Coil productivity with respect to permeability and proximity

[antigua]

This is another experiment inspired by a disagreement on a neighboring technical forum over the issue of how much flux from the magnetized guitar string reaches the lower half of a typical Fender sized coil.

The test involves using a slug coil from a PAF style humbucker to stand in for half of a Fender style pickup, width respect to coil length, and then measuring the output voltage when an exciter coil is placed over the coil when the coil is close to the exciter (upper half of theoretical Fender coil), or far from the exciter (lower half), and then again at the same distance over the coil with no pole piece, so that the degree to which the pole piece enhances the flux change through the coil, in contrast to air core, can be observed, with respect to distance.

In order to prevent the inductance of the coil from changing when air is compared to steel and AlNiCo cores, only three pole pieces are inserted into the bobbin, all to one side, and the pole piece measurements are done over the side with poles, or the side without, to measure the difference between pole materials, or "air".

The steel poles are a little longer than the AlNiCo pole pieces, so in order to keep the distances the same, I set it up so that 7.1mm of the pole piece protruded from the coil when the "far" measurements were made. This means that on the other side of the coil, the AlNiCo poles were flush, but the steel pole pieces poked out an extra 1 to 2mm. 

As usual, Ken Willmott's integrator is used to remove the +6dB/oct Faraday slope kenwillmott.com/blog/archives/152 

Other questions about the setup might be answered by looking at the pics at the bottom of the post. 


Steel pole piece:
length: 19.0mm
diameter: 5.0mm

AlNiCo 5 pole piece:
length: 17.5mm
diameter: 5.0mm


Coil far, over steel slug, black:   6.0dBV
Coil far, over air, red:           -3.6dBV
Coil near, over steel slug, green: 10.6dBV
Coil near, over air, gray:          4.7dBV
Noise, pink:                      -15.9dBV




Coil far, over AlNiCo 5 slug, black:  -1.6dBV
Coil far, over air, red:              -3.3dBV
Coil near, over AlNiCo 5 slug, green:  5.8dBV
Coil near, over air, gray:             4.9dBV
Noise, pink:                         -16.0dBV





It turns out that the permeability of the pole piece largely determines how much flux makes its way from the string the lower portion of the coil. Placing the exciter coil such that the coil is 10mm above the bobbin, there is a 2.7dBV (-3.3 to -1.6) increase when AlNiCo 5 replaces air, but with the more permeable steel poles, there is a whopping 9.6dBV (-3.6 to 6.0) difference when steel replaces air. In other words, the steel caries a lot more flux to the lower portion of the coil than the AlNiCo. Just by eyeballing the graphs, it's plain to see that the AlNiCo 5 (lower graphic) correlates closely with "air core", where as the steel plots (upper graphic) diverge widely from "air core". The AlNiCo does fairly little to carry the flux down, compared to the steel.

Another way of quantifying the difference is that, for the steel pole pieces, there is only a 4.0dBV drop when the coil is further from the exciter (10.0 down to 6.0), but for the AlNiCo, there is a 7.4dBV drop (5.8 to -1.6).

The steel pole pieces also dramatically increase the flux change through the top half of the coil as well. With the steel poles, the dBV jumps by 5.9dBV (4.7 to 10.6), where as the AlNiCo poles cause an increase of only 0.9dBV (4.9 to 5.8). What this means is that regardless of distance, steel poles make any pickup a lot louder than a comparable pickup with AlNiCo, on account of the degree to which the permeable steel magnifies the flux of the moving string.

The ironic thing is that Fender pickups, with their AlNiCo pole pieces, stand the least to gain from their longer coils, while Gibson P-90's and PAF's stand more to gain from longer coils, but they choose to use short coils instead.



Another artifact you can see, unrelated to the test, is that the steel pole piece plot shows a lower Q factor when the exciter is over the steel pole piece, and a higher Q when the exciter is over "air", showing the eddy currents are doing their thing in the steel pole pieces. It's especially apparent where the gray plot line (air) eclipses the black line (steel) at 3 to 4kHz. Compare this to the AlNiCo plots, the Q factor is about the same for air and AlNiCo, showing very low eddy current activity. 


Pictures of the test:

First, I made a new Popsicle stick exciter coil using 44AWG so that the coil would have less length. I've got it from around 5mm down to 2mm. This should have a return path that is more circular in orientation, like a guitar string, rather than a long one.




Here is the equipment, three steel pole and three AlNiCo 5 poles:



Coil far, over pole piece:

Coil far, over air:


Coil near, over pole piece:


Coil near, over air:

[ms]

Nice job!  I remember that modeling with FEMM can be used to show similar results, but I forget if there were any careful numerical comparisons.  What  you can see with FEMM is how much flux "escapes" out the sides of the pole piece rather than continuing straight on through.  I appreciate your  irony. I used to think that Fender pickups did an OK job of getting the flux to the lower part of the coil until I realized that the low permeability of alnico is important.

I find it remarkable that there are so few pickups using ferrite rods to replace the steel in a magnet backed pickup, or ferrite rod magnets by themselves.  Then you can high high permeability and low loss.  In fact the Q might be a bit too high, but that is easily fixed with a resistor.  (But I have been told that guitarists do not want to see any external components in their replacement pickups!)  But the result can be a higher output single coil pickup or a humbucker with a higher resonance and no steel sound.

[antigua]

Feb 27, 2017 7:22:42 GMT -5 ms said:

Nice job!  I remember that modeling with FEMM can be used to show similar results, but I forget if there were any careful numerical comparisons.  What  you can see with FEMM is how much flux "escapes" out the sides of the pole piece rather than continuing straight on through.  I appreciate your  irony. I used to think that Fender pickups did an OK job of getting the flux to the lower part of the coil until I realized that the low permeability of alnico is important.

I find it remarkable that there are so few pickups using ferrite rods to replace the steel in a magnet backed pickup, or ferrite rod magnets by themselves.  Then you can high high permeability and low loss.  In fact the Q might be a bit too high, but that is easily fixed with a resistor.  (But I have been told that guitarists do not want to see any external components in their replacement pickups!)  But the result can be a higher output single coil pickup or a humbucker with a higher resonance and no steel sound.

I know most people like to think of magnetic circuits as lines of flux being directed, but knowing that a magnetic field is really just the sum of magnetic moments for every moving electron seems easier for me to conceptualize. Since the steel has a higher permeability that AlNiCo, the alignment of magnetic domains, or pockets of atoms that are willing to coalign, cascade down through the material, more densely than it will through AlNiCo. I wasn't what percentage of flux density there would be at the top half of the pole relative to the bottom, but since it doesn't have a relative permeability of infinity, it's going to be weaker at the bottom of the pole piece to some extent. 

I never had any desire to start a pickup company, but over time it's becoming apparent that there are a lot of permutations that aren't available. A lot of pickup concepts have come and gone since the 50's, but the difference is that most of those were the product of trial and error, and the technical benefits were often dubious. I'm starting to see a collection of data form that strongly suggests new pickup designs where the technical benefits are well accounted for. There is a lot of unexplored potential in correctly executed coil tapping, coil forming, material selection, eddy current mitigation, and passive tone shaping. I like my day job too much, though. I wish forward thinking companies like Tonerider or Zexcoil would get on board with some of this stuff. I'd rather be the guy buying these pickups, not making them.

[stratotarts]

In fact, I am considering getting into pickup manufacturing. I want to collaborate with a woodworker friend to build complete guitars. So I am thinking of trying to integrate all the neat stuff that we've learned. I want to toss out almost everything traditional and start from scratch.