need a JFET transistor? *And if I cannot find that particular part in England, I can use a*BF245A in place of.

        "No need for FETs really, an op-amp circuit based on the TL071 or TL072 should do the job." - Steve Conner.

        This is really basic stuff to you guys but it's hard to put your foot on the ladder when you are at the bottom!
        TLO71 and TLO72 are op amps.
        I will build two of these so I have individual control of my piezos' volume.
        Now I need a bit of help choosing my circuit.
        I found this one but I don't think it has volume control. *Is this the sort of thing I should be following?
        Any comments, adjustment or even a link to a more appropriate diagrams will be excellent!

        Discrete FET Guitar Preamp


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