Unmuddy my 'bucker or dramatic slug swap results

[Antigua]

I pay a lot of attention to screw and slug alloys -- they definitely matter.

There's some analysis of pickups out there, and general info about the significance of air gap with respect to the permeability of the core, that suggest that because such a large amount of the flux path is air, that the various permeabilities of these steel alloys can't stand to make much of a difference, at least not compare to AlNiCo, which has such a low permeability, that the degree to which it alters the inductance and the reluctance path is still worth distinguishing.

[Tone Cam]

Saturation effects can also contribute significantly to the tonal properties of soft ferrite slugs. Not sure what material you are working with but the high and medium perm ferrites that I have tried saturate significantly when you put them close to an alnico bar magnet. You can see if this is happening with the slugs you are using by disconnecting the slug coil from the pickup and looking at its isolated inductance with an LCR meter. If you then take a fully magnetized humbucker magnet and move it towards the slugs the measured inductance will decrease with distance if they are saturating. The percentage reduction gives a qualitative indication of how bad the saturation is.

[Tone Cam]

I am hoping there is still some interest in ferrite poles. I wasn't around when this thread first came up but am very interested in nontraditional pole materials.

When I was first starting working on pickups I tried several different ferrite compounds. The tone was very clean due to their low ferromagnetic loss coefficients but I stopped working with them because their permeability seemed to saturate in the presence of low field magnetic fields. When I watched the inductance of a ferrite-loaded humbucker coil with an LCR meter, it would decrease significantly anytime I moved an Alnico magnet near the slugs. At the time I didn't think that saturation was good for tone.

Did you guys see/hear saturation effects with your ferrites? If so, what do you think saturation does to the tone of a pickup?

[Helmholtz]

When I was first starting working on pickups I tried several different ferrite compounds. The tone was very clean due to their low ferromagnetic loss coefficients but I stopped working with them because their permeability seemed to saturate in the presence of low field magnetic fields. When I watched the inductance of a ferrite-loaded humbucker coil with an LCR meter, it would decrease significantly anytime I moved an Alnico magnet near the slugs. At the time I didn't think that saturation was good for tone.

This is a very interesting observation. And it shows that AC and DC permeabilities are two different and independent parameters. While the ferrite cores may still carry a substantial (DC or permanent) B and thus produce satisfactory Gauss values on their tops, the AC permeability (aka "reversible permeability") may drop from initial values of thousands down to just above 1 in the presence of a sufficiently high permanent field. (Something similar happens in the strings directly above the PU.)
It is known from magnet literature that the AC permeability decreases with increasing DC fields. High µ ferrites are especially sensitive to this effect.
Inductivity is influenced by the AC-µ of the cores and thus shows the change - albeit strongly alleviated by the airgap.

Now if the AC-µ drops to very low values, two things happen. Firstly, inductivity drops by up to 30%, resulting in a higher rersonant frequency. Secondly, the poles lose their ability to collect and focus the AC flux (mainly concerning the returning part) created by the moving strings which should influence the effective aperture length.

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

[Antigua]

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

You're talking theory, but without any actual values, it's hard to say whether the effects are of a magnitude that permits it to be audible. In general, there's a lot of talk about theory, but not much hard data, so whether the rubber ever actually "meets the road" is not truly known. That's why I like gathering data, but test that involve exotic materials are harder to conduct.

[Antigua]

When I was first starting working on pickups I tried several different ferrite compounds. The tone was very clean due to their low ferromagnetic loss coefficients but I stopped working with them because their permeability seemed to saturate in the presence of low field magnetic fields. When I watched the inductance of a ferrite-loaded humbucker coil with an LCR meter, it would decrease significantly anytime I moved an Alnico magnet near the slugs. At the time I didn't think that saturation was good for tone.

Were these soft ferrites that required an permanent magnet such as AlNiCo in order to function, or hard ferrites with their own sustained B field? How much did the inductance of the pickup drop compared to having air gapped coils, which is to wonder, how close would the ferrite come to unity when the AlNiCo bar was near?

As for how this effects tone, there is a lot of vague talk on forums about how one factor or another impacts the "sound" or the "tone", but unless there is a time dependence aspect to that factor (like reactance), then the impact of that factor will be nearly or completely uniform with respect to frequency, resulting in little more than a variation in output voltage. A difference in output level itself might be perceived as a tonal change to some people's ears, but whether it should be considered such in a technical sense is another question.

I think one factor that goes overlooked due to it's complexity, and the uniqueness of the problem domain, is precisely how string pull effects tone. It's possible that the B field over the ferrite poles either produces a particularly strong magnetic pull upon the strings, or a very weak pull. You'd need a gauss meter on hand to figure out which is the case, which can be had for about $100 these days. I think a lot of the strong opinions about things like magnetic strength and pickups height come down to personal preference with regard to magnetic interference upon the movement of the guitar strings, and it's possible that if you didn't like the outcome of ferrite poles, it might have something to due with that. Another reason the subject of string pull is so opaque is that, because the effects are so complex, effecting various harmonics in difference ways, differently depending on where the pickup is located, the difference in terms of tone is difficult to state in plain English. If you ask people how adjusting the height of pickups changes the tone, you will get a lot of different answers.

[Helmholtz]

You're talking theory, but without any actual values, it's hard to say whether the effects are of a magnitude that permits it to be audible. In general, there's a lot of talk about theory, but not much hard data, so whether the rubber ever actually "meets the road" is not truly known. That's why I like gathering data, but test that involve exotic materials are harder to conduct.

Not really a factual comment.

If you read my post carefully, you will find some valid data based on (own) measurements and soft ferrite data. I used to work with ferrites and especially analyzed saturation behaviour.
A 10%-15% increase in resonant frequency is audible, at least to me.

[darkfenriz]

I am hoping there is still some interest in ferrite poles. I wasn't around when this thread first came up but am very interested in nontraditional pole materials.

When I was first starting working on pickups I tried several different ferrite compounds. The tone was very clean due to their low ferromagnetic loss coefficients but I stopped working with them because their permeability seemed to saturate in the presence of low field magnetic fields. When I watched the inductance of a ferrite-loaded humbucker coil with an LCR meter, it would decrease significantly anytime I moved an Alnico magnet near the slugs. At the time I didn't think that saturation was good for tone.

Did you guys see/hear saturation effects with your ferrites? If so, what do you think saturation does to the tone of a pickup?

Your observation is obviuosly technically correct if you consider a strong () magnet in a humbucker with ferrite pole pieces.

Ferrite pole pieces will typically saturate at below 400mT and start their saturation 'knee' around maybe 250mT.

On the other hand alnico magnets will typically be 300-900mT (correction:30-90mT) strong while ferrite aka ceramic magnets are weaker (correction:stronger) at typically 200-350mT, some are weaker than that.

So yes, the alnico magnet is (not) very likely to saturate ferrite pole pieces (same as) ceramic magnet (which I used).

Regarding the pickup working at pole piece saturation I don't see that as a problem at all. After all this is what actually goes on with magnetized alnico slugs on a strat single coil.

[Helmholtz]

Your observation is obviuosly technically correct if you consider a strong alnico magnet in a humbucker with ferrite pole pieces.

Ferrite pole pieces will typically saturate at below 400mT and start their saturation 'knee' around maybe 250mT.

On the other hand alnico magnets will typically be 300-900mT strong while ferrite (aka ceramic) magnets are weaker at typically 200-350mT, some are even weaker than that.

So yes, the alnico magnet is very likely to saturate ferrite pole pieces but ceramic magnet (which I used), well, not really, maybe a bit.

Regarding the pickup working at pole piece saturation I don't see that as a problem at all. After all this is what actually goes on with magnetized alnico slugs on a strat single coil.

Sorry, there seems to be some confusion of magnet units. 100mT correspond to 1000G. A fully charged alnico V bar magnet has around 800G= 80mT in the center of the pole faces. The ceramic bars I measured produced around 1400G=140mT and thus were considerably stronger than the alnicos.
This said, there are many different ceramic magnet grades and virtually hundreds of different soft ferrite types.

Regarding the pickup working at pole piece saturation I don't see that as a problem at all. After all this is what actually goes on with magnetized alnico slugs on a strat single coil.

Completely agree. Good point, not the same but somehow comparable.

[Tone Cam]

I think that Helmholtz is correct about the units.

Since my first post I have spent some time reading and doing tests.

There is a lot of tech data out there from the ferrite manufacturers that directly addresses questions of saturation, frequency dependence, etc. (see, for example, https://www.mag-inc.com/Design/Techn...Core-Documents). There is also a pretty good book on the subject by Goldman (https://www.springer.com/us/book/9780387281513). When I looked through the data sheets I realized that the saturation flux varies over a significant range and many compounds have values greater than 3 kGauss. I also looked for a definition of saturation flux and found one at https://product.tdk.com/info/en/cont...ory=10_ferrite. Based on that definition and the associated hysteresis curve I would expect the inductance to roll off before you get to the tabulated value but that the rolloff should be small at fields below 1 kGauss in the compounds with the highest saturation flux values.

I found some HB-sized rods in my box of old ferrites that did not show saturation behavior with an Alnico bar magnet. They showed a 10% reduction in inductance when I stuck a 3/16" tall stack of NdB discs on each pole but the fields generated by the discs (estimated from the K&J magnet calculator) were significantly greater than 2 kG.

There were major differences in transfer function between a coil that was loaded the ferrite slugs and the same coil with conventional steel slugs. (Tried to post a JPEG of the spectrum analyzer trace but got a message saying I didn't have permission to do so - send me an email if you would like a copy of the PDF version). The ferrite slugs significantly increased the height of the resonance peak and putting a fully magnetized alnico 5 bar in contact with the slugs (in the same way as it would be mounted in a HB) dropped the peak by about 50%. Mounting the same magnet the ends of the slugs dropped the peak to about 25% of its original value.

A set of pickups with the same ferrite slugs sounded good. The bridge had 5500 turn machine wound coils, an alnico 2 magnet and 1010 screws. The neck had 5000 turn coils, an alnico 4 magnet and 1010 screws. I put them in a old Epi LP Classic that I often use for testing and I was pleasantly surprised with the tones that I got from my Carr Skylark. They sounded very clear but the Alnico bars and steel screws had enough loss to keep them from being sterile. I expected the bridge to be harsh and ice-picky but found that the top end was well behaved. Overall I liked the tone better than the the Classic 57's that I took out of the guitar to test the ferrites.

It will be interesting to see what I can do to the tone with field modifiers and pole caps - the low core loss of the ferrites should make the slug coil a good 'blank canvas' for tone mods.

[Tone Cam]

Spectrum analyzer trace for previous post

Analyzer output 072818.pdf

I think that I figured out how to upload the analyzer trace.

[darkfenriz]

Thanks for catching a mistake in my statements, I corrected my post above so that it's not too confusing now

[darkfenriz]

Let's see the effects of a magnet proximity to the coil:
1. no magnet

[darkfenriz]

2. Humbucker type ceramic magnet at one side of the coil results in virtually no inductance drop

[darkfenriz]

3. However with two stronger single coil type arrangement ceramic magnets it looks quite a bit down

I hope to measure the B of the magnets tomorrow, but my measurement range is 130mT only (approx)

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)