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  • #16
    Rethinking, for the string through coil PU, a bar magnet magnetized like a PAF magnet, placed flat under the coil should work.
    I.e. north- south axis parallel to the strings.
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    • #17
      Hi, there. Looking at your post, nice work. Reminded me of an old patent (US #4,378,722). Looks like it was placed close to the bridge for the obvious reason that it would interfere with playing, but it also looks to alleviate the issue of excessive string movement as well, a problem you mentioned. Wondering if you could measure and post output (a/c voltage, millivolts) as a comparison to standard designs. Thanks.

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      • #18
        Originally posted by edwardta View Post
        Hi, there. Looking at your post, nice work. Reminded me of an old patent (US #4,378,722). Looks like it was placed close to the bridge for the obvious reason that it would interfere with playing, but it also looks to alleviate the issue of excessive string movement as well, a problem you mentioned. Wondering if you could measure and post output (a/c voltage, millivolts) as a comparison to standard designs. Thanks.
        One more thing to consider is string grounding. There is a commercial product called “Plate Mate” for acoustic guitars to prevent wood gouging under the wood plate. I have suggested simply soldering a ground wire to this copper plate to minimize pickup noise by grounding the strings to prevent them from picking up induced noise while acting as ungrounded antennas to the acoustic pickups primary signal.

        Pickup output signal is always very important but more so compared to the noise it also picks up (pun intended).

        Any pickup needs to have all metal parts at a ground level near the string ground level to minimize noise pickup (pun not intended).

        Bottom line: Ground strings for the lowest noise pickup!

        Joseph J. Rogowski
        Last edited by bbsailor; 10-26-2021, 10:13 PM.

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        • #19
          Fender made those in the 1940s used on their dual pro stgeel guitars- the pickup is commonly reffered to as the Fender trapazoid. The bass strings sound unusually clear
          https://music-electronics-forum.com/...obbyist/37745-

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          • #20
            Long thin permeable structures tend to magnetize along their length for a wide range of magnet locations, making a field that passes through the coil that falls off as the distance from the permeable structure increases. So if you pass the string exactly through the center of the coil, I would expect frequency doubling only (by symmetry), but the fraction of doubled signal should decrease as the string is located further from the center.

            I am wondering if this could be used to improve upon Zollner's method for measuring string magnetization using a tiny moving coil. Instead, use a small coil that is fixed for each measurement, but make a set of measurements where for each the coil is moved a bit along the string, changing its position with respect to the magnet, and so sampling the magnetization at increasing distances from the magnet.

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            • #21
              Originally posted by Lollar Jason View Post
              Fender made those in the 1940s used on their dual pro stgeel guitars- the pickup is commonly reffered to as the Fender trapazoid.
              https://music-electronics-forum.com/...obbyist/37745-
              Well there ya go.

              So, I don't have any equipment to measure output and all the rest, just my aging ears. I played around with it some more, including using both coils arranged like a stack on its side with the strings going thru for humbucking.

              Definitely needs magnets above and below, otherwise volume changes a lot with up-down movement.

              Without any magnet the volume was very low, lower than it seemed in my first test, in which i probably just had the amp volume set higher. But still there was some sound.

              Tried as best I could to compare it with the X2N in it's normal configuration held above the strings. Of course it wasnt a fair comparison, as I had to remove the string and then reassemble the pikcup in between comparison, and I didn't record it. And I didn't even mark where the amp volume was set... But with that said, it definitely seemed to be at least as loud in string thru mode, if not louder. And possibly clearer sounding, probably due to lower inductance without those big rails in the cores.

              I'll probably use the string thru design on my next build, and once it's fully installed ill be able to compare directly by holding a second pickup over the strings, with a switching device wired up.

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              • #22
                Originally posted by madzub View Post
                ....
                But with that said, it definitely seemed to be at least as loud in string thru mode, if not louder. And possibly clearer sounding, probably due to lower inductance without those big rails in the cores.
                ....
                I think this has good output because the coil is sensitive to the field from the string produced by the magnetization along the string while the normal pickup is sensitive to the change in magnetization along the string.

                Might be interesting to use a separate coil for each string.

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                • #23
                  My explanation:

                  No signal (induced EMF) without a changing net flux through the coil.

                  With a normal PU, the change of flux is essentially a consequence of the varying air gap between the strings and the PU poles, as the airgap contributes the major part of the varying reluctance. This works even if the string is in saturation.

                  With a string-through coil PU, the signal is produced by the changing flux through/along the string (again caused by the varying airgap of the magnetic circuit).
                  As a consequence the magnet(s) shouldn't be too strong in order to avoid string saturation.

                  .
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                  • #24
                    For the string through:
                    Consider a simple case with these assumptions:
                    1. A single layer circular coil.
                    2. String located at the center of the coil.
                    3. String saturated with magnetization along the string.

                    There is a field external to the string pointing in the direction of the string; this field gets weaker further from the string. When the string vibrates in any direction the flux through the coil decreases because the flux that leaves the coil is denser than the flux that enters it. Relax the above assumptions, and it still works in a similar way, but it is more complicated to analyze.

                    For the normal pickup:
                    The string could be saturated away from the pole on either side, and the magnetization points along the string (almost precisely) but in opposite directions on the two sides of the pole piece. There is a region over the pole piece where the string is certainly not saturated because the magnetization is transitioning from one direction to the other. In this transition region there is a field component perpendicular to the string and thus through the coil. This field weakens with distance from the string. Therefore, when the string vibrates, the flux through the coil changes.

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                    • #25
                      Originally posted by Mike Sulzer View Post
                      For the string through:
                      Consider a simple case with these assumptions:
                      1. A single layer circular coil.
                      2. String located at the center of the coil.
                      3. String saturated with magnetization along the string.

                      There is a field external to the string pointing in the direction of the string; this field gets weaker further from the string. When the string vibrates in any direction the flux through the coil decreases because the flux that leaves the coil is denser than the flux that enters it. Relax the above assumptions, and it still works in a similar way, but it is more complicated to analyze.
                      .
                      The flux through a longitudinally saturated string is essentially constant. The coil is not sensitive to any radial fields.
                      Any outside longitudinal flux density should be several hundred times smaller than the flux density in the string.
                      So I don't think a strong signal could be produced.

                      I'd expect a much stronger signal with a bar magnet (having its magnetic axis parallel to the strings) above or below the coil and non-saturated strings, because that would allow for flux variation within the strings.

                      For the normal pickup pole piece. There is a region over the pole piece where the string is certainly not saturated because the magnetization is transitioning from one direction to the other. In this transition region there is a field component perpendicular to the string and thus through the coil.
                      True for single coil PUs. Not for side-by-side humbuckers.

                      But as said, the major part of the flux change through the coil should be caused by the modulation of the (distributed) air reluctance, being much larger than the reluctance of the string.
                      Last edited by Helmholtz; 10-30-2021, 12:50 AM.
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                      • #26
                        Originally posted by Helmholtz View Post

                        The flux through a longitudinally saturated string is essentially constant. The coil is not sensitive to any radial fields.
                        Any outside longitudinal flux density should be several hundred times smaller than the flux density in the string.
                        So I don't think a strong signal could be produced.

                        I'd expect a much stronger signal with a bar magnet (having its magnetic axis parallel to the strings) above or below the coil and non-saturated strings, because that would allow for flux variation within the strings.



                        True for single coil PUs. Not for side-by-side humbuckers.

                        But as said, the major part of the flux change through the coil should be caused by the modulation of the (distributed) air reluctance, being much larger than the reluctance of the string.

                        OK, I think some calculations are necessary to see how this works.

                        On the difference between single coil and humbucker: They both work essentially the same way. This is what the plots in Zollner's 5.4.3 show. If we consider coil A, then the magnets in coil B contribute some field, producing an asymmetry over the pole pieces. The reversal of the direction of magnetization occurs for hbs as well as scs, it just does not pass through zero right over the pole.

                        modulation of the (distributed) air reluctance: This is too vague for me, as these things can be. For example, near the beginning of 5.4.3 we have an attempt to define the scale size of the source for the alternating field:

                        "The source of the static field is the magnet; its two poles are separated by 1 – 2 cm which results in a relatively large path of flux. The main source of the alternating flux, on the other hand, is the air gap reluctance in front of the pole plate, this gap being variable due to the string oscillation. Since the associated dimensions are significantly smaller, the extent in space of the alternating flux is also limited to a smaller sector."

                        If this is true in general, it must be true when the relative permeability of the pole is unity. Then the pole has no significance in the magnetic circuit, and so how can a space bounded on one side by it be defined? The source of the alternating field is the string magnetization. The relevant scale size is the distance over which the magnetization changes direction, that is, approximately the pole width. So the point he its making is correct, but the explanation is either too vague to be useful, or maybe incorrect.


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                        • #27
                          Originally posted by Mike Sulzer View Post

                          If this is true in general, it must be true when the relative permeability of the pole is unity. Then the pole has no significance in the magnetic circuit, and so how can a space bounded on one side by it be defined? The source of the alternating field is the string magnetization. The relevant scale size is the distance over which the magnetization changes direction, that is, approximately the pole width. So the point he its making is correct, but the explanation is either too vague to be useful, or maybe incorrect.
                          In my words (for normal PUs):

                          Even if the AC permeabilty (aka recoil permeability) of the pole/core is low, it carries a static flux (otherwise it wouldn't be a pole).
                          The upper part of the magnetic circuit consists of the pole (emitting the static flux), the string and the air between the pole and the string.
                          Signal is induced by the change of the net flux inside the coil, i.e the variation of the static flux in the pole.
                          Flux change is caused by the change of total circuit reluctance.
                          As the reluctance of the string is typically small compared to the series reluctance of the air, the major change of flux is caused by the variation of the airgap.

                          I consider the string as a passive (though non-linear) "impedance" in the magnetic circuit.
                          Last edited by Helmholtz; 11-01-2021, 10:52 PM.
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                          • #28
                            Originally posted by Helmholtz View Post

                            In my words (for normal PUs):

                            Even if the AC permeabilty (aka recoil permeability) of the pole/core is low, it carries a static flux (otherwise it wouldn't be a pole).
                            The upper part of the magnetic circuit consists of the pole (emitting the static flux), the string and the air between the pole and the string.
                            Signal is induced by the change of the net flux inside the coil, i.e the variation of the static flux in the pole.
                            Flux change is caused by the change of total circuit reluctance.
                            As the reluctance of the string is typically small compared to the series reluctance of the air, the major change of flux is caused by the variation of the airgap.

                            I consider the string as a passive (though non-linear) "impedance" in the magnetic circuit.
                            Here are two reasons why this explanation does not work:
                            1. The permanent magnetic pole piece does not need to be inside the coil. Since the magnetization excited in the string is almost perfectly longitudinal, the permanent magnetic pole can be located elsewhere with almost no change in pickup operation. Your explanation requires the magnetic pole to be part of a magnetic circuit and located inside the coil.
                            2. The system composed of the pickup and string does not meet the conditions required to define a magnetic circuit composed of two terminal devices characterized by reluctance, connected in at least one loop, and driven by MMF. This system is described by a differential equation, requiring solution by an appropriate technique. Application of circuit concepts is not such a technique.

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                            • #29
                              Originally posted by Mike Sulzer View Post
                              Here are two reasons why this explanation does not work:
                              1. The permanent magnetic pole piece does not need to be inside the coil. Since the magnetization excited in the string is almost perfectly longitudinal, the permanent magnetic pole can be located elsewhere with almost no change in pickup operation. Your explanation requires the magnetic pole to be part of a magnetic circuit and located inside the coil.
                              2. The system composed of the pickup and string does not meet the conditions required to define a magnetic circuit composed of two terminal devices characterized by reluctance, connected in at least one loop, and driven by MMF. This system is described by a differential equation, requiring solution by an appropriate technique. Application of circuit concepts is not such a technique.
                              1. My explanation above is explicitly for the „normal“ PU (see headline).
                                It's the model Zollner and other authors use and works well enough for me.
                              2. The magnetic circuit model can especially be used when the symmetry allows treatment as a 2-dimensional problem and the circuit components as well as the source(s) can be identified.
                              3. I can imagine an arrangement where the magnetized string is the only (local) source of static flux. This means a different magnetic circuit. It doesn't mean the magnetic circuit model can't be used for a qualitative analysis.
                              Last edited by Helmholtz; 11-02-2021, 04:33 PM.
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