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Shielding a single coil

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  • #16
    Voodoochild, do a quick search of the term "eddy currents" to get the lowdown.
    You don't want a continuous loop of metal around a pickup. An overlapping loop that's insulated between the ends is going to still give you 100% coverage (on the sides anyway) and there won't be a major sucking path for the eddy currents to form. (They will still be there in the foil but you will have stymied them somewhat). Eddy currents = significant losses at higher frequencies. Not so sure about your low-mids -though that may be an auditory sensation from losing highs...

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    • #17
      Originally posted by voodoochild View Post
      I'm not sure I follow you. I wrapped the coil with paper tape and then I wrapped copper foil around that and soldered it to ground. Are you saying I should leave a gap or a break in the copper somewhere around the coil?
      Exactly. The usual method is to have an insulated overlap.

      If so wouldn't that defeat the purpose of shielding it? I'm a newbe when it comes to shielding pickups themselves but I have been shielding guitars for 20 years. It sounds like there is a different method for pickups?
      A shield on the pickup is far closer to the action than the shielding in the guitar pickup cavity, so even if there are eddy currents in the copper foil cavity shield, it's too distant from the pickup to make much difference.

      A nickel-powder loaded conductive paint cavity shield has too high a resistance (~20 ohms) for eddy currents to matter as well, even though the resistance is low enough to be totally effective against electrostatic fields.

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      • #18
        Originally posted by Joe Gwinn View Post
        A shield on the pickup is far closer to the action than the shielding in the guitar pickup cavity, so even if there are eddy currents in the copper foil cavity shield, it's too distant from the pickup to make much difference.
        And it is very easy to understand why the distance makes a difference. What you need to do is "add up" all the field lines that pass through the conducting loop. Field lines are continuous: those that pass through the core loop around and come back in the other end. But if your loop is big enough to include a field when it loops around as well as when it passes through the core, the effect of that line cancels out. That is because the line points "up" once and "down" the other time. If the loop is big enough, nearly all lines are included both times.

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        • #19
          So it's better to shield the cavity than the coil? Or it's OK to shield the coil but you just have to be careful about how you go about it?

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          • #20
            Originally posted by David King View Post
            So it's better to shield the cavity than the coil? Or it's OK to shield the coil but you just have to be careful about how you go about it?
            If you shield the cavity, you also protect the wiring and pots and switches, more or less for no added cost.

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            • #21
              Thanks for all the good info. I always like to keep it old school and simple, and this time I stepped out on a limb and broke it. But now I know. I'll have to try it again soon and do it right next time.

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              • #22
                Originally posted by David King View Post
                So it's better to shield the cavity than the coil? Or it's OK to shield the coil but you just have to be careful about how you go about it?
                I think it's better to shield the pickup. If you shield the pickup cavity you are only shielding the back and maybe the sides of the pickup. Shielding the control cavity is always a good idea, of course.

                Here's a good way to do it. This is a Duncan MM pickup. Note the way the foil is wrapped around the coils... they have it closed in the back, and open in the front where the poles come out. This grounds the poles, and the magnets in this case are glued to the foil. Of course if you are using rod magnets, than that doesn't apply.

                This shields the coil, while not being a closed loop. It doesn't even go around the coil in the same direction the wire does.

                I completely shield my pickups, but I do something different for shielding. I can hold them a few inches from my CRT monitor without all that usual buzzing you get.
                Attached Files
                It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein


                http://coneyislandguitars.com
                www.soundcloud.com/davidravenmoon

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                • #23
                  So I tried this again on my own time to see what effect it had. I finally got the technique right by insulating the ends so it didn't create a loop. I found that it still took something away from the pickup that made it sound different in a worse sorta way. It was even more noticeable when I shielded across the magnets. I think I would rather stick with the old school single coil design without shielding. It just plain sounds better to me. On a positive note I think this will get me experimenting with dummy coils isolated further away from the single coils to reduce hum without noticeably changing the tone.

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                  • #24
                    Originally posted by voodoochild View Post
                    It was even more noticeable when I shielded across the magnets. ... On a positive note I think this will get me experimenting with dummy coils isolated further away from the single coils to reduce hum without noticeably changing the tone.
                    Shielding across the magnets provides a better path for the current than an unconnected loop around the coil; so your result makes sense.

                    A dummy coil can work well, but it is not the same as a shield. The shield is for electric fields; the coil for canceling magnetic fields. In my experience, a minimum amount of shielding is adequate for the electric fields, and the cancellation of the magnetic field is more important. So I think that you are headed in the right direction.

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                    • #25
                      Voodoo,
                      I say skip the foil and try the paint, you might just like that better.

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                      • #26
                        Originally posted by voodoochild View Post
                        So I tried this again on my own time to see what effect it had. I finally got the technique right by insulating the ends so it didn't create a loop. I found that it still took something away from the pickup that made it sound different in a worse sorta way. It was even more noticeable when I shielded across the magnets. I think I would rather stick with the old school single coil design without shielding. It just plain sounds better to me.
                        You may have raised the self-capacitance of the coil enough to matter. Some layers of insulating tape between top of coil and the shield foil can help.

                        On a positive note I think this will get me experimenting with dummy coils isolated further away from the single coils to reduce hum without noticeably changing the tone.
                        As others have mentioned, humbucking and shielding solve different problems

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                        • #27
                          Originally posted by Joe Gwinn View Post
                          A nickel-powder loaded conductive paint cavity shield has too high a resistance (~20 ohms) for eddy currents to matter as well, even though the resistance is low enough to be totally effective against electrostatic fields.
                          BTW Joe - I tried that MG Chemicals Nickel Super Shield you recommended .. WOW! I'm now on my fourth can. I made a batch of guitars this week with P-90's, played them sitting on top of the amp, with a 4 ft floresant light over head and I couldn't detect hum at medium volume. I was blown away at how quiet the guitar was. I had the guitars setup RWRP and it really didnt need it. Thanks for the recommendation.

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                          • #28
                            Originally posted by belwar View Post
                            BTW Joe - I tried that MG Chemicals Nickel Super Shield you recommended .. WOW! I'm now on my fourth can. I made a batch of guitars this week with P-90's, played them sitting on top of the amp, with a 4 ft floresant light over head and I couldn't detect hum at medium volume. I was blown away at how quiet the guitar was. I had the guitars setup RWRP and it really didnt need it. Thanks for the recommendation.
                            Quite welcome. One thing to be aware of is that depending on how and how well one sprays the shield paint, one can end up with the paint on the bottom of the routed cavity insulated from the paint on the sidewalls, reducing shielding effectiveness. The test is easy. Using an ordinary DC ohmmeter, probe around, trying all combinations of surfaces. If you always get ~20 ohms or less, no problem. If you get a much higher value anywhere, inspect under bright light to see where the painting technique needs improvement. Once you have developed a good painting process, the ohmmeter tests become unnecessary.

                            Priming the raw wood cavity before applying shield paint may reduce shielding paint consumption enough to be worthwhile.

                            The dried paint film is thin and somewhat delicate, so for long life the electrical connection to the film should involve a wide area of contact, such as a brass washer, versus just a wire and a small wood screw. Another good approach is where the film extends out on the surface of the guitar body, with area contact being made all around the routed cavity by some metal foil attached to the underside of the pickguard.

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                            • #29
                              Vodoochild, my 2 cents.

                              this is an article I wrote for an italian magazine concerning Stratocaster shielding, including pickups. It' s in italian, but you should easily translate it using Google translator. Pictures are also quite eloquent

                              Hope it can help you.

                              How to shield a Strato



                              "We' ve been making overclocked pickups since 1983"

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                              • #30
                                ...

                                There is a website somewhere that is mostly about making strats quiet, they use something called star grounding I think it is. I did some strat pickups for an up and coming "star" and his tech used star grounding and some other technique he wouldn't tell me about and got the guitar dead silent without screwing with the pickups. I found it:
                                GuitarNuts.com - The Internet Home for Wayward Pickers
                                http://www.SDpickups.com
                                Stephens Design Pickups

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