Eddy currents and slug pole pickups

[FunkyKikuchiyo]

I've been trying to do some more reading on theory, and I'm getting a bit perplexed by one point.

I understand that iron transformer cores bring great havoc if they are solid as eddy currents abound. Hence the idea of making laminations that are electrically insulated, the point being that you do not want conductivity perpendicular to the magnetic field, and the charges are suppressed in proportion to how many laminations you choose to have. However, pickups (like most things in the world) are three dimensional, so I'm getting a bit lost on what "perpendicular" is, since there are two different ways for it to be perpendicular. The theory stuff that I'm finding is only good to a point if you can't get the geometry right. Take for example a strat pickup using steel slugs and ceramic magnets. On one axis the slugs are perfectly insulated from one another. On another axis there is no insulation from one edge of a pole to another, but since the poles are so narrow in comparison to the coil size, it seems to have little effect. Geometrically it is the opposite of a laminated blade. Functionally we know that slug coils don't have the same loading as a solid blade coil. Is this just a function of the multiple cores (in this case six pole pieces) having relatively little mass, or is this type of isolation effective for tamping down the eddy currents?

Another example would be this, presumably from some intro to physics course: http://physics.usc.edu/demolab/em_eddy.html

The first example catches my eye, principally because it tells us that eddy currents are about a lot more than just having continuity from one spot to another, because obviously that comb device has continuity throughout. From any one point to another it would simply be an open circuit. Moreover, I suspect running alligator clips between the outside pieces wouldn't negate the effect, but I can't say for sure.

Experience is telling me that it probably has more to do with things like surface area and shape... or, maybe I'm just jerking off.

[fieldwrangler]

Speaking for myself I have all of the same questions. Thanks for putting this query together.

Bob Palmieri

[Joe Gwinn]

In the case of pickups, the simplest rule is that the eddy currents that affect pickup operation will be more or less parallel to the coil, so cuts perpendicular to this "coil image" will block (well, reduce) eddy currents. The most important eddy currents are those circulating closest to the coil.

With slug (rod) cores, the eddy current circulates around the circumference. If one slotted the core lengthwise, such that the cross section looked like an asterisk, eddy currents would be greatly reduced. Alternately, if one uses a bundle of smaller rods (like black iron wire) insulated from one another and in parallel to replace the slug, eddy current is greatly reduced. In round numbers, eddy current effects increase with the square of the rod/wire diameter. The oxide layer of black iron wire (sometimes plus a thin layer of varnish) is sufficient insulation. Wire-bundle cores were used in the hybrid transformers of telephones for many years. I had a telephone that used such a transformer.

With a blade, the eddy currents follow the periphery of the blade, in parallel to and just under the coil windings. If one laminates the blade, replacing one thick sheet with multiple thinner sheets insulated from one another, the eddy current is greatly reduced. Again, the effect is proportional to the square of the lamination thickness. Likewise, a thin layer of varnish or epoxy-soaked epoxy is sufficient insulation.

The voltage driving the eddy current is proportional to the voltage of a single turn. So, if we have a 5,000-turn coil with peaks at 2 volts (one can do this on the attack), the voltage across the insulation is 2/5000= 0.4 millivolts, call it a millivolt.

[FunkyKikuchiyo]

So, the answer to "which perpendicular" is "both" essentially? Mainly because the coil is in fact round?

Are the currents parallel to the coil as a rule, or is that just how it works in pickups? Some things I read said that they were (if I remember correctly) "perpendicular to the magnetic field created" and that effectively would be the same thing since magnets are typically perpendicular and under the coil. We also are creating our own magnetic field instead of letting one be created by the coil, so I guess that is another thing to get tripped up on.

Oh, and by the way.... thanks Joe!

[Mike Sulzer]

I think that there is no doubt that eddy currents affect the sound of a pickup in an interesting way. (Of course there is the obvious effect of reducing the Q of the resonance, but more interesting is what happens at lower frequencies.) If you want to eliminate the effect of eddy currents, it would be a lot easier to replace the steel slug with a ceramic slug than to make laminated slugs. On the other hand, if you want to just reduce the effect some, then using a two-piece slug (separated lengthwise with insulation between) would be a very practical and useful way to go.

[fieldwrangler]

If you want to eliminate the effect of eddy currents, it would be a lot easier to replace the steel slug with a ceramic slug than to make laminated slugs.

Ferrite??

Bob Palmieri

[Mike Sulzer]

Ferrite??

Bob Palmieri

Exactly. Hi permeability ferrite can be slightly conductive, but many times less than steel. You can get such slugs similar in size to an hb slug. (They have a small hole through the middle since they are sold as ferrite beads for putting on wire to damp oscillations.) Mine have a slightly larger diameter than hb slugs, but you can drill out an hb bobbin just a bit. I will try to post some impedance measurements on an eddy current thread that has been inactive for a couple of weeks tonight.

[FunkyKikuchiyo]

Funny you should bring that up Mike - I've been reading about ferrites and trying to figure out a way to exploit them for guitar pickups but haven't gotten that far. There are fewer things in the world than I thought that are ferromagnetic but NOT good conductors.

I'm more curious about the specific geometry than eliminating them completely. Eliminating them is good if we decide eddy currents = bad, but that isn't always true, for all of that art vs. science and happy mistake stuff we all ramble about in other threads. It seems like if we have a better idea of how they work, we can control their proportion and use them more wisely.

I'm also thinking out loud because of a revelation I had. The general understanding is that it is a current running in a loop around the coil within the core. Separate the sides (laminate them) and it goes away. By that method of thinking, short out a laminated core on both sides, and you're back where you started with a solid core, but that just isn't true. I tried this, and reading ACR@1khz on the coil with my extech, I could barely see a difference; it was around 2-3% change which is about what it was drifting anyway, so it seemed to debunk that overly simple assessment. Eddies in water are small little swirls rather than large currents, so I guess it is a matter of compressing them down and making them smaller rather than opening and closing circuits. Does that sound about right?

I also have no idea whether these currents are fundamentally AC or DC and whether or not capacitance within layers is important, but that's another matter I guess.

[Mike Sulzer]

I also have no idea whether these currents are fundamentally AC or DC and whether or not capacitance within layers is important, but that's another matter I guess.

They are ac. The same magnetic field from the vibrating string that induces the voltage around the pickup coil also induces voltages around the core by the same law of magnetic induction. Then the current flows because of the conducting path. "Eddy currents" is kind of a fanciful term. It is really not very much like eddies in a flowing stream of water.

[David Schwab]

Exactly. Hi permeability ferrite can be slightly conductive, but many times less than steel. You can get such slugs similar in size to an hb slug. (They have a small hole through the middle since they are sold as ferrite beads for putting on wire to damp oscillations.) Mine have a slightly larger diameter than hb slugs, but you can drill out an hb bobbin just a bit. I will try to post some impedance measurements on an eddy current thread that has been inactive for a couple of weeks tonight.

Mike, I seem to remember that you place a small neo magnet on top of the ferrite slug? If you place a magnet on the bottom, do they conduct any magnetism? Just curious.

I've been wanting to try these since you posted about using them several years ago.

[Possum]

Joe: so following this logic, will a humbucker cover have the most eddy currents on the sides of the cover? I guess a good test would be to use one of those topless covers and take frequency measurements for treble loss. Why would anyone want to make the slug poles brighter? They are already the brightest coil in a humbucker; in a steel pole single coil eddy currents are a great benefit to keep the thing from being as shrill as alnico rods are.....

[FunkyKikuchiyo]

Dave - I really only started the thread as an intellectual exercise to try to put together some "when I do this, that happens" sort of stuff with some real engineering, where it wasn't matching up on the level of instruction I was encountering. So, I have no desire to make slug coils brighter (I was actually thinking more in terms of steel slug strat pickups), I just want to know what is going on.

[copperheadroads]

Slugs add the midrange (muscle)
screws add the sharper (more treble) tone
Great combination I think. for a wide range of music

[Mike Sulzer]

Mike, I seem to remember that you place a small neo magnet on top of the ferrite slug? If you place a magnet on the bottom, do they conduct any magnetism? Just curious.

I've been wanting to try these since you posted about using them several years ago.

Yes they do, so you can put a magnet either on top or bottom. You need a bigger one on the bottom of course. The ferrite might satrate at a lower field lvel than steel (although I have not proven that myself), and so you wnat to check out the results careefully.

[Possum]

No, slugs are THE treble sounding coil, to control the treble of a humbucker you must focus your attempts to darken it on the slug coil. The slugs have the shortest magnetic return, a "quicker" circuit. The pole screws have a long magnetic return, skinny poles and the complication of a pole keeper as well. If you want a bucker to sound darker you RAISE the pole screws up, and slightly lower the pickup so you make the pole screw coil dominate the tone hear more of the strings. If you want a bucker to sound its most trebly you lower the screws flat, in which case the slug coil will dominate the tone. Dig out your Extech and you can see this happen in real numbers. Its exactly opposite of how everyone thinks it is. This applies to PAF's only in my own experience, StewMac kits and Duncans etc. might be a different story since the alloys are way different but I doubt it, slugs are still short little solid things. A real good way to think about this is with P90's; you have a single coil by itself, it has pole screws and a keeper, these are pretty nice warmish pickups when wound hot. Now replace those pole screws and keeper with steel slugs and you'll get a much sharper sounding pickup. Gibson did make steel slug P90's but they were only used on lap steel guitars. The original PAF prototype pickup was a double slug pickup, I made a copy of it and its very bright and sharp sounding, great for neck pickups....