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Shielding question

Discussion in 'Pickups & Electronics [BG]' started by KramerDon, Mar 25, 2014.


  1. KramerDon

    KramerDon

    Joined:
    Jun 24, 2011
    Location:
    Southwestern Michigan
    I see the great lengths people go to when shielding the control cavity and pickup routes.I've noticed my Nordstrand fatstacks have four conductors that aren't shielded and it brought a question to mind.Does anyone try and shield their pickup wires between the pickups and control cavity? I've got a couple inches between my pickups and control cavity where the conductors are just in a drilled hole/pathway,does it matter? (this is a new bass I'm building)
    Thanks for any input/advice!!
     
  2. line6man

    line6man

    Joined:
    Jun 20, 2008
    Location:
    Close to Los Angeles, CA
    The wires are supposed to be shielded if you intend for the shielding to do any good. Think of it like springing a leak in a submarine. It doesn't matter how small the leak is, the submarine becomes useless.

    If you're not using shielded wire, you need to wrap copper foil around the conductors.
     
  3. BlueTalon

    BlueTalon Happy Cynic Supporting Member

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    Endorsing Artist: Turnstyle Switch
    You don't need to shield the wires if the wire channels are shielded. Line6man is right, a little leak can torpedo your efforts (pun realized after the fact). But shielding your wires, and not shielding the wire channels, can leave some exposure at the holes where the channels meet the cavities.

    You can use self-adhesive copper shielding reverse-wrapped around a straw or something, slide it in, and then solder it at the cavities or just bend it back if the adhesive is conductive. Personally, I use copper tubing just because it's a little easier for me to deal with. (Sticky adhesive and tight spaces and my fingers don't get along very well.)
     
  4. ex-tension

    ex-tension

    Joined:
    Jun 11, 2009
    Also, if you have access to the connections inside the pickup you can replace the wires with a shielded 4-conductor cable.
     
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  6. uOpt

    uOpt Supporting Member

    Joined:
    Jul 21, 2008
    Location:
    Boston, MA, USA
    Yes.

    I do not shield pickup cavities (that's just silly) but if there are unshielded wires between pickups and cavity I shield that. When I feel like it.

    Those wires are often supposed to be twisted which according to people like DiMarzio reduces the noise but that doesn't work well with asymmetric setups.
     
  7. BlueTalon

    BlueTalon Happy Cynic Supporting Member

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    Why is it silly? Why is it any sillier to shield the pickup cavity than the pickup wires or any other cavity? Many pickup makers offer pickups with shielding in them, so the concept of shielding in close proximity to the pickup can't be too awfully silly.
     
  8. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Gold Supporting Member

    Joined:
    Dec 22, 2010
    Location:
    Saskatoon, Saskatchewan
    I find some jazz basses need the pickup cavities shielded in order to complete the job.
     
  9. uOpt

    uOpt Supporting Member

    Joined:
    Jul 21, 2008
    Location:
    Boston, MA, USA
    Shielding the pickup cavities doesn't give benefit but it has cost.

    The cost is that any conductor floating around in an oscillating magnetic field induces Eddy Currents, which dampen the resonance peak of the pickup. It works a bit like using a lower value volume pot. Now, this doesn't have to be bad, in fact many people want it and put e.g. baseplates under their pickups. However the point is, there is a small sound change that might or might not be audible. Shielding the pickup cavities is about the same as putting a baseplate.

    The supposed benefit is reduction in noise.

    But if you have a humbucker with shielded wire leading to it anyway there is no noise to reduce from shielding the pickup cavities.

    If you have a single coil with no hum canceling then you remove a bit of electric field interference, but you still get hit with full amounts of magnetic field interference which the shielding does nothing about. But the shielding still has Eddy Currents and can change sound.

    The way that I do it is aluminimum foil around the unshielded wires of Fender pickups and a ground wire, but I do not lay out the cavities with foil. The latter is useless and potentially harmful. The former is just shielding unshielded wires.

    If I like the dampening from the Eddy Currents I put a baseplate, not foil.
     
  10. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Gold Supporting Member

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    I am definitely not as educated on thr subject but my Jaco artist bass had buzz that did not go away until I shielded the pickup cavities. There was a grounded brass plate in there that did not suffice. When I shielded the cavities I did leave the brass plates in since they were already there. My VM fretless did not stop buzzing until the pickup cavities were shielded. There were no brass plates in it.

    I find shielding the cavities necessary. I have also never noticed a significant tonal change with any shielding projects I have done. I am sure there is some but I question whether anybody actually percieves it or it is placebo. We need another tone wood style quiz.
     
  11. bassbenj

    bassbenj

    Joined:
    Aug 11, 2009
    I have not noticed any tonal change as well. However, I am assured (and it makes sense) that wrapping foil around pickup coils INSIDE the plastic covers will indeed change the tone. The more distance you can keep the better it is.

    But given that for some reason a number of pickup makers don't even bother the ground the poles or use shielded wire, it's typical that SOMETHING needs to be done with pickups. To ignore the problem is no answer.

    A base plate (or copper foil in the bottom of the pickup cavity) is always a good start. If possible I like to run foil up the sides as well but often it's too tight a fit to do that. Grounding poles is good too. All my G&L MFD pickups come with a copper baseplate but they don't bother to ground the poles. Go figure. So when you touch a pole it injects loud hum right into the center of the coils. Nice. I Used silver conductive paint to ground them to the copper baseplate . And of course they don't use shielded wire so I run the copper tube thing. As BlueTalon says, sticky copper foil is a pain, but I never thought of just running some small copper tubing. I did the full pickup cavity shield on all my G&Ls and now they are MUCH quieter than your average L2500.

    Of course you can't shield the top of the pickups unless you do that guitar pickup thing with poles through a cover with holes, but I'm pretty confident that such a shield WOULD change the tone much more than even copper inside the pickup case. I have never see normal shielding of a pickup cavity change the tone. I HAVE however, noticed that ash trays and pickup covers (magnetic ... out of steel) DO change the tone of pickups slightly. Personally I like the tone better without the covers, but I like the LOOK better with the covers. In the end for me looks wins over tone. In any event the tone effect is quite minor.
     
  12. uOpt

    uOpt Supporting Member

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    What kind of pickups does it have?
     
  13. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Gold Supporting Member

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    No clue, Fender stock pickups. Single coils. It is the MIA Jaco Sig, I have the custom shop version that the hum was not removed from when I shielded everything but the cavities. I intend to shield the cavities next, and ideally complete the shielding. It does have brass plates. The VM fretless I had was the Duncan Design pickups. Oh and I shield the pickup covers, including the tops of them.
     
  14. uOpt

    uOpt Supporting Member

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    If they are single coils, how could the hum possibly have stopped completely by shielding the cavities? You will still pick up magnetic interference that the shielding doesn't do anything about.
     
  15. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Gold Supporting Member

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    I am not an electrical engineer but both the basses were silent, any noise coming through the amp at this point could just as easily be amp noise or wall power. There is no difference in touching the strings or not, and (so far) I have never gotten buzz when soloing one pickup. I am thinking the latter is probably due to the places I have played.

    They might not be single coils, I really don't know how to tell I am just assuming a 62 jazz bass has single coils. Since that is essentially what this bass is.
     
  16. uOpt

    uOpt Supporting Member

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    Is that just in one environment or does it stay completely silent when going e.g. to a bar?
     
  17. Audere

    Audere Supporting Member

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    I will answer this like an engineer :rollno: :D

    The shielding will not help the magnetic field hum and noise issues...

    Recently I have taken data related to shielding of pickup cavities and how it affects the response of single coil pickups in level and phase response.

    The shielding was around the outside of the cavities and not internal to the covers.
    The pickups were Duncan Designed 4 strings from a VM Squier which have less internal capacitance than most pickups so the affects of the shielding is larger than typical.

    Magnetic scans of pickups in a shielded cavity of a bass vs. on a table top
    In Mid Z-Mode (aka like passive or with standard preamps)
    www.audereaudio.com/images/MidZ-shield-raw.png
    In High Z-Mode where the change in resonance peak would be the biggest and the extra capacitance would change the peak location to a lower frequency (data taken with our default setup in a Pro JZ preamp)
    www.audereaudio.com/images/HighZ-shield-raw.png

    The higher curves are the unshielded pickups in all cases but the changes (IMO) are small.

    This is consistent with LCR meter readings at 20 KHz
    The change in capacitance for a series model was Raw 119.7p vs. Shielded 123.3p

    If you want to reduce the eddy current affects more - you could cut a small slit in the shielding so the loop is not closed around the pickup body.
    You would still get a high percentage of shielding coverage.

    Note: With some hum cancelling pickups depending on how they are made the changes due to shielding is bigger but I do not have any handy data to present.

    David
     
  18. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Gold Supporting Member

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    So far it has been silent in the handful of places I have played it, homes and jamspace. Have not gigged it yet. I sold the VM fretless some time ago to another TBer, I don't know how it performs these days but it was silent for me.
     
  19. uOpt

    uOpt Supporting Member

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    Jul 21, 2008
    Location:
    Boston, MA, USA
    OK. But it really is true that you foil shielding doesn't work against the electromagnetic interference. That is way in basic physics and not on those borderlines where musicians physicists argue all the time.

    Your claim that you turned a noisy single coil completely quiet with shielding is problematic, especially if it already had a properly grounded baseplate under the pickups.

    You can have environments that have very little magnetic interference, so maybe it's just that.

    Or your standards are different. You say that there is no difference touching the strings. Maybe your strings weren't grounded and it buzzed a whole lotta lot before and now it sounds comparably quiet.
     
  20. DiabolusInMusic

    DiabolusInMusic Functionless Art is Merely Tolerated Vandalism Gold Supporting Member

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    I am not thinking shielding defeats 60hz hum, I am just saying I have had clear shielding issues (hum that goes away when you touch metal) that was not removed until I shielded the pickup cavities as well. I had done many P basses before I had done a jazz, since the control cavity and pickup cavity are continuous on P basses it was never an issue. When I did my first jazz bass I did not shield the pickup cavities and the hum remained. After shielding the pickup cavities that hum has been removed. I am sure it is there in some miniscule level, but there is no discernible difference in sound whether you have your hand on the strings or not. As far as I am concerned that is silent.
     
  21. uOpt

    uOpt Supporting Member

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    Ahhh, and there it is.

    Normally the strings are grounding the player and the player acts as a shield. If you stop touching the strings it buzzes and hums. That is expected behavior the way Les and Leo designed these things. I don't like it either.

    Yes if you want to stop this madness (I'm all for it) then you need a complete shield.

    You should have mentioned that detail about not touching the strings before.
     

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