Electronics Problem

Posted by Steve Tyssen on April 19, 2010 at 4:15am
Hey guys how's everything goin ??

I'm just wondering if anyone could help me out...  I've wired up a few CBG's successfully but lately i've been having a few problems with humming / feedback..

I'm just using a piezo buzzer with a volume control and of course a 1/4" input jack.   For some reason when you plug the guitar in, there is a slight humming sound.  When you touch the volume knob with your hand the humming sound is really maximized.  And what seems even weirder is that when I touch the metal on the guitar lead / input, there is a dead silence with perfect response from the Piezo.  

I'm just wondering if anyone has experienced this or if it's a common issue!??  I've got a couple of pictures on my page that show you exactly how I've been wiring them up.  Thanks in advance!!

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Replies

  • Murray May 31, 2010 at 8:14pm
    Has the hum problem been solved?
  • Mark April 20, 2010 at 2:28pm
    It shouldn't matter which way around you wire the piezo - they don't have a ground connection like a magnetic pickup. If I've been looking at the right photo then your wiring looks OK as far as I can see (although I can't judge the quality of the soldering from a photo so it's definitely worth checking for cold solder joints as Sam advised - I check all my soldering with a multimeter as I go).

    Can you explain more clearly what you mean when you say there's dead silence and a perfect response when you touch the metal on the guitar lead? Which bit of metal are you touching? I'm presuming you're somehow touching something that's connected to the 'ground' part of the lead but it's not entirley clear to me.
    Have you ruled out problems with the lead? Have you tried other leads or have you tried other guitars with that lead?

    I'm particularly puzzled by the fact that the hum gets worse when you touch the control knob. That sounds very wrong (for a start the knob ought to be insulated from any of the circuitry and so merely touching it should have no effect). Does the effect happen when you touch the knob lightly or does it only happen if you press on the knob or turn it a little? I'm wondering if there's something damaged or defective in the pot.
  • Kevin Lawton April 19, 2010 at 10:29am
    To be honest, from your pix, I cannot verify which way around you have wired the jack. If the hum gets REALLY loud when you touch the pot body, then that would be a sign that you have the wiring to the jack reversed. If a general low level of hum gets less when you ground yourself, then that is a sign of insufficient shielding. If you are using metal strings, then you need them grounded via a wire between the pot body or jack, and a metal brige or tail-piece(s).
    Bear in mind that if you want to use a volume pot with a piezo, then you need one which is properly matched in impedance with the piezo - not a usual guitar pot.
    I'm not sure exactly how you are mounting your piezo, but it is important to get it right in the 'line of fire' with the vibrations off the strings, so that you get maximum signal off it. With max signal, then you don't need so much gain on the amp and hum won't show up as much. Best method I know of is to mount the piezo(s) inside or under the bridge - so the string vibrations actually pass right thru it. Worst method is to leave the piezo in its plastic box and just glue that inside the cigar box.
    Feedback has a whole different cause to hum, and is usually the result of having a weak string signal coming off the pickup so you have to use more amp gain which leads to enough sound from the amp for the piezo to pick some of it up - so the amplified signal gets fed back into the amp again. Once again, the solution is in the way you mount the piezo. You really need to maximise the string sound and minimise picking up any other sounds.
  • Wichita Sam April 19, 2010 at 8:33am
    on a piezo, grounding isn't the issue as on a wound pup.... if you don't have any stray leads... and if you have a good jack and if you had a good pot, then the most likely problem is a cold solder connection. Have the iron nice and hot, use thin solder wire, get the solder rreal fluid then let it cool completely before moving the connection.......

    with a wound pup you must also have to have posi/neg correct, plus ground the strings...

    the best,

    Wichita Sam
  • Elmar Zeilhofer April 19, 2010 at 6:51am
    Hi Steve,
    I had the same problem too. As I'm not an expert, I started trying... After changing the Hot and the Ground wire connection to the jack, the hum decreased. Mine had a single coil pup, not a piezo as in your setup.
    I suppose in the original wiring the hum was amplified.
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