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Thread: Measuring pickups Capacitance?

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    Measuring pickups Capacitance?

    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.

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    Last edited by okabass; 04-12-2018 at 10:45 PM.

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    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.

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    Last edited by Helmholtz; 04-12-2018 at 03:02 PM.

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    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

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    Quote Originally Posted by okabass View Post
    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

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    Last edited by bbsailor; 04-12-2018 at 08:03 PM.

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    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.

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    Last edited by okabass; 04-12-2018 at 10:01 PM.

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    Quote Originally Posted by bbsailor View Post
    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.

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    Quote Originally Posted by Helmholtz View Post

    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.

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    Quote Originally Posted by Antigua View Post
    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 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.

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    Quote Originally Posted by Helmholtz View Post
    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 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:

    Click image for larger version. 

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    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

    Click image for larger version. 

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    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.

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

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    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.

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    Quote Originally Posted by Helmholtz View Post
    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?

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    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.

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    Quote Originally Posted by Helmholtz View Post
    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?

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    Quote Originally Posted by Antigua View Post
    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".

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    Quote Originally Posted by Helmholtz View Post
    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 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.
    The lower apparent inductance measured at frequencies above about 100 Hz is mostly due to eddy currents in metal pickup parts. For example, the cores act like the secondary of a poorly coupled transformer. I write "apparent" because the meter is capable of measuring two numbers, the real and imaginary parts (or amplitude and phase) of the impedance, and therefore can be used to model one reactive component (L or C) and one resistance (series or parallel). A pickup is a more complicated circuit, with the eddy current losses involving loss (resistance) as well as inductance. (The Q of some pickups is determined mostly by the eddy current losses rather than wire resistance.) Therefore the measured inductance above 100 Hz is somewhat in error because a correct measurement requires more than two numbers. At a high enough frequency, the the inductive reactance of the eddy current effect should dominate, and therefore the capacitance as measured at 100 KHz should be very close. But i think it is important to evaluate the error with all this in mind.

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    Regarding capacitance measurements with the DE-5000 at 100kHz, I mentioned some problems in this thread http://music-electronics-forum.com/t46007-3/#post493453 where an SSL-1 measures the same as what could be derived by measuring the inductance and the peak resonant frequency (about 100pF), but a Fender Fat 50 pickup had given a reading that was much too low (about 60pF instead of 120pF). I'm moving this over to this thread since this one is about capacitance and that one is about tapped single coils, which is have issues that go beyond capacitance.

    I created extended impedence plots for both the SSL-1 and Fat 50, directly driving the pickup with a function generator, as opposed to using an external inducer coil.

    Fat 50
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    SSL-1
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    This shows that very near the test frequency of 100kHz, the Fat 50 has some sort of secondary resonance at 98kHz. The SSL-1 has a similar secondary resonance, but it's 153kHz. It appears that the overlap of the DE-5000's test frequency and the secondary resonance prevents the Fat 50 from measuring correctly. Any idea what the source of that secondary inductance and resonance is? Could it be related to the lead wires?

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    Quote Originally Posted by Antigua View Post
    This shows that very near the test frequency of 100kHz, the Fat 50 has some sort of secondary resonance at 98kHz. The SSL-1 has a similar secondary resonance, but it's 153kHz. It appears that the overlap of the DE-5000's test frequency and the secondary resonance prevents the Fat 50 from measuring correctly. Any idea what the source of that secondary inductance and resonance is? Could it be related to the lead wires?
    Without disturbing the lead dress, short the coil and use the DE-5000 to measure the inductance of its own leads. Given that number, how big must the capacitance be to explain the observed resonant frequency? This may be a clue.

    Change the lead dress: First time, with leads twisted together. Second time, with leads as far apart as possible.

    Does bringing a piece of soft ferrite, steel, copper, stainless steel close to the coil have any effect? And so on.

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    What are all these needle artefacts? I don't see them in my impedance measurements. Also the high and low frequency slopes appear not to be correct. Did you measure as I proposed without the field coil?
    The zigzag anomality is the result of an additional series and parallel resonance with higher resonant frequencies. I could show in simulations that such behaviour can be the result of partially shorted windings. Another explanation could be a very sloppy wind, where the winding is not carefully layered but outer turns are used to fill lower spaces thereby causing an uneven distribution of the distributed capacitance. I have not found a way yet to prove this idea wrong or right, as I am not winding.

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    Last edited by Helmholtz; 04-21-2018 at 08:31 PM.

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    Quote Originally Posted by Helmholtz View Post
    What are all these needle artefacts? I don't see them in my impedance measurements. Also the high and low frequency slopes appear not to be correct. Did you measure as I proposed without the field coil?
    The zigzag anomality is the result of an additional series and parallel resonance with higher resonance frequencies. I could show in simulations that such behaviour can be the result of partially shorted windings. Another explanation could be a very sloppy wind, where the winding is not carefully layered but outer turns are used to fill lower spaces thereby causing an uneven distribution of the distributed capacitance. I have not found a way yet to prove this idea wrong or right, as I am not winding.
    I don't know what causes the spikes, I assume some sort of extraneous noise, but patters such as resonances are evident regardless of the noise, so it's not preventing me from conducting tests.

    Those are both interesting possibilities: 1) a particularly uneven layer distribution causing a non uniform distributed capacitance, or 2) an internal short that would essentially create a small shorted coil within the larger coil. I'm not so sure about #1 because the SSL-1 has this second peak too, and they're known to be machine wound.

    #2 seems like an attractive explanation, a short creating a smaller shorted coil within the coil, but that raises more questions, such as how does the short occur, and why would there only happen to be one of them per each tested coil?

    I'll test some more single coils and get more data points on the second high freq. resonances.

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    Please do not use the field coil. It seems to distort the impedance frequency response. The field coil coupling only makes sense for plotting the transfer response, but tends to introduce EMI effects and distorts especially high frequency response.

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    Quote Originally Posted by Helmholtz View Post
    Please do not use the field coil. It seems to distort the impedance frequency response. The field coil coupling only makes sense for plotting the transfer response, but tends to introduce EMI effects and distorts especially high frequency response.
    These recent plots are direct, with a 1meg resistor, no inducer coil.

    I'm gathering more data points now, I just want to keep it all to one post.

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    Last edited by Antigua; 04-21-2018 at 08:58 PM.

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    Here are some of my measurements of strat PUs, some showing anomalies:Click image for larger version. 

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    Please note the straight +/- 6dB/octave slopes below and above resonance. If the anomaly lies at 100kHz, the LCR meter will give a wrong result, as it can only read impedance/admittance at the single 100kHz frequency.
    I used a 100k series resistor, as I did not care for open loop Q and 100K is close to the loading with two 250K pots and 1M amplifier input impedance.

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    Last edited by Helmholtz; 04-21-2018 at 10:04 PM.

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    Quote Originally Posted by Helmholtz View Post
    Here are some of my measurements of strat PUs, some showing anomalities:Click image for larger version. 

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    Please note the straight +/- 6dB/octave slopes below and above resonance. If the anomality lies at 100kHz, the LCR meter will give a wrong result, as it can only read impedance/admittance at the single 100kHz frequency.
    Thanks for providing the plots. It's too bad arbitrary test frequencies can't be specified with the affordable meters. I'd think you could test a handful of higher frequencies and achieve good accuracy that way.

    I'm trying to determine if it could be the lead wires could be involved, but so far it doesn't seem likely.

    I notice your plots show about one prominent anomaly per pickup, all somewhat close together, with similar Q factors, and all above 100kHz.

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    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.)

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    Quote Originally Posted by Helmholtz View Post
    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.)
    I'm testing a Lollar Blackface neck, so far I'm up to 200kHz with no anomaly. The Fralin and the Lollar and hand guided pickups, where as the Fat 50 and SSL-1 are high volume production pickups, there might be something to that.

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    Quote Originally Posted by Joe Gwinn View Post
    Without disturbing the lead dress, short the coil and use the DE-5000 to measure the inductance of its own leads. Given that number, how big must the capacitance be to explain the observed resonant frequency? This may be a clue.
    The anomalies are observed strictly with the Velleman bode plotter, and seem to vary from pickup to pickup, so I think the rig itself is mostly ruled out.

    Quote Originally Posted by Joe Gwinn View Post
    Change the lead dress: First time, with leads twisted together. Second time, with leads as far apart as possible.

    Does bringing a piece of soft ferrite, steel, copper, stainless steel close to the coil have any effect? And so on.
    That's a good idea about twisting the lead wires. I gave that a try and it didn't change the frequency at which the anomalous peak occurred.

    I tried putting 470pF across the pickup, the resonant peak dropped form ~7kHz down to 3.9kHz, but the anomolous peak only appeared to drop very slightly, from 98kHz down to around 94kHz, with a Q that was lower by about half. Those higher frequency figures are sort of rough estimates, as you can see from the rather low resolution of the plot images. Cap values higher than 470pF seem to drown out the anomalous resonance.

    As for placing permeable materials around the pickup, with steel Tele baseplates I've only ever been able to increase the inductance by about 150mH, so I don't think that wouldn't alter the circuit much. If theres value in seeing how inductance changes the anomalous peak, the better trick would probably by to find an steel pole Strat pickup with the anomaly, and then remove the pole pieces. That's easier said than done though, so I'd only do that if there were a hypothesis in place first.

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    Quote Originally Posted by Helmholtz View Post
    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.)
    The pickup is loaded with about 500 pf and played through a system with about 5KHz bandwidth. It is hard to believe that anomalies at about 100 KHz have any effect on the sound.

    It is also hard to believe that old Fender pickups have any particular special qualities. The brilliance of Fender pickups as compared to for example, humbuckers, is the result using Alnico cores that have lower conductance than steel. I suppose you could argue that the Alnico produced then has different eddy current losses than that produced now, but I have my doubts that this is a significant effect.

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    The pickup is loaded with about 500 pf and played through a system with about 5KHz bandwidth. It is hard to believe that anomalies at about 100 KHz have any effect on the sound.
    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.

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    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:

    Click image for larger version. 

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    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.




    Quote Originally Posted by Helmholtz View Post
    Right, the sound difference described can hardly be explained by the shown frequency responses. But I thought I should mention it nevertheless.
    You have to account for cognitive bias in order to draw a conclusion with respect to hearing.

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    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).

    Quote Originally Posted by Antigua View Post
    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:

    Click image for larger version. 

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    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.

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    Quote Originally Posted by Helmholtz View Post
    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.

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    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.

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    Quote Originally Posted by Helmholtz View Post
    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.

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    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.

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