Inductive balancing of cores and windings

[fieldwrangler]

Folks -

All else being equal (in terms of bobbin & core dimensions) we know that we can achieve a target inductance (let's say 5 henries for this discussion) by manipulating permeability of core material and number of turns of a given wire gauge.

What differences would we expect from maximizing core permeability (say by using 1010 steel) and minimizing the winding count as opposed to using less permeable cores and more windings? (We'll assume that saturation isn't an issue here.)

All perpectives (real world experience, theory and speculation) welcome.

Bob Palmieri

[bbsailor]

Folks -

All else being equal (in terms of bobbin & core dimensions) we know that we can achieve a target inductance (let's say 5 henries for this discussion) by manipulating permeability of core material and number of turns of a given wire gauge.

What differences would we expect from maximizing core permeability (say by using 1010 steel) and minimizing the winding count as opposed to using less permeable cores and more windings? (We'll assume that saturation isn't an issue here.)

All perpectives (real world experience, theory and speculation) welcome.

Bob Palmieri

Bob,

My gut reaction from tinkering with this stuff since I was a teenager is this. As the coil inductive reactance is raised more than the typical low Q of a guitar pickup and the reactance departs from DCR of the coil, the pickup coil has more frequency dependent characteristics in the mid range 2KHz to 3KHz range typically due to winding capacitance of about 80pf together with guitar coax cable capacitance of about 325pf for a total in the 400pf range.

Using the rule of thumb that the pickup load should be or has historically been 40 times the DCR, this gives the resonant frequency room to peak without too much loading allowing the pickup a chance to demonstrate its full flavor. A Fender single coil pickup of 6,250 ohms uses a 250K ohms pot while a Humbucker pickup in the 10K ohms range uses a 500K pot but is really closer to 400K range when the 1M amp loading is put in parallel with the 500K ohm pot.

Now, when you use a higher permeability core material, you raise the coil inductance to a higher value than using cores of lower permeability and the coil inductance coil resonance peak shape changes to being a higher Q and more limited bandwidth however subject to the increased loading of the pot value and the resonant effects of the coil capacitance and coax cable capacitance. To really hear the difference you need to put a 10M ohm FET buffer right after the pickup to isolate it from the pot and amp input loading as well as the cable capacitance resonance shift.

Different core types and core thicknesses will also have some eddy currents circulating in them so some energy will be absorbed by these eddy currents and affect the shape of the pickup response. In the old days, guitar amps were a direct evolution and modification of PA amplifiers with a speaker put in the same box for the ease of transport. These early amps used high impedance microphones with single conductor coax so guitars of that day needed to be built to have an output high enough to feed these amps typically around 100 millivolts. The signal needed to be high enough to have a good signal to noise ratio. This is what caused Leo Fender to wind thousands of turns of AWG 42 or 43 wire onto a bobbin to produce enough voltage to drive these amps but the consequences of this type of design is the resonant hump in the human hearing spectrum where the human ear tends to be the most sensitive.

Your question has quite a historical background that affects pickup design to this day. Unfortunately, today with all the electronic stuff around us, pickups have become a good noise antenna and techniques need to be used to minimize the induced noise pickup. Just try using a Fender single coil pickup near your computer to hear what I mean.

I hope this helps?

Joseph J. Rogowski

[fieldwrangler]

I'd say this provides an excellent opening perspective on this issue.

It also sets the tone for a number of valuable related issues.

[Mike Sulzer]

As the relative permeability of the cores is raised from unity, the inductance increases from the 'no core' values by about a factor of three when the permeability is about 10 or so, but does not increases further significantly as the permeability is raised more. This is a result of short open cores. The same effect means that the exact permeability of the guitar string is unimportant as long as it is not low. The differences between cores is mostly a matter of different conductivity, and thus eddy currents.

Folks -

All else being equal (in terms of bobbin & core dimensions) we know that we can achieve a target inductance (let's say 5 henries for this discussion) by manipulating permeability of core material and number of turns of a given wire gauge.

What differences would we expect from maximizing core permeability (say by using 1010 steel) and minimizing the winding count as opposed to using less permeable cores and more windings? (We'll assume that saturation isn't an issue here.)

All perpectives (real world experience, theory and speculation) welcome.

Bob Palmieri

[fieldwrangler]

As the relative permeability of the cores is raised from unity, the inductance increases from the 'no core' values by about a factor of three when the permeability is about 10 or so, but does not increases further significantly as the permeability is raised more. This is a result of short open cores. The same effect means that the exact permeability of the guitar string is unimportant as long as it is not low. The differences between cores is mostly a matter of different conductivity, and thus eddy currents.

This is really interesting. Now that you've kindly re-stated this I seem to recall that you've mentioned this effect before.

So, am I correct in assuming that there's likely very little (if any) difference in conductance between, say, 1010 & 1018 steel cores?

Bob p

[Antigua]

All else being equal (in terms of bobbin & core dimensions) we know that we can achieve a target inductance (let's say 5 henries for this discussion) by manipulating permeability of core material and number of turns of a given wire gauge.

What differences would we expect from maximizing core permeability (say by using 1010 steel) and minimizing the winding count as opposed to using less permeable cores and more windings? (We'll assume that saturation isn't an issue here.)

Thanks for thinking to ask this question, it's often been just on the edge of my mind.

The formula for determining the peak resonance is "Frequency is 1 divided by ( 2 times pi times the square root of inductance times capacitance)", and adding more winds should increase capacitance as well as inductance, whereas a more permeable core should only increase inductance, but since the resonant frequency only seems to care about the product of the two, it seems to me that you should be able to achieve the same end result through either means.

[ScottA]

Different core types and core thicknesses will also have some eddy currents circulating in them so some energy will be absorbed by these eddy currents and affect the shape of the pickup response.

As the relative permeability of the cores is raised from unity, the inductance increases from the 'no core' values by about a factor of three when the permeability is about 10 or so, but does not increases further significantly as the permeability is raised more. This is a result of short open cores. The same effect means that the exact permeability of the guitar string is unimportant as long as it is not low. The differences between cores is mostly a matter of different conductivity, and thus eddy currents.

I'm going to be spilling some of my guts on a lot of this stuff very soon, might as well start here.

Inductance can be changed both by increasing winds on a coil, and by increasing the permeability of the core.

As Joseph observed, eddy currents in the core can cause loses that affect the shape of the response. Generally, a higher effect of eddy currents in the core will lower the Q of the resonant peak. I have observed this experimentally very consistently. There are also some other implications of the "lossiness" of the core on the position of the resonant peak. I'm actually in the middle of writing up some blog posts about this stuff, but I don't think I'll get to that one for a bit. I also doubt anyone will still be reading them by then! It gets pretty thick.

While it's true that the increase in inductance does roll off at higher values of permeability (as Mike notes), it's not true that increasing permeability is unimportant after a point. It still matters. It should also be noted that permeability and conductivity are equally important in determining the magnitude of the eddy current effect.

In a nut shell, adding turns increases the inductance while maintaining the basic voice or timbre of the pickup. Changing the core multiplies inductance while also changing the voice, potentially significantly.

Air cores tend to behave in a more ideal fashion than permeable cores, so increasing turns might be thought of as a "cleaner" way to increase inductance. Adding turns maintains the basic shape of the response, but shifts it due to the increased capacitance and inductance as mentioned. Higher turn coils are lossier than lower turn coils, as can be seen by the decrease in Q with turns (an example of which is given below - this example is with a relatively efficient core installed but the trend is the same with air coils).

Coils with permeable cores can be significantly higher in inductance than the corresponding air coil, and the Q can be manipulated over a wide range, depending on the selection of the core. The highest Q will always be obtained with an air coil.

Here are some examples, and these are figures from my next blog post. The point of the blog post is to introduce the concept of just how important the pole piece is in shaping the frequency response (something that many here realize but many laypeople do not), and a lead in to some more detail about how I engineer that response with both material and configuration changes.

It is all (well, mostly) about designing the eddy current response in the core. The title of the blog series is "It's All About The Pole Piece".

Here's a single, isolated 9000 turn Zexcoil coil with various cores installed.


Here's the "Fat 5" pole piece in various coils.


These are the data associated with the pole variation:


and the coil variation:


The Q referenced in the tables is not the classical measure, but something I derived that is specific to the shape of response at resonance. I'll be talking more about that going forward as well.

And here are the measured frequency responses of a range of conventional Strat-style singles and PAF-style humbuckers.



The similarity in the range of response and the shape of response observed in the conventional pickups and pole variants in the Zexcoil coil supports the proposition that the pole piece is the primary driver in determining frequency response. Much more than subtleties of the wind, coil geometry or any other design variable. And specifically it is the effect of eddy currents in the core which determine that response.

[Antigua]

What accounts for the voice of a pickup, and why does changing the core alter the voice of a pickup, while changing the number or windings does not, or at least to a lesser extent?

[Mike Sulzer]

...it's not true that increasing permeability is unimportant after a point. It still matters. It should also be noted that permeability and conductivity are equally important in determining the magnitude of the eddy current effect.

When the inductance stops increasing significantly with increasing permeability, that means that the induced magnetic field is not increasing. That would seem to put a hold on any significant effect. For example, are you sure that the permeability is as import as the conductivity for skin effect with short open cores? I have my doubts. I think that effect limits out just like the inductance, but I do not know for sure since I have neither computed nor measured this. But I am having trouble seeing where the effect comes from if the field does not keep going up with the permeability.

[fieldwrangler]

I'll just jump in at this point to say that my general feeling has been that achieving a desired inductance by maxxing out my options with the cores results in lower Q, maxxing out my options with the wire produces more perceived ringing due to the increased capacitance. The increased resistance of the extra wire doesn't seem to dampen the response as quickly... sometimes....

Without distracting from the present path I'll just say for the moment that I have my own ideas about the primary agent influencing the character of the iconic designs, which is the geometry of the magnetic elements. Drastic movement of resonant frequency & Q can certainly obscure this personality but it can be quite persistent.

[Antigua]

I can appreciate that the core material will have the secondary effect of changing the way the magnetic flux lines intersect with the guitar strings, but as far as I know, different grades of AlNiCo pole pieces in a Strat style pickup would perform similarly in that regard, where as pole pieces versus an air coil, or slugs with with an under mounted bar magnet would make for a totally different flux line arrangement. If it's the case that the flux arrangement dictates the voice of a pickup, then would it be said that choosing a different grade of AlNiCo doesn't change the voicing of the pickup?

[ScottA]

What accounts for the voice of a pickup, and why does changing the core alter the voice of a pickup, while changing the number or windings does not, or at least to a lesser extent?

The voice, or at least what I think of as the voice, of a pickup is reflected in the shape of the frequency response. Also to some extent the position, but mostly the shape.

For example, the frequency response of a Strat-style single coil is typified by a high Q. You can change the number of windings (hence the inductance) over a very wide range and it's still going to sound and feel like a Strat.

The shape of that response is determined by the core, and specifically how the magnetic flux gets "filtered" due to eddy current effects in the core.

With something like AlNiCo 5, the eddy current effect is almost negligible, and the flux gets filtered very little. This, more than any other single factor, is what gives this type of pickup it's characteristic sound.

With something like the low carbon steel that is used in humbucker style pickups, the effect of eddy currents is dominating. In the audio frequency range with steel of the dimensions typically used in pickups, the skin effect is in full force, you could say. That results in the more round, low Q resonant peak and also the characteristic tone and response.

When the inductance stops increasing significantly with increasing permeability, that means that the induced magnetic field is not increasing. That would seem to put a hold on any significant effect. For example, are you sure that the permeability is as import as the conductivity for skin effect with short open cores? I have my doubts. I think that effect limits out just like the inductance, but I do not know for sure since I have neither computed nor measured this. But I am having trouble seeing where the effect comes from if the field does not keep going up with the permeability.

It's not really about field strength or density, or even absolute permeability (not that those things aren't important in pickup design and function). It's about how the flux gets filtered through and by the core material.

Stay tuned. I will lay out my model in some detail with supporting data. I'm sure you'll be interested and have a lot to say about it.

[ScottA]

If it's the case that the flux arrangement dictates the voice of a pickup, then would it be said that choosing a different grade of AlNiCo doesn't change the voicing of the pickup?

It does, but it's more subtle. The frequency response differences of the AlNiCo alloys are also driven by eddy current effects but they exist in a regime where the effects are not as significant.

I can quantify and account for the differences between the AlNiCos in my model. That will be a subject of a future blog post.

In fact, that's the reason I did all of the work to figure this stuff out in the first place. My "5" is not AlNiCo 5. It's a totally different material. AlNiCos don't work well in my design because of the opposing fields between the D and G string. I had to figure out another way to get the same tones, i.e. shape of the frequency response.

[Mike Sulzer]

The voice, or at least what I think of as the voice, of a pickup is reflected in the shape of the frequency response. Also to some extent the position, but mostly the shape.

For example, the frequency response of a Strat-style single coil is typified by a high Q. You can change the number of windings (hence the inductance) over a very wide range and it's still going to sound and feel like a Strat.
....
With something like the low carbon steel that is used in humbucker style pickups, the effect of eddy currents is dominating. In the audio frequency range with steel of the dimensions typically used in pickups, the skin effect is in full force, you could say. That results in the more round, low Q resonant peak and also the characteristic tone and response.

Such as in this comparison:

AlAndSteelCores.pdf

The permeability ratio between the two is probably greater than 10, the inductance ratio (low frequency) is only about two, and the ratio of pickup resonant frequencies is a lot less than implied by that since the eddy currents lower the imaginary part of the impedance (the yellow/gold line that curves). But the Qs are quite different, also a result of the eddy currents.

[Antigua]

That's interesting that the eddy currents have the particular effect on the peak's shape. It sounds like you're talking about "bandwidth" being effected, though



I tend to think of the "voice" as being the grand sum of all the factors involved. I think it's fair to say that narrow band widths result in a more shrill pickup, and could be said to determine it's shrillness, or lack there of. If the resonant peak is lower, a narrow bandwidth causes a nice midrange gives way to a nasally honk, which I readily witness when using tone caps between .022 and .003 uF. You can hear all these same things if you mess around with a parametric EQ while listening to music.