Thread: Use Multi-Section Windings to Reduce Self-Capacitance

1. Use Multi-Section Windings to Reduce Self-Capacitance

The classic way to greatly reduce the self-capacitance of a deep and wide multi-turn thick coil is to partition the winding into multiple adjacent deep but narrow coils connected in series. The adjacent narrow coils are magnetically coupled the same as when they were one wide coil, but the capacitance of the individual narrow coils is both reduced and in series. The reduction in overall capacitance can be quite dramatic. This method or approach has been around for about a century, as have banked windings.

For example, take a dual-winding ferrite pot core bobbin https://www.norwe.eu/products/en/006/E01.

The coil windings in each section of the bobbin have the same winding depth, but about one half the total winding width. The self-capacitance of each of the two section windings is maybe one half that of what we would have if we used a single-section bobbin https://www.norwe.eu/products/en/006/D01.

Because the two sections are adjacent, the magnetic flux threads the individual wire turns the same as before, so we get the full N^2 effect on self-inductance of the total winding turns N. But the electric fields are different, and each section has about half the self-capacitance of the one-section full-width coil, and because these individual self-capacitances are in series, the total overall self capacitance is about one quarter of that of the full width winding.

Because self-resonant frequency varies as the square root of the product of inductance and capacitance, the resonant frequency of the two-section coil will be double that of the one-section coil.

If one uses three sections, the resonant frequency will be tripled, in theory at least. The limiting factor is the loss of coil windings due to the physical bulk of the separators between the sections.

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2. Sort of like the old P bass and G&L pickups?

For some types of pickups, perhaps a good idea. But don't we (guitar players) rather enjoy and take advantage of the resonant peak character of electric guitar pickups as they are? In this event it's a solution looking for a problem. But for acoustic instruments or maybe even bass guitar it could be the bees knees. At the very least it's a good theoretical topic.

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3. What effect does this have on the "Q" of the coil?

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4. Originally Posted by Joe Gwinn
The classic way to greatly reduce the self-capacitance of a deep and wide multi-turn thick coil is to partition the winding into multiple adjacent deep but narrow coils connected in series. The adjacent narrow coils are magnetically coupled the same as when they were one wide coil, but the capacitance of the individual narrow coils is both reduced and in series. The reduction in overall capacitance can be quite dramatic. This method or approach has been around for about a century, as have banked windings.
I take it this works on a similar premise for why the Jazzmaster pickup had a lower capacitance, as seen in this other thread, that it puts more distance between portions of coil that have a higher voltage difference, reducing capacitance that way. So it seems that this is theoretically similar to pancaking Jazzmaster shaped coils in order to end up with a coil that is tall, but still has a low capacitance.

Originally Posted by Chuck H
Sort of like the old P bass and G&L pickups?

For some types of pickups, perhaps a good idea. But don't we (guitar players) rather enjoy and take advantage of the resonant peak character of electric guitar pickups as they are? In this event it's a solution looking for a problem. But for acoustic instruments or maybe even bass guitar it could be the bees knees. At the very least it's a good theoretical topic.
I can think of a couple benefits, one would be to cram more turns of wire onto a pickup in order to get a higher voltage output, without also pushing the resonance peak to a low frequency. The other would be to start with a very high peak for a similar output voltage, and then dial it down to taste with a selection of capacitors. A counter point this might be that the winds that are further from the guitar string produce less voltage anyway, and so it might make more sense to avoid a "tall" pickup all together.

Originally Posted by mozz
What effect does this have on the "Q" of the coil?
It shouldn't make much difference, since the core material and series resistance would be the same.

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5. This brings to mind those Fishman coils that are a stack of PCBs each with a spiral etched.

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6. Originally Posted by David King
This brings to mind those Fishman coils that are a stack of PCBs each with a spiral etched.
David, Joe, and any MEF member

This also brings to my mind the possible advantage of making a CSE187L Triad style current transformer with three sections rather than just the top with the single turn primary and the lower section with the 500 turn secondary.
The three section transformer would have the primary in the center section with two secondary sections with 1000 turns each but tapped at the 700/300 turn point to allow a variety of output tones with a variety of series or parallel output connections. By using a square copper wire equal to the size of the center primary opening area, one could minimize the leakage inductance and have a variety of sounds that would target a typical 150 ohm XLR input with a bridging impedance of 2400 ohms.

Given that the upper and lower secondaries are stacked per Joe Gwinn's post #1, the capacitance would also be reduced, depending on how the secondaries are wired, and could result in some interesting acoustic effects if some current transformer manufacturer would or could make a three layer current transformer with the primary in the center layer.

I believe the guitar community has been locked into the old tube based high impedance input requirements of pickups that has resulted in the typical "electric guitar sound" with the resonance point being in the area where the human ear is most sensitive (about 3 to 5Khz). Anyone choosing to look beyond this "electric guitar sound" needs to consider how low noise, lower impedance inputs allow for a different variety of sound. The electronic theory still equally applies to all of these design variants.

This is just a little audio food for thought.

Joseph J. Rogowski

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7. Originally Posted by David King
This brings to mind those Fishman coils that are a stack of PCBs each with a spiral etched.
That's definitely a low impedance coil that uses active tone shaping to mimic the response curve of a high impedance pickup, so while the pancaked circuit board might result in a lower capacitance, it's probably irrelevant anyway. The capacitance in that case might also depend on the size of the printed traces, which I'd suspect are larger than 42AWG.

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8. Originally Posted by mozz
What effect does this have on the "Q" of the coil?
As Antigua says, probably not a lot of difference.

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9. You might not want to do anything that reduces the number of turns you can fit in the available space because you lose output, that is, unless there is something else to gain, and decreasing capacitance is not an important issue because the cable capacitance is much larger than the coil capacitance. Thus the contribution of the coil capacitance to the actual resonant frequency in use is not large.

However, the inductance with the split bobbin is interesting. Since the cores are short, all the flux from one coil does not pass through the other even if the permeability is high. It is not obvious to me whether the inductance changes with a split bobbin, or which way it changes if it does. Suppose it decreased with the split bobbin. Then you could add some more turns and get higher output for the same inductance.

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10. Originally Posted by Mike Sulzer
You might not want to do anything that reduces the number of turns you can fit in the available space because you lose output, that is, unless there is something else to gain, and decreasing capacitance is not an important issue because the cable capacitance is much larger than the coil capacitance. Thus the contribution of the coil capacitance to the actual resonant frequency in use is not large.

However, the inductance with the split bobbin is interesting. Since the cores are short, all the flux from one coil does not pass through the other even if the permeability is high. It is not obvious to me whether the inductance changes with a split bobbin, or which way it changes if it does. Suppose it decreased with the split bobbin. Then you could add some more turns and get higher output for the same inductance.
One can compensate for reduced winding space by using the next finer wire size. Unless the separators are fairly thick, inductance won't be much affected, so long as the same total number of turns is used. Cable capacitance is of course unaffected, but reducing the self-capacitance of the pickup increases our wiggle room.

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11. Originally Posted by Bagpipe
What is the base of your assumptions - other than paranoia?
"Paranoia?" Hunh?
Oh my god, they're conspiring to change my inductance!
Quick! Don the tinfoil hats and run to the fallout shelter!

“Just because you're paranoid doesn't mean they aren't after you.”
- Joseph Heller, Catch-22

-rb

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12. One could wind the narrow coils sequentially between movable winder plates with just a thin paper or mylar separator. Each layer could even be fused with wax, lacquer or epoxy before winding the next, adjacent layer. Not the fastest way to wind but it would minimize the lost volume/turns. This would allow one to maximize the self-capacitance reduction by experimenting with different coil layer thicknesses and intra-winding dielectric barriers and thicknesses.
For mass production the coil layers could be wound simultaneously using fusible insulation magnet wire on a mandrel with rigid separators, the coils then heat fused, the mandrel slipped out of the assembly and the rigid separators removed and replaced with thinner dielectric material. Finally inserting a magnet or iron core and wrapping the whole thing in tape would tidy it up.

Alternatively one could sell traditional pickups with a low self-capacitance cable, about \$20 from Kimber cable...

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13. The classic way to greatly reduce the self-capacitance of a deep and wide multi-turn thick coil is to partition the winding into multiple adjacent deep but narrow coils connected in series. The adjacent narrow coils are magnetically coupled the same as when they were one wide coil, but the capacitance of the individual narrow coils is both reduced and in series. The reduction in overall capacitance can be quite dramatic. This method or approach has been around for about a century, as have banked windings.
If you make a Jazzmaster type guitar pickup with six coils to reduce the pickup's capacitance, wouldn't this pickup be impossible to use 'normally' aka in a passive instrument's wiring environment? I think you would have to use a preamp with some strong toneshaping to sound anything like its passive counterpart.

You might not want to do anything that reduces the number of turns you can fit in the available space because you lose output, that is, unless there is something else to gain, and decreasing capacitance is not an important issue because the cable capacitance is much larger than the coil capacitance. Thus the contribution of the coil capacitance to the actual resonant frequency in use is not large.
This is interesting. Would the cable capacitance of whatever the guitar cord is made of make a bigger difference to the sound than the pickup capacitance? I remember some people using coily guitar cords for this reason.

Ken

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14. The coily guitar cords were actually among the worst offenders IIRC. If they were popular it's because they killed off some treble and added crackly noises to the mix.

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15. Originally Posted by Bagpipe
Sorry to be so clear to you, but since we are not talking about Neumann u47 Transformers, but a shitty e-guitar pickup,...
Thanks for finally making your point clear.

There is a difference between saying "you are making much ado about nothing" or even "you are picking fly shit out of the pepper" than saying "you suffer from a mental illness".

-rb

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16. Bagpipe,
Please understand that we're not to be taken too seriously here. We've run out of things to talk about re conventional pickups at least a decade ago but can't give up the bonhomie so we keep hoping to stumble into some esoteric corner of coilwinding lore to rediscover and pontificate upon with no intention of actually experimenting with it. It's a way for us to show off what we think we used know and to keep our narrow minds somewhat limber as we lumber off into our dotard-age. Talking "reason" at us won't change a thing.

I might point out that Snake-oil seems to sell to wanna-be musicians a whole lot better than EE-Facts.

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17. Originally Posted by David King
The coily guitar cords were actually among the worst offenders IIRC. If they were popular it's because they killed off some treble and added crackly noises to the mix.
I can see the "charm" in using those awful coily cords. It's sort of like an editor who adds scratches, stains & jitter along with an umber tint to make video images seem antique. Similarly hiss, rumble & crackle are now digitally available to make a new recording sound like an old 45. There are times to use these things, and times to avoid them.

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18. Originally Posted by Mike Sulzer
You might not want to do anything that reduces the number of turns you can fit in the available space because you lose output, that is, unless there is something else to gain, and decreasing capacitance is not an important issue because the cable capacitance is much larger than the coil capacitance. Thus the contribution of the coil capacitance to the actual resonant frequency in use is not large.
If turns per available space becomes an issue, you just use a finer wire. If this design reduced capacitance from, say 130pF to 30pF, that 100pF savings is in the realm of becoming worthwhile.

Originally Posted by Mike Sulzer
However, the inductance with the split bobbin is interesting. Since the cores are short, all the flux from one coil does not pass through the other even if the permeability is high. It is not obvious to me whether the inductance changes with a split bobbin, or which way it changes if it does. Suppose it decreased with the split bobbin. Then you could add some more turns and get higher output for the same inductance.
Wouldn't the inductance just be reduced slightly due to the imperfect magnetic coupling? It should be less inductance than if there were no spacer in between, but higher than if you had two separate inductors in series.

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19. Originally Posted by ken
If you make a Jazzmaster type guitar pickup with six coils to reduce the pickup's capacitance, wouldn't this pickup be impossible to use 'normally' aka in a passive instrument's wiring environment? I think you would have to use a preamp with some strong toneshaping to sound anything like its passive counterpart.
The Jazzmaster analogy is just in terms of shape and not size. You would stack six miniature Jazzmaster coils, so that the resulting coil is still tall, like a more typical Fender pickup, but segmented into six sections that put a greater distance between the parts of the coil(s) where the difference in voltage is the greatest.

Originally Posted by ken
I
This is interesting. Would the cable capacitance of whatever the guitar cord is made of make a bigger difference to the sound than the pickup capacitance? I remember some people using coily guitar cords for this reason.

Ken
A 10' guitar cable usually features about 400pF-500pF capacitance, and a typical guitar pickup has around 100pF, so the cable capacitance is larger, but the pickup's capacitance is not so little as to not matter at all.

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20. Originally Posted by Bagpipe
rjb: Sorry for the "shitty"
I take no offense to the "shitty".
I don't really take offense to the "paranoia". It just struck me as an odd word choice.

Originally Posted by Bagpipe
My point is, that i want to point on EE-Facts - rather than snake oil.
EE Fact 1: A long, curly, high-capacitance guitar cord will lower a pickup's resonant frequency enough to make an audible tone difference. Whether the resulting tone is good, bad, or indifferent is a matter of taste.

EE Fact 2: A long, curly, microphonic guitar cable with crappy right-angle connectors will crackle. Period.

-rb

EDIT: Wow, a lot of posts here since my last visit. That's what I get for taking a ten minute shower....

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21. Originally Posted by Bagpipe
What about horizontal field arrangement? North left/South right
I think Gittler tried that on his "fishbone" guitars. Long spiral pickup windings paralleled the strings. One of these days I may have to deal with such a thing. A customer has one of 3 Gittler basses ever made, in pieces in his bureau drawer. It may fall to me to make its electronics work and I'm not looking forward to it. If the instrument can be restored, he may be able to flog it for beaucoup bucks.

I think the Gittler pickups were a subject of discussion on these pages many years ago. If I'm not mistook, the bottom line was, they were a terrible design - no wonder nobody carried on making anything like them. They got the Darwin Award for pickups iow.

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22. Originally Posted by Bagpipe
Now you are beginning to suggest multi-layer winding and such nonsense. Sorry - but...See:
Maybe you are able to reduce 130pF to 100pf or MAYBE to 82pF.
Based on the capacitance observed in a Jazzmaster pickup, I'd expect a pancaked coil design to possibly drop as low as 30pF.

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26. Originally Posted by Bagpipe
Anti-Gua:
reducing wire diameter results in higher DC_Resistance if you want to achieve same inductance. OR: You use a stronger magnet.
The series resistance is fairly trivial. Even another 10k series resistance isn't going to decrease the output by any significant amount.

Originally Posted by Bagpipe
Inductance wouldn't be decreased AT ALL. At least not in common measurement ranges. The difference is SO SMALL, that it plays no role. It gets swamped practically with differing magnet-specs, string/pole distance and so on. I can assure you, that you wouldn't notice the change from single to split-bobbin AT ALL!

You wouldn't even notice the loss due to smaller wire Diameter.

Why is it so?

Because a pickup is so damn NON-IDEAL in every way. Ever considered the amount of leakage flux? Especially in connection with closed HumBuggers?
A guitar pickup has rather tight winding-to-winding magnetic coupling, even if the core is so open as to be nearly air, which is why the inductance is as high as it is despite the imperfect core. So if the windings were broken into two halves, I'd expect some drop, but maybe it would only be a matter of millihenries.

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27. Originally Posted by Bagpipe

And STILL:

30pf Or 130pF plays STILL no role. Why? You tell me...
http://music-electronics-forum.com/t46404-5/#post495799

Originally Posted by Bagpipe
30pf Or 130pF plays STILL no role. Why? You tell me...
Two reasons: 1) that's like 2.5 feet less guitar cable with the volume on 10, 2) as you turn the volume down and the capacitance of the cable is separated from that of the guitar pickup, the pickup's lower intrinsic capacitance plays a greater role, and so a lower C works to preserve top end as the volume is rolled off.

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28. Originally Posted by Bagpipe
" because you lose output"
Do you? I mean: Significantly?

I hope it is clear, that you decrease Copper-Loss at the same time, you unwind your coil.

Inductance falls. This is all. And with this, a lot of side-effects (including losing a LITTLE output) come....

P.S. Thanks a lot (*NOT*) for your reply to my request, Mr. Sulzer. How friendly of you...
That made sure that I will not be replying to the request you made in a private message.

I think the issue is that the dividers take up space.

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31. Originally Posted by Bagpipe
you (mike) write:"inductance changes with a split bobbin"

???
You mean in 1/1000 measurements, or what? Your coil gets thicker, with the same number of turns (split bobbin vs. normal bobbin) - the field should be huge enuff - i expect inductance changes in the "micro" range. Sounds a little paranoid, what you suggest here.
A split bobbin is just one: Not needed, because self-capacitance plays no role, in the face of Cable Capacitance.

Why should a different Bobbin change inductance <<<in a SIGNIFICANT>>> way?? What is the base of your assumptions - other than paranoia?
Inductance is greatly influenced by the flux created by current in one turn passing through each other turn. That is the source of the n^2 effect. Separating one coil into two or more coils in series can affect this except when you have a high permeability closed loop core. As I said, I do not know how much it affects it, or which way, but my point is that if it reduces it, that is useful because you can't the same output with a higher resonant frequency or more output with the same.

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32. Originally Posted by Bagpipe
- What about horizontal field arrangement? North left/South right
1940s-1950s Electromuse "Eye-Beam" lap steel pickup. Next.

-rb

PS- The sun is shining here for the first time in 2 weeks. See you guys later.

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33. Originally Posted by ken
If you make a Jazzmaster type guitar pickup with six coils to reduce the pickup's capacitance, wouldn't this pickup be impossible to use 'normally' aka in a passive instrument's wiring environment? I think you would have to use a preamp with some strong toneshaping to sound anything like its passive counterpart.

This is interesting. Would the cable capacitance of whatever the guitar cord is made of make a bigger difference to the sound than the pickup capacitance? I remember some people using coily guitar cords for this reason.

Ken
Yes, the cable capacitance is more important the the coil capacitance.

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35. Originally Posted by Bagpipe
Antigua:

Winding to Winding magnetic coupling? What are you talking about?
???, Magnetic coupling between turns is what makes high value inductors (and thus, audio transformers) possible, and so is also responsible for limiting the high frequencies in pickups..

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