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  • Refections (steel in the concrete of the nearby walln ?) can cause coherent sources.
    Objects that are microscopically small compared to the wavelength could hardly produce reflections. Rather steel parts will conduct the magnetic field and change its direction but will not influence phase. Remember we are not dealing with propagating electromagnetic waves. Rather we have independent magnetic and electric near fields.

    Anyway a guitar can and must be "sufficiently" silent without shielded cavities.
    Don't agree. Interference by electric fields can ONLY be reduced by shielding. Especially with Hi-Z PUs and when strings are not grounded good shielding is essential.
    Fluorescent lamps mainly produce electric interference fields (low current - high voltage). Electric field interference typically manifests as higher frequency noise.

    Humbucking coils can only compensate magnetic interference - not electric interference.
    Last edited by Helmholtz; 09-13-2019, 08:40 PM.
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    • But the axis of maximal length are rotated in these planes.
      This shouldn't make a noticeable difference.


      (the cancellation effect of the problem loop was best when the plane of the compensaton loop and the planes of the pickup coils were at an angle of 45°. Which should not happen.)
      Except if you are over-compensating.
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      • BTW, Ilitch advises against thick metal shielding (https://www.ilitchelectronics.com/wp...bpncsumii1.pdf) with their system.
        I assume the reason is that highly conductive shielding reduces the high frequency content in the compensating signal by Eddy currents.
        So I would go with conductive paint or very thin metal foil. Electric shielding does not require high conductivity.
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        • mhmm, but that does not explain, why one loop humbucks and the other doesn't.
          I still wonder if it is a question of loop geometry.

          Comment


          • On the scope the noise signals from the loop and from the pickup seem completely unrelated.
            If you wired the CT in series with the PU, you can just connect the scope to the output jack and alternatingly short the CT or the PU (using a short wire with clips) to see the noise signals independently.
            I would do this with both basses placed in the same position and compare noise signals (best with controls disconnected).
            Last edited by Helmholtz; 09-14-2019, 04:23 PM.
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            • Originally posted by bea View Post
              The Sharkfin is Bass 1. Primavera is Bass 2 and looks like that:

              [ATTACH=CONFIG]55198[/ATTACH]

              last question first: the cavities and the channels are unshielded. Still unshielded. Will be done later when i make the active circuits. Anyway a guitar can and must be "sufficiently" silent without shielded cavities.

              Strings are not grounded - grounding strings becomes useless as soon as soon as nylon tapewounds are used.

              Refections (steel in the concrete of the nearby walln ?) can cause coherent sources.

              But anyway - i am testing pretty close to computer, and there are many switching power supplies nearby. but there are also 3 lamps which can be clearly and uniquely identified as sources of electromagnetic fields. And moving the guitar in these fields has a strong impact on the noise. With or without compensation.

              I checked the wiring, and the schemes are identical and correct. Also the DC resistance of the CTs are identical. The length of the primary wire varies ba about 20%.

              LCR-Meter: one of my Multimeters has L and R ranges. At least large capacites seem to be measured well.

              Angle: the planes of the coils are parallel to eacch other exept of course the windings in the CT. But the axis of maximal length are rotated in these planes.

              (the cancellation effect of the problem loop was best when the plane of the compensaton loop and the planes of the pickup coils were at an angle of 45°. Which should not happen.)

              For ever future step it is necessary to remove the coil from Primavera. Which i will do now.

              EDIT: Measuring L gives 0.7 H for the loop, measured at the seconday ot the CT.
              And something around 1.1-1.2H for the pickup. So the CT system should work, and the specs should be close enough for at least some humbucking.

              bea,

              Look closely at the round string end and see the center string core wire wrapped around the round string end. Even if the outer wrappings are not metal, the center is attached to the round string end and the tail piece that electrically connects to the string end should be grounded. Strings with metal cores act a long antennas and pick up any noise induce into them. Grounding even the string cores, will minimize the noise. Give it a try. Let us know if there is any noise change? A simple alligator clip wire from the metal tailpiece to the output cable ground will work for this test.

              Joseph J. Rogowski

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              • Strings with metal cores act a long antennas and pick up any noise induce into them.
                I think the bigger source of electric noise is the body of the player when not grounded via the strings.
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                • Originally posted by Helmholtz View Post
                  I think the bigger source of electric noise is the body of the player when not grounded via the strings.
                  Yes. That is why when touching ungrounded strings the noise can increase. Some noise can be due to the orientation of the instrument to the noise source. However, in some cases, a whole area can be noisy no matter what orientation the instrument is in.

                  Joseph J. Rogowski

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                  • Try a wrist grounding strap?

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                    • Some noise can be due to the orientation of the instrument to the noise source.
                      As magnetic interference is polarized/directional and electric interference is not, a strong noise dependence on the instrument's orientation indicates dominating magnetic interference.
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                      • Originally posted by David King View Post
                        Try a wrist grounding strap?
                        Sure, but good electric shielding is more convenient.
                        - Own Opinions Only -

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                        • Originally posted by Helmholtz View Post
                          If you wired the CT in series with the PU, you can just connect the scope to the output jack and alternatingly short the CT or the PU (using a short wire with clips) to see the noise signals independently.
                          I would do this with both basses placed in the same position and compare noise signals (best with controls disconnected).
                          Whis is exactly what i did. My description on the differences between the signals were based on these. Even for the Primavera, where the loop is larger, overcompensation is not an issue.

                          These days i concentrated more on completing the sharkfin (which is sufficently noise free) electrically, actually just a micro switch with an RC parallel to the pickup and a shortcut function. This is mostly a design issue - to integrate a switch into the design without disturbing it too much and to cut a small opening in the wood. Although fine tuning parallel RC values also takes its time.

                          But of course, humbucking the Primavera remains on the ToDo list - i'm going to play with the loop orientation as well as with an unused high Z PU as the compensation coil.

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                          • Thank you

                            Hi there!

                            First post even though I follow this thread since a couple of years. Following my records I found this thread back in January 2015 and now I feel it's time to thank you for all the information given in this thread which helped me to build some LowZ pickups based on bbsailor first post. Much of what is said is far beyond my knowledge and sometimes it is really hard to understand for me. But I know one thing: It works!

                            So, I just wanted to show you some pictures of what I did. First it started with a "LowZ-Alumitone-Version".

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                            Those Pickups using self wound coils 300 wounds each and a microphone-preamp for feeding a HighZ input of an typical guitar amp. I also used them with a 1:10 step-up transformer placed after the pickups. Does work, too. But I had a hard time to match up with the preamp signal coming from the piezos in the bridge. That's why I ended up in using preamps.


                            Next thing have been current sensor based pickups either a copper or aluminum primary and two current sensors from 500 up to 2500 wounds.

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                            It is a lot of fun playing around with this design, since it's much easier for me than winding coils. My target is always to get a flat response pickup and shape the tone afterwards. This is not the true idea behind this thread, but there are a lot of advantages you get from a LowZ design even if you only trying to feed a classical HighZ guitar amp. Even this works:

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                            It is a mixed aluminum/copper primary pulled over a standard humbucker. Two totally different sounds at the same place. I'm sure that I will put this in one of my future builds.
                            The next one is a quick & dirty build for an acoustic.

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                            I remembered this thread today, because I just finished my latest pickup.

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                            What impresses me is that there are so much more possibilities in designing a pickup than having one or two coils sitting side by side, with or without a cover. Even if I only try to copy a sound which would be easier to achive the traditional way.

                            So, again: Thanks a lot! Without you this wouldn't have been possible for me to do. Still learning.

                            Cheers,
                            Micha

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                            • Micha, great work on these, they are really well executed and imaginative designs.

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                              • Micha,

                                When making low impedance pickups with thick string loop wire, look up on the web the "skin effect" This skin effect says that higher frequencies do not penetrate fully to the core of thicker wire diameters. http://diyaudioprojects.com/Technical/American-Wire-Gauge/
                                Here are a few wires that I have used.
                                AWG 4 .2043 inches diameter 5.189mm diameter 650Hz for full current depth penetration
                                6 .162 4.1148mm 1100Hz
                                8 .1285 3.2639mm 1650Hz
                                24 .0201 .51059mm 68Khz

                                When I used very thick wire for the string loop I noticed that the output level increased due to lower resistance of the string loop with more current being generated but I also noticed that the lower frequency response was being more emphasized. In your experiments, try using multiple parallel strands of thinner wire but with the same area/resistance as thicker wire and listen for the tonal changes due to the skin effect.

                                You did a nice job on your pickups. Thanks for sharing with us.

                                Joseph J. Rogowski
                                Last edited by bbsailor; 10-13-2019, 05:58 PM. Reason: added web link

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