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Problem with piezo pickup

Discussion in 'Amps, Mics & Pickups [DB]' started by Diego Gonzalez, Jan 6, 2014.

  1. Hello folks. I have a little problem. First of all, I don't have a preamp. I want to find a DIY solution to reduce the volume of an extremely loud piezo before entering my amp (a gk mb-200). The piezo clips even with the amp volume in 3 without playing any note. The upright isn't close to the cabinet. In amplitube I have to reduce the input level to -15db to get a good sound. The gk has a -10db switch that works great. I'm searching for a cheap solution for using others amps that don't have that switch.

    Thanks in advance!

  2. Don Sibley

    Don Sibley

    Jun 27, 2005
    Fort Worth, TX
    What pickup are you using?
  3. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    It sounds like you may have something wrong with your pickup, but not knowing the type of pickup you have makes it difficult to help you. Please fill out your profile
    so we can help you. Just so you know, the GK shouldn't be doing that with most piezo pickups. I've used many a GK Combo (200MB Series I and Series II, and the MBE) and never had a problem with the Underwood pickup, Realist Copper Foil, BP-100, or Revolution Solo.

    Francis Deck who builds the HPF Buffer Pre AMP, posted the plans for a version of the box on his website. http://personalpages.tds.net/~fdeck/bass/quickand.pdf
    So if you know how to assemble and build such things you could have a the unit for your time, plus parts.

  4. Steve Dallman

    Steve Dallman Supporting Member

    Using a passive volume control will work, but may not be the best solution. A piezo needs to see a 4.7-10 meg input to maintain fidelity. But, the input of an amp is around 1meg, sometimes lower.

    A high output piezo usually has more piezo mass, and would likely do alright into a 1meg input. A 1meg audio taper pot in a box might work fine.

    Or use a series resistance. I have a few guitars with Fishman Powerbridges in them, and they are high enough output to break up most preamps or even amp inputs.

    I usually use a 470k resistor in series bypassed with a 100pF or smaller cap (to reduce dulling the resistance produces. Then I use a 1meg audio pot after the resistor/cap combo.

    I'd make a small box with a 470k resistor and a 1meg pot. I might put in a switch, to bypass the series resistor.
  5. drurb

    drurb Oracle, Ancient Order of Rass Hattur; Mem. #1, EPC

    Apr 17, 2004
    Clips without playing a note? :confused: Could you explain?
  6. First of all, thank you all for the quick replies. This is a custom mic made by an argentinian luthier. It's one of those you put under the two legs of the bridge. The luthier says that normally you don't need a preamp to use it. I don't know how to explain the problem with other words. The mic delivers too much power to the amp, and it clips at low amp volume. I think that it has a sort of preamp circuit inside the jack. Is this possible?
    I tried to lower the volume with a 500k pot. It works, but it's very noisy. Tried an 1m pot and a 330pf cap. It works better but it's still noisy.
    Steve, can you point me to a diagram of that circuit? Thank you!
  7. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I suggest a capacitor in parallel with the pickup. You can put it inside the output jack of the pickup, or even into one end of a cable for experimenting. I suggest trying 0.01 microfarads, cheap poly film type.
  8. I tried it and it didn't work. I tried several other things and settled with an 1meg pot and after that the 0.01uf cap in parallel. It makes little noise and the volume is reduced. The problem is that I note a loss of sound quality when I record the bass with amplitube. Reducing the input volume from amplitube (without using this attenuator) sounds better. I think that I have to invest in an active preamp.
    Thank you for all your help guys.

    Best regards and a happy 2014,