# Thread: Mitigating eddy current losses in pickup covers, and other metal parts

1. ## Mitigating eddy current losses in pickup covers, and other metal parts

An electrical engineer and guitar hobbyist named Ken Willmott has done some experimentation to figure out what geometries cause more or less eddy current damping in guitar pickup covers. It's very informative, and he suggests some specific modifications that can be made to existing covers in order to greatly reduce eddy currents without making a radical change in their appearance.

Here is a PDF he prepared http://kenwillmott.com/blog/wp-conte...r_Geometry.pdf

The tl;dr is that, because eddy currents require continuous metal wherein a current can move in a circular pattern, if you cut the metal and break continuity wherever current could flow in a circle, on a plane that is parallel to the coil, you greatly reduce eddy current damping.

Ken Willmott has a blog here kenwillmott.com | My projects, ideas and stories

I've seen it discussed here that a laminated cover or core would reduce eddy losses, but it appears that the audio spectrum is of low enough frequencies that it's not necessary to have a cover or core material finely laminated. Simply cutting a pole piece radially, for example, might eliminate the majority of eddy current losses that it would otherwise cause, while still performing the function of increasing inductance and bringing flux to the guitar strings.

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2. So, if the cuts to the top of a brass cover make it essentially audibly transparent, would similar cuts have the same effect on a brass baseplate (which are maligned for killing high frequencies)?

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3. In theory, yes, in practice, no. The majority of eddy current losses are caused by the flux change that is represented by the moving guitar string. There is a flux change brought about by current moving through the coil, but it's small compared to that of the moving guitar string, so the cover, screws and slugs have a significant impact, but the base plate is too far removed to impact the Q factor or the overall high end audibly.

For example, the greatest losses I've seen are in Filter'trons, and it's been proven that nearly all the losses disappear when the fillister screws are removed. It goes from having no Q factor to speak of, to having a +7 to +10dB rise at the resonance with the screws removed. The metal base isn't very consequential.

I hadn't heard this said about the base plate, but if that's a popular belief, I can do a quick test with a PAF clone, with and without a base plate to see exactly what happens.

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4. Originally Posted by Antigua
I hadn't heard this said about the base plate, but if that's a popular belief, I can do a quick test with a PAF clone, with and without a base plate to see exactly what happens.
I tried to find a specific thread to quote, but yes, that seems to be the conventional wisdom around here. I've not done A-B tests myself to observe this, but as I have begun to make my own baseplates, it's a topic that interests me.

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5. But which eddy currents affect the frequency response?

First, consider the eddy currents resulting from current flow in the coil. These eddy currents load the coil, and they damp the coil-cap resonance. They have the most effect where the pickup impedance is high and the frequency is high. It takes less energy loss to affect the response where the impedance is high.

Now consider eddy currents induced directly by the motion of the string. These currents rise at the usual 6db per octave, and thus we expect to see gradual differences with frequency with the cover/no cover cases, etc.

Now look at illustration 30 in the pdf file that you linked to. It shows the full range response for a tele neck pickup for the various cases (cover, no cover, slotted cover). The only significant differences are in the resonant region. There are no gradual 6db/octave differences. This is experimental evidence that all significant effects from the eddy currents in the cover are from the damping of the resonance resulting from currents flowing in the coil.

This implies that a base plate also has a significant effect since it is no further from the coil than the cover.

Originally Posted by Antigua
In theory, yes, in practice, no. The majority of eddy current losses are caused by the flux change that is represented by the moving guitar string. There is a flux change brought about by current moving through the coil, but it's small compared to that of the moving guitar string, so the cover, screws and slugs have a significant impact, but the base plate is too far removed to impact the Q factor or the overall high end audibly.

For example, the greatest losses I've seen are in Filter'trons, and it's been proven that nearly all the losses disappear when the fillister screws are removed. It goes from having no Q factor to speak of, to having a +7 to +10dB ride at the resonance with the screws removed. The metal base isn't very consequential.

I hadn't heard this said about the base plate, but if that's a popular belief, I can do a quick test with a PAF clone, with and without a base plate to see exactly what happens.

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6. Originally Posted by Antigua
An electrical engineer and guitar hobbyist named Ken Willmott has done some experimentation to figure out what geometries cause more or less eddy current damping in guitar pickup covers. It's very informative, and he suggests some specific modifications that can be made to existing covers in order to greatly reduce eddy currents without making a radical change in their appearance.

Here is a PDF he prepared http://kenwillmott.com/blog/wp-conte...r_Geometry.pdf

The [idea] is that, because eddy currents require continuous metal wherein a current can move in a circular pattern, if you cut the metal and break continuity wherever current could flow in a circle, on a plane that is parallel to the coil, you greatly reduce eddy current damping.

Ken Willmott has a blog here kenwillmott.com | My projects, ideas and stories
Ken W's analysis is correct so far as I can see.

What's missing is that one can make covers out of metals that don't support significant eddy currents at audio frequencies. The classic example is nickel silver. Non-magnetic stainless steel also works. The issue is to comper materials by their skin depth at say 1,000 Hertz. Both magnetic permeability and electrical resistivity of the bulk metal matter.

There were a number of threads on this, so some searching is in order. I recall publishing some tables.

I've seen it discussed here that a laminated cover or core would reduce eddy losses, but it appears that the audio spectrum is of low enough frequencies that it's not necessary to have a cover or core material finely laminated. Simply cutting a pole piece radially, for example, might eliminate the majority of eddy current losses that it would otherwise cause, while still performing the function of increasing inductance and bringing flux to the guitar strings.
Laminating the cover will not work - orientation is wrong. Each lamination will have eddy currents circulating, so the effect of laminating is nil. If the magnetic lines are perpendicular to the plane of the laminations, there will be full eddy currents. If perpendicular, no eddy currents.

The cover (and baseplate) slits will work, as shown by Ken W.

Laminating a blade core does work, although there is an acoustic optimum, to be determined using golden ears. Too few too thick and the sound is too dull. Too many too thin and the sound is too sharp. Three thin soft steel laminations seems to be the answer, based on what I've seen in ads.

One can visualize the path the eddy currents will take by regarding their path as a blurred image of the pickup coil in all nearby pieces of metal. If you can only see the coil reflection edge-on, the eddy currents are reduced. I did a thread on this some time ago, using heat maps of induction-heated steel objects as the example.

The baseplate can definitely have an effect, if it's close enough. If it's made of stainless stell, there will be little effect.

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7. Nickel silver causes eddy losses, they just aren't as steep as brass. As a matter of fact, just last night I was comparing a nickel silver cover from Tonerider with a brass Seymour Duncan cover, and this is how it broke down:

And here's a pic of me doing this

Ken's modifications take the losses down to a fraction of a decibel, with either metal.

As far as laminations making for a harsh sound, the Q can be damped with parallel resistance, so my thinking is start with a high Q and then damped in to taste. From a pickup maker's perspective, it seems to be heretical to hardwire a resistor or a cap to a pickup, so I can see why they might want to leave the losses as is.

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8. Originally Posted by Mike Sulzer
But which eddy currents affect the frequency response?

First, consider the eddy currents resulting from current flow in the coil. These eddy currents load the coil, and they damp the coil-cap resonance. They have the most effect where the pickup impedance is high and the frequency is high. It takes less energy loss to affect the response where the impedance is high.

Now consider eddy currents induced directly by the motion of the string. These currents rise at the usual 6db per octave, and thus we expect to see gradual differences with frequency with the cover/no cover cases, etc.

Now look at illustration 30 in the pdf file that you linked to. It shows the full range response for a tele neck pickup for the various cases (cover, no cover, slotted cover). The only significant differences are in the resonant region. There are no gradual 6db/octave differences. This is experimental evidence that all significant effects from the eddy currents in the cover are from the damping of the resonance resulting from currents flowing in the coil.

This implies that a base plate also has a significant effect since it is no further from the coil than the cover.
I'm willing to do a with and without base plate test, if that's helpful in coming to a conclusion. The closest thing I had done before was to compare a brass base plate of a Chinese humbucker to a DiMarzio 36th Anniversary's steel or nickel silver base plate, whatever it is they used. The difference in that case was minimal, less than 1dB.

Attachment 41435

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9. I just did a with/without base plate test, here is the result:

It looks like there is a potential for about half a decibel improvement in the resonance if you were to make the base plate completely transparent to eddy currents.

They say that one decibel is the smallest difference in amplitude that a person can discern, so if a difference is seen that doesn't exceed 1dB, there's cause to conclude that it's inaudible by itself, though several of these small factors could combine together to total more than one decibel of difference, so if you start out with a cheap, thick copper cover, as opposed to a better nickel silver base plate, and then find a way to totally eliminate eddy current damping, that combination could equal one decibel. But even then, it's hovering on the threshold of human hearing.

Here's the test setup:

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10. Originally Posted by Joe Gwinn
Ken W's analysis is correct so far as I can see.
Yes, his "slot solutions" are right and very useful, but the physical cause of the losses is not "signal attenuation", but rather damping of the resonance in the pickup electoral circuit. I have discussed the effect in illustration 30, but you also can see this in others, such as 26. Note that the unmodified brass cover wipes out the resonance nearly completely. The full slotted cover has a bit more than 1db at resonance, but the effect is almost entirely gone outside the neighborhood of the resonance, more than can be explained by a 6db/octave effect.

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11. There are some things still to understand here. According to this table: https://www.nde-ed.org/GeneralResour...ivity_Misc.pdf, brass is more than 5 times as conductive as nickel silver. There are variations amongst specific alloys, so who knows for sure, but based just on conductivity, I would expect the difference between NS and brass on the plot to be much more. So I am not convinced we understand what is really happening.

Originally Posted by Antigua
Nickel silver causes eddy losses, they just aren't as steep as brass. As a matter of fact, just last night I was comparing a nickel silver cover from Tonerider with a brass Seymour Duncan cover, and this is how it broke down:

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12. Thanks. I do not understand these results, but I wonder if the distance from the exciting coil is so important, then is its geometry also important? That is, how different would the results be with a tiny exciting coil which would make a field more like that from a string? Also I think it would be interesting to try this with pickup coils with fewer turns which would put the resonance well up above the audio range. Then it might be easier to see what the effects of eddy currents as a function of frequency are. That is, try to make the circuit effects simpler.

Originally Posted by Antigua
I just did a with/without base plate test, here is the result:

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13. Let's look at a further case of interest in regard to the loss by damping the pickup resonance. This is a case when we would expect that there could be no resonant damping, and there is not. Look at Illustration 7. Take the case with the loop below the cover, and consider it the reference. (It is not a perfect measure of "no effect", but it is close.) Now compare this to the case with the loop at the exciter coil. Notice that at about 10 KHz, this is about 2db below the reference. At 5KHz it is still about two db below the reference. Thus in this case the pickup, to a reasonable degree of accuracy, is just passing on what is happening to the coil. This is very different to what happens when the loop or cover is at the pickup; in those cases the loss is selectively higher at the resonance.

By the way, it does not look as though the loops and covers are exactly the same in what they do. Possibly the skin effect is a small factor in what the loops do, but not the covers, which are a thinner flat conductor. (However, it is very useful to have both!)

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14. Originally Posted by Mike Sulzer
There are some things still to understand here. According to this table: https://www.nde-ed.org/GeneralResour...ivity_Misc.pdf, brass is more than 5 times as conductive as nickel silver. There are variations amongst specific alloys, so who knows for sure, but based just on conductivity, I would expect the difference between NS and brass on the plot to be much more. So I am not convinced we understand what is really happening.
You're probably right, it's probably not brass. I think I saw a copper undercoat and was confused. It might even be differences in the thickness that caused the difference.

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15. Originally Posted by Mike Sulzer
Thanks. I do not understand these results, but I wonder if the distance from the exciting coil is so important, then is its geometry also important? That is, how different would the results be with a tiny exciting coil which would make a field more like that from a string? Also I think it would be interesting to try this with pickup coils with fewer turns which would put the resonance well up above the audio range. Then it might be easier to see what the effects of eddy currents as a function of frequency are. That is, try to make the circuit effects simpler.
That would take a little more work, so I'd have to test this at a later time, but I have done other tests to see if proximity between the exciter coil and pickup makes a difference, and I had found that it didn't.

For this test, the exciter coil had 1 Vpp driving it, and I've found that increasing the voltage up to 5 Vpp doesn't generally change the response curves. Perhaps the driver coil can create a stronger flux field that is able to reach and interact with the further ends of the pickup, but the flux is weak enough at that point that those effects are very small, and disappear beneath the more immediate and powerful effects caused by the more proximate cover, poles and screws. It seems that proportionately, between weaker strings or a stronger exciter coil, scale rather linearly. In fact, that's easy to demonstrate, so I will do so. If the exciter coil was causing an added distortion, if anything the 0.4dB difference would likely be even smaller.

A little array of tiny coils that resemble guitar string would be nice to have, I'll have to think about how to construct that.

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16. Originally Posted by Antigua
A little array of tiny coils that resemble guitar string would be nice to have, I'll have to think about how to construct that.
You just need one. probably have to drive it with a power amp through an 8 (or 4) ohm resistor to make a constant current source. (I have a SS power amp set up to have an output impedance well above 8 ohms, so I could probably drive such a coil directly. Have to try it; just have to keep the coil impedance low at all relevant frequencies)

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17. ... deleted ...

I was testing and retesting, and getting results that contradicted what I had said prior in this post, so I'm blowing it away.

That being said, the test results for the base plate / no base plate should be reliable. If anything, the 1Vpp exciter coil would make the differences more pronounced, rather than less. I don't think you will see even a one decibel difference between a solid base plate, and one that has been cut to break up eddy current swirls. It also doesn't appear that the choice of metal used will make a difference in the resonant peak that exceeds one decibel.

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18. Originally Posted by Antigua
You're probably right, it's probably not brass. I think I saw a copper undercoat and was confused. It might even be differences in the thickness that caused the difference.
If it has a copper coat, it is most likely made of mild steel like 1018 or 1020. The purpose of the copper is to prevent rust.

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19. Originally Posted by Joe Gwinn
If it has a copper coat, it is most likely made of mild steel like 1018 or 1020. The purpose of the copper is to prevent rust.
What if the cover is made of nickel or nickel silver? Would it need a copper coat in order to apply the chrome finish?

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21. Originally Posted by Antigua
What if the cover is made of nickel or nickel silver? Would it need a copper coat in order to apply the chrome finish?

To plate mild steel with chrome, yes a copper layer is needed between steel and chrome.

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22. Originally Posted by Antigua
What if the cover is made of nickel or nickel silver? Would it need a copper coat in order to apply the chrome finish?
I do not know if it is essential, but I have found copper underneath chrome and on top of nickel silver.

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23. Originally Posted by John Kolbeck

I think an exciting prospect would be to make ceramic Strat pickups with a pole piece that reduces eddy losses to get a Q factor that is more associated with AlNiCo Strat pickups, and possible get some vintage "quack" out of a low price point pickup.
That is easy; there are a variety of ceramic materials available that have relatively speaking no loss and permeability as high as you would like. You can even get them readily as ferrite beads with perfect size for pickup pole pieces if you do not mind a small hole down the length of the cylinder. You can use ceramic magnets or small NEOs as you like.

As for the losses, does it seem reasonable that a current induced in a non ferrous material at audio frequencies is sufficiently large to produce a magnetic field that significantly reduces the field form that source? If true, would this not imply that you could make an audio transformer, at least for the higher frequencies, with an air core?

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24. I realize this is an old thread, but I've got a question about Ken W's article and the slotting patterns of the humbucker cover.

Perusing the loaded plot lines for wire loops on the face of the pickup:

Illustration 20
Shows the greatest effect around the largest (12mm) loop around one coil.

Illustration 21
Shows the "major circuit around all poles is much more significant than the smaller eddies around the poles".

Perusing the loaded plot lines for the slotted humbucker covers:

Illustration 26 (called out as 23)
Full slot cover "to interrupt both major current flows and localized current flows around the gaps."
At resonance, no cover and slotted cover lines differ "by at most about 0.5 dB, which is considered completely inaudible under all but extremely unusual circumstances."
Let's say 0.4 dB.

Illustration 27
Half slot cover: "the major current flow around the periphery of the poles has been blocked on one side by the series of slots. These need only extend to the pole centers for this design variant to work.
Amazingly, the loaded loss is about 0.7dB, which is on the extreme threshold of audibility."
Ler's say 0.7 - 0.4 = 0.3 dB difference between half and full slot covers.

Essentially, the full slot cover is aurally transparent, while the half slot cover is barely perceptible.

My question is:
What if you cut a single horizontal slot across the center of the cover?
(It could extend only to the pole centers- although, practically speaking, its easier to cut a full slot with a jewelers' saw.)
Wouldn't that interrupt the major current flows around the coils- although not the relatively insignificant flows around the individual poles?

I sure wish Ken had tested that configuration. It would be interesting to see the difference between one slot and six slots.
Does anyone have experimental data, or a knowledgeable opinion?

So, why is this "relevent"?
Say, hypothetically, that some lazy cheapskate has a pair of stock Epi Dot pickups....

Thanks,
-rb

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25. Originally Posted by rjb
I realize this is an old thread, but I've got a question about Ken W's article and the slotting patterns of the humbucker cover.
My question is:
What if you cut a single horizontal slot across the center of the cover?
(It could extend only to the pole centers- although, practically speaking, its easier to cut a full slot with a jewelers' saw.)
Wouldn't that interrupt the major current flows around the coils- although not the relatively insignificant flows around the individual poles?

I sure wish Ken had tested that configuration. It would be interesting to see the difference between one slot and six slots.
Does anyone have experimental data, or a knowledgeable opinion?

So, why is this "relevent"?
Say, hypothetically, that some lazy cheapskate has a pair of stock Epi Dot pickups....

Thanks,
-rb
It's hard to be sure without pictures, but if I understand correctly, you mean a slot that extends from an end pole to the other end pole. Notice that in illustration #21 that the 6mm loop is the worst performer. That would be equivalent to the "bare pole" Telecaster neck covers, which don't eliminate losses due to the effect mentioned with #21, "major circuit around all poles is much more significant than the smaller eddies around the poles". In fact, I did test this with an actual cover, but as a humbucker, in illustration #24, "toaster cover".

However, if you extend a horizontal slot all the way, so that you have two electrically and mechanically separate halves, there is no path and then no losses. The obvious problem is, to support the halves without making electrical contact. You could use a plastic or other non-conductive frame. I thought about that for a while, and came to the conclusion that it would be too difficult, expensive, or unreliable from a mechanical standpoint. I looked first for configurations that don't require major redesigns of the pickup frame or construction.

Ken Willmott

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26. Originally Posted by babysnake
It's hard to be sure without pictures, but if I understand correctly, you mean a slot that extends from an end pole to the other end pole.
No, I'm envisioning a humbucker pickup in playing position.
I mean a "horizontal" slot that extends across the face of a humbucker cover, "vertically" positioned between the poles for the 3rd and 4th strings.
In other words, the "cross bar" of an H-slot, without the "goal posts" AKA "toaster slots".
My thought is that this single cut would block the major circuits around the two rows of poles- although not the smaller eddies around the poles.

Originally Posted by babysnake
I looked first for configurations that don't require major redesigns of the pickup frame or construction.
My sentiment exactly. But my goal is a configuration that requires minimal modification of existing commercial pickup covers.
In this instance, greatest benefit for least effort may take precedence over sonic perfection.
Thus my whine: "I sure wish Ken had tested that configuration. It would be interesting to see the difference between one slot and six slots."

-rb

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27. Thanks for the clarification. The asymmetry of that configuration makes it hard to predict its behaviour. It is definitely worth trying. It is not possible for me to do it now, because my cutting tools are scattered around at the moment due to a move. If you like, I can send you a blank brass cover (I think I have some but have to check), you can make any cuts you like and return it to me for electrical tests.

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28. Originally Posted by Joe Gwinn
Ken W's analysis is correct so far as I can see.

What's missing is that one can make covers out of metals that don't support significant eddy currents at audio frequencies.
Not to nitpick, but my paper does say, "Pickup designers responded in several ways. One was to use plastic covers. Another was to use a nonmagnetic
metal that has a relatively low conductivity, thus reducing but not eliminating the effect".

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29. Originally Posted by babysnake
...If you like, I can send you a blank brass cover (I think I have some but have to check), you can make any cuts you like and return it to me for electrical tests.
Sure, I could do that in the name of science. Thanks for the offer.
The resident metal smith gets back from Arizona today, and maybe she'll tell me where she hides the bench pin and jeweler's saw blades.
Otherwise, it'll be an ugly gash made with my Harbor Freight rotary tool cutting wheel.
I'll check my settiings to ensure I have PMs enabled.

Thanks again,
-rb

PS - Welcome to the club.

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30. Originally Posted by rjb
No, I'm envisioning a humbucker pickup in playing position.
I mean a "horizontal" slot that extends across the face of a humbucker cover, "vertically" positioned between the poles for the 3rd and 4th strings.
In other words, the "cross bar" of an H-slot, without the "goal posts" AKA "toaster slots".
My thought is that this single cut would block the major circuits around the two rows of poles- although not the smaller eddies around the poles.
I don't think the single through-cut between the D and G would be all that much different than no cut at all, because even though the test showed a more significant losses around the entire perimeter, there was no test comparing half of the perimeter, but if there were, I'm sure it would have been much closer to the full perimeter losses than the individual pole piece losses. I think ultimately the cut(s) have to involve all of the pole pieces from at least one approach, and in the case of a single coil pickup, also ensure there is no continuity around the perimeter of the cover.

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31. Originally Posted by Antigua
I don't think the single through-cut between the D and G would be all that much different than no cut at all, because even though the test showed a more significant losses around the entire perimeter, there was no test comparing half of the perimeter, but if there were, I'm sure it would have been much closer to the full perimeter losses than the individual pole piece losses. I think ultimately the cut(s) have to involve all of the pole pieces from at least one approach, and in the case of a single coil pickup, also ensure there is no continuity around the perimeter of the cover.
The problem is that although currents are produced in the locations where the tests revealed, the electrons are mobile, and able to follow the path of least resistance once set in motion. So the design problem is not limited to blocking current in those areas. You have to look at all the available current paths, and block almost all of them. As you suggest, the current from each set of 3 poles combine, and have a substantial return path immediately adjacent to the slot. So I fear, the single slot is not likely to make a big difference. Of course, we have always believed in experimental results to really decide such issues. If I had to wager on this, I would bet on a loss.

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32. Got it. Oh well.

-rb

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