Measuring pickups Capacitance?

[okabass]

It seems that PU's capacitance is not so easy to measure. I think normal method is to define the resonance frequency and inductance and then calculate the capacitance.
It would be nice if you could get the capacitance easier to calculate the resonance frequency and have a feeling how the PU will behave.

I thought that not so sophisticated or brutal method would be to make a tap at half point the coil. Then wind the rest. Then disconnect or cut the coil at half tap, measure the capacitance at the start and end.

So if you are winding 10000 turns, tap it at 5000 turns. Wind the rest, cut the wire at the tap, measure the capacitance, solder the wire back.
Never tried, but if someone has, please tell if it doesn't work.

[Helmholtz]

Sorry, your method will not give useful information on the capacitances that determine the PU's resonance(s). The equivalent circuit of your arrangement consists of the series wiring of two coupled windings of a transformer with distributed and reflected capacitances and a rather small interwinding capacitance (<50pF or so) in between. (Please see thread titled "Switchable additional winds"). The result will be more or less the interwinding capacitance. But this does not relate to any PU resonance.

You can measure PU capacitance with an LCR meter that allows measuring at 100KHz. With typical high impedance PUs the accuracy is very good. The absolute measuring error (always negative) caused by the presence of inductance is below 3pF for a PU inductance of 1H and decreases proportionally with higher inductances.

Generally the importance of PU capacitance on the frequency response seems overrated. Its impact on the loaded resonant frequency is much smaller than inductance. A difference of 100pF corresponds to a 3 foot change in length of a typical guitar cable. Thus cables of different lengths can be used to evaluate audible effects of different PU capacitances.

[okabass]

HI

Thank you. Really informative answer. I thought that if the measuring the cap. is so easy, it have been used.

Yes the pots, cable etc. impact is very big: several kHz. It is surprisingly big, when you first time notice the difference with lunloaded and loaded PU. Like:
http://i.imgur.com/ryNSHbQ.png

[bbsailor]

It seems that PU's capacitance is not so easy to measure. I think normal method is to define the resonance frequency and inductance and then calculate the capacitance.
It would be nice if you could get the capacitance easier to calculate the resonance frequency and have a feeling how the PU will behave.

I thought that not so sophisticated or brutal method would be to make a tap at half point the coil. The wind the rest. Then disconnect or cut the coil at half tap, measure the capacitance at the start and end.

So if you are winding 10000 turns, tap it at 5000 turns. Wind the rest, cut the wire at the tap, measure the capacitance, solder the wire back.
Never tried, but if someone has, please tell if it doesn't work.

Pickups sound a certain way due to the unintended consequences of winding enough wire on the pickup to generate enough voltage to drive the input stage of early guitar amplifiers which were derived from tube- based PA amplifiers. These amplifiers typically had a high impedance of about 1M ohm and the consequence of this high impedance design has audible effects. This early design has not evolved much. Les Paul recorded his "Recording" model guitars directly into mic inputs using low impedance pickups without the typical electric guitar sound. Below is the reason for the electric guitar sound.

1. The amp input acts as a bridging input impedance that is about 10 times higher than the source impedance to minimize loading to get the maximum input voltage. At pickup resonance, this bridging impedance affects pickup loading the most as the pickup resonance point is where the pickup impedance is the highest.

2. Using thin magnet wire AWG 42 or 43 allows many thousand turns to be placed on the bobbin to get a high enough output voltage to drive the amplifier required input level. Since this thin wire also has a very thin non conductive coating, there is a very slight capacitance between adjacent strands laying in very close proximity to each other. Multiply this very slight capacitance by the thousands of pickup turns and you begin to get enough capacitance to form a resonant peak in the region of human hearing where the ear is most sensitive. See this link https://sonicscoop.com/wp-content/up...son-curves.png

3. Hand winding pickups puts more air between the winds than machine winding where the wire density is more compact. Thus, hand winding has lass capacitance than machine winding. Typically, pickups have between 80 pf to about 160 pf of capacitance but this is heavily overshadowed by the typical guitar cable capacitance of about 35 pf per foot or about 350 pf for a 10 foot cable. To hear what a guitar sounds like look up "Tillman buffer amplifier" where you can see how to build a JFET in the guitar end of a guitar cable to put a 1Meg to 3 Meg ohm input load right at the guitar end of the cable and also isolate the pickup from the cable capacitance to hear a higher shift in the natural pickup resonance due to the elimination of cable capacitance effects.

4. Guitars have a fundamental frequency range from 82 Hz on the open low E string to 1312 HZ on the 24th fret of the high E string. However, guitar pickups respond to the way a string is strummed as the horizontal movement affects the 2nd harmonic (twice the fundamental frequency) more because when a string passes a pickup pole piece it generates a peak voltage each time is passes the pole piece. Strings typically oscillate in an oblong pattern with both horizontal and vertical movement and depending on which pickup you use, where you strike the strings and how you strike the strings can have an audible impact on the initial string movement and sound. The first 30 up to 50 milliseconds has the most hearing impact on the perceived quality of the sound. Try this simple experiment either by just listening or by listening and observing the pickup output on an oscilloscope. Turn on the neck pickup. Pinch the low E string over the pole piece and pull sideways about .125 inch and then let go. Now, observe and listen the very first few oscillations. Pinch the string again in the same place above the pickup pole piece and pull it up about .125 inch and release (or a little less for low action guitars) and now listen and observe the sound. This vertical pick should sound a little louder as the vertical string motion contributes more to string output level then horizontal string motion. Try it on the other open strings also.

As you can see, pickup capacitance, cable capacitance and other loading (amp input impedance, volume controls and tone controls) can contribute to the perceived sound but there are more variables involved. Playing at a low level where the ear is most sensitive between 3 KHz to 5 KHz and the bass frequency sensitivity is very low is a lot different than when playing in a band where the typical listening level is higher and the threshold of hearing sensitivities becomes more flat across the guitar fundamental and harmonic range up to about 5 KHz where the typical high impedance pickup response tends to quickly fall off after the pickup resonant peak.

I hope this helps?

Joseph J. Rogowski

[okabass]

HI

Thank you for your in-depth answer.

I've wind only some dozens PU as hobby, but it is nice to have theory also. My interest is now in 51 7ender Precision bass PU type. It is quite simple but sounds surprisingly good.Try to find a good balance with clarity and lo end. Now It feels that around 10000 turns 42 AWG (7,7 kΩ), 3,3 Henrys is quite close.

[Antigua]

4. Guitars have a fundamental frequency range from 82 Hz on the open low E string to 1312 HZ on the 24th fret of the high E string. However, guitar pickups respond to the way a string is strummed as the horizontal movement affects the 2nd harmonic (twice the fundamental frequency) more because when a string passes a pickup pole piece it generates a peak voltage each time is passes the pole piece. Strings typically oscillate in an oblong pattern with both horizontal and vertical movement and depending on which pickup you use, where you strike the strings and how you strike the strings can have an audible impact on the initial string movement and sound. The first 30 up to 50 milliseconds has the most hearing impact on the perceived quality of the sound. Try this simple experiment either by just listening or by listening and observing the pickup output on an oscilloscope. Turn on the neck pickup. Pinch the low E string over the pole piece and pull sideways about .125 inch and then let go. Now, observe and listen the very first few oscillations. Pinch the string again in the same place above the pickup pole piece and pull it up about .125 inch and release (or a little less for low action guitars) and now listen and observe the sound. This vertical pick should sound a little louder as the vertical string motion contributes more to string output level then horizontal string motion. Try it on the other open strings also.

I came up with a way to demonstrate the horizontal vs. vertical string movement difference. You abut the guitar string with a guitar pick (or something similar), directly over the selected pickup, and then pull and release the string with your fingers, while the string is still against the pick. The presence of the pick forces the string to only move along one axis. Of course, the friction between the pick and the string causes the string to stop moving very quickly, but for the brief transient, you can hear the differences. If the guitar pick is parallel to the face of the guitar, or "side to side", and you release the string, you hear almost nothing. If you re-orient the guitar pick so that it's perpendicular to the face, or "up and down", year hear the transient very clearly. This shows how one axis of string movement produces a clearly audible signal, while movement along the other axis does not.

[Antigua]

You can measure PU capacitance with an LCR meter that allows measuring at 100KHz. With typical high impedance PUs the accuracy is very good. The absolute measuring error (always negative) caused by the presence of inductance is below 3pF for a PU inductance of 1H and decreases proportionally with higher inductances.

How interesting. The DE-5000 has a 100kHz setting. I've calculated the capacitance of dozens of pickups based on the peak frequency. I'll give C measurements a try with a DE-5000 and see how close it comes to those peak f based measurements.

[Helmholtz]

How interesting. The DE-5000 has a 100kHz setting. I've calculated the capacitance of dozens of pickups based on the peak frequency. I'll give C measurements a try with a DE-5000 and see how close it comes to those peak f based measurements.

Just make sure to select parallel mode to measure Cp at 100kHz and serial mode to measure inductance (Ls) at low frequency.

If you find a noticeable deviation from the capacitance calculated via frequency response, the reasons are:

1) Inductance at resonance frequency is lower than the value measured at a lower frequency. This is typical for PUs with high µ steel cores, where inductance decreases with frequency caused by the magnetic skin effect. An indication is the 1kHz inductance being lower than the 100Hz value.

2)The peak frequency of a low Q parallel resonant circuit is not identical to the theoretical resonant frequency. (I think this is discussed in Terman's book)

In any case the 100kHz measuring gives the most reliable result.

[Antigua]

Just make sure to select parallel mode to measure Cp at 100kHz and serial mode to measure inductance (Ls) at low frequency.

If you find a noticeable deviation from the capacitance calculated via frequency response, the reasons are:

1) Inductance at resonance frequency is lower than the value measured at a lower frequency. This is typical for PUs with high µ steel cores, where inductance decreases with frequency caused by the magnetic skin effect. An indication is the 1kHz inductance being lower than the 100Hz value.

2)The peak frequency of a low Q parallel resonant circuit is not identical to the theoretical resonant frequency. (I think this is discussed in Terman's book)

In any case the 100kHz measuring gives the most reliable result.

It works! I don't know how you knew this would work, but you're exactly correct. I measured a Seymour Duncan SSL-1 neck pickup, and got the exact same capacitance value 103pF , reported here a few months ago Seymour Duncan SSL-1, Analysis and Review | Fender Stratocaster Guitar Forum

Here's a pic of the measurement:



Same story with a Seymour Duncan SH-1N or "'59 neck", measured 98pF here Seymour Duncan SH-1N "59" Neck, Analysis & Review | GuitarNutz 2 , showing 99pF with the DE-5000



100kHz was the only test frequency that worked. For the SSL-1, at 10kHz, it shows 4.86pF in PAR, 39pF in SET, both of which are way off, and lower frequncies are increasingly off, showing capacitance values in the nano-farad range. At 100kHz, switching between SER and PAL shows the same value to within 1pF, for bot the SSL-1 and the SH-1N, so while I assume there is a good reason to use PAL mode, SER appears to be very close as well.

Note in the picture that I have the lead wires far apart from each other, and not touching, as when they're brought close together, you instantly see the capacitance rise by a few picofarads.

Since it appears the DE-5000 reliably measures capacitance as well as inductance (only SER mode is suitable for inductance, and the lowest frequency setting of 100Hz is most accurate), and that these value agree with those derived from impedance plots, it's therefore possible to calculate the resonant peak of the pickup using nothing more than the DE-5000.

The resonant peak itself is rarely useful, since in an electric guitar, a high degree of additional capacitance will bring the true resonant frequency down to a much lower value. For the surveying of pickups I've done, I also include resonant peaks with a 470pF capacitor across the pickup, which is intended to represent a guitar cable, and give a more realistic representation of what the pickup will do in situ. I will try putting a 470pF cap across the pickup, then I'll measure the capacitance again, and see how closely the capacitances sum, as well as determine how closely the calculated resonant peak "with load" comes to the peak measured with an oscilloscope. If it turns out the DE-5000 can effectively acquire all these data points, then the only advantage that would remain for bode plot measurements is in determining how much resonant damping occurs due to eddy currents.

Thanks again for tipping me off to the fact that an LCR meter can calculate the capacitance of a pickup at 100kHz, this is very valuable information to have.

[okabass]

Thank you for the tip Hemholtz and Antigua. Just ordered DE-5000 LCR meter from eBay.

[Helmholtz]

You are welcome.

I knew that it would work before I bought an LCR meter with the 100kHz option some years ago. My considerations were based on understanding of:

- the PU's equivalent circuit
- the measuring principle of LCR meters in series and parallel modes.

The measuring error can be estimated from the formula for the apparent capacitance in parallel resonant circuits, which can be derived from the imaginary part of the admittance (1/Z). It is essential that the measuring frequency is far above the circuits resonance, where the frequency response of the impedance shows a clear -6dB/octave (= purely capacitive) behavior.
A measuring frequency closer to resonance (i.e. 10khz) will give a lower meter reading, as the meter shows apparent capacitance. The latter decreases towards resonance and = 0 exactly at resonance.

[Antigua]

You are welcome.

I knew that it would work before I bought an LCR meter with the 100kHz option some years ago. My considerations were based on understanding of:

- the PU's equivalent circuit
- the measuring principle of LCR meters in series and parallel modes.

The measuring error can be estimated from the formula for the apparent capacitance in parallel resonant circuits, which can be derived from the imaginary part of the admittance (1/Z). It is essential that the measuring frequency is far above the circuits resonance, where the frequency response of the impedance shows a clear -6dB/octave (= purely capacitive) behavior.
A measuring frequency closer to resonance (i.e. 10khz) will give a lower meter reading, as the meter shows apparent capacitance. The latter decreases towards resonance and = 0 exactly at resonance.

Since most inductors have a much lower inductance, in the mH range, and would have a very high resonant peak, is it somewhat unusual to be able to find the parasitic capacitance of an inductor with an LCR meter, even if it does have a 100kHz test range?

[Helmholtz]

I am not sure, if I understand your question.

As noted earlier, the measurement error increases with 1/L and the measuring frequency has to lie in a frequency region, where the impedance drops proportionally with increasing frequency (i.e. -6dB/octave). In practice this means that the measuring frequency (100kHz) should be at least about a factor 10 higher than the resonant frequency, as a rule of thumb.

[Antigua]

I am not sure, if I understand your question.

As noted earlier, the measurement error increases with 1/L and the measuring frequency has to lie in a frequency region, where the impedance drops proportionally with increasing frequency (i.e. -6dB/octave). In practice this means that the measuring frequency (100kHz) should be at least about a factor 10 higher than the resonant frequency, as a rule of thumb.

So if 100kHz needs to be 10x above the resonant frequency, so this means 10kHz would be the highest resonant frequency for which this would work with accuracy. AFAIK, very few inductors have a resonant frequency that is as low as 10kHz, and that guitar pickups are special in that that have a very high inductance, generally above 2 henries. In a way, that means this is sort of a "trick", in that that this method would not work for most inductors, just those with especially low resonant peaks, and so we've "lucked out", in a sense.

Thanks again for participating here, your insights have really moved the ball forward in my pickup research. Are you involved with Gitec?

[Helmholtz]

So if 100kHz needs to be 10x above the resonant frequency, so this means 10kHz would be the highest resonant frequency for which this would work with accuracy. AFAIK, very few inductors have a resonant frequency that is as low as 10kHz, and that guitar pickups are special in that that have a very high inductance, generally above 2 henries. In a way, that means this is sort of a "trick", in that that this method would not work for most inductors, just those with especially low resonant peaks, and so we've "lucked out", in a sense.

Thanks again for participating here, your insights have really moved the ball forward in my pickup research. Are you involved with Gitec?

I would not call it a trick, just applied physics. Fortunately it works fine with typical high impedance PUs. You could use the method for resonant frequencies above 10kHz but would need to calculate the error caused by inductance and compensate. Inductors with even higher resonant frequencies require meters/equipment with higher measuring frequencies.
I know GITEC and had some contact. They don't seem to be interested in exchange/communication with people who refuse to become a member of the club. But I highly appreciate Manfred Zollner's book "Physik der Elektrogitarre".