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Post by stratotarts on Dec 4, 2016 16:36:54 GMT -5
Edit: There is hum cancellation in the secondary, but not the primary and also in the primary. It took me a while to see it.
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Post by stratotarts on Dec 1, 2016 15:38:14 GMT -5
The 0.31nF is not a typo, it includes the test cable capacitance as well as the pickup capacitance. In this test, the new unloaded resonant frequency was 4425 Hz. Using the ratio method as you did, you have 6.73nF/(4425/930)^2 = 297pF which comes close to 0.31nF.
I start with a close approximation of the inductance, which can be obtained from any of the swamped measurements. In those cases, the capacitance is a small fraction, so the error is not great. Using that and the unloaded resonant frequency, I obtain an estimate of the intrinsic capacitance. Since the inductance is already known with fair accuracy, the intrinsic capacitance is therefore known with fair accuracy. I add this quantity to the capacitance of the swamping cap, to accurately reflect the load when subsequent calculations are made with the different swamping caps. Any error in the original estimate of inductance is divided by the ratio of swamped to intrinsic capacitance.
This procedure is open loop, not requiring any values that have been calculated, to replace earlier ones. Perhaps there is an algebraic method of doing it in one step, but I am not seeing it right now (my math abilities are often strained, so I generally avoid the effort unless there is a big payback). I added a column to my spreadsheet to accommodate this improvement.
I think that testing with 100nF was interesting because the error is inherently small due to swamping, and the frequency is lower, which some may prefer. The question of whether the inductance remains the same at higher frequencies remains unproven, we have not been able to verify it because there is a chicken-and-egg dependence between intrinsic capacitance and inductance at such frequencies. At the moment, my approach is to assume that inductance remains relatively constant vs. frequency. If the 6 part model predicts something different, then we do have a bit of mystery.
Without that assumption, we would not have a way to know whether we see a high inductance and a low capacitance, or a low inductance and a high capacitance at high frequencies. That is because the inductance can not be reliably isolated at high frequencies, and capacitance can not be reliably isolated at low frequencies.
Perhaps you could find out more with a network analyzer, but those are expensive.
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Post by stratotarts on Dec 1, 2016 12:22:08 GMT -5
I wanted to nail down the inductance measurements of the SH1n at different frequencies. So I bypassed the integrator, and ran some plots with different swamping capacitor values. I measured all the test caps with a meter, and then subtracted the aggregate capacitance due to cable and pickup internal capacitance (calculated from a known estimate of L and an unloaded measurement of F). It does appear that the measured inductance does not vary much when measured at different frequencies, with different chosen swamping cap values. For the benefit of non-members who can't enlarge the charts, the result is: Frequency Inductance --------- ----------- 310 4.2 420 4.1 715 4.0 930 4.2
Previously, I was not applying recursive calculation to my measurement with the integrator. When I do that, I compute 4.3H measured at 1150Hz (using the 4.7nF load which actually meters out to 4.3nF). So all in all, I feel that the integrator measurement must be quite close. For some unknown reason, the swamped readings don't work properly with the integrator at the lower frequencies. If I want wide range inductance measurements, I will probably have to build a permanant test fixture to repeat what I've done here in a convenient fashion.
The problem with understanding how the Extech meter fits into this, is not knowing exactly how it makes measurements at the settings of 100 and 1000 Hz. Although I didn't go below 310Hz, from what I saw, the Extech should read almost the same value for this pickup at the two settings. I think we are going to have to do some comparisons that will qualify the Extech readings, in order to come up with reliable inductance measurements.
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Post by stratotarts on Nov 29, 2016 13:46:18 GMT -5
The V5 integrator is not perfectly flat at 100Hz, so the "droop" there is an integrator artifact, not a property of the pickup. Until now, I didn't think much about it because sub 1dB measurements in that frequency range aren't usually important.
The larger the swamping capacitor, the more accurate the inductance measurement is, because the intrinsic capacitance will be proportionally smaller. But it will be measured at a correspondingly lower frequency. That frequency will be much lower than any frequency at which the results will be used to predict important behaviour. The outcomes of measurements and calculation should determine the choice of target measurement values. For example, when attempting to predict the loaded resonant frequency and Q, given certain external components, the L and C that you want to use is the value of L and C at that target frequency. I think we all believe that the C does not vary much. The L does vary, so the best predictor of L is a value of L that was measured at the closest frequency to the target frequency.
I've been aware that measuring L at different frequencies has been an outstanding task, and this problem makes it more urgent. I think that the impossibility of measuring L and C independently, dictates either assuming a relatively constant L with frequency, or using successive approximation. It would be more reassuring to know that at least one parameter can be measured accurately if other parameters must be computed using its value.
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Post by stratotarts on Nov 29, 2016 11:40:54 GMT -5
The V5 integrator circuit does not perform well when attempting to measure low frequency peaks such as are produced by using a large value swamping capacitor. Here is the result with a 68nF test cap: In order to avoid clipping, the cal value for the 68nF sweeps had to be lowered. In spite of that, none of the integrated sweeps were accurate. The measured loaded resonant frequency with the integrator is 170Hz and with that capacitor, the calculated inductance is 11H. The other plot with a more modest capacitance of 4.7nF indicates 4.44H. I am not sure of the reason, but my guess is that the dynamic range is always exceeded. I bypassed the integrator and took a reading (yellow plot) which yields a more believable frequency of 275Hz. That produces a result of 4.19H for the inductance (4.93H with 68nF, for those calculations I used the 81nF that my meter showed when measuring the test capacitor). The test capacitor in the integrator is measured with the same DMM. So I believe it's safe to conclude that the inductance didn't really change very much between 1150Hz and 275Hz. It has been established via successful modeling that inductance does vary with frequency. It would take more refined testing to establish the exact variation. A single-value inductance specification would be most useful when measured at the same frequency where it is applied. In most cases, that is the loaded resonant frequency. 1000Hz is closer to that than 125Hz. So I would be more inclined to take the a reading close to the 1000Hz value as the "real" or working value. Here you can see by comparison of integrated and non-integrated plots that at 1150Hz at least, the integrator is accurate:
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Post by stratotarts on Nov 28, 2016 12:12:16 GMT -5
Device Circuit under Test : SD SH-1N ‘59 model Test Date : Nov 27, 2016 DC resistance (Ohms) : 7,390 Self Resonant Frequency (Hz) : 6420 Self Resonant Peak (dB) : 6.4 Loaded Resonant Frequency (Hz) : 2980 Loaded Peak (dB) : 2.4 Inductance Test Resonant Frequency (Hz) : 1152 Inductance (H) : 4.44 Calculated Intrinsic Capacitance (pF) : 128 Loaded Parallel Q Calculated from Peak : 1.32 Loaded Peak Loss (dB) : 2.8 Gauss: slugs 450 / screws 380 Attachments:
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Post by stratotarts on Nov 25, 2016 14:05:39 GMT -5
5.36k - that has to be the highest frequency neck we've ever seen. When you add together the two ~600 Gauss poles for every string per pickup, isn't that a lot of string pull? Or is the whole string pull problem overstated?
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Post by stratotarts on Nov 21, 2016 16:39:15 GMT -5
Thanks, I've been wondering about the Jazz set for a long time.
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Post by stratotarts on Nov 10, 2016 14:58:55 GMT -5
When it's working as an electrostatic shield, small slots don't have any effect at wavelengths much greater than the slot diameters. This is because the current travels across the surface of a cage at almost light speed. At GHz frequencies, the slots will produce delays that imbalance the fields on alternate sides of the shield, thus allowing some transmission of signal. In fact, some clever microwave circuits make use of slots to transform impedances and construct virtual components. But those frequencies are so heavily attenuated by the coil and cable reactance, that none of it will get out to the amplifier anyway (also the amp will have limited susceptibility to it). So in practice, the shielding with slots should be just as good as a solid box with none.
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Post by stratotarts on Nov 10, 2016 13:12:36 GMT -5
Absolutely! First I thought it would be too hard to slice a pole in half, but as you show, it's not necessary. That could be done in a vice with a diamond saw or hacksaw. Definitely worth trying (with eye protection, of course)!
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Post by stratotarts on Nov 10, 2016 9:58:03 GMT -5
I've recently completed studies of eddy current geometry in pickups, and presented some novel prototype pickup covers that exhibit extremely low losses. It's too big to attach here, so here is a link: study (PDF format)
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Post by stratotarts on Oct 31, 2016 7:49:58 GMT -5
The magnetic field forms a node that reflects waves in both directions on the string. It's because there is an impedance change. The bridge and nut are such reflectors, but the impedance change is more radical - hence most of the energy is reflected. In the case of the magnetic field, only a small fraction is reflected. The non-harmonically related position along the string of the magnetic field causes intermodulation products between the normal and the field induced offset frequencies of the string, as soon as a non-linear part of the signal chain is reached.
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Post by stratotarts on Oct 22, 2016 12:13:12 GMT -5
Lately I've just been looking at the change in dB between the flat frequency response and the height of the peak, because it's just very easy to do and it has some correlation to what you'd actually hear. All it sacrifices is the band width, which tend to all be alike for, say a Strat pickup, then in the case of humbuckers and Filter'trons the eddy losses are so aggressive that Q factor seems to lose all meaning. Yes, I always record that in a column of my spreadsheet. The idea of my "Loaded Peak Loss" calculation is to obtain a rough figure-of-merit for losses that is more independent of pickup and load parameters. I keep such analytical figures in the right hand columns, as they are not as fundamental measurements. I also hide many columns because they are seldom directly relevant for an evaluation or comparison. I use: Loaded Peak Loss = 20*log( Q) - measured peak
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Post by stratotarts on Oct 22, 2016 11:53:20 GMT -5
I found a great formula for loaded Q that takes both coil and load resistance into account. Credit to Manfred Zollner: linkL = coil inductance C = combined internal and external parallel capacitance R1 = coil resistance R2 = load resistance Q = sqrt(L*C*(1+R1/R2)) / (R1*C + L/R2) It can be useful in estimating physical losses, by subtracting the calculated peak that this Q produces from the actual measured peak.
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Post by stratotarts on Oct 18, 2016 11:49:08 GMT -5
Very intelligent to put the connection bundle between the slug coil and baseplate instead of trying to squish them between the coils. Kudos. Although in this case it wouldn't matter because there is more space there than with fully wound coils.
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Post by stratotarts on Oct 18, 2016 8:59:26 GMT -5
Is that wax residue between the coils, or something else?
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Post by stratotarts on Oct 14, 2016 18:35:44 GMT -5
Yes, the resistance should be included. There is little difference in behaviour between the screw and slug coil, in the effect of the other coil being open or shorted: SH-1N (stock no cover)
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Post by stratotarts on Oct 14, 2016 16:23:21 GMT -5
The disabled coil still has resistance.
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Post by stratotarts on Oct 14, 2016 16:20:21 GMT -5
Yes, the wiring for a short is more convenient on on multi switch, than for an open. The loaded difference on an SD SH-1N measures about 1.5 dB:
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Post by stratotarts on Oct 14, 2016 15:16:47 GMT -5
The circuit will perform reliably and for quite a while on a battery, but I recommend using a switching regulator based 9V adapter. We found out before that some adapters are too noisy, but I did find one cheap off-the-shelf 9V adapter that is absolutely quiet - I continue to use it for all my tests.
Having said that, a small difference in the power supply voltage should not affect the measurements significantly. So it is more a matter of convenience which supply you choose.
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Post by stratotarts on Oct 13, 2016 18:21:09 GMT -5
I put the kapton tape in to prevent the slugs from contacting the cover. My theory is that the slugs formed a bypass circuit through two slugs, the baseplate and the cover. I'm not sure now why it happened, I was just happy to eliminate it because I wanted to move on to another experimental step. I suggest trying it on the screws and slugs. However I admit that the effect that you are seeing is stronger. It is a real mystery.
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Post by stratotarts on Oct 13, 2016 16:33:09 GMT -5
Strange about the double peaks. It reminds me of some results I got when I put a solid cover on the A3B1 humbucker. At first I got an extra peak in the response: Then I put kapton tape on the pole slugs and it went away:
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Post by stratotarts on Oct 12, 2016 9:26:16 GMT -5
I just realized that guests can't expand the attachments. So:
DC resistance (ohms): 7050 Inductance (H): 4.78 Calculated Intrinsic Capacitance (pF): 111
Loaded Resonant Frequency (Hz): 2800 Loaded Peak (dB): -1
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Post by stratotarts on Oct 11, 2016 7:11:58 GMT -5
Considering the eddy current losses in the screw coil side, is it possible that there is a short in the coil windings? Are the slugs the same size on both bobbins?
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Post by stratotarts on Oct 10, 2016 17:08:31 GMT -5
Here is an Ebay pickup with the SD logo: linkI don't have metallurgical tools to analyze the metal. I'm judging by comparing the colour of the cover with the gold coloured sample which is familiar to me as brass (from another pickup cover). That's the best I can do. I guess I should have mentioned that. Important Edit:
I just found out, the copper is just a plating! Under it, is the brass. I guess they use the copper plating as a base for the chrome.
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Post by stratotarts on Oct 10, 2016 10:55:23 GMT -5
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Post by stratotarts on Oct 10, 2016 10:49:58 GMT -5
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Post by stratotarts on Oct 10, 2016 10:45:55 GMT -5
This pickup is an Asian import and can be purchased for about US$7.00 from various online vendors. Some of them incorrectly list it as having an Alnico V magnet, it is in fact ceramic. The names "Andoer" and "N3X0" are not always associated with it, it is often sold as a no name product. The "N3X0" name is interesting as there is also an Asian humbucker labelled "A1B3" and thus the alpha-numeric-alpha-numeric nomenclature may be a pattern common to the same manufacturer. As dictated by the price, the design and materials severely limit the pickup's performance. The poles consist of two thick steel plates, which create a very large inductance which lowers the resonant frequency. However, the result is not too bad compared with PAF form factor humbuckers. The really serious problem with this pickup is the covers. Edited for a correction...These are the only ones that I have ever seen that are composed of copper. As cover losses are inversely proportional to conductivity, copper is arguably the worst material that you could possibly choose for this purpose, as it has the highest conductivity of any economical metal (that is why it is used for most wire). Brass is also inferior to nickel-silver, but copper is slightly worse. Strangely, the covers are chrome plated before spray painted black. Bare copper would have better adhesion for paint, but the manufacturer is likely obtaining covers in chrome, unable to source the bare covers. The inference from this, is that some other chrome plated covers on the market might also be composed of copper.
In fact, the covers are brass. I was confused by a copper base plating that I guess is used to facilitate the chromium plating. The baseplate is brass. Strangely, some of the online listings show a Seymour Duncan logo stamped on it. The ones I obtained did not have these. The price includes a steel mounting ring and screws, not shown. Attachments:
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Post by stratotarts on Oct 8, 2016 22:10:11 GMT -5
The values from exciting the pickup directly with a constant current source, and from exciting it with an external magnetic field, differ in the location of the magnetic field. In the case of electrical excitation, the field (and therefore the measured influences) are located around the pickup coil and core. In the case of the exciter coil, the measured influences are based on the field around the exciter coil. This is by design, in order to capture the asymmetric nature of the system. The interposition of a cover is a perfect example of this. It more strongly shapes the response because it intervenes between the excitation field and the pickup coils. It is important to keep in mind that although electromagnetic laws are reversible mathematically, systems composed of multiple components with intervening losses and frequency dependent coupling are generally not. However, it is true that the results from internal and external excitation correlate strongly because after all, many of the system properties are common. I've read your comment a few times, aren't you essentially like Antigua and me allude to that there are more than one voltage sources - "from internal and external excitation" - at work? I would like to present the following problem: Consider a configuration set up as in 78932Pickup under test: some strat sc pickup with alnico rod magnets We're performing the test, we measure -> test results #1 Now test #2: First we degauss the magnets We're performing the test once again, we measure -> test results #2 Do you believe like I do that test results #1 and test results #2 should be identical? I am referring to the two kinds of test, one is direct excitation of the pickup through a high value resistor. You, in 78932 above, Errede, Carson, Lawson all have used this method. The other is by external magnetic excitation as you report that you now do, and Lemme, Moore, myself and now Antigua also use. In the modeling area, I feel that Carson came the closest because of its simplicity. His model has only a fixed capacitor, and a frequency variable inductor and resistor. I found that making the inductance constant made almost no difference (probably because it acts at a low frequency where the inductive reactance is also low). So even a simple LCR model with variable resistance (it obeys an exponential law with relation to frequency), models reality very closely. I did not pursue the implementation of it because I have been focusing on refining test equipment. In these discussions about modeling I mostly sit back and read because it's gotten ahead of my full understanding. If I understand correctly, we are wondering what a good model would be. In my view, that would be one that balances simplicity with predictive power. How well it corresponds to the actual physical mechanisms inside the pickup is not as important. I believe that those mechanisms are much too complex to model directly without a quantum leap in computational methods (for example multiple discrete element simulation such as is used to emulate string vibration). I don't think of modeling itself as adding to the understanding of the internal physics. It seems to me, more about being able to analyze data and perform simulations. So it seems arbitrary to me, whether components of the model actually exist in the physical pickup. Two different voltage sources, or 12 Henry inductors, neither really exists in the pickup, right? Lastly, I think results #1 and #2 should be identical.
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Post by stratotarts on Oct 7, 2016 9:49:52 GMT -5
The values from exciting the pickup directly with a constant current source, and from exciting it with an external magnetic field, differ in the location of the magnetic field. In the case of electrical excitation, the field (and therefore the measured influences) are located around the pickup coil and core. In the case of the exciter coil, the measured influences are based on the field around the exciter coil. This is by design, in order to capture the asymmetric nature of the system. The interposition of a cover is a perfect example of this. It more strongly shapes the response because it intervenes between the excitation field and the pickup coils. It is important to keep in mind that although electromagnetic laws are reversible mathematically, systems composed of multiple components with intervening losses and frequency dependent coupling are generally not.
However, it is true that the results from internal and external excitation correlate strongly because after all, many of the system properties are common.
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