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Grounding a piezo on a polyurethane-string bass?

Discussion in 'Pickups & Electronics [BG]' started by josiah goldfish, Oct 25, 2013.

  1. Hey folks,
    How would I go about grounding a piezo on a home-made uke-bass? ATM I've got an undersaddle transducer running straight into a stereo jack plug. When I play it hums, but if I grab the jack it stops humming.

    Any thoughts?
    Thanks in advance,
  2. bassbenj


    Aug 11, 2009
    Whenever you have hum that goes away when you touch ground that means you have a lack of shielding somewhere. Question is where?

    Here's how it should be set up. Look at yours an compare.

    Under saddle transducer should have two wires out of it and it should be shielded with thin metal foil or other conductive material. The metal foil MUST connect to the ground side of the output wires. Usually in commercial under saddle transducers there is a shielded wire that in your case runs to the jack. If those wires are just a pair of insulated ordinary wires, you MUST shield them by putting braid or foil over the wires an grounding it or by replacing the wires with a coaxial shielded wire. NO WHERE should anything be visible except grounded shield from the transducer to the jack. The shield and ground side go to the BODY of the jack and the "hot" wire goes to the tab on the jack that rubs on the plug tip. As little "hot" wire should be visible at this point as possible. If worse comes to worse, you'll have to enclose the entire jack in a grounded metal box. I sometimes solder these up out of blank copper-coated un-etched circuit board material. Solder it up, epoxy board seams, sand flat and paint. looks great!

    I'm guessing your transducer isn't shielded or you are running unshielded wire to the jack. Piezos are like no load at all so it's just like the end of an unplugged guitar chord when it comes to picking up hum.
  3. lz4005


    Oct 22, 2013
    I've always wondered how that worked.
    My Ubass is reasonably quiet, considering how hot the preamp is. No real audible hiss as long as you keep the high eq at 3/4 or below.

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