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

[Mike Sulzer]

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)

[Antigua]

... 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.

[Joe Gwinn]

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.

[Antigua]

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?

[Joe Gwinn]

I looked up some old but relevant threads:

http://music-electronics-forum.com/t3724/

http://music-electronics-forum.com/t4149/

http://music-electronics-forum.com/t29324/

[Joe Gwinn]

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.

[Mike Sulzer]

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.

[Mike Sulzer]

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?

[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.

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

[babysnake]

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

[rjb]

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.

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

[babysnake]

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.

[babysnake]

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".

[rjb]

...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.

[Antigua]

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.