Impedances of a humbucker using different types of alnico

[Mike Sulzer]

Following up on discussions in another thread, here are some impedance measurements of a humbucker using different types of alnico magnets. The measurements are made with an I-V circuit into Electroacoustics Toolbox on my Mac.

Magnetizing the magnets: I have found the method using inch size neos to be inconsistent. I do not have and do not want to build and electro magnet setup, and so I developed the following method:

1. Seven 3/8" dia. by 1/4" thick neos are placed along each jaw of a drill press vice in a line near the top, packed as tightly as possible, covered by two layers of masking tape.

2. The jaws are opened wide enough so that an alnico magnet can be placed along one jaw.

3. The jaws are closed, making a very strong field through the alnico across the length.

4. The jaws are opened, and the magnet is lifted out as straight up as possible.

5. The magnet is immediately placed in the pickup.

The magnitudes of the impedances versus frequency are plotted here: http://www.naic.edu/~sulzer/ImpMagComp.png. The inductances are pretty close with about 3% difference maximum. A5 is the most different from the others. If you hear significant differences between different types of alnico, it probably is not due to the different inductances. The differences between A5 and the others are probably pretty close to the minimum audible. (Remember, the resonant frquency depends on the square root of the inductance.)

Measurements of different samples of the same types lie pretty much on top of each other over the displayed frequency range, and so the measurement gives good consistency. The resonance is near 13 KHz; this is with very small external capacitance in the system; the magnitude there is about 800 Kohm. This is of course very sensitive to external effects and so is not repeatable, and thus is not plotted.

[marku52]

alnico grades?

The various Alnicos have some different properties, based on their alloying. I got a chance to test this with a free-standing coil that I could put identical but different grade of Alnico magnets into. Apparently (based on these measurements) they differ in (at least) magnetic permeability (will affect the L measurement) and conductivity (will affect eddy current losses, and thus Q). Here they are, from an Extech at 1KHz. The base coil has no magnet inserted in the bobbin.
L Q
base coil 1.012H 1.498
with A5 1.245H 1.820
with A2 1.307H 1.902
with A3 1.299H 1.879

It looks like all the grades have a mag perm >air. and their conductivity does vary, so that the Q moves around some too, tho not a lot. I haven't been able to find this data on any magnetic vendor website, they usually give you a Br and an HC,and a demag curve and call it good. Anyone got better data?

[belwar]

Here's the analysis I did a while back. I would have included C5, but the one dave king was kind enough to send me shattered in the mail :<

[Joe Gwinn]

Many magnet vendors do quote the permeability of their alnico, under the rubrick of "recoil permeability", which is usually around three. For ceramic and rare earth magnets, it's very close to unity. However, even with a magnet in the coil, most of the magnetic path is in the air, so the net effect on coil inductance won't be large.

More informative than Q (whose computation involves the inductance) is the AC Resistance (which is directly measured). Actually, the most useful resistance value is the excess of AC resistance over DC resistance, as this difference is proportional to eddy current loss in the alnico.

[Mike Sulzer]

marku, Belwar,

Thanks for posting the measurements. I want to look at the relative inductances (later, no time now) to see how consistent we are.

By the way, there is a typo in the labeling of my plot: where it says A6, that should be A8. I got my magnets from Mojo, and thye had A8, but not A6.

[marku52]

JoeG:
More informative than Q (whose computation involves the inductance) is the AC Resistance (which is directly measured). Actually, the most useful resistance value is the excess of AC resistance over DC resistance, as this difference is proportional to eddy current loss in the alnico.[/QUOTE]

Ah, very useful, so I went back and redid the measurement on a different coil (had already assembled the other) and included the RAC numbers. This is a slotted bobbin that I can put a humbucker sized magnet inside, not on the bottom as in an actual HB.

RDC measured at 3.949K with a Fluke meter. NM means no magnet in the coil. Sorry about the way the HTML converter eliminates the extra spaces I had tried to insert to make the table more readable.

type L(mH) Q RAC (K)
NM 912.8 1.462 3.927
A5 1133.8 1.794 3.976
A3 1186.2 1.857 4.018
A2 1219.7 1.918 4.001

It is a little odd that the RAC of the coil no mag is lower than the RDC number. I guess this is a measurement discontinuity from the Extech to Fluke. The other numbers are interesting, A5 doesn't add much to the loss of the coil itself, and adds the smallest amount of inductance, so its perm must be the lowest of the 3. A2 and A3 both add more L and more loss, maybe this is part of the sonic difference between the Alnico grades?

So you might guess that A5 would sound the most "transparent", and the other two a bit "warmer"?

Thoughts?

[salvarsan]

Many magnet vendors do quote the permeability of their alnico, under the rubrick of "recoil permeability", which is usually around three.

As invented measurements go, recoil permeability isn't so bad. It attempted to address the often-nonlinear hysteresis when extracting work from a mag field. Sometimes the recoil slope is flat, and sometimes it's a little elliptical, properties that aren't accurately expressed in a bulk permeability figure.

As a predictive quality in pickup design, I have no idea as to its worth.

Permeability changes, OTOH, have expected and predictable effects on coil inductance.

[big_teee]

So, let me see if I have this straight?
Taking a humbucker that has say a 7.5k DCR, will always have a 7.5k DCR regardless of magnet?
The R will change with different magnet types at different Freqs. ?
So does that explaing why different magnets sound different in the same Pickup?
Please keep it simple!
Thanks,

[salvarsan]

So, let me see if I have this straight?
Taking a humbucker that has say a 7.5k DCR, will always have a 7.5k DCR regardless of magnet?

Yes.

The R will change with different magnet types at different Freqs. ?

Yes. The ExTech LCR meter gives you the AC resistance directly.

So does that explaing why different magnets sound different in the same Pickup?

Yes, in part, but generally true.

[Joe Gwinn]

The DC resistance depends only on the windings, not what is nearby.

The AC resistance is the DC resistance plus the reflected effect of eddy currents in nearby metal.

[Joe Gwinn]

It is a little odd that the RAC of the coil no mag is lower than the RDC number. I guess this is a measurement discontinuity from the Extech to Fluke.

Yes, it's measurement inaccuracy. The air-core Rac value is pretty close to the Rdc value, and these differ by about 0.5%, which is smaller than the claimed accuracy of the Extech (and probably the Fluke).

The other numbers are interesting, A5 doesn't add much to the loss of the coil itself, and adds the smallest amount of inductance, so its perm must be the lowest of the 3. A2 and A3 both add more L and more loss, maybe this is part of the sonic difference between the Alnico grades?

A5 has the lowest excess AC resistance, followed by A2, and then A3 (the highest).

As for inductance, A2 and A3 swap positions.

I find plotting excess AC resistance as a function of inductance and staring at the plot to be the most useful approach.

So you might guess that A5 would sound the most "transparent", and the other two a bit "warmer"?

I wish I had a precise engineering definition of these two terms. I suppose "warmer" means that the lows are boosted, but not by too much. But what does "transparent" mean? That the frequency response is more or less flat, and the phase response is roughly linear with frequency one supposes. This implies that one is best well away from resonance.

There is a large school of thought that holds that the lower the inductance the better, so one would prefer an air core (or ceramic or neo), but a metallic cover may be needed to tame the highs.

[Mike Sulzer]

Yes, it's measurement inaccuracy. The air-core Rac value is pretty close to the Rdc value, and these differ by about 0.5%, which is smaller than the claimed accuracy of the Extech (and probably the Fluke).



A5 has the lowest excess AC resistance, followed by A2, and then A3 (the highest).

As for inductance, A2 and A3 swap positions.

I find plotting excess AC resistance as a function of inductance and staring at the plot to be the most useful approach.



I wish I had a precise engineering definition of these two terms. I suppose "warmer" means that the lows are boosted, but not by too much. But what does "transparent" mean? That the frequency response is more or less flat, and the phase response is roughly linear with frequency one supposes. This implies that one is best well away from resonance.

There is a large school of thought that holds that the lower the inductance the better, so one would prefer an air core (or ceramic or neo), but a metallic cover may be needed to tame the highs.

Lower inductance will move the resonance higher, which might or might not be what a guitarist wants to hear. Move it too high out past the range of the guitar speaker, and the guitarist will say it has fewer highs, not more, compared to a pickup with a typical resonance.

Looking at my curves makes me think that the differences are mostly due permeablity differences, not conductivity. This is because the curves diverge smoothly from each other as frequency increases without changes in shape. But I will turn my mag/phase data into L and series R as the Extech does, and see what happens.

[David Schwab]

I wish I had a precise engineering definition of these two terms. I suppose "warmer" means that the lows are boosted, but not by too much. But what does "transparent" mean? That the frequency response is more or less flat, and the phase response is roughly linear with frequency one supposes. This implies that one is best well away from resonance.

Warm= less highs. More lows would be "fat".

Transparent= more highs, maybe less mids.

These are the terms as used by most bass players.

[marku52]

AFAIUunderstand it, magnets effect the sound several ways. One way is that the magnetic permeability of magnetic materials differ. Putting an alnico bar magnet (even if it is not magnetized) inside a coil will increase the inductance of the coil (just as if it were a small piece of iron). For neos and ceramics, this effect is slight to non-exisitent. for Alnicos, it is real and varies between grades. Varying the L changes the coil resonant freq, clearly a big contributor to tone.

Also, varying is the conductivity of the magnet material---the more conductive, the more eddy current losses. Again, ceramics and neos aren't very conductive, so they don't introduce these losses. And the losses are proportional to freq^2, so they roll off the highs. Alnicos all have this loss to varying degree. As I propose up thread, this may be why ceramics are regarded as "harsh", they are lacking the hi frequency loss that Alnico provides.

Now this is *fun*. Wind a coil on a bobbin so that you can put a bar magnet inside it. Now measure the L, Q, and RAC (the extech does this very well) while you put different magnets inside the coil. (this is a way to distinguish unknown grades of Alnico if you have known samples to compare to)

For another kick, set the coil on a sheet of brass and see how the numbers change. you'll never think about eddy currents the same way again....

[Mike Sulzer]

And the losses are proportional to freq^2, so they roll off the highs. Alnicos all have this loss to varying degree. As I propose up thread, this may be why ceramics are regarded as "harsh", they are lacking the hi frequency loss that Alnico provides.

Losses are not always proportional to frequncy squared. Consider an audio transformer, primary, secondary, and a resistive load. Consider using the primary as an inductor with a normal value of load resistance connected. The power dissipated in the load in this situation is an eddy current loss. Throughout the useful range of the transformer, this loss is close to constant. As the frequency get higher, the loss drops. You know it does, because the power transferred to the load drops as the frequency increases out of the range of the transformer. The reason it does is explained by the transformer model. The so-called leakage inductance is in series with the load, and as the inductive reactive increases, the voltage across the load resistor drops. The cores of a pickup sure do look like little secondaries with currents circulating near their surfaces.