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Thread: Switchable additional winds

  1. #36
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    Quote Originally Posted by David Schwab View Post
    I've said several times...

    "The secondary coil has both ends not connected to anything..."

    Now do the tests.
    I missed that, but when you have a three conductor coil tap, it is not the case that the secondary is disconnected at both ends, and so this is what I am suggesting to the OP, use four conductor cable and wire it as shown in that diagram I posted above in order to fully disconnect the tapped coil.

    With the secondary coil open at both ends, the coil will still be slightly reactive, because the secondary will capacitively couple along its length, but that effects is a lot smaller than when the coil is connected at one end, based on similar testing with humbucker coils.

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    Last edited by Antigua; 03-27-2018 at 08:23 PM.

  2. #37
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    Yep. One end connected makes it a capacitor. Same effect you get with shielded cable. The shield presents a capacitance to ground because of it's proximity to the conductor. I don't think anyone would argue that or the similarity in the circuits. Both ends disconnected from the secondary coil eliminates the problem BUT FOR ONE THING!!! If you shield an inductor with copper (grounded or not!) it changes the performance. I can't say why because I don't have the tech chops, but I can assure you that it does because of my own experience shielding pickup coils with copper tape. Copper has a unique ability to bend EMF and I have to assume it's this property that is responsible.

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  3. #38
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    Quote Originally Posted by Chuck H View Post
    Yep. One end connected makes it a capacitor. Same effect you get with shielded cable. The shield presents a capacitance to ground because of it's proximity to the conductor. I don't think anyone would argue that or the similarity in the circuits. Both ends disconnected from the secondary coil eliminates the problem BUT FOR ONE THING!!! If you shield an inductor with copper (grounded or not!) it changes the performance. I can't say why because I don't have the tech chops, but I can assure you that it does because of my own experience shielding pickup coils with copper tape. Copper has a unique ability to bend EMF and I have to assume it's this property that is responsible.
    Probably eddy currents http://kenwillmott.com/blog/wp-conte...r_Geometry.pdf conductive areas that intersect the lines of flux (covers, pole pieces, copper tape) form eddy currents which manifest as increased series resistance.

    If you were to cut the copper tape so that it doesn't have continuity around the coil, it will prevent the eddy current loop, but then if it's not grounded, it won't block electrostatic noise, but if it is grounded it will put capacitance across the coil, so copper tape shielding coils is a lose-lose proposition, in my estimation.

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  4. #39
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    DC Resistance:
    full: 15.53K
    tapped: 7.80K

    Inductance:
    full: 7.842H
    tapped: 2.173H

    Resonant Peak:
    full: dV: 10.7dB f: 4.84kHz (black)
    tapped: dV: 9.4dB f: 5.00kHz (red)

    Loaded Resonant Peak (200K ohms & 470pF):
    full: dV: 2.8dB f: 2.03kHz (green)
    tapped: dV: 4.8dB f: 3.51kHz (gray)

    Calculated C:
    full: 128pF (138-10)
    tapped: 456pF (466-10)


    This is an interesting discussion. Thanks for the measurements. (How do you measure the frequency response? By field injection via transmitter coil and integration of the output signal?)

    Such a big increase of capacitance (over 300pF) cannot be explained by capacitive coupling.

    My explanation is the following:

    When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

    This effect does not depend on electrical connection between the coil parts.

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  5. #40
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    Quote Originally Posted by Helmholtz View Post
    DC Resistance:
    full: 15.53K
    tapped: 7.80K

    Inductance:
    full: 7.842H
    tapped: 2.173H

    Resonant Peak:
    full: dV: 10.7dB f: 4.84kHz (black)
    tapped: dV: 9.4dB f: 5.00kHz (red)

    Loaded Resonant Peak (200K ohms & 470pF):
    full: dV: 2.8dB f: 2.03kHz (green)
    tapped: dV: 4.8dB f: 3.51kHz (gray)

    Calculated C:
    full: 128pF (138-10)
    tapped: 456pF (466-10)


    This is an interesting discussion. Thanks for the measurements. (How do you measure the frequency response? By field injection via transmitter coil and integration of the output signal?)

    I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.

    Quote Originally Posted by Helmholtz View Post
    Such a big increase of capacitance (over 300pF) cannot be explained by capacitive coupling.

    My explanation is the following:

    When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

    This effect does not depend on electrical connection between the coil parts.
    You might be right. It's a certainty that the unused coil will still be in circuit when disconnected at both ends, since both capacitive and inductive coupling are still in play. That can be observed with humbuckers too, and the effect is very small, though of course they're not going to couple as well as two co-axial coils. I'll see if I can model this with LTSpice, but if that doesn't give a clear picture, I'll probably have to make a four conductor tapped single coil to test out.

    Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation.


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  6. #41
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    >I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.<

    This is what I thought and described above, the method of my german physicist colleague Lemme. I have been using this for many years. But there are different possibilities to take care of the necessary 1/f (-6dB/octave) compensation: A constant current source for the excitation coil with 1/f sweep amplitude or integration of the output signal. I use a power amp driven by the PCSU 200 signal generator and a big air core choke in series with the low impedance exciter coil to provide a current source with 1/f characteristic. If you don't use a constant current source with 1/f characteristic or integrate the output signal afterwards, you will not get the true PU's frequency response as the measured frequency response will be distorted by the frequency dependance of the impedance of the exciter coil and the non-ideal coupling. An exciter coil driven by a low impedance (50 Ohms) source will also load the PU and reduce the resonance peak.

    >Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation<

    A closed copper loop around a pickup coil is not a good idea, as this is represents a shorted secondary coil in a transformer, loading down the PU. Consequently output signal, inductance and Q will be lowered. Connecting the loop to ground will strongly increase the PU's capacitance to ground in addition.

    Do you have numbers for the capacitance increase with a non-closed and non-grounded copper foil? This would be a better capacitive equivalent to the disengaged windings in a tapped coil.

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    Last edited by Helmholtz; 03-28-2018 at 09:53 PM.

  7. #42
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    Quote Originally Posted by Helmholtz View Post
    >I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.<

    This is what I thought and described above, the method of my german physicist colleague Lemme. I have been using this for many years. But there are different possibilities to take care of the necessary 1/f (-6dB/octave) compensation: A constant current source for the excitation coil with 1/f sweep amplitude or integration of the output signal. I use a power amp driven by the PCSU 200 signal generator and a big air core choke in series with the low impedance exciter coil to provide a current source with 1/f characteristic. If you don't use a constant current source with 1/f characteristic or integrate the output signal afterwards, you will not get the true PU's frequency response as the measured frequency response will be distorted by the frequency dependance of the impedance of the exciter coil and the non-ideal coupling. An exciter coil driven by a low impedance (50 Ohms) source will also load the PU and reduce the resonance peak.
    For the integration I use an integrator amp circuit designed by Ken Willmott Electric Guitar Pickup Measurements | kenwillmott.com , which is why you don't see a +6dB/oct curve in the pickup plot on the previous page. He offers the schematic for free, but if you contact him he might send you a printed circuit board, as he had a handful of them produced, if you're interested.

    As for the coupling between the exciter and the pickup, I've found that the coupling between them is so low that it's almost impossible to get the exciter to interfere with the impedance of the tested pickup, at least with the Velleman bode plotters. Maybe in other test setups it's more of an issue, though.

    Quote Originally Posted by Helmholtz View Post
    >Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation<

    A closed copper loop around a pickup coil is not a good idea, as this is represents a shorted secondary coil in a transformer, loading down the PU. Consequently output signal, inductance and Q will be lowered. Connecting the loop to ground will strongly increase the PU's capacitance to ground in addition.

    Do you have numbers for the capacitance increase with a non-closed and non-grounded copper foil? This would be a better capacitive equivalent to the disengaged windings in a tapped coil.
    I agree, I'd advice people with copper taped coils to carefully remove it, if at all possible, or at the very least snip the ground connection. I don't have other values with/without, but I can gather this info in the next week or two and send you a private message when I get it. If you don't visit this forum to often, private message me your email address and I'll send you additional test results that way.

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  8. #43
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    Thanks for your replies.

    I think I will stick to my trusted measuring setup as I have verfied its linearity to over 20kHz. It doesn't need the integrator because the 6dB/octave behaviour is already implemented in the drive current for the exciter coil.
    If you use e.g. a 3mH exciter coil in series with a 100 Ohm resistor driven by a voltage source, the drive current will decrease above 5kHz by 6dB/octave caused by the increasing reactance of the coil, respectively the RL time constant. Consequently the PU's output will show a reduced high frequency response. Of course this can be improved by a lower inductance and higher value series resistor.

    It is easy to verify my "capacitance transformer theory" indirectly by the following experiments, if you have a tapped single coil PU (which I do not):

    1) Connect an external capacitor of e.g. 100pF to the tap and the outer end of the coil (i.e. in parallel with the disengaged part of the coil).
    Measure the respective increase of capacitance in the active coil.

    2)Short-circuit the disengaged part of the coil and note the corresponding decrease of capacitance in the active coil. In this case other parameters (L,Q) of the tapped PU may change as well because of the "eddy current effect".


    I plan to keep track of interesting threads regarding pickups and tube amplifiers.

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  9. #44
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    I just did the experiment with copper foil wrapped around a strat pickup. I used 2 wraps of back insulated foil, ends not shorted to avoid eddy currents.
    The results are:

    Without foil Cp=87pF, with foil (not connected) Cp=88pF, foil connected to output Cp=89pF and with foil connected to ground Cp=123pF.
    The tapped coil situation would correspond to "foil connected to output".
    I think the results show clearly that it does not matter, if the disengaged windings remain connected to the tap or not. It also shows that a dramatic increase of capacitance in the active coil of over 300pF cannot be the result of capacitive coupling.

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  10. #45
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    Quote Originally Posted by Helmholtz View Post
    Thanks for your replies.

    I think I will stick to my trusted measuring setup as I have verfied its linearity to over 20kHz. It doesn't need the integrator because the 6dB/octave behaviour is already implemented in the drive current for the exciter coil.
    If you use e.g. a 3mH exciter coil in series with a 100 Ohm resistor driven by a voltage source, the drive current will decrease above 5kHz by 6dB/octave caused by the increasing reactance of the coil, respectively the RL time constant. Consequently the PU's output will show a reduced high frequency response. Of course this can be improved by a lower inductance and higher value series resistor.

    The Velleman PCSU has an output impedence of 50 ohms. I'll get specs on the exciter coil, it's about 100 turns of 44AWG, here's a pic http://i.imgur.com/iJ4ejoR.jpg

    I haven't come across this problem myself. For example, here is a P-Bass pickup that measured well up to 14kHz



    Quote Originally Posted by Helmholtz View Post
    It is easy to verify my "capacitance transformer theory" indirectly by the following experiments, if you have a tapped single coil PU (which I do not):

    1) Connect an external capacitor of e.g. 100pF to the tap and the outer end of the coil (i.e. in parallel with the disengaged part of the coil).
    Measure the respective increase of capacitance in the active coil.

    2)Short-circuit the disengaged part of the coil and note the corresponding decrease of capacitance in the active coil. In this case other parameters (L,Q) of the tapped PU may change as well because of the "eddy current effect".


    I plan to keep track of interesting threads regarding pickups and tube amplifiers.

    I can do that, though it might be a week or two before I can get to it. I think this is a high value area of exploration since it means endowing a single coil pickup with multiple usable electrical values, without the drawbacks of what is currently available on the market.

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  11. #46
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    <My explanation is the following:

    When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

    This effect does not depend on electrical connection between the coil parts.>
    Here
    http://www.datatronics.com/pdf/distr...ance_paper.pdf
    is a good description of the summing-up of distributed capacitances of coupled windings.

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  12. #47
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    Quote Originally Posted by Helmholtz View Post
    Here
    http://www.datatronics.com/pdf/distr...ance_paper.pdf
    is a good description of the summing-up of distributed capacitances of coupled windings.
    Thanks for the link, I'll have to look over that carefully to see if it's equations might apply to transformers that have coils with a common axis and plane.

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  13. #48
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    Quote Originally Posted by Antigua View Post
    Thanks for the link, I'll have to look over that carefully to see if it's equations might apply to transformers that have coils with a common axis and plane.
    The effect only requires sufficient magnetic coupling. Good coupling means that the windings share most of the same magnetic flux. Concentric arrangement of the windings on a common ferromagnetic core provides good preconditions for strong magnetic coupling. The coupling factor can be determined from the ratio of primary inductance with secondary shorted (Lps) to primary inductance with secondary open (Lpo). Perfect coupling (100%) means Lps/Lpo = 0 and no coupling means Lps/Lpo = 1.
    In case of weaker coupling, the transformed secondary self-capacitance will appear on the primary side in series with the transformed secondary leakage inductance, constituting an additional series resonance circuit. As a consequence the pickup's frequency response will show additional peaks and dips above the main primary resonance frequency.

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    HI
    I tried the wiring you suggested on a 51 type P bass single coil. Coils are 6,7 kΩ / 2,6 H (tapped) 7,8 kΩ / 3,4 H (full). It works very nicely. The sound difference is subtle but useful. It could be more so I'll try something like 5,7...6,5 kΩ and 7,8 kΩ.

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    Quote Originally Posted by okabass View Post
    HI
    I tried the wiring you suggested on a 51 type P bass single coil. Coils are 6,7 kΩ / 2,6 H (tapped) 7,8 kΩ / 3,4 H (full). It works very nicely. The sound difference is subtle but useful. It could be more so I'll try something like 5,7...6,5 kΩ and 7,8 kΩ.
    To which wiring diagram do you refer? The one in the original post, or Antigua’s of Feb 28?

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  16. #51
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    To Antigua's.
    As a matter of fact I just wind a 5,7 kΩ + 1,9 kΩ (7,8 kΩ) coils to an old Duncan quarter pounder frame. But there's some short in coils (reads 50Ω when it should be ∞Ω). Didn't have time to check more closely. Must perhaps rewind that.

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  17. #52
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    I'm glad to hear that worked out. I'll have to make a four lead tapped single coil and try it myself. I think the difference between two pickups tends to be pretty subtle, but a big deal is made of it, because it's the sound you have to live with. Put a switch in there and you might demand a bigger shift. I figured that was why tapped single coils presently on the market had such radical tap points, until I found out that the capacitance, and possibly inductive coupling, made it somewhat necessary, too.

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  18. #53
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    Wind 5,7 kΩ + 1,9 kΩ (7,8 kΩ) coils to normal 51 PU frame (ca. 5 mm magnets). I found that Quarter pounder magnets made it too thick sounding. There's volume difference, but it ain't bad. I like the most that you get a 50's and a modern fuller sound without character change.

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    Last edited by okabass; 04-16-2018 at 12:44 AM.

  19. #54
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    In a sense, you are all correct - all these effects do happen, independently of one another, so it's a matter of relative magnitude, and experiment is the only way to sort it out.

    The floating open-circuited coils gets both electrostatic and magnetic coupling. The floating coil increases the self-capacitance of the nearby connected coil. This happens even if the connected coil is only connected at one end, so no metallic currents flow. What does flow through the capacitances are displacement currents due to changing electric fields. Currents through the connected coil will induce voltages in the floating coil, which is capable of resonance with its own self-capacitance, and this effect will be reflected back into the connected coil. It can get pretty complicated.

    How to reduce the effects? Two main remedies. First, lay down a few layers of mylar or waxed paper tape on top of the inner winding, before starting the outer winding. This will decrease the capacitance between coils, but will not have much effect on the transformer coupling between the coils. One can suppress a lot of the effect by loading the floating coil with a fixed resistor, thus reducing the effects of resonance. The best way to find the correct resistor value is a pot and a golden ear.

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    Thank you for the post.
    Mylar plastic. Do you mean clear mylar only or with metal foil (like space sheet)? How about this? https://www.ballisticproducts.com/Th...ctinfo/MYTHIN/

    Fixed resistor? Parallel to the floating coil? What value pot would be a good starting point if the floating coil is around 2 kΩ ? The other coil is 5,8 kΩ, total 7,8 kΩ.

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    Last edited by okabass; 04-16-2018 at 10:15 AM.

  21. #56
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    Quote Originally Posted by Joe Gwinn View Post
    In a sense, you are all correct - all these effects do happen, independently of one another, so it's a matter of relative magnitude, and experiment is the only way to sort it out.

    The floating open-circuited coils gets both electrostatic and magnetic coupling. The floating coil increases the self-capacitance of the nearby connected coil. This happens even if the connected coil is only connected at one end, so no metallic currents flow. What does flow through the capacitances are displacement currents due to changing electric fields. Currents through the connected coil will induce voltages in the floating coil, which is capable of resonance with its own self-capacitance, and this effect will be reflected back into the connected coil. It can get pretty complicated.

    How to reduce the effects? Two main remedies. First, lay down a few layers of mylar or waxed paper tape on top of the inner winding, before starting the outer winding. This will decrease the capacitance between coils, but will not have much effect on the transformer coupling between the coils. One can suppress a lot of the effect by loading the floating coil with a fixed resistor, thus reducing the effects of resonance. The best way to find the correct resistor value is a pot and a golden ear.

    While the mylar foil might help to better separate the windings (even though its epsilon is around 3) and thus reduce interwinding capacitance, this will have neglegible effect on the active coil's resonance. The reason is that interwinding capacitance is not in the circuit as long as the outer coil is floating (and not shielded by a grounded outer copper foil). The primary (active) coil resonance only changes if its effective capacitance to ground changes.

    Loading the floating coil with a resistor will as well load the active coil with the reflected resistance (and - depending on coupling - a leakage inductance), thus reducing Q and resonance peak. This might not be desirable.

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  22. #57
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    Quote Originally Posted by okabass View Post
    Thank you for the post.
    Mylar plastic. Do you mean clear mylar only or with metal foil (like space sheet)? How about this? https://www.ballisticproducts.com/Th...ctinfo/MYTHIN/
    Yes, clear mylar. Actually, kapton would work just as well. Or scotch tape.


    Fixed resistor? Parallel to the floating coil? What value pot would be a good starting point if the floating coil is around 2 kΩ ? The other coil is 5,8 kΩ, total 7,8 kΩ.
    Connecting the ends of the floating coil, to compete the circuit. I have no idea what value is best. It may be that shorting the ends together is best, given the large DC resistance of the floating coil. Start with a 10 K pot and fiddle.

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    Quote Originally Posted by Helmholtz View Post
    While the mylar foil might help to better separate the windings (even though its epsilon is around 3) and thus reduce interwinding capacitance, this will have neglegible effect on the active coil's resonance.
    Agree.


    The reason is that interwinding capacitance is not in the circuit as long as the outer coil is floating (and not shielded by a grounded outer copper foil). The primary (active) coil resonance only changes if its effective capacitance to ground changes.
    Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.


    Loading the floating coil with a resistor will as well load the active coil with the reflected resistance (and - depending on coupling - a leakage inductance), thus reducing Q and resonance peak. This might not be desirable.
    It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.

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  24. #59
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    >Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.<

    Not convincing. In high frequency transformer applications open (non-terminated) floating windings are typically avoided.
    A grounded interwinding shield effectively reduces interwinding capacitance (capacitive coupling between windings) but inevitably increases ground capacitance for both windings, which seems counterproductive in PUs.
    Can you show measurements and/or an equivalent circuit?
    BTW: I generally assume the inner start of the active pickup winding to be grounded.


    >It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.<

    In my experience most guitar players prefer some coloration i.e. resonance peak. Otherwise they could as well back the the tone control a bit to get a similar result.

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  25. #60
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    Interesting thread, but here's something I never usually see discussed.

    We tend to think of coil-tapping in terms of "additional winds". That is, wind enough to make a viable coil, then add some more turns to make a hotter coil, but retain the selectable connection to the point where the added turns begin. Because the additional turns are on the outside, the added circumference makes for greater resistance per turn.

    But how does that compare to making the additional turns on the inside? For example, put 1500 turns snuggled up against the polepieces, run a lead out to a solder terminal, then add another, say, 6500 turns. I won't pretend to understand Eddy currents or magnetism in general, nearly as well as my colleagues here. But at the very least, the scenario I describe moves the "start" of the default coil outwards, and obviously changes the circumference of the turns.

    So what I'm wondering is: how does the relocation of the additional turns to the "inside" change things with respect to inductance, resonant peaks, etc., and which would be the preferred arrangement - added coils on the outside, or the inside?

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    Quote Originally Posted by Mark Hammer View Post
    Interesting thread, but here's something I never usually see discussed.

    We tend to think of coil-tapping in terms of "additional winds". That is, wind enough to make a viable coil, then add some more turns to make a hotter coil, but retain the selectable connection to the point where the added turns begin. Because the additional turns are on the outside, the added circumference makes for greater resistance per turn.

    But how does that compare to making the additional turns on the inside? For example, put 1500 turns snuggled up against the polepieces, run a lead out to a solder terminal, then add another, say, 6500 turns. I won't pretend to understand Eddy currents or magnetism in general, nearly as well as my colleagues here. But at the very least, the scenario I describe moves the "start" of the default coil outwards, and obviously changes the circumference of the turns.

    So what I'm wondering is: how does the relocation of the additional turns to the "inside" change things with respect to inductance, resonant peaks, etc., and which would be the preferred arrangement - added coils on the outside, or the inside?
    I can't answer this question directly, but I can tell you that when the inner and outer coils are the same turn count, the plots overlap almost identically:



    Based on this, I'd think it wouldn't make much difference if you tapped inward versus tapping outward.

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    Quote Originally Posted by Antigua View Post
    I can't answer this question directly, but I can tell you that when the inner and outer coils are the same turn count, the plots overlap almost identically:



    Based on this, I'd think it wouldn't make much difference if you tapped inward versus tapping outward.
    Inductance of the windings is essentially determined by the square of the turns (with a small additional influence of dimensions, forget resistance). Inductive coupling forces the resonant frequencies of both coils to be identical (see link on coupled windings) even if their inductances differ.

    @Antigua, you can improve S/N ratio of your plots by choosing the Automatic Voltage Scale option in the bode plotter of the PCSU 200.

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    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.

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    3 wire vs. 4 wire tapped pickups

    Perhaps this has been mentioned already but using the lower resistance tap with a pickup using 3 wires instead of 4 would leave the outer winds connected to the hot signal output but not terminated by a path to ground, a situation which I suspect would make the pickup more susceptible to noise as the unterminated winding would act as an antenna. Which would be another reason to use 4 eyelets and leads instead of 3.

    Correct me if I am mistaken about that.

    Thanks!

    Steve A.

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    Quote Originally Posted by Helmholtz View Post
    >Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.<

    Not convincing. In high frequency transformer applications open (non-terminated) floating windings are typically avoided.
    We are talking audio transformers here, not high-frequency (like RF) here. And in RF floating windings are used, such as for feedback in oscillators.

    A grounded interwinding shield effectively reduces interwinding capacitance (capacitive coupling between windings) but inevitably increases ground capacitance for both windings, which seems counterproductive in PUs.
    True. People add some insulation between to reduce capacitance to ground (versus the other coil) if it's a problem.

    Can you show measurements and/or an equivalent circuit?
    BTW: I generally assume the inner start of the active pickup winding to be grounded.
    No, I haven't built this recently. But what I'm saying is straight out of books on designing audio transformers. It's actually a complicated bunch of balanced compromises.


    >It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.<

    In my experience most guitar players prefer some coloration i.e. resonance peak. Otherwise they could as well back the the tone control a bit to get a similar result.
    Then use a larger resistor, to allow some coloration from the floating coil through. As I said, the correct instrument here is a golden ear.

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    Mark Hammer,

    I can tell you that some pedal steel pickups designs leave an airgap of .015-.020" between the magnets and the inside (start) of the coil to kill off some highs. I'd imagine that switching out the inner coil tap would have the same effect. Never tried it but it might have a use on an otherwise too bright guitar.

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    We are talking audio transformers here, not high-frequency (like RF) here. And in RF floating windings are used, such as for feedback in oscillators.
    No new arguments here. I am familiar with scientific literature on transformers. And I know and understand the detailed equivalent circuit, which is the same for RF and LF applications. My assessments are based on this knowledge as well as basic physics.

    If a winding is used for feedback purposes, it has to be connected/terminated. This creates a situation that is not comparable to a tapped PU with an open winding.

    I repeat: I do not see how the interwinding capacitance (which is rather low for a cleanly wound PU anyway) could influence the actice coil's resonance. It is not in the circuit when the secondary is floating. And it is essentially shorted when the two windings are connected at the tap (3 wire type).

    My general resume regarding tapped singlecoils, based on theory and the measurements of Antigua, is that tapping inevitably introduces unwanted additional capacitance in the active coil and thus tends to prevent getting really different sounds (i.e. resonant frequencies) from different configurations (full vs tapped).
    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.

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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.
    You state that the shape of the coil can have an influence on sound. The question is why. Is it really the shape of the coil or just caused by different resonances?
    The sound difference between a Jazzmaster and Strat PU can be explained by different resonances (frequency and Q). I you want to study the single influence of coil shape, all other influencing factors - especially resonance - need to be equal for the two PUs. This can be achieved by varying the number of turns and - if necessary - additional capacitors and resistors.

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    Quote Originally Posted by Helmholtz View Post
    No new arguments here. I am familiar with scientific literature on transformers. And I know and understand the detailed equivalent circuit, which is the same for RF and LF applications. My assessments are based on this knowledge as well as basic physics.

    If a winding is used for feedback purposes, it has to be connected/terminated. This creates a situation that is not comparable to a tapped PU with an open winding.

    I repeat: I do not see how the interwinding capacitance (which is rather low for a cleanly wound PU anyway) could influence the actice coil's resonance. It is not in the circuit when the secondary is floating. And it is essentially shorted when the two windings are connected at the tap (3 wire type).

    My general resume regarding tapped singlecoils, based on theory and the measurements of Antigua, is that tapping inevitably introduces unwanted additional capacitance in the active coil and thus tends to prevent getting really different sounds (i.e. resonant frequencies) from different configurations (full vs tapped).
    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.
    Well, as I said upthread, all these effects are present to one degree or another, simultaneously. Various ways to increase or decrease this or that effect was discussed. So, one listens, and uses these tricks to steer the pickup towards a desired sound. The role of the theory is to give us clues as to what alterations are likely to move things toward or away from a desired direction.

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    Well, as I said upthread, all these effects are present to one degree or another, simultaneously. Various ways to increase or decrease this or that effect was discussed. So, one listens, and uses these tricks to steer the pickup towards a desired sound. The role of the theory is to give us clues as to what alterations are likely to move things toward or away from a desired direction.
    Yes, Sir. Right you are.

    I love and support experimenting and listening tests. Experience and theoretical analysis of the dependencies may help to concentrate on the more promising experiments.
    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.

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