Unmuddy my 'bucker or dramatic slug swap results

[Helmholtz]

Meanwhile if we revisit the ferrite pole material specification it looks like it looses quite a bit of initial permeability at fileds as low as 100mT:
https://www.ferroxcube.com/upload/me...le/MDS/3b1.pdf

(it is not very obvious to read from the B-H curve provided as you need to look at the B-H curve slope versus B value)

To be clear, AC µ cannot be read from the B-H curve. It is NOT identical to the slope dB/dH. What you want is a curve like the one called (reversible) Permeability vs. DC Bias shown in this document:
https://www.mag-inc.com/Media/Magnet...7.pdf?ext=.pdf

For this special high permeability ferrite, µ drops from the initial value of around 20000 to below 10 at an H-field of 1000A/m, corresponding to only 13Gauss or 1.3mT. But these measurements are taken in a laboratory situation, where the material is placed in a uniform H-field.
The actual B and H values inside a PU core vary from point to point and are practically impossible to measure. Consequently core sections closer to the magnet may already be in saturation while regions further away don't.

Of course it makes no sense to use such extremely high µ materials in PU cores. A change from µ=20000 to µ=2000 will barely be noticeable. But a change from µ=200 to µ=20 will.

[Helmholtz]

B=µH
dB/dH=µ
I must be missing something here, care to explain?

Sorry, this is really complicated stuff which cannot be explained in a few words. I would refer you to some standard literature on ferromagnetics. You may want to look up "incremental permeability" and "reversible permeability".

Just so much: Your typical hysteresis or B(H) curve is taken under steady-state conditions and cannot give information for AC fields. Small AC fields superimposed on a DC field produce minor loops. These minor loops have a lesser slope than the major loop/curve. And the slope of the minor loops determines the reversible permeability, which is relevant for the inductance. So you have to take µ=dB/dH from the minor loops, which is typically smaller than the dB/dH of the major loop.

Actually there are many different permeabilities, as can be seen from the list of section 221-03 of this international standard:
http://www.electropedia.org/iev/iev....nform&part=221
Which of them is relevant depends on the specific situation/problem.

[darkfenriz]

Sorry, this is really complicated stuff which cannot be explained in a few words. I would refer you to some standard literature on ferromagnetics.

wow, some great attitude there

[Tone Cam]

Helmholtz is right about the 'complicated' part. Chap 3 of the Goldman book talks about AC effects in ferrites but only briefly mentions reversible permeability. He reprinted a figure from Bozorth (https://www.wiley.com/WileyCDA/Wiley...eCd-IEEE2.html) that explains the differences between initial perm (small field variations in unmagnetized material), dc perm (slope of the major hysteresis loop) and reversible perm (dB/dH for DC biased material) and states that reversible perm differs from dc perm due to eddy current effects.

It seems like reversible perm is the closest match to the slugs in a pickup but I can't figure out if it is appreciably different than DC perm at audio frequencies. I'm guessing that reversible perm (like most other ferrite properties) will depend on the material.

The fact that I could find a material that didn't saturate at Alnico field levels and sounded good in a set of pickups indicates that ferrites could be a useful alternative to traditional pole materials. Like everything else in a pickup there is enough variation in the properties of available materials that it will take some trial and error experimentation to figure out what compounds sound the best. I've got copies of Goldman, Bozorth and Cullity and Graham (https://www.wiley.com/WileyCDA/Wiley...eCd-IEEE2.html) but believe that I would have to start looking at journal references to come up with a clear idea of whether the published manufacturers data will provide anything more than a qualitative indication of what materials would be useful to try.

Ferrite compounds are optimized for specific applications and it's probably easier to get samples from a few different classes and see how they sound in a pickup. I am guessing that low perm power cores are not going to sound the same as higher perm materials that are optimized for RF applications.

The most useful thing that I am taking away from this discussion is a realization that the negative opinions that I have had about ferrites for the past 8 or 9 years were wrong. Darkfenriz' results and the pickups I put together this week indicate that at least some of the ferrite materials sound good when they are used as slugs in conventional HB designs.

I am really glad that darkfenriz told us about his results with the Ibanez pickups.

[darkfenriz]

Glad I could be some help/inspiration.

My background with ferrites is mainly SMPS-related (conversion/filtering) and more recently operation in mild (and isotropic) DC magnetic bias (nearby MR imaging units) so I believe I have my imagination on hysteresis and magnetization, but I probably look at things from a different perspective and a different feeling on what does and doesn't matter.

Out of curiosity - what ferrite materials did you try?
My initial try has been ferroxcube 3B1
https://www.ferroxcube.com/upload/me...le/MDS/3b1.pdf
for moderate permeability and good availability, but I agree the results could be different with power compounds like 3C90...

[Helmholtz]

It seems like reversible perm is the closest match to the slugs in a pickup but I can't figure out if it is appreciably different than DC perm at audio frequencies. I'm guessing that reversible perm (like most other ferrite properties) will depend on the material.

It might be interesting to note that reversible permeability in the presence of a DC or permanent field is always lower than initial permeability, while DC permeability is not.

[Tone Cam]

I unfortunately don't know what the material is. I am >99% sure that it is medium perm material but the folks that sold it to me could not tell me anything else about it. It was probably surplus material from from a discontinued R&D project. I bought it in 2009 and was attracted to the low price and the fact that it was the right size to go into a humbucker bobbin. I have some rods of Fair Rite material 77 that are too fat to fit into a humbucker bobbin and slugs that were fabbed from a different Fair Rite material. The paperwork/emails on the slugs references a customer part number but doesn't identify the material.

[Antigua]

2. Humbucker type ceramic magnet at one side of the coil results in virtually no inductance drop
[ATTACH=CONFIG]49902[/ATTACH]

A change of 4.08 henries to 4.07, ~10 mH, is very tiny. That's not going to move the resonant peak by an audible amount. But it does show how the core permeability changes slightly as it moves closer to saturation. I'd suppose that it's not fully saturated through, if you have a neodymium on hand, it would be interesting to see if the stronger B field pushes the inductance even lower.

[Helmholtz]

Soft ferrite pole PUs will show a comparatively strong sound dependance on magnet strength.

The above statement from my post #18 should be replaced by:

>High perm soft ferrite pole PUs may show a comparatively strong sound dependency on magnet strength.<