Measuring pickups Capacitance?

[Mike Sulzer]

The capacitance of a coil is a surprising subject. The capacitance of a single layer coil is a transmission line effect, not what you might think. A multilayer coil is complicated, and there is certainly some effect from electrostatic coupling between layers. But it might be with certain winding techniques you effectively get a multiple component circuit perhaps involving more than one transmission line (with capacitive reactance).

I measured four other machine wound Fender pickups, three from a Mexican Strat with a DCR or 6.9k, and one from Japan with a DCR of 5.4k, the three Mexican pickups showed single secondary peaks of 114kHz, 125kHZ and 150kHz, and the Japan made single coil have a second peak at 150kHz.

I made a LTSpice model that seems to result in a similar plot, it has another resonant coil in series with the lumped capacitance. Maybe this models what is happening, maybe not:

[ATTACH=CONFIG]48582[/ATTACH]

Another possible clue is that this doesn't appear to be something that effects inductors in general, as far as I can tell from Google searches, so aspects that make a pickup like most ordinary inductors can probably be ruled out, leaving qualities that are more specific to Stratocaster pickups.

The second peak frequency is definitely specific to the pickup, if I test the same pickups a second time, they show the same second peak. I've tried fiddling with the lead wires, but moving them around, twisting them, etc. doesn't make any difference.

All four tested pickups have steel pole pieces, and the three Mexican pickups are more or less identical in shape and size and probably turn count. If the pole pieces were a factor, I'd think the frequencies would all be a lot closer.

It's interesting that the second resonance varies from 100kHz to 150kHz from pickup to pickup. These machine wound pickups have coils that are rather flat, so it's likely the traversal is fairly uniform, and in fact the "hand guided" pickups don't seem as likely to have the anomalous peak. So oddly enough, the pickups with a uniform manufacturing method show a randomness in this second peak, where as the hand guided pickups, with randomly laid wire, are possibly more uniform, insofar as they don't have this peak, though it could be the case that's it's just at a much higher frequency and/or the resonance is suppressed to the point of being unobservable.






You have to account for cognitive bias in order to draw a conclusion with respect to hearing.

[Mike Sulzer]

Right, the sound difference described can hardly be explained by the shown frequency responses. But I thought I should mention it nevertheless.

If it wasn't for such hard to understand sound effects, I would not waste my time (and some money) with countless parameter and response measurements, material analyses, simulations, literature researches, listening tests and so on. The standard PU filter response and parameter measurements can get quite boring and frustrating over time, as they often don't vary much and can explain only part of the PU's sound. But I am a physicist and want to find out.

An excellent goal, IMO. My own bias is to look near the resonance for effects that can alter the harmonic content from what one might expect. For example, a pickup with steel cores must have a somewhat non-standard impedance shape around the resonance because of frequency varying eddy current losses (and reactance). I have not yet tried to compare the measured shape to what one expects with no eddy current losses, but it is a project for the future. My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.

[Helmholtz]

The capacitance of a coil is a surprising subject. The capacitance of a single layer coil is a transmission line effect, not what you might think. A multilayer coil is complicated, and there is certainly some effect from electrostatic coupling between layers. But it might be with certain winding techniques you effectively get a multiple component circuit perhaps involving more than one transmission line (with capacitive reactance).

Thanks, this is exactly the direction/background of my second possible explanation. Now we need a volunteering winder who is willing to produce some extreme samples for measuring and verification purposes. In the meantime I will try to master transformers with varying degrees of coupling in LTSpice (being a beginner still) to simulate differently coupled parts of the PU coil.

[Antigua]

BTW, the one with almost no anomaly (Fralin) sounds by far best to me. It is the only one that has the great brilliance of a good vintage strat PU. (I own a set of original '59 strat PUs for reference.)

You have the Velleman setup as I do, it would be cool if you could share plots of the 59's you have on hand. I'm also curious to know what the inductance values are.

Someone mentioned the AlNiCo grades varrying in the 50's, I've received some pickups from China that supposedly had AlNiCo 5 pole pieces, but showed a Br value that was about 10% below expected, and yielded a higher Q factor than expected, due to higher resistivity. I don't know what made that Chinese AlNiCo different, be it a composition difference, or a production difference (though I'd think composition on account of the resistivity issue). Maybe the AlNiCo in your 59's has a similar sort of difference. I'd also be curious to know what the flux reading is at the tops of the pole pieces.

Regarding the notion that they sound subjectively better, there is an obvious bias to favor all things vintage, which might owe to nostalgia, or simple scarcity or it's perceived worth to others, or the famous guitarists who promote vintage gear, such Eric Clapton and Keith Richards. This bias impacts nearly every aspect of electric guitar, including caps, pots, hookup wire, body wood, the finish coat, steel hardware, even guitar amps and effects pedals. It seems that guitar manufactures just can't make a great guitar anymore, can they? It's such a pervasive and enduring bias that will power alone can't assure impartiality.

[Helmholtz]

You have to account for cognitive bias in order to draw a conclusion with respect to hearing.

I am well aware of possible cognitive bias. My ears and the attached computer are certainly biased by my personal sound preferences. But I have absolutely no desire to fool myself. More than often my listening tests did not confirm my (biased) expectations. Still, what counts in the end is sound and not measurements. I am not trying to persuade anybody to trust my assessments, instead I encourage everyone to do his own listening comparisons/evaluations and not rely on data only. But of course having and understanding measurement data helps to narrow down the variety of candidates.
It would be naive, though, to assume the a PU's total transfer reponse could be completely descibed by a simple linear passive low pass filter composed of lumped elements. A PU is essentialy a non-linear transducer. Sorry for going astray.

[Helmholtz]

Someone mentioned the AlNiCo grades varrying in the 50's, I've received some pickups from China that supposedly had AlNiCo 5 pole pieces, but showed a Br value that was about 10% below expected, and yielded a higher Q factor than expected, due to higher resistivity. I don't know what made that Chinese AlNiCo different, be it a composition difference, or a production difference (though I'd think composition on account of the resistivity issue). Maybe the AlNiCo in your 59's has a similar sort of difference. I'd also be curious to know what the flux reading is at the tops of the pole pieces.

Give me some time to look up my notes. Not going to dissect my old strat at the time, so no frequency plots. But I took resistance, inductance, capacitance and B values. Will send PM eventually.

[Antigua]

It would be naive, though, to assume the a PU's total transfer reponse could be completely descibed by a simple linear passive low pass filter composed of lumped elements.

It's not naive, it's a real possibility. I'd even argue it's the more probable possibility. You're well versed in scientific principles regarding physics, but you're not giving much regard to the necessity of double blind testing.

[Helmholtz]

You're well versed in scientific principles regarding physics, but you're not giving much regard to the necessity of double blind testing.

How could you know? I consider this a disrespectful allegation and a personal offense. Of course in the end my pickups have to appeal to me.

I hope you do extensive (double blind) listening tests with trained ears and experienced players- and not only rely on measurements.

[Antigua]

Here are extended impedance plots of four pickups I had made at home, which were intentionally wound with low uniformity and varying degrees of tension, including one which is a loose mess. Each is 8,000 turns with a DC resistance right around 6k ohms.










(the big messy pickup)


The earlier theory that a less consistent pickup might exhibit fewer or no anomolous high secondary resonant peaks appears to be squashed, as some of these pickups show several such peaks. The large, bushy Strat pickup which could never fit a cover over the top, has two very prominent secondary resonances, one at 86kHz and another at 153kHz. Two others show a single dominant secondary resonance, with additional peaks that look like ripples. Only one of the four, #3, appears to have no secondary resonance, although there is possibly a subtle knee at 64kHz.

Based on this information, I wondering if there is some sort of event that occurs randomly in the production of a pickup, one that causes multiple, smaller resonant circuits to appear within the coil.

Maybe it has to do with a segment of wire being laid, which comes physically very close to a segment of the coil that is many turns removed. For example, sometimes at the ends of the coil, you get crevices between the coil and the flat work, because the wire doesn't come right up to the edge of the flat work as it traverses back and forth. Then, at some random point, a segment of wire will manage to slip down into the crevice, which would cause that segment of wire that slipped down to be side be side with a portion of the coil that is maybe several hundred turns removed from itself.

While it's true that these higher peaks don't manifest in the audible frequency ranges, I think it's important to figure out what causes them, since it is a feature of the pickup all the same. It might even serve as a clue to indicate how a coil was made, without having to disassemble the coil to look at it directly.

[Antigua]

How could you know? I consider this a disrespectful allegation and a personal offense. Of course in the end my pickups have to appeal to me.

I hope you do extensive (double blind) listening tests with trained ears and experienced players- and not only rely on measurements.

In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.

The easiest way to marry specs with subjective experience would be to, not only have a blind fold and a friend help you conduct an test free of extrinsic factors, but to also have as many specifications as possible about those pickups. Unfortunately, only the DC resistance is readily available, so I'm working towards providing other guitarists more extensive specs for popular pickups, including capacitance measures.

[Helmholtz]

Maybe it has to do with a segment of wire being laid, which comes physically very close to a segment of the coil that is many turns removed. For example, sometimes at the ends of the coil, you get crevices between the coil and the flat work, because the wire doesn't come right up to the edge of the flat work as it traverses back and forth. Then, at some random point, a segment of wire will manage to slip down into the crevice, which would cause that segment of wire that slipped down to be side be side with a portion of the coil that is maybe several hundred turns removed from itself.

This is what I ment. I suppose that the effect requires a contiguous portion of the winding with a discontinuity in magnetic coupling and/or distributed capacitance. Magnetic coupling decreases with distance from the center/core. Distributed capacitance changes with the arrangement of the turns.
I tend to think that the effect would show strongly (while at lower frquency) in a bifilar wound coil with the two windings wired in series. But this is just speculation at this point.

[Helmholtz]

In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.

I don't quite understand what you are talking about. Probably due to my limited command of English. Not sure if I really need to know. Anyway, I prefer technical arguments/discussion.

But it definitely does not sound like an excuse to me.

[okabass]

Great info on the thread. Not that witchcraft or caveman level physics, which is so common when you read web pickup forums.
Thanks.

[Helmholtz]

My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.

This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?

[Antigua]

My favorite way to measure pickup impedance is to put a resistor (say 2K) in series and drive this series combination with a signal with a broad frequency spectrum, sampling across the series combination, and across the resistor. Then with suitable processing, you can get the voltage across the pickup and the current through it. The ratio is the impedance with essentially no loading effects. You can measure hundreds of frequency points at once, giving a very good measurement of the frequency variation.

This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?

This sounds like what we've been doing in this thread, using the VElleman bode plot and function generator, similar to what is described here http://www.syscompdesign.com/assets/...ar-pickups.pdf but using a 1meg resistance instead of 56k.

Another option is to use the Velleman's frequency sweep with persistance, which yields the peak like this:



I though there was a way to feed it white noise with the function generator, which would reveal a peak in the FFT view even more quickly, but I'm not seeing the option.

This is what I ment. I suppose that the effect requires a contiguous portion of the winding with a discontinuity in magnetic coupling and/or distributed capacitance. Magnetic coupling decreases with distance from the center/core. Distributed capacitance changes with the arrangement of the turns.
I tend to think that the effect would show strongly (while at lower frquency) in a bifilar wound coil with the two windings wired in series. But this is just speculation at this point.

I remember you mentioning this possibility. If this is what is going on, it should be possible to model with LTSpice somehow. I'll work on that more later.