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Thread: Switchable additional winds

  1. #71
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    Quote Originally Posted by Helmholtz View Post
    The easiest way to study the influence of the interwinding capacitance and other parameters of the tapped SC would be to simulate the appropriate equivalent circuit in LTSpice.
    I use LTSpice quite a bit. If you could propose an equivalent circuit for the apparent high capacitance of a tapped coil I would certainly give it a go. I did read the PDF you linked to discussing transformer capacitance, but I'm not 100% clear on how to translate that to the tapped coil model.

    You said further up that you tested foil tape around a single coil, and did not get as high of a capacitance as a was apparent with the tapped coil, but isn't a secondary winding of a tapped pickup extremely close to the primary coil? Even foil tape will contain a little more distance between the windings than will the continued winding of the secondary coil. Being able to model this in LTSpice and see the "apparent" high capacitance without actually assigning a high capacitance would be very informative.

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  2. #72
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    Quote Originally Posted by Mark Hammer View Post
    Not to be too much of a pest, but would you expect those plots to be similar if we were talking about a flatter coil, like a Jazzmaster, as opposed to a coil where inner and outwer turns have very similar circumference?

    I ask because the overall shape of the coil can have an influence on tone, so naturally I'm curious about whether coil shape/dimensions also have an impact on the inner/outer turns aspect. Like I say, I don't know enough about this stuff to have any opinion. I'm just going on superficial aspects of pickups.
    The belief that the shape of the coil effects the tone isn't well founded. The coil merely translates flux change into a voltage. If anything about the geometry of a single coil were to effect the tone, it would have to be the pole piece, because that along with the string determine the flux change, and while guitar strings are geometrically simple and uniform, pole pieces can be had in a variety of shapes and sizes. For example, if the pole piece is a long "blade", even harmonics caused by side to side movement are suppressed. If the pole piece is very narrow, they're increased. If the pole piece were very wide enough,like half an inch or more, comb filtering would cancel out higher harmonics, when the field is about twice the width of those harmonic divisions along the string. Humbuckers do this, insofar as having two rows of small pole pieces is similar to one very wide pole piece.

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  3. #73
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    Quote Originally Posted by Helmholtz View Post
    If you just want an additional sound with a lower resonant frequency it is much more efficient (and easier) to hardwire a suitable capacitor across a non-tapped PU. And I would start out with a lower wind non-tapped PU to allow for a wider range of useful resonances.
    The thing about that is that if you use a cap to get the "full" sound, you wouldn't get an associated voltage boost for having more turns of wire in series. Maybe an inductor across the pickup can be used to "unwind" it, but the issue with that is that the load decreases the resonance. Maybe if you used a 1meg volume and no-load tone pot, you could retain enough resonance to allow a parallel inductor to work effectively, but then you'd need to replace it with parallel resistance when you switched the inductor out of the circuit, which isn't too much to ask. The harder task is finding a suitable inductor. The Bill Lawrence "Q Filter" is an inductor that supposedly works well, but I don't know too much about it.

    OTOH, even with just a cap, I think you can get really close to P-90 sound with a Strat if you pick a cap value that brings the resonant peak down to about 2kHz. I suspect the reason that's not often done is that people associate tone with the guitar as a whole, they don't expect a Strat with three little pickups to have a big, warm P-90 tone. Similarly, you rarely see Les Pauls fitted with Fender single coils.

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  4. #74
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    I am well aware of all this and if the loudness step is what you are after, my proposal won't do.

    My point is, that the unavoidable inductive coupling between the winding parts prevents a tappable overwound SC from convincingly reproducing the brighter sounds a a lower or "standard" wound PU, no matter how you split the coil or separate the windings, as the unused part of the coil always increases the effectice capacitance across the active coil by a considerable amount. As a result the two sounds available won't differ much - except when using very long guitar cables. But find out yourself.

    The parallel capacitors method, instead, allows selective tuning of the resonance and thus for really different sounds without loudness steps, which I consider favorable.

    In my experience the sound and dynamics of a good P-90 cannot be emulated by a Strat type design with alnico cores. The passive filter characteristics of a PU cannot explain the total response of a PU, even if the aperture is equal. But this would be a completely different topic.

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  5. #75
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    I'm the last one to impart magical properties to shape. However, coil shape is a "marker" for other things that can be predicted to affect the electrical properties, as in Helmholtz' example of resonance. Certainly the properties of the magnet or polepieces (or slogs) play a significant role, however their role is not in spite of the coil properties, but in concert with them.

    I am assuming that if I used normal Strat polepieces, and a conventional number of turns with very conventional wire and tension, but inserted a spacer between the polepieces and coil, such that the coil really only "started" 1/8" or 3/16" out from the polepieces (which, of course, would not fit inside the usual plastic pickup cover), it would likely end up sounding different, if only because of the distance of so many turns from the polepieces themselves. If a pickup has 800 additional turns that are switch-selectable, I would not expect that pickup to sound much different when those turns are added from the location closest to the polepieces vs the outside of the overall coil, simply because so few turns creates a negligible difference in distance from the polepieces.

    But when one has a flatter pickup, like a Jazzmaster or even P90, it doesn't take as many turns to create distance from the polepieces (or center bar and slugs). And I would think that where perhaps it might not make that much difference if additional turns were situated on the inside or the outside of a Strat-type pickup, they just might for a flatter wider profile. Again, taking into consideration that there are more, and less, consequential numbers of additional turns.

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  6. #76
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    You said further up that you tested foil tape around a single coil, and did not get as high of a capacitance as a was apparent with the tapped coil, but isn't a secondary winding of a tapped pickup extremely close to the primary coil? Even foil tape will contain a little more distance between the windings than will the continued winding of the secondary coil. Being able to model this in LTSpice and see the "apparent" high capacitance without actually assigning a high capacitance would be very informative.
    My measurements with the foil (I have measured several PUs) show that it has no noticeable effect on the resonance of the active winding, as long as the foil is not grounded (or shorted). While the interwinding capacitance between the two winding parts will be somewhat higher than the capacitance between coil and foil (my guess: around 50pF, can be easily measured in a 4 wire PU; in a 3 wire PU it is shorted anyway), there will be no influence on the active coil's resonance as well, as it acts like a capacitor with one lead connected and the other lead up in the air. The active coil's resonance can only be influenced by capacitances that take effect to ground.
    Please note that in the pdf article on coupled windings the interwinding capacitance is not even mentioned, even though there is some also in toroidal transformers with separated windings. The reason is that it has absolutely no effect as long as not more than one of the windings are grounded.
    IMHO the increased effective capacitance across the active coil is not caused by the interwinding capacitance but by reflected capacitance.

    I may assist with the LTSpice model but fear that this would mean hijacking this thread. We seem to have lost the OP a while ago.

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    Last edited by Helmholtz; 04-18-2018 at 09:58 PM.

  7. #77
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    Quote Originally Posted by Mark Hammer View Post
    I'm the last one to impart magical properties to shape. However, coil shape is a "marker" for other things that can be predicted to affect the electrical properties, as in Helmholtz' example of resonance. Certainly the properties of the magnet or polepieces (or slogs) play a significant role, however their role is not in spite of the coil properties, but in concert with them.

    I am assuming that if I used normal Strat polepieces, and a conventional number of turns with very conventional wire and tension, but inserted a spacer between the polepieces and coil, such that the coil really only "started" 1/8" or 3/16" out from the polepieces (which, of course, would not fit inside the usual plastic pickup cover), it would likely end up sounding different, if only because of the distance of so many turns from the polepieces themselves. If a pickup has 800 additional turns that are switch-selectable, I would not expect that pickup to sound much different when those turns are added from the location closest to the polepieces vs the outside of the overall coil, simply because so few turns creates a negligible difference in distance from the polepieces.

    But when one has a flatter pickup, like a Jazzmaster or even P90, it doesn't take as many turns to create distance from the polepieces (or center bar and slugs). And I would think that where perhaps it might not make that much difference if additional turns were situated on the inside or the outside of a Strat-type pickup, they just might for a flatter wider profile. Again, taking into consideration that there are more, and less, consequential numbers of additional turns.
    Mark,
    Do not forget to take into account the strength of the magnetic field on the inner turns versus the outer turns voltage generated. Using a metal plate to help spread the magnetic field under the coil is one way to make the outer winds more efficient voltage generating turns.

    The ultimate test is to wind Jazzmaster or P90 type pickup with an inner coil with independent start an ending leads or connections and then add an outer coil with the same number of winds with independent start and ending leads. With no metal plate under the coils attach the inner and outer coils to a dual trace oscilloscope and visualize the output levels of each independent coil. Now add a metal plate to help spread the magnetic field and do the same thing to see the output levels of each coil change due the this added metal plate variable being observed.

    Wider coils detect a wider string length window and will have a different harmonic component than a narrow coil with a shorter string length window due to harmonic cancellations of certain frequencies over the pickup coil area.

    If you do this experiment you will see results and add to a scientific answer to this question.

    Joseph J. Rogowski

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    Last edited by bbsailor; 04-18-2018 at 11:10 PM.

  8. #78
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    Quote Originally Posted by Mark Hammer View Post
    I'm the last one to impart magical properties to shape. However, coil shape is a "marker" for other things that can be predicted to affect the electrical properties, as in Helmholtz' example of resonance. Certainly the properties of the magnet or polepieces (or slogs) play a significant role, however their role is not in spite of the coil properties, but in concert with them.

    I am assuming that if I used normal Strat polepieces, and a conventional number of turns with very conventional wire and tension, but inserted a spacer between the polepieces and coil, such that the coil really only "started" 1/8" or 3/16" out from the polepieces (which, of course, would not fit inside the usual plastic pickup cover), it would likely end up sounding different, if only because of the distance of so many turns from the polepieces themselves. If a pickup has 800 additional turns that are switch-selectable, I would not expect that pickup to sound much different when those turns are added from the location closest to the polepieces vs the outside of the overall coil, simply because so few turns creates a negligible difference in distance from the polepieces.

    But when one has a flatter pickup, like a Jazzmaster or even P90, it doesn't take as many turns to create distance from the polepieces (or center bar and slugs). And I would think that where perhaps it might not make that much difference if additional turns were situated on the inside or the outside of a Strat-type pickup, they just might for a flatter wider profile. Again, taking into consideration that there are more, and less, consequential numbers of additional turns.
    I don't want to question your findings regarding the coil to polepiece distance. But from the point of physics the only things that change with a wider "blind" core are an increased inductance because of greater mean circumference of the coil for a given number of turns, resulting in a lower resonant frequency and a slightly lower resonance peak because of the increased resistance. Altogether this will make the sound somewhat mellower. But there is another good reason to not let the wire touch the polepieces, i.e. prevention of chemical corrosion.
    Again, different PU designs should be compared via samples having identical inductance/resonance (and not identical resistance!) to avoid that one only hears the sound difference caused by different resonance. Resonance/inductance of most designs can be easily adapted by the number of turns.

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  9. #79
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    That's pretty much what I was suggesting, though perhaps in a manner that wasn't as clear as it needed to be. In essence, you have confirmed my initially query: that if one placed the additional turns closest to the polepieces - without any other modifications to compensate (as Joseph suggested) - there could be a change in the inductance and resonance of the default/basic coil, compared to if the basic coil was on the inside, by virtue any difference in the circumference of the turns in that basic coil.

    Again, all of this is qualified by whether the number of turns making up the "additional" part yields any sort of appreciable difference to the circumference of the majority of turns on the basic coil. I say "majority of turns" because, even if the additional turns constituted 2000 out of 8000, and were situated on the inside rather than outside, many of the 6000 turns would be the same distance from the polepieces, and have the same circumference they would have had if the coil were only 6000 turns. Some, however, would be farther away from the polepieces than they might have been without those inner 2000.

    Joseph's point about the influence of polepiece proximity suggests that placing additional turns on the inside mght result in a more noticeable difference in tone if one switch-selects those additional turns, compared to if the switch-selected addition was on the outside. Or have I misunderstood?

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  10. #80
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    Quote Originally Posted by Helmholtz View Post
    I am well aware of all this and if the loudness step is what you are after, my proposal won't do.

    My point is, that the unavoidable inductive coupling between the winding parts prevents a tappable overwound SC from convincingly reproducing the brighter sounds a a lower or "standard" wound PU, no matter how you split the coil or separate the windings, as the unused part of the coil always increases the effectice capacitance across the active coil by a considerable amount. As a result the two sounds available won't differ much - except when using very long guitar cables. But find out yourself.
    I dunno, you know you stuff, I won't deny that, but I'm getting pretty close to winding a four conductor tapped single coil to see for myself. I only wind pickups for testing, so it's an ordeal to get everything set up.

    Quote Originally Posted by Helmholtz View Post
    In my experience the sound and dynamics of a good P-90 cannot be emulated by a Strat type design with alnico cores. The passive filter characteristics of a PU cannot explain the total response of a PU, even if the aperture is equal. But this would be a completely different topic.
    I'm willing to entertain that topic. The AlNiCo poles have strong magnet pull, but a lower permeability. The lower permeability means the magnetic reluctance is higher, despite the stronger pull of the magnetic pole pieces. I don't believe magnetic reluctance really effects the tone, I don't see any plausible cause for that to be the case, but it is a fact that stronger magnetic pull upon the string interferes with how the string vibrates. I think the key to making the Strat pickup sound like a P-90 is, aside from matching the RLC characteristics, is to set it very low, so that the magnetic pull between the Strat pickup and strings are similarly weak. The overall output might be lower, but that should only effect the S/N ratio, and not the transfer function. A complication with testing this notion is that few Strats have P-90s, and few Gibsons have Strat pickups in them, so it's hard to compare apples to apples.

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  11. #81
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    Quote Originally Posted by bbsailor View Post
    Wider coils detect a wider string length window and will have a different harmonic component than a narrow coil with a shorter string length window due to harmonic cancellations of certain frequencies over the pickup coil area.
    The coil can't be the window, because the coil doesn't determine what flux change occurs. The coil size and shape only matters for how efficiently it sees flux change that is cast off by the interaction between the pole piece and the guitar string. If a wide section of the string is magnetized, it will cancel higher harmonics as you said, but the coil doesn't determine that, it's merely a receives what has already been determined, with varying degrees of efficiency.

    For example, with a coil as wide as the Jazzmaster pickup, it's not only seeing flux change from from the primary magnetic path between the guitar string and pole piece, but the wider loops of the coil are also receiving a flux change from the return path of that same magnetic field. Though the flux density of the return path is weaker than the primary path, the return path is opposite polarity, so it represents a cancellation of signal, and so the wide coil is just an inefficiency, and little more. The job of the coil is just to take the flux change and turn it into a voltage. The question is just how efficiently it does the job.

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    Last edited by Antigua; 04-19-2018 at 08:49 AM.

  12. #82
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    Quote Originally Posted by Mark Hammer View Post
    That's pretty much what I was suggesting, though perhaps in a manner that wasn't as clear as it needed to be. In essence, you have confirmed my initially query: that if one placed the additional turns closest to the polepieces - without any other modifications to compensate (as Joseph suggested) - there could be a change in the inductance and resonance of the default/basic coil, compared to if the basic coil was on the inside, by virtue any difference in the circumference of the turns in that basic coil.

    Again, all of this is qualified by whether the number of turns making up the "additional" part yields any sort of appreciable difference to the circumference of the majority of turns on the basic coil. I say "majority of turns" because, even if the additional turns constituted 2000 out of 8000, and were situated on the inside rather than outside, many of the 6000 turns would be the same distance from the polepieces, and have the same circumference they would have had if the coil were only 6000 turns. Some, however, would be farther away from the polepieces than they might have been without those inner 2000.
    I just tested a tappable SSL-4 with the DE-5000, and got these values from the two halves of the pickups (Ls at 100Hz, Cp at 100kHz). Note that this is a three conductor tapped pickup, so the both coils are always connected at at least one end.

    white to blk (whole coil) DCR: 13.62k Ls: 6.833H Cp: 179pF
    white to red (inner coil) DCR: _6.57k Ls: 1.675H Cp: 384pF
    black to red (outer coil) DCR: _7.05k Ls: 1.992H Cp: 310pF


    As for how the inner verses outer coil geometry impacts the inductance, the outer coil has both a higher DC resistance and a higher inductance, but the inductance is not that much different, considering.

    Regarding the capacitance, the DE-5000 is showing different values from what I calculated from the f and L using an oscilloscope, but it is still showing a higher capacitance for either individual half, compared to the whole.

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  13. #83
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    Thanks for your diligence. The extra mile is much appreciated.

    True, the differences are not dramatic. But then I suspect the differences existing between pickups X, Y, and Z, in those same parameters, that lead a player to prefer one over the others may be similarly "undramatic". Sometimes, what looks small on the measurement devices is a nuance that attracts a player. It may be small, but if they hear it (reliably), and it makes a difference to them, then I suppose we can say that it matters. Maybe more than it should, but it matters.

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  14. #84
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    Quote Originally Posted by Antigua View Post
    The coil can't be the window, because the coil doesn't determine what flux change occurs. The coil size and shape only matters for how efficiently it sees flux change that is cast off by the interaction between the pole piece and the guitar string. If a wide section of the string is magnetized, it will cancel higher harmonics as you said, but the coil doesn't determine that, it's merely a receives what has already been determined, with varying degrees of efficiency.
    This is correct. German electroacoustics professor Manfred Zollner could show by lab measurements that the string lenght window (aka aperture) only depends on the distribution and strength (to a lesser extend) of the PU's magnetic field. No influence of the coil's shape. The effective aperture length of a single coil is around 0.5''max. Humbuckers have two discrete windows, one over each coil. Humbuckers are "blind" right in the middle over the coils, because in this region the string saturates and its a.c. permeability drops to 1.
    He also showed that the inner and especially the upper windings (closest to the strings) contribute most to the signal voltage, as the a.c. magnetic field is strongest in and around the upper part of the poles. The alternating part of the magnetic flux takes paths different from the permanent flux. The reason for this is that the permeabilities and consequently the magnetic conductivities of ferromagnetics (strings, pole pieces) differ vastly between a.c. and d.c. fields, a.c. permeabilities beeing generally much lower.

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  15. #85
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    Regarding the capacitance, the DE-5000 is showing different values from what I calculated from the f and L using an oscilloscope, but it is still showing a higher capacitance for either individual half, compared to the whole.
    As mentioned earlier, calculating the capacitance from the resonance peak of a PU's impedance is not very precise.

    1)There are additional parasitic capacitances from the setup: Wiring, input capacitances of scope/probe/meter etc.
    2)The peak frequency of the impedance of a parallel resonant circuit is always somewhat lower than the theoretical resonant frequency given by 2*pi*fres = (L*C)^(- 0.5), depending on damping. If you use the peak of the transfer response instead of the impedance response, the error will be even greater.
    3)The inductance drops with increasing frequency and is always lower at resonance than at 100Hz. This effect depends on core materials and eddy current paths. It is typically small for strat type PUs with alnico cores not having metal covers.

    But - apart from these more academical considerations - I don't think an error of 10% or +/- 20pF really matters. A difference of more than 100pF might be audible, though.

    (Did you try the "Automatic Voltage Scale" option? It will allow you to see the straight high frequency -12dB/octave slope in the transfer response (resp. -6dB/octave in the impedance response) which is masked by the noise floor in your plots.)

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    Last edited by Helmholtz; 04-19-2018 at 02:56 PM.

  16. #86
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    The ultimate test is to wind Jazzmaster or P90 type pickup with an inner coil with independent start an ending leads or connections and then add an outer coil with the same number of winds with independent start and ending leads. With no metal plate under the coils attach the inner and outer coils to a dual trace oscilloscope and visualize the output levels of each independent coil. Now add a metal plate to help spread the magnetic field and do the same thing to see the output levels of each coil change due the this added metal plate variable being observed.

    Can you post scope pictures?
    I don't have tappable SCs, but I am curious because lab measurements (Zollner) have shown that the a.c. magnetic field at the bottom of a PU is only a very small percentage of the a.c. field at the top, which means that there is not much to be distributed. The baseplate of a tele bridge PU increases inductivity by 6% but the output is increased only by 0.6dB, which is close to nothing.

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  17. #87
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    I'm willing to entertain that topic. The AlNiCo poles have strong magnet pull, but a lower permeability. The lower permeability means the magnetic reluctance is higher, despite the stronger pull of the magnetic pole pieces. I don't believe magnetic reluctance really effects the tone, I don't see any plausible cause for that to be the case, but it is a fact that stronger magnetic pull upon the string interferes with how the string vibrates. I think the key to making the Strat pickup sound like a P-90 is, aside from matching the RLC characteristics, is to set it very low, so that the magnetic pull between the Strat pickup and strings are similarly weak. The overall output might be lower, but that should only effect the S/N ratio, and not the transfer function. A complication with testing this notion is that few Strats have P-90s, and few Gibsons have Strat pickups in them, so it's hard to compare apples to apples.
    I agree, a true comparison is hard to do.
    As an experienced player I notice major differences in the attack response between different core materials, even with equal B values at the strings and equal resonances. I think the clue is that the effective a.c. permeability of steel is not a constant but depends strongly on a.c. and d.c. field strenghts as well as on frequency. I have been doing measurements for some time that seem to support my theory. Don't want to go into more detail at the time.

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  18. #88
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    Quote Originally Posted by Helmholtz View Post
    I agree, a true comparison is hard to do.
    As an experienced player I notice major differences in the attack response between different core materials, even with equal B values at the strings and equal resonances. I think the clue is that the effective a.c. permeability of steel is not a constant but depends strongly on a.c. and d.c. field strenghts as well as on frequency. I have been doing measurements for some time that seem to support my theory. Don't want to go into more detail at the time.
    Attack response would generally correspond to transient signals. You know a lot about transformer issues, is there any analogous consideration is transformer design, especially when you choose to use steel core versus ferrite? Modeling the guitar pickup and string as a loosely coupled transformer seems to draw an endless number of parallels otherwise.

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  19. #89
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    Quote Originally Posted by Helmholtz View Post
    Can you post scope pictures?
    I don't have tappable SCs, but I am curious because lab measurements (Zollner) have shown that the a.c. magnetic field at the bottom of a PU is only a very small percentage of the a.c. field at the top, which means that there is not much to be distributed. The baseplate of a tele bridge PU increases inductivity by 6% but the output is increased only by 0.6dB, which is close to nothing.
    I've done similar tests with Tele base plates using the Vellemen and an inducer coil over head, and found the different with and without the base plate to be in the area of 1dB, give or take. One thing I did not try, whoch I suspect would make a difference, is if the pole pieces were to be replaced with steel instead of AlNiCo, there would be a lower magnetic reluctance between the strings and base plate, and so it might be realistic to see the output climb higher when the pole pieces are steel.

    Regarding the magnetic path though, the metal base of the P-90 is either brass or nickel silver, which are not magnetic / permeable, so it does not have much effect to speak of on the magnetic circuit anyhow, and a Jazzmaster pickup of course uses fiberboard typical of Fenders.

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  20. #90
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    Quote Originally Posted by Helmholtz View Post
    As mentioned earlier, calculating the capacitance from the resonance peak of a PU's impedance is not very precise.

    1)There are additional parasitic capacitances from the setup: Wiring, input capacitances of scope/probe/meter etc.
    2)The peak frequency of the impedance of a parallel resonant circuit is always somewhat lower than the theoretical resonant frequency given by 2*pi*fres = (L*C)^(- 0.5), depending on damping. If you use the peak of the transfer response instead of the impedance response, the error will be even greater.
    3)The inductance drops with increasing frequency and is always lower at resonance than at 100Hz. This effect depends on core materials and eddy current paths. It is typically small for strat type PUs with alnico cores not having metal covers.

    But - apart from these more academical considerations - I don't think an error of 10% or +/- 20pF really matters. A difference of more than 100pF might be audible, though.

    (Did you try the "Automatic Voltage Scale" option? It will allow you to see the straight high frequency -12dB/octave slope in the transfer response (resp. -6dB/octave in the impedance response) which is masked by the noise floor in your plots.)
    Regarding point #1, I determined the rig capacitance to be 10pF, so I subtract that from the final value.

    I haven't set the Velleman up again yet, but I will try automatic scale when I do. I'd like to create a four lead tapped pickup this weekend, and do it all at once.

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    Attack response would generally correspond to transient signals. You know a lot about transformer issues, is there any analogous consideration is transformer design, especially when you choose to use steel core versus ferrite? Modeling the guitar pickup and string as a loosely coupled transformer seems to draw an endless number of parallels otherwise.
    Requirements for transformer and PU core materials are vastly different. In PUs technically inferior materials often give preferable sound results. A PU has more and different functions. I don't consider PUs as transformers. Both use induction, but so do other technical devices.

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    Are the coils stacked or winded with two wires in the same time ?

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    Last edited by catalin gramada; 04-19-2018 at 11:05 PM.
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    Quote Originally Posted by Helmholtz View Post
    As mentioned earlier, calculating the capacitance from the resonance peak of a PU's impedance is not very precise.

    1)There are additional parasitic capacitances from the setup: Wiring, input capacitances of scope/probe/meter etc.
    2)The peak frequency of the impedance of a parallel resonant circuit is always somewhat lower than the theoretical resonant frequency given by 2*pi*fres = (L*C)^(- 0.5), depending on damping. If you use the peak of the transfer response instead of the impedance response, the error will be even greater.
    3)The inductance drops with increasing frequency and is always lower at resonance than at 100Hz. This effect depends on core materials and eddy current paths. It is typically small for strat type PUs with alnico cores not having metal covers.
    I've been measuring the capacitance of more Strat pickups with the DE-5000, and for some pickups, I'm getting very different results than I had based on the measured inductance and the resonant peak, like twice as little, or twice as much. Do you believe the DE-5000's measurements are more likely to be accurate? You mentioned that the inductance measure at 100Hz might not be accurate at the resonant frequency, though especially in the case of Strat pickup where eddy currents are low, the inductance tends to read the same value whether testing at 100Hz or 1kHz.

    One example, I have a Fat 50 for which I measured 148pF from 2.4H inductance and a 7.9kHz resonance. The DE-5000 in Cp @ 100Hz is giving me back 66pF. This disparity is alarming because it means one method or the other is liable to be rather off.

    One fun thing I was able to do with the DE-5000 was, while measuring capacitance, I pressed in on the coil with my fingers and I could see the capacitance climb by about 2pF when squeezed.

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    Quote Originally Posted by Helmholtz View Post
    Requirements for transformer and PU core materials are vastly different. In PUs technically inferior materials often give preferable sound results. A PU has more and different functions. I don't consider PUs as transformers. Both use induction, but so do other technical devices.
    I'm not suggesting the pickup is a transformer by itself, but some others who I've talked to about this describe the pickup as a primary and the guitar string as being like a secondary, with poor coupling between them. In fact I think the idea has been discussed on this forum once or twice. Where has a real transformer's coils would have a mutual inductance near unity, the pickup and the string would have a linkage that is a small fraction of unity, due to large air gap separation. Based on your broader understanding of transformers, do you think this analogy is useful, or not similar enough to draw a comparison?

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    Quote Originally Posted by Antigua View Post
    I'm not suggesting the pickup is a transformer by itself, but some others who I've talked to about this describe the pickup as a primary and the guitar string as being like a secondary, with poor coupling between them. In fact I think the idea has been discussed on this forum once or twice. Where has a real transformer's coils would have a mutual inductance near unity, the pickup and the string would have a linkage that is a small fraction of unity, due to large air gap separation. Based on your broader understanding of transformers, do you think this analogy is useful, or not similar enough to draw a comparison?
    No, a magnetic PU is a transducer. It is actually an electrical machine (including the strings), an electrical power generator followed by an integrated lowpass filter.

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    Last edited by Helmholtz; 04-20-2018 at 03:11 PM.

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    One example, I have a Fat 50 for which I measured 148pF from 2.4H inductance and a 7.9kHz resonance. The DE-5000 in Cp @ 100Hz (??) is giving me back 66pF. This disparity is alarming because it means one method or the other is liable to be rather off.
    I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

    One fun thing I was able to do with the DE-5000 was, while measuring capacitance, I pressed in on the coil with my fingers and I could see the capacitance climb by about 2pF when squeezed.
    Two reasons:
    1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
    2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.

    I feel that this topic better belongs to the thread "Measuring capacitance..."

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    Maybe you guys could consider a Private Message, PM.
    Since You're going over, & over, the same ground?

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    Quote Originally Posted by big_teee View Post
    Maybe you guys could consider a Private Message, PM.
    Since You're going over, & over, the same ground?
    Thank you Terry! I thought the same thing and even unsubscribed to the thread since it was looping for so long and I got tired of it popping up in my box. Not that some great stuff isn't being covered. What doesn't go over my head has been hashed and rehashed.

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    Quote Originally Posted by Helmholtz View Post
    No, a magnetic PU is a transducer. It is actually an electrical machine (including the strings), an electrical power generator followed by an integrated lowpass filter.
    The thing is you can't model mechanics with a modeler like LTSpice, and since the interaction between string and pickup is electromagnetic, I think there is value is being able to abstract the relationship in terms that can be electrically modeled.

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    Quote Originally Posted by big_teee View Post
    Maybe you guys could consider a Private Message, PM.
    Since You're going over, & over, the same ground?
    I've found a ton of useful info on MEF, and I mean lots, through Google searches. If this is a PM, the info will not be accessible to anyone else who pursues this subject later.

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  31. #101
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    Quote Originally Posted by Helmholtz View Post
    I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

    Two reasons:
    1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
    2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.
    This in a nutshell is why it is not considered a good idea to attempt to measure coil capacitance a at ten times the self-resonant frequency. Too fiddly. Also, all manner of effects that are irrelevant at the normal operating frequency range can become significant, causing errors. The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.

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    I see very little pickup winding relevance!
    I'll go back to my yard mowing!
    T

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    Last edited by big_teee; 04-20-2018 at 08:02 PM.
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    Quote Originally Posted by Joe Gwinn View Post
    This in a nutshell is why it is not considered a good idea to attempt to measure coil capacitance a at ten times the self-resonant frequency. Too fiddly. Also, all manner of effects that are irrelevant at the normal operating frequency range can become significant, causing errors. The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.
    Quote Originally Posted by Helmholtz View Post
    I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

    Supposing even 100kHz is prone to error due to secondary resonances in that range, what if a meter was able to test at 1 MHz? It looks like such meters cost well over a thousand dollars, but maybe a hand held that can test 1MHz will hit the market eventually.

    Here are plots using the auto voltage scaling, and yes, they fix the noise floor issue amazingly well. Thanks again!

    These first two are without the integrator amp, just the pickup into the Velleman PSU200:


    closer detail


    This plot is with the integrator amp designed by Ken Willmott in between the pickup and the Velleman



    It appears that there are three impedance anamolies past the resonance, a small one at 48kHz, and a taller one at 93kHz, and another at 308kHz. I'm guessing that second resonance at 93kHz is that of the 100 turn exciter coil connected to the integrated function generator, due to it's prominence. I'm not so sure what accounts for the other smaller peaks beyond the resonance.


    Quote Originally Posted by Helmholtz View Post
    Two reasons:
    1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
    2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.

    I feel that this topic better belongs to the thread "Measuring capacitance..."
    I believe it's #1, because if it were #2, simply touching the pickup should induce the difference, but the increase corresponded to pressure. It demonstrates in real time the relationship between tension and capacitance, which is really cool.

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    Last edited by Antigua; 04-20-2018 at 09:56 PM.

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    The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.
    "Terman's method" is fine, at least as long as inductance does not change with frequency, one takes care of parasitic capacitances of the measuring setup and the system (PU) has only one resonance. Maybe a little time-consuming. The theory behind is simple. Always nice to have different options. But each method has its drawbacks. I am not going to revive an old, fruitless discussion, though. Prefer being pragmatical instead, as PU capacitance is not a primary influencer of sound.

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    The easiest and more accurate way to plot a PU's impedance response is to directly drive the PU from the generator with a series resistor of 100k to 1M ( the higher, the better). The idea is measuring the voltage across an impedance driven by a constant current source.

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