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Post by ms on Feb 22, 2017 6:15:26 GMT -5
I've been reading about coil capacitance, and I came across an interesting revelation here coil32.net/theory/self-capacitance.htmlWhat this is saying is that the windings only capacitively couple with their neighbors who are perpendicular to the axis. So if you have a pickup sitting on a table, windings capacitively couple with windings to their left or two their right, but not above and below. The interesting thing about that is that it means a single layer coil has about the same capacitance as straight wire, because such a coil only has neighboring wire above and below. Of course a pickup is multilayer, but the point of it all is that the winding capacitance is limited to one plane throughout the coil. A practical application is that if you want to manufacture low C coils, some sort of layer spacers, or spacing agent, might do the job, such as a layer of lacquer or wax. You wind, then dip, then wind, then dip, etc. Straight wire? Equations 3 and 4 in your reference contain the diameter of the coil, not the length of the wire. I do not understand that relationship. Any way, this is for single layer coils; very different for a guitar pickup.
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Post by ms on Feb 21, 2017 6:54:13 GMT -5
It looks as though you have applied enough H so that the magnetization has moved some along the hysteresis curve in the direction of saturation. That is, the change is B you are getting is not as large for the same applied change in H as it was with smaller magnets. I thing there is still a considerable way to go before reaching complete saturation.
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Post by ms on Feb 19, 2017 6:39:00 GMT -5
I'm not saying this proves of disproves anything that been said, but this all conforms to what I understand about Ohms and Faraday's laws, that for a given induced current, if you halve the resistance, you get half the voltage, as seen by adding and removing a parallel dummy coil, but by doubling up on the current by restoring the pickup to two "productive" coils, you retain the voltage on just one coil by itself, despite having halved the resistance across the whole pickup. When you connect the second coil but do not excite it, it acts as a load equal to the source impedance of the excited pickup, and so you get half the voltage of the excited coil by itself with no load. When the excited coil operates by itself with no load, there is (ideally) no current flowing (at low frequencies). The change in resonance frequency probably indicates that you have some capacitance in the measurement setup.
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Post by ms on Feb 18, 2017 17:20:29 GMT -5
I just posted some new data a few minutes ago regarding tapped coils that is now one page behind us, just i n case anyone misses it guitarnuts2.proboards.com/post/80442/threadAn analogy to this is with two 1.5V dc battery cells. Put them is series for x2 voltage ie 3V, but put them in parallel and you get the same voltage 1.5V. But isn't the voltage maintained because you have twice the current? If one of those two batteries dies, then the dead battery becomes a parallel resistor, dropping the overall voltage, depending upon the resistance of the dead battery. So the point I'm getting at is it seems that the two sides of the parallel humbucker only behave as two batteries because both coils generate a current, as opposed to a parallel dummy coil, like a stacked humbucker, where the dummy coil is not a substantial contributor to signal current, not behaving like a battery. If you put two ideal batteries in parallel, the total current stays the same. It is a function of the load on the batteries, which is the same in both cases.
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Post by ms on Feb 18, 2017 6:44:31 GMT -5
Here's another data point to consider; a PAF knock off is series, parallel, and split to slug and screw: As you can see, the series mode has an easy 5.0dBV higher output than the parallel or split modes, but interestingly there is an equivalence between parallel and split. So with the parallel mode, obviously the resistance is halved, and the inductance is nearly halved, so the voltage should drop, but wait! you get EMF from both coils instead of just one or the other, so the voltage output is doubled, bringing it back up to what either coil puts out alone. Something that seems obvious when you think about it, but is fun to see happen in a practical demonstration. One thing I don't really understand too well is the distinction between the inductance and resistance in terms of defining the output voltage. Is this more V = I*R, or V = I*Z? So if the output for both parallel and split is, rounded to 1.20Vrms, the resistance for each coil alone is about 3.7k ohms, so the current would therefore be 324uA, and in parallel the resistance is 1.85k, which means the current would be 648uA, twice the current for twice the total area of coil. So in parallel you get twice the current, but half the resistance, or reactance, and so the total output voltage end up being a wash. ~~~*~*~*~~~*~**~*~~~~~ In other news, I received my first guitar with P-90s, and I played if for about fifteen minutes so far, but I can tell you the P-90's obviously have a low resonant peak and an overall dar character, as I expected they might. But it raises a question: why can a P-90 neck pickup get away with a 6 henry / 2kHz P-90 sound good as a neck pickup, but if you use this same recipe for a PAF you get a muddy pickup? Most neck PAF's are closer to 4.5 henry / 2.kHz. Maybe the higher voltage output of the humbucker format, or the wider sampling of harmonic nodes creates a composition of harmonic content that is simply less flattering with you have a lower resonant peak. I'll do a bode plot of those P-90's soon so that we can see exactly how their response curves look. Neither resistance nor inductance directly defines the output voltage. The results from combining the two coils in series or parallel are very much consistent with the individual measurements, but to see this you need a better understanding of the fundamentals of circuit analysis.
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Post by ms on Feb 16, 2017 19:15:38 GMT -5
I doubt that you get the direct eddy current affect right unless you use a coil causes a field very much like the field the string makes.
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Post by ms on Feb 16, 2017 15:08:34 GMT -5
It looks like that small exciter coil has a horizontal axis. Should it not be vertical to align with the most sensitive direction of the pickup coils? The relative sensitivity of the pickup to other axes may not be consistent with their main axes. I'd suspect the single pickup is being relatively under reported in this case compared to the Hb. Yes, the pickup coil is sensitive to the field component pointing through the coil. The magnet magnetizes the the string in this direction right over the center of the pole piece. As you go along the string away from the center, the direction of the magnetization changes, and so the contribution to the signal drops. Results should be more consistent using a vertical coil also since a small tilt should have less effect.
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Post by ms on Feb 15, 2017 15:16:39 GMT -5
For relative levels within 3 db, I think you could just use careful measurements made with your small exciter coil. And having made all these measurements, I think you have the data to show that, or to find out what needs to be done in order to make it work. That's a good point; for comparing gives types of pickups, like low output HB versus high output, the exciter method should be good enough. And it just occurred to me that I can probably compare a single coil and HB by just orienting the exciter over the single coil as though it were one half of a humbucker, at 90 degrees with only half the exciter over the single coil. That should impose a similar flux field. It will be interesting to see how close those measures come to the hard strumming approach. I think the right way to excite a hum bucker is with two small coils, over corresponding pole pieces in each of the pickup coils. (Exciter oils must be out of phase, of course.) The string is magnetized most strongly and in the right direction (field pointing through the coil) right over the pole piece, and the coil should do the same thing.
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Post by ms on Feb 15, 2017 6:13:25 GMT -5
A idea I was wondering about, which I think Ive heard of elsewhere, would be to mount a length of piano wire eccentrically in the chuck of a drill, and run the drill at a consistent height and speed over the pickups. Im not sure if it would be fast enough and would need to be at least 6000rpm to cteate 100 hz or more. Along these lines, I saw someone tape a length of guitar string to a speaker and then place it over a pickup. Not bad, but the string is not in situ. I wouldn't have thought it mattered until I saw such variation between bridge, middle and neck positions. The way a pickup relates to the strings seems to differ in the bridge versus the neck, it's not a linear thing. For example, a humbucker bridge will be louder than a single coil bridge because the humbucker reaches out towards the neck, but this "handicap" doesn't exist for the neck position, where both types of pickups are more or less on equal ground (save for the neck anti node). Complicating matters even more, if the pickup has a coil that is close to the top of the pickup (Lace Sensor), this bridge/neck disparity appears to be magnified. For my purposes, all I really need are ball park figures, because if two pickups are within, say 3dB, you can close that gap by just adjusting the heights of the pickups a little. What's important are conditions that represent huge differences, such as three more henrys of inductance, or two side by side coils instead of one. For relative levels within 3 db, I think you could just use careful measurements made with your small exciter coil. And having made all these measurements, I think you have the data to show that, or to find out what needs to be done in order to make it work.
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Post by ms on Feb 13, 2017 8:33:29 GMT -5
The comb filtering affects the very high harmonics only which contribute very little to the total power. The comb filtering is audible on the D string, but to a lesser extent that the other two wound strings, but even on them it does not affect the power much..
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Post by ms on Feb 8, 2017 15:33:16 GMT -5
One thing (ok one amoung many!) that I'm not understanding is how you are getting the output plots derived from impedance measurements. When I calculate such things from equivalent components, I need to end up with reducing the system to two impedances (each with real and imaginary parts) forming a voltage divider. Then I can get a ratio of input to output voltage given a virtual voltage source with a flat response. I cant get the result with just one overall impedance. John, here is the answer from an earlier post. This is exactly what I am doing: So start with the measured impedance; it includes all four of the elements in the above representation. Suppose we find the C value somehow. (It seems practical to do this at the resonance by fitting to a model that incorporates the eddy current effects over a narrow frequency range so that Rse can be assumed constant over that range.) This C can be "unparalleled" leaving an impedance, call it Zu. The real and imaginary parts of this impedance are shown in the impedance plots shown earlier (green and yellowish lines), and the accuracy of the C measurement is shown by the removal of the shape of he peak from Zu. Now Zu and C can be used to make a voltage divider. That is, Zu replaces the three series elements in the above pickup model. Then the frequency response can be directly computed from the impedances of Zu and C. This is the idea I am trying, but it is necessary to measure the frequency response directly in order to check the results. I can do that with the same system used to measure the impedance, modified a bit. Here is the Python routine used to do the calculation: # Function to compute the frequency response from the impedance # Inputs are the frequency array, the complex impedance, without C, the C # value, and the R value (load, across the C) def frfZ(freqs, Zu, C, R): pt = pupc.pupc(fext = freqs) Zc = pt.fZC(C) return pt.vdiv(Zu, pt.par(Zc, R)) vdiv calculates the response of a voltage divider. It is a method in the class I listed here earlier.
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Post by ms on Feb 8, 2017 7:58:02 GMT -5
Sometimes looking at extreme cases can help build insight into what is happening. So here is the gain comparison (derived from impedance measurements and from exciter coil measurements like the other plots) for the SD hot rails. This is a very high output (2 V p-to-p on the initial transient of a power chord) single coil sized pickup with a very low unloaded resonance peak frequency. That is, if you use it with a FET buffer in the guitar unloaded, it sounds much like a normal humbucker loaded with pots and cable. It is about 13H and 17K. The direct eddy current loss (that is, the loss from cancelation of the magnetic field from the vibrating string or exciter coil from currents in the steel) is very large as the plot shows. The reason that it does not display the dip in response at the lower frequencies is that the resonance is so low that the consequent rise in response covers it up.
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Post by ms on Feb 6, 2017 19:42:34 GMT -5
The attached plot is the slug coil from the foreign humbucker, stock results above. I took out the stock slugs and replaced them with some I bought a while ago, probably from StewMac. There is much less dip; the resonant peak is higher in level and its frequency is higher. This is consistent with significantly lower eddy current losses, at least partly through lower permeability. (I seem to be picking up a bit of noise tonight that should not be there; have not been able to find the source.) My conclusion is that materials matter, and that buying random materials will get you random results. I just the slug coil being analyzed, or is the screw coil still in series with it? I wonder if the difference between the 59 and Jazz steel properties is intentional or not. I have swapped screws in slugs between pickups in other tests and found that the differences were pretty small in those cases. It's hard to say what all factors were involved, but one thing is for sure, with added loaded, the Q factor drops to around 2 to 5dB amplitude, the resonance frequency is closer to 3kHz, and you see less pronounced differences with respect to eddy currents, whether it be from a cover or whatever. This is just the slug coil. Certainly the load tends to hide the differences, and so using the unloaded coil gives more accurate measurements. We just have to verify that using such results to model the loaded case is accurate.
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Post by ms on Feb 6, 2017 19:07:01 GMT -5
The attached plot is the slug coil from the foreign humbucker, stock results above. I took out the stock slugs and replaced them with some I bought a while ago, probably from StewMac. There is much less dip; the resonant peak is higher in level and its frequency is higher. This is consistent with significantly lower eddy current losses, at least partly through lower permeability. (I seem to be picking up a bit of noise tonight that should not be there; have not been able to find the source.) My conclusion is that materials matter, and that buying random materials will get you random results.
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Post by ms on Feb 6, 2017 18:56:33 GMT -5
Hmm, I get resonant peaks well below 10kHz for these same pickups, and I don't think I'm imposing a whole lot of capacitance, although.. are you bypassing the stock hookup wire? I've seen up to 70pF from the braided hookup wire. I am using the stock cable, about 13". My two measurement methods are consistent.
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Post by ms on Feb 6, 2017 16:13:28 GMT -5
It's probably a lot to ask, but you could try swapping the slugs and poles and see if the impedance plots follow them around. I'm willing to test different screws and slug alloys if there is a supplier. I'd like to do this, I just have a back log of testing and other related projects. BTW, it looks like your plots show resonant peaks well beyond 10kHz, am I interpreting that wrong? I was just thinking that that is the next test to do! Yes, I do see peaks above 10 KHz, but not always. Here is the SD SH4: Note that I have not plotted above 10KHz. This pickup is is a 16K bridge pickup, often sold in a set with the SH2N. I suspect that the steel is the same in the two pickups. The impedance method I use should not load the pickups with any significant C. I made an effort to get the input C in the HiZ buffer down pretty low, but could not get below 11.6pf. Thus when computing the response from the impedance, I add 11.6pf for comparison purposes with the gain measurements.
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Post by ms on Feb 6, 2017 15:23:37 GMT -5
In the case of the impedance measurement, we measure eddy current effects that result from current in the coil; that is, the eddy currents that affect the operation of the electrical circuit. Using the exciter coil, we see those effects and another kind. The exciter coil (or string) makes a time varying magnetic field. This field induces currents in the cores (or other metal). These induced currents make a magnetic field. It is the sum of the two that induces voltage in the pickup coil. (We expect the total field to be less than the original, but I doubt that the phase between the two is exactly 180 degrees.) If you sell something as a PAF replacement, I would think you should get the materials right, so it does not surprise me that the steel is different from other pickups. Here is the comparison for the slug coil of the foreign PAF type replacement I have mentioned before: It shows a similar dip, but even more pronounced. OK, I think I understand, and this is why I have used an exciter coil for testing, in order to get a realistic response as it relates to the strings, and not just that of the coil as a standalone RLC low pass. Since the "direct" method is a little easier to arrange than the exciter method, I think it's also a convenient way to get the resonant peak really quickly, if you're not concerned with an impedance plot that depicts all of the eddy current related losses. I'm not sure what you mean by the sum being effected by the eddy currents being less than 180 degrees out of phase with the coil. Due to Lenz'slaw, I would expect that they should be perfectly oppositional. What would cause the eddy current field to be an imperfectly opposed phase? Regarding the materials, I can believe they would make a '59 more authentic and the Jazz less so, but I would be surprised if they designs called for specific grades of steel, or rather, were specifically different for these two models. I know that the MEF pickup makers occasionally discuss the differences to be had with different grades of steel, but it's all very vague, and IIRC all the popular choices are very similar with respect to conductivity and permeability. I would sooner suspect that the differences were unintended on their part. The current that flows in response to a magnetic field could be flowing in a circuit that has inductance and/or capacitance. Thus the phase can be shifted. But it must dissipate energy, not create it, and that limits how it can shift. There is at least one pickup maker, not posting so much any more on MEF, who has claimed that he has done the research to determine what the steel was in PAFs and that it matters. In any case materials do matter. How else do you explain the dip in response present in the response of some pickups, but not others?
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Post by ms on Feb 6, 2017 12:51:43 GMT -5
Mike, can you run both impedance and gain tests with, say, JohnH's, stratotarts and antigua's standard load of 200K ohms and 470 pF, and have both tests work computationally? I apologize if I missed any dicussion on this previously. I'm curious to see if for the frequency range of interest in current passive instruments, basically 6 KHz and below, how the eddy current loss results jibe with JohnH's measurement methodology and parametrization of modeling data. I think they are leading to the same place, based on JohnH's reworking of some of his data into real and imaginary plots. It is good to have both modeling and measurement, and from there we can not only characterize existing magnetic pickups, but also have a means of guiding new design and evaluating new designs against old. I think that would be a good check. If you have the impedance measurement and the gain difference as illustrated i those plots, it should be possible to compute the gain with the load. And that is exactly what would be checked!
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Post by ms on Feb 6, 2017 12:46:17 GMT -5
Here is the comparison for the SH1N (PAF type neck pickup): The dip at the lower frequencies is interesting and probably audible. Presumably the steel in this pickup is not the same as in the SH2N. I would expect the slugs to be different, but also the pickups do not use the same screws. Those in the SH1N are just a bit fatter and very slightly longer. So we're seeing two plot lines; one plot line is response as measured through the wires, and the other is response as measured through the exciter coil, so as to see which eddy current losses effect to just magnetic field of the coil, and those which effect the fields of both the coil and the guitar string? I didn't think they would use different parts for the Jazz and the '59 neck models. I'm surprised to hear that they would use separate screws for the two models. It seems like a strange distinction to make. One thing I do know if that the '59 that comes with the braided one conductor wire is different in various ways from the '59 that comes with the 4 conductor wire; the two conductor version as a real wood spacer and longer mounting legs. In the case of the impedance measurement, we measure eddy current effects that result from current in the coil; that is, the eddy currents that affect the operation of the electrical circuit. Using the exciter coil, we see those effects and another kind. The exciter coil (or string) makes a time varying magnetic field. This field induces currents in the cores (or other metal). These induced currents make a magnetic field. It is the sum of the two that induces voltage in the pickup coil. (We expect the total field to be less than the original, but I doubt that the phase between the two is exactly 180 degrees.) If you sell something as a PAF replacement, I would think you should get the materials right, so it does not surprise me that the steel is different from other pickups. Here is the comparison for the slug coil of the foreign PAF type replacement I have mentioned before: It shows a similar dip, but even more pronounced.
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Post by ms on Feb 5, 2017 7:53:51 GMT -5
Here is the comparison for the SH1N (PAF type neck pickup): The dip at the lower frequencies is interesting and probably audible. Presumably the steel in this pickup is not the same as in the SH2N. I would expect the slugs to be different, but also the pickups do not use the same screws. Those in the SH1N are just a bit fatter and very slightly longer.
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Post by ms on Feb 4, 2017 19:08:54 GMT -5
The coil calculator at 66pacific.com says my coil is .383 microH, or about .048 ohms at 20 KHz, and so the impedance probably changes only couple parts per thousand from low frequencies to 20KHz.
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Post by ms on Feb 4, 2017 19:01:14 GMT -5
Here is the comparison for my tele bridge pickup: The effect is small until the frequency is high as expected for alnico magnets used as cores.
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Post by ms on Feb 4, 2017 18:38:43 GMT -5
With the Extech on 1KHz, the coil measures the same as with the leads shorted, about 3 microH. If it were as big as 1 microH, which I doubt, the magnitude of the impedance would be .126 ohms at 20KHz. That would be small enough, and I think it is smaller.
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Post by ms on Feb 4, 2017 16:01:05 GMT -5
Hi ms Just looking at your pickup exciter-coil circuit, there seems to be a difference between what you show and how antigua and stratotarts have been doing it. Apologies first if I have misunderstood! It looks like you are driving the exciter through a low impedance, and what that means is that the current through the coil (and hence the field that it creates) is then influenced by the exciter coil impedance which can add its own influence on the final result, varying with frequency. In their system based on stratotarts circuit design, the exciter is fed through a higher value resistor, which swamps the effect of exciter coil impedance. Feeding a constant voltage vs frequency into this creates a constant current at all frequencies and hence a consistent flux which rises linearly with frequency. Then the circuitry integrates this to take out the linear rise. This makes the specific characteristics of the exciter coil unimportant, for a more consistent overall result. Last year, it was fascinating to follow the development and testing of this method and I think it is the right way to do it. The coil is quite small and has only a few turns (seven), and so the 7 ohms is much larger than the coil impedance. This is why I need a power amp to run it. Also I am using a voltage and current feedback circuit in the amp which raises its dynamic output impedance to about 15 or 20 ohms. So this is very much a current source as far as the coil knows. I checked the interaction by starting with the coil flush against the top of the pickup. I observed a small shift in the resonant frequency until the spacing was raised to about 1/8 inch. So I stopped there. That interaction could be caused by capacitive or magnetic effects; the object is just to make it very small. If the current through the coil did vary from changes in its impedance, it would not cause a measurement error because the coil current is measured and then divided into the cross spectrum so that the effects of variations in current are taken out. In fact the current does vary perioidicly with frequency because of the anti-aliasing filter (I think) and non periodically because of the digital low pass filter on the codes. The first would be visible in the measured frequency response if it had a significant effect.
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Post by ms on Feb 4, 2017 12:07:35 GMT -5
Mike, You should think about doing an AES paper on your measurement methodology. I, for one, really appreciate your theoretical insight and persistence. I do want to point out that it's at least possible that the ESR of the 470 uF electrolytic capacitor might be enough at higher frequencies (say, 10-20 Khz) to slightly change the very low AC impedance to ground of the bottom end of the pickup in your buffer. Since you went to a lot of trouble to get a completely "neutral" buffer, and you are measuring some pretty subtle eddy effects, it wouldn't be a bad idea to bypass the 470 uF with a .1 to .22 uF polyester film (good) , polypropylene or polystyrene (best) film capacitor, to make sure that you have a darn good "AC ground" for the pickup in your measurements. Probably no difference, but maybe best to eliminate or minimize any possible sources of inaccuracy for these measurements. -Charlie Thanks, Charlie. Here is the impedance of that pickup: You are right, bypassing the electrolytic is the right thing to do. In this case it likely does not matter since the magnitude of the impedance at the top is between 100K and 200K, but next time I open the box, I will put a film cap across it
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Post by ms on Feb 4, 2017 7:50:11 GMT -5
The major effect of eddy currents on the response of a steel cored pickup is the damping of the pickup resonance. The effect can be greater than the damping from the coil resistance. However, there is another, smaller effect. This is the partial direct cancelation of the varying field from the vibrating string (or test coil) resulting from currents set up in the cores (or other metal). That is: the varying field induces current; the induced current produces a magnetic field; the total of this field and the original field is less than that of the original. (The new field must oppose the buildup of current as Maxwell's equations show, although this opposition is often referred to as Lenz's law, which I describe as "There ain't no free lunch.") Stating this and measuring it are two different things: to measure two things which cause similar effects, we need two different kinds of measurements. These two can be impedance measurements and response measurements. As shown earlier, impedance measurements can be converted to response measurements. However, such response measurements do not show the second effect because there is no exciting magnetic field, just a current. However, response measurements using a current driven coil show both effects. Therefore it should be possible to look at the effect from direct inducement of current by comparing the two measurements as a function of frequency. Therefore I have added response measurements to the setup. The same cross spectrum and data accumulation software is used, but the post processing is different, and of course the hardware setup is different. This is shown in the first attachment: The output of the Tascom feeds a power amp which provides power to a series resistor driving a small coil with a current sensor resistor to ground. The coil is driven from an impedance much higher than its own to help minimize coupling effects and keep the current reasonably independent of frequency. The exciting signal is similar to the one used in the impedance measurement, Golay complementary sequences, but the signal is digitally low pass filtered before conversion. This is to compensate for the pickup's increasing response with frequency.* (As described in various discussions on this and other forums, the law of magnetic induction describes a process which naturally increases with frequency.) The buffer is a single low noise FET used as a source follower. There is no separate bias resistor; bias is through the pickup coil. Therefore the input resistance of the circuit is about a gazillion ohms. The input capacitance is low because the ~20pf gate source capacitance is greatly reduced by the feedback of the source follower, and so the gate-drain capacitance of about 5pf is the dominant effect from the FET. Then there is capacitance from the wiring, etc. The input capacitance of the buffer was measured: the cores were removed from a hum bucker coil (to remove the eddy current effects), the resonant frequency frequencies measured by response and impedance were compared, and the input capacitance was derived. The measured value was 11.6pf. In the following measurements, when the response for the two kinds of measurements are compared, 11.6pf is added to the total capacitance in the conversion from impedance to response. We expect to see different effects from different materials. But for now, this post ends with one "run of the mill" measurement made from an SD SH2N (a Seymour Duncan neck pickup with medium response often paired with a high output bridge pickup): The gain measurement was adjusted to be the same as the response form the impedance measurement at low frequencies. Then you can see the gain drop with increasing frequency. It is not a big effect, about 1 db at 5KHz, the top end of a guitar speaker response. The effect might be just audible. Some other pickups have larger effects, including a rolloff before the resonance, as has been shown on this forum. * This filtering is for SNR purposes. It does not actually take out the rise since the spectrum of the current measurement is divided into the cross spectrum. The rise after the division is taken out digitally by dividing by a ramp function.
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Post by ms on Feb 2, 2017 6:48:24 GMT -5
I think that he proper length for the screws is that which gives best hum cancelation. In general, since the screws have smaller diameters than the slugs, leaving a bit of extra length should help compensate for the missing permeable magnetic material. I have not tested this, but have always thought that this is why the screws stick out a bit. Knowing what I know about the design priorities of the first PAF, my guess would be that this happened to be the length of screws they had on hand, perhaps for the P-90's or something. I think Seth Lover wanted to use slugs for both coils. When that was changed for marketing reasons, I think he would have made an effort to keep the hum cancelation as good as reasonable. When you take the screws out one at a time while listening, you can hear the hum increase as each one is removed, and so at least a reasonable balance between the effects of slugs and screws is required to get good cancelation. Of course opinions vary, and few bother to actually get the facts. For example, the guy who ran the electronics sub forum on MMIF years ago insisted that a coil with no steel balanced one with steel just fine. He knew because he tried it. Well, some reduction and good cancelation are two different things.
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Post by ms on Feb 1, 2017 12:07:00 GMT -5
I think that he proper length for the screws is that which gives best hum cancelation. In general, since the screws have smaller diameters than the slugs, leaving a bit of extra length should help compensate for the missing permeable magnetic material. I have not tested this, but have always thought that this is why the screws stick out a bit.
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Post by ms on Jan 26, 2017 14:59:14 GMT -5
Hi John, I just have time for a quick comment now. The skin effect probably only matters for the cores, but they are important since they are in the coil.
Also I think the eddy current effects seen with the impedance measurement are not all of the effects, most in the case of a hb without a bad cover, but not all. I believe the dip in response with rising frequency seen with some humbuckers is not seen with the impedance measurements, and therefore you need both kinds of measurements (impedance and response) to be able to sort out what all the effects are. This might or might not be right, but I think we can find out.
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Post by ms on Jan 26, 2017 7:27:53 GMT -5
Thanks for posting this. It looks like it can be very valuable for making generalized models of eddy current losses for simulation purposes. It's hard to find mathematical treatments of eddy current losses on the 'net in general, so this is valuable information. I'm not smart enough to apply this math now, but I hope to be in the near future. Thanks. I am going to keep going with this with the goal getting the eddy current effects into as simple form as possible. This stuff is not very easy!
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