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Thread: Laminated Core

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    Member Chris Turner's Avatar
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    Laminated Core

    So, I'm thinking of trying my hand at making a laminated core from layers of sheet metal.

    What would suffice as a good material for electrically isolating the layers? Would plain old double-sided Scotch tape work, or maybe some sort of thin PVC-based tape?

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    Quote Originally Posted by Chris Turner View Post
    So, I'm thinking of trying my hand at making a laminated core from layers of sheet metal.

    What would suffice as a good material for electrically isolating the layers? Would plain old double-sided Scotch tape work, or maybe some sort of thin PVC-based tape?
    The voltage between laminations is of order a millivolt. Anything mechanically strong enough will work, even a layer of varnish obtained by dipping the laminations and allowing them to dry before assembly.

    Mylar film and epoxy is one option, as is onionskin paper and epoxy. The laminations must be completely degreased for the epoxy to stick.

    Be sure to deburr the edges of the laminations, so you won't get accidental current loops.

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    Once you glued the lams together you are all done with cutting, grinding drilling etc.
    I'd try some thin transfer tape. I love the stuff from Sellotape/scapa in CH but I can't find it in the States. It's Scapa 4456. Thinner than 3M.
    Dick Blick sells something similar i'm sure:
    Double-Coated Tapes - BLICK art materials

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    If you're making a prototype, how about simple mylar packaging tape? Once you put tape on both sides of the lams, just glue them together.

    ken

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    Member Chris Turner's Avatar
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    Quote Originally Posted by ken View Post
    If you're making a prototype, how about simple mylar packaging tape? Once you put tape on both sides of the lams, just glue them together.

    ken
    Yeah, it's just for a prototype at this point. I'll probably just use double-sided Scotch tape for now.

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    Pickup Maker David Schwab's Avatar
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    I've used double sided tape, and also paper and super glue.

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    Some Asian-made guitars, such as some Univoxes, would use humbuckers where one or both coils looked like a sort of "staple" coil from the top, but was two soft iron (?) slugs butted up against each other, looking like a pair of "combs", such as you'd see along the outside of a Jaguar pickup. Of course, there would be a ceramic magnet linking those slugs and another that might have adjustable screws in it.

    Would you consider this a "laminated" core or does lamination have to involve more than two blade-like pieces?

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    ToneOholic! big_teee's Avatar
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    So what is the benefit of a laminated core?
    Is it something you can hear, or very mynute?

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    Member Chris Turner's Avatar
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    Quote Originally Posted by big_teee View Post
    So what is the benefit of a laminated core?
    Is it something you can hear, or very mynute?
    It reduces eddy currents, therefore you can get more highs from the same size core (compared to a solid core)

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    Quote Originally Posted by Chris Turner View Post
    It reduces eddy currents, therefore you can get more highs from the same size core (compared to a solid core)
    Exactly. Vary the effect by varying the lamination thickness. Power transformer laminations are in the range 0.010" to 0.014".

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    Are we suggesting laminating parallel to the long or short dimension of the pole bar (as viewed from above) for optimum effect?

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    Quote Originally Posted by fieldwrangler View Post
    Are we suggesting laminating parallel to the long or short dimension of the pole bar (as viewed from above) for optimum effect?


    Probably long; it is easier. Short would give smaller loops for the same thickness, and so less eddy currents.

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    In fact, this is what I've often thought about laminated cores; if you really wanted to minimize eddy currents you'd hafts face up to the proverbial "inconvenient truth" about making lots of little strips.

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    Quote Originally Posted by fieldwrangler View Post
    In fact, this is what I've often thought about laminated cores; if you really wanted to minimize eddy currents you'd hafts face up to the proverbial "inconvenient truth" about making lots of little strips.
    I do not see the need for a laminated core. If you want no eddy currents, but need a sufficiently high permeability material to carry the permanent field to the strings, use ferrite.

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    Quote Originally Posted by Mike Sulzer View Post
    I do not see the need for a laminated core. If you want no eddy currents, but need a sufficiently high permeability material to carry the permanent field to the strings, use ferrite.
    The issue is to get the desired amount of eddy current loading, to get a desired sound.

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    Quote Originally Posted by fieldwrangler View Post
    Are we suggesting laminating parallel to the long or short dimension of the pole bar (as viewed from above) for optimum effect?
    The loading effect effect varies with something like the square of the thickness. The strips can be long way or short way, so long as the thickness dimension is perpendicular to the axis of the coil. Long way is a lot less work. Kinman has a patent on doing it the short way, but he isn't building a blade pickup.

    Cutting transverse slits in a thick blade won't work - it's the smallest dimension that's perpendicular to the coil axis that counts.

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    Quote Originally Posted by Joe Gwinn View Post
    The issue is to get the desired amount of eddy current loading, to get a desired sound.
    That's not the issue fieldwrangler brought up, the one I was addressing. On the other hand, if you do want to get a controlled reduction in eddy currents over a solid blade, say 2, 3, 4, 5... times less, why use laminations? That is difficult. Just cut the blade into pieces in the obvious way along the axis of the pickup, and make sure that they are insulated from each other.

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    That's basically what Bartolini does.

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    Quote Originally Posted by Mike Sulzer View Post
    That's not the issue fieldwrangler brought up, the one I was addressing. On the other hand, if you do want to get a controlled reduction in eddy currents over a solid blade, say 2, 3, 4, 5... times less, why use laminations? That is difficult. Just cut the blade into pieces in the obvious way along the axis of the pickup, and make sure that they are insulated from each other.
    I see now; what I am suggesting would only be good for fine tuning; if you want to reduce it a few times, laminations in the usual way are easier.

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    Quote Originally Posted by Joe Gwinn View Post
    The loading effect effect varies with something like the square of the thickness. The strips can be long way or short way, so long as the thickness dimension is perpendicular to the axis of the coil. Long way is a lot less work. Kinman has a patent on doing it the short way, but he isn't building a blade pickup.

    Cutting transverse slits in a thick blade won't work - it's the smallest dimension that's perpendicular to the coil axis that counts.
    Maybe I am still confused. If you make thin laminations twice as thin (and thus have twice as many), the resistance of each stays almost the same, and the voltage around the loop is half as much. So the power dissipated in each drops by a factor of four (p = (v^2)/R). But you have twice as many laminations, and so the total power has been reduced by half. That makes the variation look linear.

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    Quote Originally Posted by Mike Sulzer View Post
    Maybe I am still confused. If you make thin laminations twice as thin (and thus have twice as many), the resistance of each stays almost the same, and the voltage around the loop is half as much. So the power dissipated in each drops by a factor of four (p = (v^2)/R). But you have twice as many laminations, and so the total power has been reduced by half. That makes the variation look linear.
    I was speaking of individual laminations. If you have more than one lamination, their effects will add, as you note.

    The shape of the loss versus frequency will follow the thickness of the individual laminations, not the number of laminations, so one adjusts both individual thickness and number to suit.

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    Quote Originally Posted by Joe Gwinn View Post
    The shape of the loss versus frequency will follow the thickness of the individual laminations, not the number of laminations,
    I think if you pull out half the laminations, the frequency at which 3db is lost would rise. In a pickup, the onset of skin effect in the individual laminations might also be a complicating issue.

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    Quote Originally Posted by Mike Sulzer View Post
    I think if you pull out half the laminations, the frequency at which 3db is lost would rise. In a pickup, the onset of skin effect in the individual laminations might also be a complicating issue.
    No, the height of the peak will also change, preserving the shape.

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    Quote Originally Posted by Joe Gwinn View Post
    No, the height of the peak will also change, preserving the shape.
    This is complicated; for exact results, one must consider a specific situation, but consider this somewhat specific situation: an air coil into which one can insert one or two identical laminations, thin enough so that they do not affect the inductance significantly. We apply changng flux to induce a voltage. The voltage across the coil is determined by a voltage divider in which the coil inductance (+ resistance) is the series element and the shunt element is the impedance resulting from the loading due to the lamination. This impedance as a function of frequency can be determined by the theory described elsewhere, but never mind, it is what it is. One lamination results in a certain attenuation versus frequency. Two laminations halves the shunt impedance. If the shunt impedance were purely resistive, this would cut the 3 db frequency in half, assuming the coil is purely inductive at this frequency. The shunt impedance is not purely resistive, while the series impedance is not purely inductive, and so it will not do that, but if the former is partly resistive and the latter mostly inductive, I expect the frequency to drop, how much depending on the specifics.

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    Quote Originally Posted by Mike Sulzer View Post
    This is complicated; for exact results, one must consider a specific situation, but consider this somewhat specific situation: an air coil into which one can insert one or two identical laminations, thin enough so that they do not affect the inductance significantly. We apply changng flux to induce a voltage. The voltage across the coil is determined by a voltage divider in which the coil inductance (+ resistance) is the series element and the shunt element is the impedance resulting from the loading due to the lamination. This impedance as a function of frequency can be determined by the theory described elsewhere, but never mind, it is what it is. One lamination results in a certain attenuation versus frequency. Two laminations halves the shunt impedance. If the shunt impedance were purely resistive, this would cut the 3 db frequency in half, assuming the coil is purely inductive at this frequency. The shunt impedance is not purely resistive, while the series impedance is not purely inductive, and so it will not do that, but if the former is partly resistive and the latter mostly inductive, I expect the frequency to drop, how much depending on the specifics.
    I have no idea if your proposed model circuit is an adequate representation for what's actually going on. I'd start with trying it in a test coil, and then fit the math to the experimental results.

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    Quote Originally Posted by Joe Gwinn View Post
    I have no idea if your proposed model circuit is an adequate representation for what's actually going on. I'd start with trying it in a test coil, and then fit the math to the experimental results.
    What circuit model were you basing your statement ("No, the height of the peak...") on, if any? Clearly, the coil capacitance effect is explained by the series-shunt divider model, coil in series, shunt capacitor. Why would the impedance from eddy currents be different? The loading from eddy currents appears across the coil in the same way; it is like the secondary of a transformer.

    Fitting to experimental results is what I have been doing in that other discussion. But also, starting with the experimental impedance measurements, it is possible to "unparallel" the coil inductance and construct the series-shunt circuit model. Preliminary results for steel show the small dip in the middle like some humbucker response measurements, but before I show this I want to get frequency response measurements from the same coil-core combinations (using a test exciter coil), but this requires some slightly different software that I need to write.

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    Quote Originally Posted by Mike Sulzer View Post
    What circuit model were you basing your statement ("No, the height of the peak...") on, if any? Clearly, the coil capacitance effect is explained by the series-shunt divider model, coil in series, shunt capacitor. Why would the impedance from eddy currents be different? The loading from eddy currents appears across the coil in the same way; it is like the secondary of a transformer.
    It's based on physics reasoning, which may or may not translate well to lumped-parameter circuit designs, although approximations are often good enough to be useful.

    The reasoning is that if a single lamination of thickness t produces a given eddy-current loss curve (with respect to frequency), two such laminations next to one another will produce the same curve, only with twice the loss at each and every frequency. In other words, the curve goes up and down, but does not change shape.

    By contrast, if a pair of laminations of thickness t is replaced by a single lamination of thickness 2t, one will get a different frequency response than the lamination of thickness t.

    Now, it must be noted that the above rule applies only when the skin depth is no less than t and 2t or so. If the skin depth is far less than t, varying the lamination thickness will have little effect.

    Fitting to experimental results is what I have been doing in that other discussion.
    True, and a very useful thing to do.

    But also, starting with the experimental impedance measurements, it is possible to "unparallel" the coil inductance and construct the series-shunt circuit model. Preliminary results for steel show the small dip in the middle like some humbucker response measurements, but before I show this I want to get frequency response measurements from the same coil-core combinations (using a test exciter coil), but this requires some slightly different software that I need to write.
    This is the part I'm less sure of, as it seems to have cart before horse. The issue is that the physics of eddy current is not fully expressible by networks of lumped-parameter components. Although one can come up with useful approximations, one usually starts with measurements on the system in question, and then back-fits a circuit diagram that yields and adequate approximation, where "adequate" is decided in the context of what one will use the system in question for.

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    Quote Originally Posted by Joe Gwinn View Post
    It's based on physics reasoning, which may or may not translate well to lumped-parameter circuit designs, although approximations are often good enough to be useful.
    True, physics reasoning is more general, but this statement of the physics does not describe the situation completely:
    Quote Originally Posted by Joe Gwinn View Post
    The reasoning is that if a single lamination of thickness t produces a given eddy-current loss curve (with respect to frequency), two such laminations next to one another will produce the same curve, only with twice the loss at each and every frequency. In other words, the curve goes up and down, but does not change shape.

    By contrast, if a pair of laminations of thickness t is replaced by a single lamination of thickness 2t, one will get a different frequency response than the lamination of thickness t.
    The effect of the coil inductance is missing, and it is necessary for determining the losses in the pickup and its response.

    Quote Originally Posted by Joe Gwinn View Post
    This is the part I'm less sure of, as it seems to have cart before horse. The issue is that the physics of eddy current is not fully expressible by networks of lumped-parameter components. Although one can come up with useful approximations, one usually starts with measurements on the system in question, and then back-fits a circuit diagram that yields and adequate approximation, where "adequate" is decided in the context of what one will use the system in question for.
    The concept of impedance is more general than that of lumped elements restricted to Rs, Ls and Cs. Equation 8 in the pdf file referred to in the current pickup measurement discussion is for impedance, but you cannot represent that impedance exactly by such lumped elements except in certain limiting cases. However, that impedance is exact in a case where the stated conditions are met. Then the question is, how closely are these conditions met in a real pickup? This is why I measured an artificial case, shorted turn, and showed that it compares well, and two pickups, one that compares well and another for which the model is of some use, but clearly not fully accurate.

    I do not think that representing the eddy current losses as an impedance across the coil is perfectly accurate, but I doubt that it is any less accurate that saying that a coil has "capacitance", when the reality is much more complicated. To the extent that this is true, one can take out the coil impedance, leaving that due to eddy currents. The accuracy of this can be checked by measuring the frequency response of the pickups by use of an exciter coil and comparing that to predictions using the impedance concept just described. I doubt that they will agree perfectly, but we shall see.

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    Quote Originally Posted by Mike Sulzer View Post
    True, physics reasoning is more general, but this statement of the physics does not describe the situation completely.
    Well, it is certainly a back-of-the-envelop kind of analysis, but I don't understand your point.

    The effect of the coil inductance is missing, and it is necessary for determining the losses in the pickup and its response.
    Yes, there will be an effect, but how big? Earlier, you had in effect concluded it would be minimal: "This is complicated; for exact results, one must consider a specific situation, but consider this somewhat specific situation: an air coil into which one can insert one or two identical laminations, thin enough so that they do not affect the inductance significantly." In any event, this assumption deserves a direct test.

    In a pickup with one coil and two laminations, currents in the two laminations can and will interact with one another.

    The concept of impedance is more general than that of lumped elements restricted to Rs, Ls and Cs. Equation 8 in the pdf file referred to in the current pickup measurement discussion is for impedance, but you cannot represent that impedance exactly by such lumped elements except in certain limiting cases. However, that impedance is exact in a case where the stated conditions are met. Then the question is, how closely are these conditions met in a real pickup? This is why I measured an artificial case, shorted turn, and showed that it compares well, and two pickups, one that compares well and another for which the model is of some use, but clearly not fully accurate.
    I think you are restating my point, and it's true that one always has impedance, even if there are no lumped-parameter elements.

    For the audience: A lumped-parameter circuit is one where there are things like recognizable inductors, capacitors, and resistors, each being a object one can hold in the hand, and each being a fairly pure representative of the breeds L, R, or C. A common example of a non-lumped parameter is the stray capacitance of an inductance coil.

    I do not think that representing the eddy current losses as an impedance across the coil is perfectly accurate, but I doubt that it is any less accurate that saying that a coil has "capacitance", when the reality is much more complicated. To the extent that this is true, one can take out the coil impedance, leaving that due to eddy currents. The accuracy of this can be checked by measuring the frequency response of the pickups by use of an exciter coil and comparing that to predictions using the impedance concept just described. I doubt that they will agree perfectly, but we shall see.
    I agree, but submit that it can get pretty complicated, which is why one normally starts with measurements and the simplest plausible lumped-parameter circuit that emulates the measured behaviour, adding components only when and where the approximation is insufficient for the intended use. There is a whole literature on such things, and many a researcher has become famous for finding a nice approximating model. where nice means simpler (more tractable mathematically) than anything else that accurate.

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    If you want to know the change in the frequency response of a pickup from adding a laminated core, you must include the effect of the coil inductance because it is an important part of the circuit. I did not conclude that the effect the inductance would be negligible, as you said I did. All I did was consider a case where the effect of the laminations on the inductance would be small, just for simplicity. Read my statement (that you quoted) again. I do not think that there is any point in commenting on the rest of what you wrote until this misunderstanding is cleared up.

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    Quote Originally Posted by Mike Sulzer View Post
    If you want to know the change in the frequency response of a pickup from adding a laminated core, you must include the effect of the coil inductance because it is an important part of the circuit. I did not conclude that the effect the inductance would be negligible, as you said I did. All I did was consider a case where the effect of the laminations on the inductance would be small, just for simplicity. Read my statement (that you quoted) again. I do not think that there is any point in commenting on the rest of what you wrote until this misunderstanding is cleared up.
    The tone of voice seems to be getting sharper, so maybe it's best to give this a rest.

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mark Hammer View Post
    Some Asian-made guitars, such as some Univoxes, would use humbuckers where one or both coils looked like a sort of "staple" coil from the top, but was two soft iron (?) slugs butted up against each other, looking like a pair of "combs", such as you'd see along the outside of a Jaguar pickup. Of course, there would be a ceramic magnet linking those slugs and another that might have adjustable screws in it.

    Would you consider this a "laminated" core or does lamination have to involve more than two blade-like pieces?
    Those were the Maxon pickups. They did that because they were stamping out the parts, not because they anted them laminated. Same thing with the "keeper" in the Hagstrom Bi-Sonic/Dark Star pickup. It's just easier to make that way since they could stamp out a thinner piece of steel and stack them up.

    Since the pieces went insulated from one another they weren't really laminated.

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    Quote Originally Posted by David King View Post
    That's basically what Bartolini does.
    They also used to have the laminated core humbucker, which was a guitar pickup with exposed coils and laminated blades. The ones I had heard back in the 80s were wound pretty hot and had an enhanced midrange tone.

    I used laminated cores in one of my bass pickups. Compared to a solid core (used with the same bobbin/windings/magnet) they had a brighter, edgier tone.

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  34. #34
    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Mike Sulzer View Post
    If you want no eddy currents, but need a sufficiently high permeability material to carry the permanent field to the strings, use ferrite.
    EMG is doing that on some of their guitar pickups:



    One reason to use, or not use ferrite poles is steel loads the pickup differently from ferrite. Depends on the tone you are looking for.

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    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein


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  35. #35
    ToneOholic! big_teee's Avatar
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    Those EMGs look Sharp!
    Black Chrome Maybe?

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