1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  

Hum when volume is turned down?

Discussion in 'Pickups & Electronics [BG]' started by Will_White, Feb 2, 2017.


Tags:
  1. Will_White

    Will_White

    Jul 1, 2011
    Salem, OR
    I have a bass that hums when you turn the volume below 8 and the hum gets louder down to about 5 then stays constant until the volume is off. It doesn't have any shielding, and the pot casing isn't grounded, the overall noise is pretty low with the volume all the way up. What could cause it to act like this?
     
  2. megafiddle

    megafiddle

    May 25, 2011
    This is a single pickup bass?

    And the hum remains even with the volume control at zero?

    Also, are you still getting some pickup signal out of the bass, even with the volume at zero?

    If so, it sounds like a bad ground connection to the volume pot. One of the outer pot terminals connects to the pickup, the other outer pot terminal connects to ground. So this terminal will either have a wire connected to it and running to a circuit ground somewhere, or it will be soldered to the pot case. If soldered to the case, then the case needs to be grounded somehow. This grounding is often done by mounting the pots and output jack on a metal control plate, or a pickgaurd lined with metal foil. In the latter case, the connection is made through the pot and jack mounting bushings.

    So somehow or another, you need to have a good electrical connection between the ground terminal of the pot, and the ground terminal of the jack.

    Ohmmeters are very helpful for checking the connections. But even a low cost "bug light" will work.

    -
     
  3. Will_White

    Will_White

    Jul 1, 2011
    Salem, OR
    It doesn't hum with the volume turned off, rereading I notice that was unclear. The volume does have a good ground. I can post the wiring diagram and a picture of the wiring tonight if it would be helpful.
     
  4. megafiddle

    megafiddle

    May 25, 2011
    No hum with the volume at zero would indicate that the ground connection to the pot terminal is good.

    What happens right around a lowest volume settings?

    Does the hum remain strong all the way down and then suddenly stop right at zero? Or does it decrease gradually?

    And same question for the pickup signal (while playing notes), does that decrease gradually or abruptly?

    If the decrease in hum and signal is abrupt, right at or near zero, it could be a broken resistive track inside the pot.

    If it is gradual, it could be a poorly shielded cable, or a very strong hum source close to the cable.

    If neither of the above turn out to be the problem, then it could be some shielding is needed.

    A photo and diagram could be helpful then.

    If you adjust the volume so that the hum is present, is the hum reduced if you set the bass down and move away from it?

    Or while holding it, does touching a good ground (strings, bridge, output jack bushing) have any effect?

    -
     
  5. Will_White

    Will_White

    Jul 1, 2011
    Salem, OR
    The hum decreases gradually around one or two, the signal has a smooth sweep the full range of the pot.

    Touching the strings, the jack, or the plug housing has no effect on the hums level, and I've tried it with other cables and in other places and it still happens.
     
  6. megafiddle

    megafiddle

    May 25, 2011
    The pot itself is good then.

    Something not mentioned yet, how loud is the hum? I have assumed that it is loud enough to cause a problem, making the bass unusable at those particular volume settings.

    A small amount of hum increase is normal at volume control mid settings. Should be insignificant though.


    Increased hum at a mid setting of the volume control is due to the high imedance of the bass output circuit when the control is set there. The impedance is very low at a setting of '10', and zero at a setting of '0'. Impedance is much higher at a mid setting. This is when the circuit is most sensitive to noise. The circuit is least sensitive when the imedance is low; that is why all is quiet at '0' and '10'.

    This circuit in question runs from the wiper (center terminal) of the volume control to the amp input. The only portion of this circuit that is unshielded is the wire running from the pot to the jack inside the bass. Since grounding yourself didn't have any effect, it seems very unlikely that the hum is being picked up there in that short length of wire. So that leaves the cable and amp input.

    Do you have a 1/4" plug (without a cable on it), or a very short cable, like those for connecting pedals together? Plug either of these into the amp instead of the bass cord, and note the hum level. You might want to start with the amp volume down and bring it up slowly to your normal setting. It should be pretty quiet. If there is hum as loud or louder than the hum in question, then that is the problem.

    -
     
    Last edited: Feb 3, 2017

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.