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Thread: Moving a Coil Farther from the Magnets...?

  1. #1
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    Moving a Coil Farther from the Magnets...?

    <late night musing>Just out of curiosity what happens when you move a coil farther away from the magnets? To illustrate this, we read a decent amount about tapped coils, however, everyone is always leaving the INSIDE windings on when they tap. What could be expected, tonally, of doing the same but instead leaving only the putter could on?
    </late night musing>
    Chris

  2. #2
    Senior Member Jim Darr's Avatar
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    To answer your question my suggestion would be to conduct some experiments for yourself. You can get interesting tonal effects by doing this. I have used the outside portion of the tap to vary the tonal response. The results will vary because of coil geometry, turn pattern/ count, gauge, magnet type and strength, and proximity of the coil to the magnetic source. Best to try some simple experiments at first with some tapped coils and come up with some formulas that work for you.

    Several of the older, 60s/70s, import pickups actually had the coil not touching the magnets, while others were wound directly on, or near, the magnet(s).
    =============================================

    Keep Winding...Keep Playing!!!

    Jim

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    With a tapped coil, the most obvious difference would come from the fact that the inner and outer coils would produce different inductances, and possibly capacitances (depending on whether the unused portion of coil is still connected or not). For equal wind counts, the outer core would be lower inductance, and therefor sound brighter and quieter as a result.

    The less obvious difference is how the harmonic content will change for expressing a flux change across a wider area of coil. I suspect it would change very little. The difference in terms of inductance and resonant peak would probably be so much more prominent than any harmonic difference, that I don't think you'd ever be able to discern it specifically. I think it's the kind of thing you'd have to figure out on paper, and take it on faith from there.

    Quote Originally Posted by Jim Darr View Post
    I have used the outside portion of the tap to vary the tonal response.
    How did it sound different?

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    Would love to hear sound sample of anyone's got them? Obviously having context as to the pickup type, etc. would be useful too.
    Chris

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    Old Timer J M Fahey's Avatar
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    With a tapped coil, the most obvious difference would come from the fact that the inner and outer coils would produce different inductances, and possibly capacitances (depending on whether the unused portion of coil is still connected or not). For equal wind counts, the outer core would be lower inductance, and therefor sound brighter and quieter as a result.
    Sorry but no.
    Inductance depends only on number of turns, not on distance to core.The only difference you will readily find is that outer coil will be fatter, have longer wire which means it will have more DC resistance.
    Thatīs the basic reason why the inner coil is preferred.

    Something that will change but probably not changing the stakes by much , is coil capacitance towards the core.
    Juan Manuel Fahey

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    A pickup's inductance is complicated by the fact that a pickup has core that is partially air and partially permeable metal, which is an unusual arrangement, but it seems to me that if the coil is wider, the flux will not link up with that permeable core as well. Are you saying that the geometric relationship between the coil and it's partially permeable core are irrelevant?

    Another factor I forgot to mention is that the unused portion of coil will cause a strong eddy current if it's shunted, rather than disconnected. So it's like this:

    - if the unused coil is full disconnected, it's almost but not entirely inert.
    - if the unused coil is connected at only one end, it will capacitively couple with the active coil
    - if the unused coil is grounded at both ends, current will flow, and this will cause eddy current resistance in the active coil

  7. #7
    Senior Member Jim Darr's Avatar
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    Quote Originally Posted by John Kolbeck View Post
    How did it sound different?
    The sound will vary depending on all the winding variables we talk about here on this forum. But, when the coil tap is right in the middle of the total turn count, essentially making two coils with the same turn count and winding pattern, the outside coil will have a longer length of wire per turn and therefore higher resistance. The outside coil, by itself will have a more dominate bass response than the inner coil.

    To really shape the sound we can use a different coil geometry, wind pattern, and vary the turn count. As I suggested in my previous post in this thread, try winding a few and see for yourself. Try to come up with some formulas that work for the sound you are after.
    =============================================

    Keep Winding...Keep Playing!!!

    Jim

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    I guess I probably should have provided a framework for my hypothetical question and some parameters. For example: let's say you have a strat pickup with much bigger top and bottom forbon so you can put as much wire on it as you want. You then wind 6k of 42AWG and have two leads attached to that. You then wind another 6k of 42AWG on top of that previous coil and give it its own 2 leads. What would the tonal difference be between these two coils all other things remaining constant?
    Sounds like I got my answer anyways though haha: more dominant bass response.
    I do indeed have some plans to 3D print some bobbins to allow me to play with this idea firsthand but it's nice to get a yes/no evaluation as to whether I'd just be wasting my time first.
    Thanks,
    Chris

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    Inductance is proportional to the area of the coil, so the outer coil must be "stronger", but not because of wire lenght or DCR.
    A coaxial humbucker by VOX incorporates a little of this principle.
    SkinnyWire likes this.

  10. #10
    Member Alberto's Avatar
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    Things may not be so simple. The coil is also farther from the magnets, shouldn't it be weaker than the inner coil?

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    The main problem I'm having with this is that I can't find any good info on how half solid, half air cores effect inductance. They tend to either be air, or solid. A permeable core increases the inductance by a non trivial amount, so I think that's going to have a greater impact than cross sectional differences alone.

  12. #12
    ToneOholic! big_teee's Avatar
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    I vote that the outside coil will be weaker and darker (muddier), than the inside coil.
    I suspect there will be some coupling between the two coils.
    IMO if you're going to wind two coils, make mine side by side and call it a humbucker!
    T
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    Terry

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by kmensik View Post
    Inductance is proportional to the area of the coil, so the outer coil must be "stronger", but not because of wire lenght or DCR.
    A coaxial humbucker by VOX incorporates a little of this principle.
    Thatīs true on an *air* core coil.

    Here we have a magnetic core which has 100 to 1000 times higher permeability (depending on purity) than air so it is the defining factor, period.

    Whether the coil is tightly wound around the iron core or is separated from it by, say, 5mm woud change inductance, worst case, around 1%, since 99% of magnetic flux would still go through the magetic core and, say, 1% would have difficulty negotiating the air path .... which is in parallel with the iron one, not in series.

    If we have 2 resistors, magnetic resistors in this case, in parallel, the low value one will be the main factor, rather than a 100X higher one in parallel.

    Thatīs why I plain wrote that inside and outside coils would have basically same impedance, and only important parameter that would vary wouldbe resistance, coupled with a small change in parasitic capacitance, both because coil would be physically ņarger and wire would be longer, not because of a "magnetic" problem.
    Juan Manuel Fahey

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    So I have a tapped SSL-4. Based on looking at how the coil wires approach the eyelets, and by comparing inductive and capacitive coupling between the outside of the coil and an LCR probe, it appears that the black lead is the start of the outer coil, the red lead is the mid point, and the white wire is the end of the inner coil. Seymour Duncan advises that you send black to ground, and tap between the white and the red, which means when tapped, the outer coil is active.

    The outer coil has a DC resistance of 7.0k, the inner portion measures 6.6k. The outer coil reads an inductance of 2.00 henries at 120Hz . The inner coil reads 1.68 henries. Inductance and DC resistance aren't linearly proportionate, but the outer coil does seem to have somewhat more inductance relative to DC resistance. Even though the outer coil has a lower ratio of permeable material to "air" in it's core, it looks like the inner coil is so thin that the ratio is not all that much different.

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    I had a discussion related to this topic in this post: Why do different magnet type produce different sounds? see number 42. Generally, the outer coil turns are farther from the stronger magnetic field and thus produce less voltage per coil turn than the inner coil turns. I outlined an experiment to observe and measure the inner and outer coil output differences.

    Joseph J. Rogowski
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    Quote Originally Posted by bbsailor View Post
    I had a discussion related to this topic in this post: Why do different magnet type produce different sounds? see number 42. Generally, the outer coil turns are farther from the stronger magnetic field and thus produce less voltage per coil turn than the inner coil turns. I outlined an experiment to observe and measure the inner and outer coil output differences.

    Joseph J. Rogowski
    It's true that a coil with more area has a higher inductance, and that the outer coil has slightly more area, so you're assuming that the slight increase in flux density for the inner coil will outweigh the slightly higher inductance of the outer coil. It sounds to me like it might be a wash, with respect to output voltage. Whichever configuration produces the higher inductance would have a slightly lower resonant peak, though.

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    Quote Originally Posted by John Kolbeck View Post
    A pickup's inductance is complicated by the fact that a pickup has core that is partially air and partially permeable metal, which is an unusual arrangement, but it seems to me that if the coil is wider, the flux will not link up with that permeable core as well. Are you saying that the geometric relationship between the coil and it's partially permeable core are irrelevant?

    Another factor I forgot to mention is that the unused portion of coil will cause a strong eddy current if it's shunted, rather than disconnected. So it's like this:

    - if the unused coil is full disconnected, it's almost but not entirely inert.
    - if the unused coil is connected at only one end, it will capacitively couple with the active coil
    - if the unused coil is grounded at both ends, current will flow, and this will cause eddy current resistance in the active coil
    Would these effects be true of a humbucker or a stacked humbucker?

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    Quote Originally Posted by Richard View Post
    Would these effects be true of a humbucker or a stacked humbucker?
    It's still true, but to a smaller extent. When the coil pickup is side-by-side with the primary, it will be mostly inert if it's fully disconnected or connected at one end. But if the second coil is shunted to ground, it will have continuity within itself, and so it will be generating a voltage internally that does not effect the rest of the circuit, but still creates an opposing magnetic field due to Lenz's law, and that opposing field can be though of as eddy current losses against the primary.

    A stack would be the same; does nothing if fully disconnected, or connected at one end, but the lower coil would be further from the strings, so it would generate less eddy current losses when it's shunted.

    If one end is connected in either case, there will be some capacitive coupling with the primary coil, but it's much a smaller amount than a tapped coil, where the two coils are in very close proximity to on another.
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    Quote Originally Posted by J M Fahey View Post
    Here we have a magnetic core which has 100 to 1000 times higher permeability (depending on purity) than air so it is the defining factor, period.

    An open short core has much less effect than implied by its high permeability. Remember even a small air gap in the otherwise complete magnetic circuit has a significant effect on the operation of a transformer. A magnetic circuit that is mostly air gap cannot have a high effective permeability, maybe something like 10 at most.

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    I have a ceramic Strat pickup, I measured the inductance fully assembled, it was about 4 henries. Then I broken off the magnet and popped the steel slugs out and it read about 1.5 henry, so the steal pole pieces roughly triple the inductance in the context of a guitar pickup. The same Strat pickup with AlNiCo will usually read around 2.2 henries, because AlNiCo is less permeable than the steel.

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    Yes. The ratio of inductances is significantly less than the ratio of permeabilities. The same effect holds true for guitar strings. As long as the permeability of the string is high, the exact value is not so important.

    The steel core SC has a lower resonant frequency and higher eddy current losses than the Alnico SC. Not the same thing at all, more like a hum bucker. You might use fewer turns to get back part of the difference.

    Quote Originally Posted by John Kolbeck View Post
    I have a ceramic Strat pickup, I measured the inductance fully assembled, it was about 4 henries. Then I broken off the magnet and popped the steel slugs out and it read about 1.5 henry, so the steal pole pieces roughly triple the inductance in the context of a guitar pickup. The same Strat pickup with AlNiCo will usually read around 2.2 henries, because AlNiCo is less permeable than the steel.

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    One difference in using turns further from the core: The field lines induced in the core from the vibrating string go through the core and then loop back around to the string. If a loop of wire encloses the same field line coming from the string and going back to it, the output from that field line is lost since its signal cancels. This is because the field points in opposite directions in the two passes through the loop. The bigger the loop, the more field lines cancel.

    Since the same effect holds true for the core by itself, a larger loop contributes less to the inductance.

    This is not a really big effect for loops with the width of a guitar pickup, but if you used a loop several inches in diameter, you would get very little. But there is some loss with individual pole pieces because of the length of the coil.

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    Quote Originally Posted by John Kolbeck View Post
    It's still true, but to a smaller extent. When the coil pickup is side-by-side with the primary, it will be mostly inert if it's fully disconnected or connected at one end. But if the second coil is shunted to ground, it will have continuity within itself, and so it will be generating a voltage internally that does not effect the rest of the circuit, but still creates an opposing magnetic field due to Lenz's law, and that opposing field can be though of as eddy current losses against the primary.

    A stack would be the same; does nothing if fully disconnected, or connected at one end, but the lower coil would be further from the strings, so it would generate less eddy current losses when it's shunted.

    If one end is connected in either case, there will be some capacitive coupling with the primary coil, but it's much a smaller amount than a tapped coil, where the two coils are in very close proximity to on another.
    I have a guitar with stacked single coils (Fender Vintage noiseless) that I am about to rewire, this and the thread about the single humbucker and the 5 way switch has me thinking about some other options. The pickups seem to have a falloff in response in the upper range cutting some chime out. When I ground the lower coil they are a bit over the top. When I first wired it up I had a bad pot so I believe my signal was passing through my treble bleed network which had a nice thin sound. Fortunately the guitar has plenty of room to work inside.

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    Quote Originally Posted by John Kolbeck View Post
    It's still true, but to a smaller extent. When the coil pickup is side-by-side with the primary, it will be mostly inert if it's fully disconnected or connected at one end. But if the second coil is shunted to ground, it will have continuity within itself, and so it will be generating a voltage internally that does not effect the rest of the circuit, but still creates an opposing magnetic field due to Lenz's law, and that opposing field can be though of as eddy current losses against the primary.

    A stack would be the same; does nothing if fully disconnected, or connected at one end, but the lower coil would be further from the strings, so it would generate less eddy current losses when it's shunted.

    If one end is connected in either case, there will be some capacitive coupling with the primary coil, but it's much a smaller amount than a tapped coil, where the two coils are in very close proximity to on another.
    I have a guitar with stacked single coils (Fender Vintage noiseless) that I am about to rewire, this and the thread about the single humbucker and the 5 way switch has me thinking about some other options. The pickups seem to have a falloff in response in the upper range cutting some chime out. When I ground the lower coil they are a bit over the top. When I first wired it up I had a bad pot so I believe my signal was passing through my treble bleed network which had a nice thin sound. Fortunately the guitar has plenty of room to work inside.

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    I found this online inductance calculator. I'm ignoring permeability of the core as the question is about the coil itself changing geometry keeping other variables constant. The basic inductance of the coil seems to change in a linear fashion (5000 turns, 42AWG wire) at various core diameters, I used quarter inch, half, three-quarters, one, and two inches.

    So a '5000 turn' coil with a one inch core has 2.25 times the Inductance of a half inch core.

    online calculator: https://www.eeweb.com/toolbox/coil-inductance (another is CalcTool: Solenoid properties calculator)

    henrychart.jpg

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    Be aware that calculators for solenoids are not all that accurate for coils that don't resemble solenoids, which are far longer than their diameter.

    Pickup coils are very short, typically about the length of the shortest thickness (end on) or a fifth the length of the widths (broadside).

    There are formulas for short coils that are more accurate, but even these are approximate for something shaped like a pickup coil.

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