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  • #16
    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 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.
    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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    • #17
      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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      • #18
        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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        • #19
          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.
          Last edited by Helmholtz; 04-21-2018, 07:31 PM.
          - Own Opinions Only -

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          • #20
            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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            • #21
              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.
              - Own Opinions Only -

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              • #22
                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.
                Last edited by Antigua; 04-21-2018, 07:58 PM.

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                • #23
                  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.
                  Last edited by Helmholtz; 04-21-2018, 09:04 PM.
                  - Own Opinions Only -

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                  • #24
                    Originally posted by Helmholtz View Post
                    Here are some of my measurements of strat PUs, some showing anomalities:[ATTACH=CONFIG]48574[/ATTACH]
                    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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                    • #25
                      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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                      • #26
                        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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                        • #27
                          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.

                          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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                          • #28
                            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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                            • #29
                              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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                              • #30
                                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.




                                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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