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piezo pickup = earth loop nightmare!

Discussion in 'Recording' started by teddybarrington, Jul 19, 2006.

  1. Hi everybody

    newbie here :oops:

    just joined up to "pick your brains"

    I have just got myself a piezo pickup for my kalimba - if I plug it into my bass amp, or effects chain, or into the 1/4" jack mic input of my BOSS loopstation, I get a dreadful earth hum. this can be fixed by attaching the casing of the amp, or the casing of the cable, to the body of yours truly.

    Not the ideal solution.

    Now, I'm guessing the impedance is all wrong anyway, so

    question

    what is the easiest fix - a D.I. box, or somehow altering the cable?

    I'm erring towards a D.I. box with an isolator / ground lift, but I'm pretty unsure.

    thanks in advance.

    ed :wink:
     
  2. moonbaby

    moonbaby Mmmmmm Well-Known Member

    Piezo pick-ups require a preamp. Even if the sales person tells you otherwise. This is not the same as a DI box, although there are preamps on the market that have that capacity. You need to shop around for the right one. Fishman and LR Baggs make some decent models. Check them out online. Most any basic pre will work to run it to your bass amp, but the DI issue will come into play if you want to run it down a snake to a remote mixer, say 20 feet (6meters) or more away from you.
     
  3. thanks!

    I just found this out when the pickup manufacturers emailed me. I think also that it is NOT an earth loop problem, but a shielding problem (no doubt made worse by no pre-amp), so am going to create a tin foil shield.

    Can't explain the hum dissapearing when the ground is touched (either casing of cable or any part of the ground area, i.e. metal part of amp)

    Does anyone know the physics behind this?

    ed
     
  4. guitarbill

    guitarbill Guest

    Yikes, instrument grounding! Always a bone of contention. If your preamp fix and shielding with foil dosn't work consider this: Some folks say don't ground directly because it's a total safety hazard, and they are mostly right. Some other authorities on the subject, including Adrian Legg and Dan Erliwine, say ground the bridge/strings through an isolating resistor of about 220k - 1/2 watt- in paralell with a .001 mfd./ 500volt ceramic disc capacitor. Others, including major manufacturers say ground away directly at the bridge(strings; in your case the tines of your kalimba.) This is the approach that I have settled on with one huge caveat: You must, absolutely must, make sure you are safe from leathal shocks by ensuring your equipment is properly grounded and you have a working GFCI powering all of your equipment (from one outlet) that may touch you while touching your instrument.

    The safest way to power your rig no matter how large is from one outlet. That eliminates voltage drops from different outlets which all by itself could put a least a few volts of voltage at you lips through your hands to ground. Not necessarily leathal but uncomforatable and noisey! A GFCI might catch that if all the grounds were the same. However grounds from different outlets are most likely not at the same potential!

    Be careful, be safe and if you are unsure how to properly protect yourself consult with a licensed electrician who hopefully knows how to check for true grounds, proper GFCI installation and voltage drops at different outlets you may be tempted to use together.

    good luck and get a GFCI! It won't help with hum but it will keep you alive[/u]
     

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