Eddy Currents & Coil Size, Shape, Wire Diameter...etc...

[raygun85]

I've done a lot of research on this topic and I just can't seem to figure it out.

Is there any direct correlation between eddy currents and the size/shape of a coil? For instance, comparing taller and narrower coils like Strats and Teles to shallower and wider coils like P-90's, Jazzmasters, and other soapbar pickups.

Also, is there any bearing on eddy currents when using a wide and shallow coil with individual rod magnets (i.e. Fender Jazzmaster) versus bar magnets with slug, blade, or adjustable screw pole-pieces (i.e. P-90 or similar)?

[Possum]

This is a matter of experience. Screw anything that you read, its not a good way to learn about this stuff, you have to prove things to yourself. There was a point where I quit believing anything I read online and decided that I didn't know ANYTHING, and then did experiments to prove or disprove things I'd read. You're talking about years worth of locking yourself in a garage and testing things out for yourself ;-) but its the only way to go. Even now, I am finding things I used to think I were written in stone aren't true. It helps to have some technical gear to get readings off your pickups to see what is really going on. Its not about "eddy currents" because you are isolating something from everything else that happens in a pickup, like capacitance, wire resistance, wire diameter, magnet type and strength, etc. etc. etc. You pretty much have to make everything you are talking about and then test it and see what it does. Blade poles generally tend to be bass boomy and don't like alot of wire on them, alnico rod pole pickups are very bright, pole screw/keeper pickups (P90's) warm things up, coil shapes/design have a huge effect on overall tone. The thing I like about pickup making is that there seems to be no end into understanding everything about the subject, which also means no end to the amount of frustration too :-)

[marku52]

I don't think that eddy currents are key to the sonic difference between tall and skinny VS short and fat. I look at it this way (and I'm sure someone will point out if I am wrong)

Only magnetic flux that cuts a coil produces output (or is linked, in the technical sense). The flux lines are concentrated around the pole pieces, and especially around a rod magnet, ala Fender. So a tall and skinny coil effectively links a lot of those flux lines.

OTOH, with a short and fat coil, like a P90, quite a lot of the flux lines complete (link from N to S) without cutting the outer coils at all. So those turns aren't linked to the inner flux, where the flux density is the highest. This makes the outer turns more like an LCR filter, changing the tone without adding a proportional amount of output.

Someday, when my ship comes in, and I cross the river, and reach the highest mountains, blah, blah, I'll do an experiment where I use the same magnet and change only coil aspect ratio to see the sonic difference. Of course, you can't just change the aspect ratio. Even keeping equal # turns, the fat coil will have higher R because the outer loops are longer, and I would suppose have higher L because the loop area is larger as well. Maybe you'd have to wind to equal L, which would put fewer turns on the fat coil. It's an interesting thought experiment because "all things being equal" is basically impossible. My bet is that the fat coil has a lower Fres, and a rounder tone, and the skinny coil, well, sounds like a Fender.

[David Schwab]

I don't think that eddy currents are key to the sonic difference between tall and skinny VS short and fat. I look at it this way (and I'm sure someone will point out if I am wrong)

I agree. Eddy currents are more a product of conductors in the pickup, such as blades, covers, baseplates, etc. I suppose pole pieces too, but they have a smaller surface area. The larger the surface are, and the thicker the conductor, the more of an effect.

Mike S can correct me if I'm wrong. But that's my understanding on how they impact pickups.

[Possum]

A taller coil also spreads the coil out so there is much less capacitance, inductance etc. etc. etc. Trying to quantify things works to some extent but then falls apart many times if you look too closely or try to isolate one effect and think its the main cause of things. I'm using the Syscomp gizmo with two "identical" pickups, but one sounds noticeably brighter than the other, the Syscomp frequency sweep shows no difference, the resonant peak only shows about a 30hz difference, but the ear is hearing a very noticeable difference between the two pickups. High tech gear is really helpful but in the end it fails to describe what you hear with your ears. For instance I always thought loading a pickup up with alot of metal would make the pickup sound nice and warm so i tried with a humbucker and it didn't work, I used a wide pole keeper and metal shims. I have a Guild humbucker that has a 1/4" wide pole keeper and square slugs, so there's more metal in it than a regular humbucker but these things are really bright. In the end you just have to build one and try it out, paper pickups don't sound like real pickups ;-)

[raygun85]

I don't think that eddy currents are key to the sonic difference between tall and skinny VS short and fat. I look at it this way (and I'm sure someone will point out if I am wrong)

Only magnetic flux that cuts a coil produces output (or is linked, in the technical sense). The flux lines are concentrated around the pole pieces, and especially around a rod magnet, ala Fender. So a tall and skinny coil effectively links a lot of those flux lines.

OTOH, with a short and fat coil, like a P90, quite a lot of the flux lines complete (link from N to S) without cutting the outer coils at all. So those turns aren't linked to the inner flux, where the flux density is the highest. This makes the outer turns more like an LCR filter, changing the tone without adding a proportional amount of output.

Someday, when my ship comes in, and I cross the river, and reach the highest mountains, blah, blah, I'll do an experiment where I use the same magnet and change only coil aspect ratio to see the sonic difference. Of course, you can't just change the aspect ratio. Even keeping equal # turns, the fat coil will have higher R because the outer loops are longer, and I would suppose have higher L because the loop area is larger as well. Maybe you'd have to wind to equal L, which would put fewer turns on the fat coil. It's an interesting thought experiment because "all things being equal" is basically impossible. My bet is that the fat coil has a lower Fres, and a rounder tone, and the skinny coil, well, sounds like a Fender.

Ok, so this is where I keep getting hung up in my circular thinking (sorry! ) - what you're describing about the outer windings in a soapbar pickup forming an LCR circuit. If the outer windings are more-or-less "dummy windings", laying outside the lines of the magnetic field, then haven't we added more metal (i.e. copper wire), decreasing the PUP's efficiency by eddy current losses?

[marku52]

If the outer windings are more-or-less "dummy windings", laying outside the lines of the magnetic field, then haven't we added more metal (i.e. copper wire), decreasing the PUP's efficiency by eddy current losses?

No, because the outer turns don't make a "shorted turn". That's what eddy current losses are, they are losses from a conductive path that can form a shorted turn coupled to the magnetic flux. That's why they are reduced by reducing the conductivity of the material of the shorted turn (like the ferrite dust in a toroid core, it is basically a non-conductive ceramic. Or using low-conductivity nickel silver instead of brass for a pickup can). The outer windings of a pickup aren't shorted, which is why I argue that they make a sort of LCR filter that is still loosely coupled to the pickup flux.