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  • Whaddya get when....

    You pull one of the bar magnets on a P90 and only provide the one primary sensing area? First, is this even feasible and nondestructive? I have no idea if the bar magnets are glued in or removable once installed.

  • #2
    Magnets are not glued in. Total magnetic flux will be halved, resulting in less sensitivity/output. Sensing width (aperture) will be somewhat reduced resulting in a little more treble.
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    • #3
      Some cheap P90s that come in some cheap imports?
      Only have one ceramic maget. North to the Pole screws, best I remember.
      They don't sound too bad, but I always put two Alnicos in, when I rebuilt them.
      T


      "If Wishes Were Horses, Beggars Would Ride!" Scottish Proverb 1600s
      Terry

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      • #4
        Full acknowledgement and head nods to previous posters. If no other options are possible it'll do a nice trick on a stuffy, snotty mid-range focused P90 in the neck position for sure.

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        • #5
          Originally posted by Mark Hammer View Post
          You pull one of the bar magnets on a P90 and only provide the one primary sensing area? First, is this even feasible and nondestructive? I have no idea if the bar magnets are glued in or removable once installed.
          The sensing area is dictated by the screws, far more than the magnets, because the steel screws have a much higher permeability that the magnets, and the screws are much closer to the strings, so there is only ever one primary sensing area. The end result of removing one of the magnets won't be much different from lowering the pickup a few millimeters, it's just two different ways of getting a weaker magnetic flux between the pickup and strings. I'd try lowering the pickup before going through all that trouble.

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          • #6
            Actually I don't understand the concept of a primary sensing area (what would be the secondary one?).
            A PU's aperture (string sensing length) is essentially the magnetized section/length of the string above the PU. With 2 magnets the aperture will be symmetrical with reference to the screws, with only one magnet it will be single sided.
            The same flux that enters the string above the screws will leave it after travelling several mm along the string and then return to the magnet's opposite pole.

            Magnetic flux acts like electric current: It always flows in loops (from one pole of it's source to the opposite one), taking the path of least "resistance" and eventually re-enters it's source (magnet). Nothing gets lost.
            (But other than electric current, magnetic flux can't be stopped as there is no magnetic insulator.)
            Last edited by Helmholtz; 04-28-2020, 11:25 PM.
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            • #7
              Originally posted by Helmholtz View Post
              Actually I don't understand the concept of a primary sensing area (what would be the secondary one?).
              A PU's aperture (string sensing length) is essentially the magnetized section/length of the string above the PU. With 2 magnets the aperture will be symmetrical with reference to the screws, with only one magnet it will be single sided.
              More or less what I meant.
              When I say "secondary", I am assuming that there is some small field, flux, or whatever one calls it, between the top of the screws and side of the keeper bar that the bar magnet is coupled to on one side.

              I should probably explain why I asked the question in the first place. A friend has an Epiphone Casino - a true hollow body (no centre block) with two P90 pickups. He wants to remove the neck pickup and try using a Telecaster neck pickup in its place, with an adaptor that employs the same screw holes as the P90 cover. I said "Well why don't you just pull one of the magnets on the neck pickup?", thinking that it would yield the thinner sound he is thinking of trying.

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              • #8
                He wants to remove the neck pickup and try using a Telecaster neck pickup in its place, with an adaptor that employs the same screw holes as the P90 cover. I said "Well why don't you just pull one of the magnets on the neck pickup?", thinking that it would yield the thinner sound he is thinking of trying.
                Ok, that makes it clearer. Removing one magnet from the P-90 won't give you the treble response of a Tele PU and it won't make it much brighter.
                Actually the aperture (string sensing length) of a P-90 with both magnets isn't much wider than that of a Fender type PU. The main difference is in the PUs' filter responses, in other words the different resonant frequencies. To get in the ballpark of a Tele PU you would need a P-90 with a much lower turns number, maybe 5,000 instead of the typical 10,000 turns.
                Last edited by Helmholtz; 04-28-2020, 11:02 PM.
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                • #9
                  Originally posted by Mark Hammer View Post
                  I should probably explain why I asked the question in the first place. A friend has an Epiphone Casino - a true hollow body (no centre block) with two P90 pickups. He wants to remove the neck pickup and try using a Telecaster neck pickup in its place, with an adaptor that employs the same screw holes as the P90 cover. I said "Well why don't you just pull one of the magnets on the neck pickup?", thinking that it would yield the thinner sound he is thinking of trying.
                  Hello,

                  Here is my humble attempt to help:

                  https://forum.seymourduncan.com/show...arrowfield-mod

                  FWIW (my 2 cents). :-)

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                  • #10
                    Very interesting AND helpful. Thanks for that!

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                    • #11
                      Originally posted by freefrog View Post
                      Hello,

                      Here is my humble attempt to help:

                      https://forum.seymourduncan.com/show...arrowfield-mod

                      FWIW (my 2 cents). :-)
                      Interesting measurement results.

                      While I don't see a basic difference in (averaged) frequency response between 2 magnets and one magnet on neck side (except for a level difference of roughly 6dB), a single magnet on bridge side seems to reveal an additional effect.
                      I think it can be explained by a shift of the string sensing center towards the bridge, which would be equivalent to a PU position closer to the bridge, resulting in less fundamental frequency content.

                      It is no surprize that inductance only changes slightly, because the permeability of Alnico is very low. So the resonant frequency won't shift perceivably.
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                      • #12
                        Originally posted by Mark Hammer View Post
                        Very interesting AND helpful. Thanks for that!
                        You're welcome. :-)

                        Interesting measurement results.

                        While I don't see a basic difference in (averaged) frequency response between 2 magnets and one magnet on neck side (except for a level difference of roughly 6dB), a single magnet on bridge side seems to reveal an additional effect.
                        I think it can be explained by a shift of the string sensing center towards the bridge, which would be equivalent to a PU position closer to the bridge, resulting in less fundamental frequency content.

                        It is no surprize that inductance only changes slightly, because the permeability of Alnico is very low. So the resonant frequency won't shift perceivably.
                        Thx for your comment. I had vaguely meditated on this empirical experiment in the meantime and your analysis about the string sensing area converges with the ideas passed through my mind.

                        I EDIT this message to add a few precisions about the test that I’ve shared, if ever it can help future readers to have a clearer perception of my modest contribution... :-)

                        -How and where strings were plucked has generally been carefully controlled in the soundtracks that I use to compare pickups. The same sequences are played in all these tracks with the same gear, settings, cable capacitance, input impedance and so on;

                        -A magnifier and a precision ruler are generally used to check where is any pickup relatively to the strings if it has been swapped or modified. In the test guitar used for my experiments, pickups can be loaded from the back, through a hole in the body, so I don’t even have to unstring/restring the instrument nor to change anything else than the pickup;

                        -To overcome the usual ADSR morphing, ALL the frequential peaks produced along each soundtrack are “stacked” in a synthetic screenshot, in order to capture the typical frequency response of each pickup under strings.

                        This method is not a panacea since it leaves aside the dynamical behavior of transducers (reason why measurements of their resonant peaks are also dramatically incomplete when it come to predict their tone, IME/IMHO).

                        Now, stacking the frequencies of a whole soundtrack is useful because even the most inconsistent playing would finally generate the same synthetic spectrum in such a case.

                        The test that I’ve shared above is based on this approach. The related method is so well trained now that if I stack upon each other two screenshots captured at several years of distance with the same pickup in a same guitar, the graphs will be almost exactly aligned if they have been captured in the same conditions.

                        In the quick test that I've done with mags in a P90 and that I’ve shared with Mark Hammer, screenshots were captured in conditions similar enough to make the comparison meaningful. Obviously, I wouldn't have contributed to this thread it if it wasn't the case... :-))
                        Last edited by freefrog; 04-30-2020, 11:17 AM.

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                        • #13
                          Originally posted by Helmholtz View Post
                          I think it can be explained by a shift of the string sensing center towards the bridge, which would be equivalent to a PU position closer to the bridge, resulting in less fundamental frequency content.
                          I'd not draw conclusions too quickly about the sensing window. The AlNiCo has a permeability that is so low that the overall inductance only drops slightly when it's removed, and if it's insubstantial in that regard, it makes it hard to believe that it would be so much more substantial in another, closely related regard. A good way to test a change in the sensing width/window would be to take the magnet out, and orient the pickup lengthwise along a string, rather than the usual perpendicular orientation, and see if the output is higher when the plucked guitar string is over the side of the pickup with the magnet versus without. This sort of testing has a lot of variables, such as how hard the string is plucked, where it's plucked, how high the pickup is set, the moment in time represented by the snapshot, etc. It's really a lot of work to account for all of it. The change in sound is very subtle to the human ear, and so it's also very subtle in the data representation, and so the smallest oversight can spoil the whole analysis. A person might say "this change makes a softer sound", but plucking softly makes a softer sound too, you'd have to have some way of making sure the picking wasn't softer.

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                          • #14
                            Scientific hypothesizing is great fun isn't it? I mean no disrespect to be clear, but...pull a mag, plug into an amp, and listen to what happens with your own ears. Report back once you have. The effect is apparent and distinguishable, moreso with a nickle silver baseplate.

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                            • #15
                              The AlNiCo has a permeability that is so low ...
                              I can't think of a reason why the magnet's permeability should have a significant influence on the spatial distribution of the static magnetic flux. After all the magnet is the source of all flux. Any flux the magnet injects into the screws must re-enter the magnet via its other/outer pole. And according to physics the flux always takes the shortest or most attractive return path.
                              The signal AC flux is just a modulation of the static flux and can't take a path completely different from the static flux.
                              So with a single magnet a P-90 will have an asymmetrical aperture, sensing the string only at the side where the magnet is.
                              Last edited by Helmholtz; 04-29-2020, 09:52 PM.
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