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Question about shielding

Discussion in 'Luthier's Corner' started by bassist4christ, Oct 24, 2005.

  1. bassist4christ

    bassist4christ Banned

    May 26, 2005
    I was reading the other posts and they keep talking about grounding, is that just the regular ground for the wires, or a new ground for the shielding? Also what would happen if I used alummin foil and it touched the (passive) electronics inside my bass?
  2. jwymore


    Jul 26, 2001
    Portland, OR
    All the shielding must have a path to ground to be effective. There are various ways to do this but the bottom line is it has to be grounded somehow back to th ground side of the input jack.

    Aluminum foil will work but it can't really be soldered to make good connections. Copper is better.

    Since the shielding is grounded you don't want any signal carrying or hot wires touching the shielding.
  3. bassist4christ

    bassist4christ Banned

    May 26, 2005
    How is a easy way to ground it back to the input jack? Thanks for your time.
  4. Worshiper


    Aug 13, 2004
    New York
    wait...I'm confused. If you don't want any "hot wires" touching the shielded surfaces, what is the point of shielding?
  5. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    Shielding the electronics cavity is the same idea as having a coaxial cable. The grounded outer braid reduces electromagnetic pickup in the "hot" wire.

    If you were to connect the hot signal to the shielding (which is by definition connected to ground) that there would be no output. It'd be exactly like having the volume knob fully down all the time.
  6. Dugz Ink

    Dugz Ink

    Oct 23, 2005
    To kill RFI/EMI by connecting it to the ground.

    I use thin sheets of copper, which can easily be formed to fit the cavity/space, and then I solder a wire to the copper. (The wire connects to the output jack's "ground" pin.)
  7. I successfully combined aluminum (aussies say "aluminium") foil and cotton-mesh, double-sided carpet tape (the kind used to tape down carpets) into a very usuable shielding material...

    not as good as copper, but way more usable than plain foil...
    i've done two basses and a homemade outboard preamp box with it and it works very well.
  8. actually, Geoff, shielding the electronics cavity it is the same idea as having "shielded" cable (this is also known as a Faraday shield).

    Coaxial cable is a little different in that the outer sheath forms a waveguide for the high frequency signal that is being launched down the cable...it is used to contain or "guide" the HF waveform down the cable...

    but with that said...shielding works by draining off low frequency (audio spectrum) noise that could otherwise enter the signal path (through capacitors and jacks, etc..)...basically this noise cannot enter into the cavity because the shield (which is grounded) drains it away before it can enter the signal conductor (which is sent to the amp through a shield cable).
  9. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    I used coaxial as most people should have a better mental image of this type of a setup, because it is "coaxial" wires in that both share the same axis (one surrounds the other). It is true that in technical speak a coaxial cable is a waveguide (like the RG-58 that we use for all our data acquisition in our lab). I doubt most people even know what a waveguide is (or TEM, TE or TM modes). Any, technically you are the correct one.
  10. I know I was splitting hairs. But, I figured..."why not?"....:D

    How's the LP coming? I can't wait to see the finished product. How's the weather in Canada? It's starting to get a bit sticky here in Tropical Queensland.
  11. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    It's fine for clarity's sake. Anything rigorous is always good in my book.

    The LP's coming along nicely. I should really get a picture or two up. Another coat of poly is drying right now, so maybe I'll wet sand it and then snap some photos after this coat (which I'm hoping will be the last). I have to get into the machine shop this week so I can resaw some padauk to make my pickguard. Other than that I think everything is good to go, the cavity cover and pickup rings are drying with the neck right now and the electronics are wired and ready to go in.

    Here in Canada change is in the air, the temperatures falling along with the leaves (which are making a good mess as well). I bet things are nice in Queensland. On comes the winter here. :scowl:
  12. bassist4christ

    bassist4christ Banned

    May 26, 2005
    So how is a good and easy way to ground the shielding?
  13. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    What I do is that I take all of my other ground wires (volume pots, pickups, bridge) and bring them (soldered) to an o-ring terminal that I screw into the cavity that presses against the shielding. Then I run a single wire from there to the ground lug on the output jack. It depends how you are doing it. If you have grounded the back of any of the pots and the pots are touching the wiring then you might already be grounding the shielding. I don't like doing it that way though because you typically wind up with ground loops which make things noisier (since the wiring starts to act like an antenna for you).
  14. what Geoff is describing is a "star ground" one of the best grounding techniques (if not THE best).

    If I were building up an electronics cavity from scratch, I'd do it this way, too.
  15. bassist4christ

    bassist4christ Banned

    May 26, 2005
    I have a cheaper bass, it only has one ground wire leaving the tone pot going under the bridge. Can I take that ground wire and ground it to foil?
  16. leave the wire going to your bridge...you'll need that

    best thing is to solder a new wire to your tone pot, and then attach THAT wire to the foil.

    if you tin your wire first, it will solder much easier to the pot..

    your pots have little eyelets or solder spots that are used to connect the solder tabs of your pots to the internals


    make sure that you insulate these from the foil...you can paint them with clear nail polish...or apply a little bit of electrical tape (the nail polish is a much neater solution)...

    if these touch your foil, chances are your bass will cease producing sound...
  17. Dugz Ink

    Dugz Ink

    Oct 23, 2005
    Hmmm... I don't want to start a fight, but I would completely disagree with that; I would keep all grounds that are for the purpose of RFI/EMI control seperated as much as possible from the pots.

    The problem: Shielding that picks up high levels of RFI/EMI can also saturate the pot chassis with RFI/EMI, thus putting it in close proxinity to the electrics as it passes to the ground.

    Of course, I did most of my wiring in radio stations, where we had a lot of RFI/EMI, so what we did might be over-kill for a guitar... unless it's being played near a 5,000 watt AM transmitter, at which point the foil won't cut it.

    Anyhow... this is what I do:

    1) Bridge ground to shielding to "ground" terminal on output jack, via a seperate (dedicated) wire... usually white insulation, so it stands out when I'm working.

    2) Pot chassis to "ground" terminal on output jack, via the shielding on a shielded cable. NOTE: If you do this, do not allow the pot chassis to touch the shielding; it will cause a ground loop. If there's a diference in resistance, it will produce a light hum.

    3) The "ground" wire for the caps and pots (not the chassis) to the "ground" terminal on the output jack via one of the wires that's inside the shielded cable.

    Yes, all grounds connect to the same spot. But the RFI/EMI from the shielding does not pass by/through the chassis of any of the pots, and the RFI/EMI from the chassis does not pass through the "ground" for the audio... at least not until you're past all of the susceptable devices.

    Resistance at a point of connection (caused by oxidized connectors, dirty contacts, bad solder, etc.) is highly susceptable to RFI/EMI... like a noise-magnet. Surrounding a device that has resistance at connections with RFI/EMI guarantees trouble. So I do what I can to keep RFI/EMI as far from connections and contacts (like pots) as possible.

    Add the usual disclaimers... my opinion, results may vary, etc.

  18. Worshiper


    Aug 13, 2004
    New York
    ok...here's the thing that I'mhung up on. If the pots are made of metal and only the hot signal is supposed to be on the pot, but the pot is touching hte sheilding surface, why doesn't the shielded surface recieve the hot signal.

    Sorry if I'm being thick headed here, electricity is not my forte. I'm more of a magnets and math kind of guy.
  19. Geoff St. Germaine

    Geoff St. Germaine Commercial User

    The hot signal from the pickups should not be going to the chasis of the pot. The pot chasis is isolated from the internal components and the lugs so there is no connection between them.

    Do you understand how the internals of the pot work?
  20. Worshiper


    Aug 13, 2004
    New York
    to be honest...No. hehe :help: