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2 circuits, 2 output jacks and hummmm

Discussion in 'Pickups & Electronics [BG]' started by Lasermatch, Mar 14, 2013.

  1. Lasermatch


    Feb 28, 2012
    I added a separate passive pickup, volume pot and output jack to my passive Spector Performer bass (bridge and neck passive EMG pickups). Each original pickup has a separate volume and tone pot, so 5 pots all together and 3 pickups. The share the common ground, which is the bridge.

    When I plug one output jack to one amp and second output jack to the second amp, I get a ton of hum and buzz. It does not appear that anything is inappropriately touching anything it shouldn't. Any ideas what the problem could be or how to fix?

    The bass amps do not have polarity switches...

  2. Crater


    Oct 12, 2011
    Dallas, TX area
    My guess is the common ground is the problem. You've created the classic "ground loop" situation, because you have two grounds and two ground paths. Remove the bridge ground for the separate pickup, it should electrically isolated if you're going to play it through a separate amp.

    Another solution would be to use a DI or iso box to lift the ground on signal to one amp.
  3. Lasermatch


    Feb 28, 2012
    Thanks, Crater... No question the hummmmm is classic ground loop hummm.

    I thought about disconnecting the ground to the single pickup, but I read that cutting the ground to the bridge basically makes ME the ground, does it not? Or would this make me a ground conduit to the sceond circuit's ground? If I become the conduit, then I suppose there could be humm until I touch the strings/bridge?

    I am a little concerned about doing that, though I can't pretend to have enough electronic knowledge to tell you exactly why I am...
  4. Crater


    Oct 12, 2011
    Dallas, TX area
    The added-on pickup does not need it's ground tied to the bass bridge. Keep that ground disconnected from the other pickup circuit entirely. Should be wired as:

    Pickup hot --- pot terminal/ pot terminal ---> output jack TIP terminal
    Pickup ground ---> volume pot case ---> output jack SLEEVE terminal.

    There should be no continuity between the grounds of the two output jacks. The ground of the original pickups (including the bridge ground) should work for noise elimination purposes.

    I know there are commercial basses with two outputs, "Ric-O-Sound" and others, and they apparently don't have isolated circuits, so I don't know how they're supposed to work with a two-amp setup without causing a ground loop.
  5. Use a transformer type DI with ground lift in the signal path of the second amp is one way. It will sever the common ground between amps. Disconnecting the bridge from the second output is not dangerous as long as amp number one is properly grounded. The guitar strings are still grounded through one amp just not through both amps simultaneously. This also eliminates the need for a DI.
  6. Lasermatch


    Feb 28, 2012
    Thanks guys. I know the 4001 stereos have both outputs stemming from both pickups, so collectively it is all one circuit.

    The first amp is an old SVT - which does not contain a grounded plug, while the second amp is a GK that is fully grounded. So, it looks like separating the bridge ground from #2 is likely not the best option. Instead, I will look into the transformer DI with ground lift as a viable option on the 2nd amp... I suspect the best place to put it is at the end of the effects chain before the head? Since the DI output is an XLR, can I use one of those XLR to 1/4" converters to plug into the amp input?

  7. mech

    mech Supporting Member

    Jun 20, 2008
    Meridian, MS, USA
    Lift the ground from the PU that feeds the STV. You want the metal parts and strings connected to a good earth ground for shielding. The SVT achieves it's chassis grounding by a small value capacitor that is connected to (hopefully) the low side of line cord and this is not a good ground to use for your bass.


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