Measuring pickups Capacitance?

[Antigua]

The difference of coercive force between AlNiCo and steel would cause the shape of the magnetic field around them to differ, because when the coercivity is low, the material more readily gives way to the shape of the magnetic field of the aggregated magnetic circuit, whereas if the coercivity is high, the flux contributed by the pole piece will be less influenced by the external fields, and more a function of it's own physical shape. An example would be a PAF that has it's slug replaced with AlNiCo pole pieces. The slugs would have a magnetic field that is influenced by the other magnetized parts of the humbucker, but the AlNiCo's magnetic field would be relatively independent of them. That being said, since the aperture is too narrow to cause audible comb filtering in either case, it probably doesn't matter.

Another thing occurred to me, and it probably explains why the outer tapped coil generated more voltage than the inner tapped coil; if the aperture is wider than the wire loop, then the flux that is outside of the loop will have a cancelling effect. You get voltage when flux of a given polarity passes through the loop, but if that same polarity is also outside of the loop, it generates opposite phase voltage because it's pushing into the return path of the loop. Therefore, the most productive turns of wire might not be the ones that are closest to the core, but would be those that are most optimally sized for the aperture, which also changes in size as the string is nearer or further from the string. So the most productive coil loops would not only change as the pickup is raised and lowered, but as the string moves around. This could also mean that a more optimal pickup would have a buffer between the pole pieces and the coil, which plastic bobbins have by default, while Fender style AlNiCo pickups don't.

[Helmholtz]

The difference of coercive force between AlNiCo and steel would cause the shape of the magnetic field around them to differ, because when the coercivity is low, the material more readily gives way to the shape of the magnetic field of the aggregated magnetic circuit, whereas if the coercivity is high, the flux contributed by the pole piece will be less influenced by the external fields, and more a function of it's own physical shape.

Your conception of the coercive force (Hc) seems completely wrong. Hc has absolutely nothing to do with the PU's magnetic field. What matters is flux distribution and ac µ. Hc is a constant, a specific parameter of the ferromagnetic material. It does not show in magnetic circuits.

Another thing occurred to me, and it probably explains why the outer tapped coil generated more voltage than the inner tapped coil; if the aperture is wider than the wire loop, then the flux that is outside of the loop will have a cancelling effect. You get voltage when flux of a given polarity passes through the loop, but if that same polarity is also outside of the loop, it generates opposite phase voltage because it's pushing into the return path of the loop. Therefore, the most productive turns of wire might not be the ones that are closest to the core, but would be those that are most optimally sized for the aperture, which also changes in size as the string is nearer or further from the string. So the most productive coil loops would not only change as the pickup is raised and lowered, but as the string moves around. This could also mean that a more optimal pickup would have a buffer between the pole pieces and the coil, which plastic bobbins have by default, while Fender style AlNiCo pickups don't.

As already mentioned, the measurements of Zollner show that the signal yield of turns increases with the width of the turns within the first millimeters from the core. As a consequence, the outer turns of a strat PU contribute somewhat more than the inner ones. The local ac flux density was found to remain positive up to a distance of around 6mm from the magnet, from where it changes polarity. As long as the inner flux density does not change polarity, wider loops see more total flux and produce more voltage.

[Antigua]

Your conception of the coercive force (Hc) seems completely wrong. Hc has absolutely nothing to do with the PU's magnetic field.

I'm just curious, why do you feel it necessary to use hyperbolic adjectives like "completely wrong" and "absolutely nothing"? It makes this discourse less pleasant.

My understanding stems from this:

https://www.duramag.com/techtalk/aln...from-alnico-5/

One obvious question that may be asked is why does an Alnico 5 magnet have a similar Residual Induction as some Rare Earth magnets like Neodymium Iron Boron, but are apparently weaker than the Rare Earth magnets? This is because the Rare Earth magnets have a much higher Coercive Force. As described above, their self-demagnetization impact is much lower when the alloy has a high Coercive Force. Large magnetic fields can be generated with Alncio 5, but the magnet must be very magnetically long and use iron / steel elements to help reduce the effects of self-demagnetization.

That I'm seeing in this explanation, is that the lack of coercivity of AlNiCo causes it to be weaker because it magnetically interacts with itself, resulting in a lower sum of flux density out of the top of the magnet's polar faces. But that Br flux has to go somewhere else instead, I would suppose it must fan outward, away from the polar axis. It appears to me that they generally construct AlNiCo so that the magnet's length will be much longer than it's width in order to mitigate the consequences of a low Hc.

As already mentioned, the measurements of Zollner show that the signal yield of turns increases with the width of the turns within the first millimeters from the core. As a consequence, the outer turns of a strat PU contribute somewhat more than the inner ones. The local ac flux density was found to remain positive up to a distance of around 6mm from the magnet, from where it changes polarity. As long as the inner flux density does not change polarity, wider loops see more total flux and produce more voltage.

Maybe so, I'm just corroborating the observation with reasoning. Maybe I'm just talking to myself.

[Helmholtz]

Yes, but it is this statement that needs some modification: "Aperture corresponds to the magnetized string length." Magnetize a short section of the string half a meter from the pickup, and it contributes very little to the signal. Move the magnetized section closer, and it contributes more. The coil and the pole piece determine how much, and so they are involved in this component of the aperture, although this is not a big effect for magnetization over the coil.

Our discussion made me curious. So I did some radial flux density measurements along the (PU facing) under side of the strings above the PUs of a strat and a P-90 equipped LP. I used a Gaussmeter and also checked with a flux detector foil, which shows the areas where the flux runs parallel to the string.

The results were:
- The aperture lenghts as measured by Zollner roughly correspond to the extension of the primary flux as determined from the points of zero radial flux density.
- The returning flux, having much lower B values than the primary flux, extends much farther and can be detected positively up to distances of over 1 inch from the center of the poles on each side.

I have to realize that „magnetized string length“ is not a useful measure for the aperture, but I am glad I did these measurements. They provide another piece of the puzzle and learning something always makes me glad.

I understand your point that the aperture (as defined by the length of string the PU can read) must be influenced by the PU's directional sensitivity characteristic. But it seems that coil width and core material have no strong influence in real guitar use at least between the PU types measured, while magnet strength and PU/string distance do.

My original point was that aperture is not given by the width dimension of the PU.

I need to mention that I measured DC (static) flux. The paths and distribution of the AC (alternating) flux content may differ.

[Antigua]

Here's a practical question: based on the information at hand, will an 8,000 turn Stratocaster pickup produce a higher voltage if a spacer is placed between the pole pieces and coil? This way, the first wind is not directly against the pole piece, but instead a couple millimeters away from the pole piece. If so, this could constitute a genuine improvement over existing products. 43AWG or a more carefuly wind pattern might be required in order to still fit the cover over the pickup, due to a wider coil diameter.

[Helmholtz]

I would say yes, if you think around 2dB more output justifies the effort. But I guess that making the coil shorter with the magnets protruding from the bottom by 2 or 3mm or so is a more effective way to increase output with the same number of turns. Alternatively you could use double or triple bottoms.

[Antigua]

I would say yes, if you think around 2dB more output justifies the effort.

It certainly does, if you consider that something like a Texas Special is billed as being a hotter pickup, but with only perhaps 500 more turns of wire, just barely realizes a ~2dB difference. This would achieve the same boost without also making the pickup darker (so long as the inductance doesn't climb too much as a result of the wider loop areas).

[rjb]

...Texas Special is billed as being a hotter pickup, but ... barely realizes a ~2dB difference.
This would achieve the same boost without also making the pickup darker...

How much output do you need?
If I'm not mistaken, simple height adjustment can typically realize a >2dB change....

I would wager that, whether they realize it or not, most "players" <snark>(versus "physics weenies")</snark> who opt for "hotter" pickups actually choose them for their "warmer" or "raunchier" (AKA "darker") tone- not for a potential 2dB increase in output level.

-rb

[Antigua]

How much output do you need?
If I'm not mistaken, simple height adjustment can typically realize a >2dB change....

I would wager that, whether they realize it or not, most "players" <snark>(versus "physics weenies")</snark> who opt for "hotter" pickups actually choose them for their "warmer" or "raunchier" (AKA "darker") tone- not for a potential 2dB increase in output level.

-rb

I agree completely, it really is just about getting a darker tone. But if the customer, in their customerly wisdom, says they want output, let them have their output, I say.

[Antigua]

Here is a bode plot comparison between an SD SSL-1, Lollar Special S Middle and a Fender Jazzmaster pickup. The Jazzmaster has a wide and flat coil, while the SSL-1 and the Special S have an average Strat shaped coils. In this test, an external coil drives the pickups, with the coil placed dead center above the bobbin, where there is a whole in the fiber board to accommodate the winding machine's mandrel, so that the vertical distance between the coils is about the same in each case.



The Jazzmaster pickup achieves a higher inductance for a lower DC resistance than the Strat pickups, presumably due to the wider coil area. The Jazzmaster and the SSL-1 are comparable in terms of DC resistances, both being around 6.5k, while the Jazzmaster and Special S are comparable in terms of inductance, both being about 3.1H. The Special-S has a higher DC resistance than either the Jazzmaster or the SSL-1, at 7.5k ohms.

Based on the test, the Special-S only appears to generate 0.9dBV greater output than the SSL-1 at 1kHz, while the Jazzmaster pickup generates 3.1dBV greater voltage than the SSL-1, or 2.2dBV greater than the Special-S. So it seems that when either DC resistance or inductance are matched, the wide and flat Jazzmaster coil still generates more voltage output than the Strat pickup in either case. This makes sense, given that more turns of the Strat coil are located further from the alternative magnetic source, but despite it maybe being obvious, at least this represents a quantifiable demonstration.

Aside from the difference in output, an interesting aspect of the Jazzmaster pickup is that it has a much higher resonant peak, despite also having a higher inductance, which means the intrinsic capacitance is a lot lower. The SSL-1's capacitance calculates out to about 100pF, the Lollar about 90pF, while the Jazzmaster's only comes out to only 30pF. I'm not sure why the capacitance is so much lower. Is a tall, thin coil conducive to capacitance, while a short, fat coil is not?

[Helmholtz]

Is a tall, thin coil conducive to capacitance, while a short, fat coil is not?

Yes, this is a general principle. Short, fat coils have more layers than thinner, longer coils with a similar number of turns. This keeps inner and outer windings farther apart thus reducing distributed capacitance.

[Antigua]

Yes, this is a general principle. Short, fat coils have more layers than thinner, longer coils with a similar number of turns. As a consequence the partial voltage between layers is lower and the total voltage spreads over more layers. Another way to view this is that short, flat coils keep inner and outer windings farther apart thus reducing distributed capacitance.

So greater distances between portions of coil lowers capacitance; a tall, thin coil also puts a lot of distance between the top-most and bottom-most winds, so are you saying that you get less capacitance with the short-flat coil because the starting winds and the ending winds are set farther apart, as opposed to placing the winds-per-layer farther apart?

[Helmholtz]

So greater distances between portions of coil lowers capacitance; a tall, thin coil also puts a lot of distance between the top-most and bottom-most winds, so are you saying that you get less capacitance with the short-flat coil because the starting winds and the ending winds are set farther apart, as opposed to placing the winds-per-layer farther apart?

Yes, it can be shown that the partial capacitances between adjacent turns having larger voltage difference contribute most. The direct neighboring turns within the same layer almost have the same voltage, so their contribution is small. The contribution of the capacitances between neighboring layers is much stronger.