|
Post by stratotarts on Jan 6, 2017 12:37:09 GMT -5
I find what you're doing with these signals very interesting, but what is the reason to consider using them vs. the common swept sine wave?
|
|
|
Post by stratotarts on Jan 5, 2017 11:02:42 GMT -5
I think it's a case where Occam's Razor applies... a typical humbucker has a notably lower Q and loaded resonant frequency than a typical single coil. It's amply sufficient to explain the difference. It's also easy to demonstrate, unlike the aperture phenomenon which is a perfectly plausible theory with almost no experimental validation. What we need to know, is the magnitude of the effect. There are too many physical variables feeding into it to trust math alone. Of course most of the difference is the result of different frequency response in the pickup circuit. But the effects of the two regions of sampling in a humbucker are both measurable and audible in a consistent way. After all, bridge and neck pickups do sound grossly different because they sample the string in different locations and thus have different ratios of harmonics. Considering closer spacing is just an extension of this to more subtle effects. Yes, but subtlety can extend to imperceptibility. I've taken an unmounted pickup and held it over the strings, moved it up and down the strings. I think if you try this, you will not be able to detect a difference of one inch.
|
|
|
Post by stratotarts on Jan 5, 2017 6:12:57 GMT -5
I think it's a case where Occam's Razor applies... a typical humbucker has a notably lower Q and loaded resonant frequency than a typical single coil. It's amply sufficient to explain the difference. It's also easy to demonstrate, unlike the aperture phenomenon which is a perfectly plausible theory with almost no experimental validation. What we need to know, is the magnitude of the effect. There are too many physical variables feeding into it to trust math alone.
|
|
|
Post by stratotarts on Jan 4, 2017 19:16:19 GMT -5
In fact, inductance via permeable core is superior to inductance via wind count, because you don't suffer the capacitance increase with a permeable core. DiMarzio patented the idea of putting permeable material in the "dead space" between the slugs and screws to get a higher inductance for fewer winds, which is a shame, because it's pretty much an all around good idea. One way to go, if you have the means, is the have the entire bobbin be a permeable body, like a pot core inductor. You can get the same inductance, probably with a shockingly low number of turns of wire. One reason I'm not thrilled about the idea of inventing a space aged pickup is that with all the patents accumulated over the past 70 years, chances are the idea was patented in the 70's or 80's and then I'd have to pay some amount of money to a dead beat, who's probably on oxygen, who never went anywhere with the long forgotten idea he'd patented. Besides, and this is something else to take note of, guitarists have by and large rejected new concepts. DiMarzio employs some of their patented methods in their more recent pickups, but doesn't mention the fact at all in the product copy, likely out of fear that they would scare away guitarists. For example, the "PAF Master" is supposed to invoke a mental picture of vintage authenticity, and yet its an "airbucker" with enhanced ferrous cores. Companies like Lace seem to flail with their highly visual futuristic ideas. I gotta be honest, Strats, Tele and LP's look very 50's. Futuristic (80's-present) looking pickups clash with them. I've begged, as a consumer, for Lace and Zexcoil to give their new pickup designs a more vintage look, as I'm on the fence about them otherwise. Sure. Look what happened to Dialtone Pickups: before and after
|
|
|
Post by stratotarts on Jan 4, 2017 6:40:38 GMT -5
Not to say that it wouldn't be an acceptable design, but using steel pole pieces goes against your design goal. The parts that are closest to the string make the biggest difference when it comes to eddy currents. For what you're talking about, magnetic pole pieces are "de rigeur" - have a look at the Gibson Firebird: link
|
|
|
Post by stratotarts on Jan 3, 2017 20:56:19 GMT -5
Those calculations assume an open string. Play the chromatic scale ascending on a string. Does each note sound "different"? I think not very much. Yet according to this theory, completely different harmonic nodes are present over the fixed pickup. There is an obvious discrepancy between theory and practice here. I actually think that the tone does change as you fret higher, and becomes a purer tone with more emphasis on the fundamental. I can think of three reasons: 1. The main reason would be because as the vibrating length gets shorter due to fretting, and the pickup is at a fixed distance from the bridge, it is becoming at a greater % of the vibrating part of the string and so getting relatively more fundamental. 2. The same effect is happening to the picking distance which, if you keep picking in one place, is getting nearer to the centre of the vibrating string as you fret higher. 3. There's possibly a third effect too due to bending stiffness of the strings which rends to limit the highest frequencies since the string resists bending so tightly. These frequencies represent a lower order of harmonics being curtailed if you fret higher. Actually, I got entangled in my point. I realize that the tone changes as you go up. What I meant is that it seems to do it fairly proportionally, without huge peaks and valleys. I'm wondering why those aren't clearly audible if the harmonic nodes are positioned over the pickup aperture. The audible effect is a smooth, linear change of tone in proportion to the increase in fret. As a rough analogy, the human vocal tract modulates the harmonics in different ways which sound like vowel expressions. That is what I'd expect to hear from filtering different harmonics. Kind of like a vocorder. On a related note, one would expect the effect to be stronger with a single pickup because the nodes overlap and "fill each other in". But I've never noticed any difference like that between single and both pickups engaged.
|
|
|
Post by stratotarts on Jan 3, 2017 20:24:03 GMT -5
Here is an update to the testing document, only because C1, C2 and D1 had to be updated in the schematic. I have also finally gotten around to including the slope calibration procedure. link to Pickup_Measurement_Procedure
|
|
|
Post by stratotarts on Jan 3, 2017 6:55:12 GMT -5
Those calculations assume an open string. Play the chromatic scale ascending on a string. Does each note sound "different"? I think not very much. Yet according to this theory, completely different harmonic nodes are present over the fixed pickup. There is an obvious discrepancy between theory and practice here.
|
|
|
Post by stratotarts on Dec 31, 2016 7:33:37 GMT -5
I purchased two Cavalier Phoenix (Gibson Firebird replica) pickups from Cavalier Pickups, for the bridge and neck of my kit Tele. This will be a report of electrical characteristics only, as it is not practical for me to reassemble them if they are torn down. I have seen pictures of the internals elsewhere. The Firebird has a unique construction which lends it a very bright sound, with relatively high output and low eddy current losses. Unfortunately, this makes it harder to build and thus more expensive. I believe that is why most other mini humbuckers use a scaled down PAF-like design with a single bar magnet and steel bars, slugs and/or screws. The two types are as different as night and day. The Firebird uses two bar magnets, one inside each humbucking coil. Although the cover is solid metal, it is chrome plated nickel silver and so has no significant losses that would muffle the tone. As the inductance and losses are quite low, the tone is very single coil like, and pairs well with single coil pickups. Hence it is popular as a replacement Tele neck pickup. As is often reported, Cavalier's Rob DeStefano discussed my order at length with me on the phone, and provided a lot of useful information about pickups as we narrowed down the build to an identical set of stock (thus wound for neck) Phoenix. Rob can build with different options and the only major differences between Firebird and these Phoenix that I know of, is the lack of reflector plates (stock, also my choice) and two wire grounded leads to allow for phase reversal. The build and shipment were prompt and perfect. I'm definitely wishing I had done this sooner. Device Circuit under Test : Cavalier Firebird Test Date : Dec 30, 2016 DC resistance (Ohms) : 6,790 Self Resonant Frequency (Hz) : 7770 Self Resonant Peak (dB) : 7.2 Loaded Resonant Frequency (Hz) : 4060 Loaded Peak (dB) : 5.0 Inductance Test Resonant Frequency (Hz) : 1630 Inductance (H) : 2.17 Calculated Intrinsic Capacitance (pF) : 184
|
|
|
Post by stratotarts on Dec 28, 2016 20:47:24 GMT -5
These remind me a lot of the cheap Alnico strat set I tested earlier:
Device Circuit under Test : LJ-50 retest Test Date : Dec 9, 2016 DC resistance (Ohms) : 5,870 Self Resonant Frequency (Hz) : 9830 Self Resonant Peak (dB) : 13.1 Loaded Resonant Frequency (Hz) : 4920 Loaded Peak (dB) : 7.5 Inductance (H) : 1.56 Calculated Intrinsic Capacitance (pF) : 153
The thin coils with poly tape are a common factor, but the bobbins and covers are only superficially similar.
|
|
|
Post by stratotarts on Dec 28, 2016 20:32:54 GMT -5
Indeed, it seems that all the economy pickups from China have plated brass covers. It's really unfortunate for them, because they have learned all the other lessons so well.
|
|
|
Post by stratotarts on Dec 23, 2016 18:51:54 GMT -5
Here is a very promising vari-C circuit:
|
|
|
Post by stratotarts on Dec 22, 2016 20:42:56 GMT -5
They are a very special ceramic, a so-called ferrite. Although it is true that ceramic (ferrite) permanent magnets tend to have low permeability, the soft magnetic versions can have very high permeability. They also can be very lossy at higher frequencies (used to suppress rf pickup) or not. In the audio range ferrites in general are quite low loss, and permeabilities can be as high as several thousand. My original thoughts were regarding ceramic permanent magnets in the shape of slugs. Since they aren't electrically conductive, the eddy current issue is mitigated. And the low permeability suggests that the inductance would be lower than that of steel and perhaps lower than AlNiCo? But the idea of highly permeable ferrite as pole-pieces with external magnets sounds interesting as well. I think that would be really great, but the question is, where to find them? Or any magnets in the right size, for that matter. The strat magnets fit the holes, but they're way too long. I'd like to ditch the bar magnet because I think it would be great to make pickups flatter.
|
|
|
Post by stratotarts on Dec 20, 2016 6:58:56 GMT -5
Our threads here have definately taken the science of guitar and pickup electronics further than has been presented anywhere before on a public site. On the modelling, the derivation and use of impedance curves is promising so long as it also comes with a voltage vs frequency curve. You need real and complex impedance plus voltage vs frequency to capture it. An impedance curve that varies with frequency will still give a flat-line response into a high impedance input unless voltage change is also modelled. The method of making a 6-part model, although it works quite well, is tricky to form an algorythm for, since there are 6 variables and the result is a close fit rather than an exact match. Ive been using 3 resistors, 2 inductors and 1 cap. To find 6 values you need 6 pieces of info, which are usually highly interactive. The easiest one to get right is to match the measured dcr. So i make the main coil resistance a function of tbe other two and tbe dcr. This then leaves 5x variables. These varaibles cant be fully seperated but each has a more primary effect on certain aspects of the response. It takes many interations to home in. In our loaded and unloaded models, the single cap most directly affects the frequency ratio of the two peaks, in combination with the cable capacitance. Tbe load resistor (in parallel with the whole model) is most effective in changing the height of the unloaded peak. The main inductance gets the peak frequencies where they should be. The second inductor with its series resistor shapes the curves, getting the width of tbe peaks and any mid dips modelled. These are the trickiest ones to select and equally reasonable modells can have a range of values here and still be a good (though not perfect) match. I'm still not sure what the "eddy current" resistor and inductor actually do. I understand inductive/capacitance resonance, but I'm not sure how the parallel resistor and inductor manage to create a second low pass filter that somehow leaves the original inductive/capacitance resonance mostly untouched, as opposed to, say, a resistor by itself, which purely dampens the original resonance, or just an inductor by itself, which acts as a high pass filter. Put the two together in series, and it does neither of those things that they do by themselves. AFAIK, actual eddy current attenuation is external to the pickup circuit; it's within the "AC source" black box. In LTSpice I have to live with the fixed voltage for the AC sweep, but can you just model the eddy currents as an attenuation of the "AC source" with GuitarFreak? Lastly, I don't quite understand how the impedance curve differs from the voltage by frequency bode plots. The impedence is reactance and resistance, we know the resistance is whatever it is, so if you drive the pickups with a constant voltage in, the voltage curve that comes back out must owe to the variable impedance with frequency. So, how is an impedance curve different from what we're doing now? Well, I think the first questions are still open for more research and head banging. To the last question, the impedance curve that we're measuring now doesn't capture phase information, only amplitude. We can see phase on the plots, but because it doesn't convey much more information than the amplitude by just eye-balling it, it isn't of much interest. But when modeling, a precise phase measurement would capture the effect of all the L's and C's in the pickup across the entire spectrum. Here is a document I'm finding useful to understand impedance measurements: Agilent Impedance Measurement Handbook
|
|
|
Post by stratotarts on Dec 19, 2016 23:29:20 GMT -5
... While it impresses me, I like to bear in mind that like any simulation the results are only as good as the model. The characteristics of pickups and other guitar electronic components are simplified to linear mathematical models of inductance, capacitance and resistance by using inductors, capacitors and resistors. As Antigua indicated, these are often distributed things and not really fully correct when lumped (it would be terribly complex to even begin to model them as distributed components, so I am not suggesting we ever try). ...... John's 6-component pickup circuit model does manage to track measured results to fractions of a decibel. There are also some interesting models in Zoeller's work, yet untranslated to English. So there is a lot more hope for it than you suggest. But it bears repeating that it is possible to capture a plot of complex impedance instead of only a composite of the real and imaginary impedance as everyone seems to be doing now (including the V5 integrator). That is what Mike Sulzer's rig can do. Such a data set allows perfectly accurate modeling without having any composite ideal component model at all, since it represents exactly the impedance that is presented at the pickup terminals, no matter what component configuration exists inside it. I am not sure that it can work well with external excitation, so I think there is still a place for the V5 and similar approaches, as a complimentary method. For one thing, I have a hard time believing that eddy losses can be measured accurately by exciting the pickup coil itself, since the influence of the string field is asymmetrical with respect to the coil, and materials such as covers that stand between the two may prove to have a larger influence than would be indicated by a coil based (conducted) measurement. Since the pickup is a two terminal device, if the complex impedance at a certain frequency is known, what you measure is the superposition of arbitrarily complicated linear circuits inside that do not need to be known. The vector can be plugged directly into a simulation of a load network, and the response plotted. It is not necessary to know anything at all about the internal circuits in order to do this. There is no limit to this principle, as far as I know, as long as the components remain linear. It is also easy to derive basic parameters such as inductance and capacitance this way, Mike has apparently already done that. By refining the analytical methods, it might be possible to also automatically translate the data set into a matching 6-component model, or other models that are more complex than the basic LCR one. The main usefulness of such reductionism is to aid in classifying and comparing different designs. This really is an unprecedented "stone soup" gathering of interest. I think it is really great how many of us have specialized and attacked different aspects of the problem. It's been about half a year since I started the integrator project, and if the next half year brings the same intensity of research, it will contribute to the largest effort to understand pickups that has happened since they were invented. The free and open exchange of opinions and information has been one reason that it has motivated so many knowledgeable and hardworking people to contribute. We have had mostly positive feedback and many thanks from others who were curious about our findings. A few people didn't get it, but that always happens.
|
|
|
Post by stratotarts on Dec 19, 2016 12:04:11 GMT -5
Steel is a lot more permeable than any of the AlNiCos. Bear with me. Just some noob musings to follow . . . I get the impression that steel is the real culprit in ceramic pickups. I have to wonder if ceramic magnets in the same shape as the AlNiCo slugs might not be so offensive in terms of shifting the resonant peak to a lower frequency. I would imagine it's much easier to machine steel into a slug and slap a magnet or two on the backside of the flatwork. But it doesn't seem as if it would be all that difficult to bake ceramic magnets in the same shape as the slugs. Yet we don't see that. Any thoughts on this? Sure. In the case of humbuckers, there are many reasons. I believe the main reason for using steel is to reduce the number of magnets to only one. That reduces material and assembly cost. Ferrite and Ceramic magnet material is brittle, it can't readily be made into screws. The number of players that actually adjust the filister screws is miniscule, yet there they are. It's my impression that the reason they are used instead of slugs is mainly historical and aesthetic. IIRC, Seth Lover's prototype PAF had no screws, and they were added to make it look more like a P-90. In the case of the Fidelitron, they help to secure the coils. It would be possible to either use magnets directly, as with Firebirds or Strat/Tele, or to replace steel with Ferrite, which has high permeability, while retaining the single under-slung magnet. Ferrite pole pieces would definitely be feasible, but are not currently available in the correct dimensions. A pickup made with those would have to have them in both coils, which would lead to a flat, shiny, screwless top. That would be a hard sell to consumers who don't have any understanding of poles, and may not even realize that there are a second, hidden set of poles under the cover. Any such pickup would appeal to a smaller market, since many players have become accustomed to the tonal response of the steel based designs, and would have to follow a learning curve in order to benefit from the extended range of an improved design, while still being able to reproduce the tones that they already have.
|
|
|
Post by stratotarts on Dec 17, 2016 8:31:13 GMT -5
Curious. In "B" you measure the voltage across the voltage source. Don't you already know the value, since you defined it and it's not affected by the load because the load is in parallel with it?
|
|
|
Post by stratotarts on Dec 14, 2016 16:22:18 GMT -5
What about the amp? I have a ProCo MusicMan 10 foot cable that measures 380pF. When I plug that into the Cube 80x, I see 1,900pF. A tube amp would be about 50 pf, with the Miller effect amplifying the grid plate capacitance of the 12AX7. A solid state could be anything, including near zero. I cannot see why anyone would make it over 1000 pf. Measuring with an input resistor in parallel should be possible on the par setting of the Extech. I cannot see why either. When I read your replies I thought that I forget DC isolation, since the cap meter I'm using (just a setting on the DVM) is skewed or disabled by resistance. But when I put a 1uF in series to eliminate that, I get the same reading. One possible reason could be a clumsy attempt at RFI suppression. Edit - I think now that it's just the simplistic way that the meter reads capacitance. So I guess this is not the true input capacitance value, it's just the (possibly 1M) input resistance playing with the meter.
|
|
|
Post by stratotarts on Dec 14, 2016 7:51:10 GMT -5
What about the amp? I have a ProCo MusicMan 10 foot cable that measures 380pF. When I plug that into the Cube 80x, I see 1,900pF.
|
|
|
Post by stratotarts on Dec 12, 2016 23:17:50 GMT -5
Great stuff! I will have to find one of those Roland cables somewhere... I tested all the ones I have, and they all have ridiculously high capacitance. I prefer a simple thing like a properly designed cable to fancier solutions, like preamps and such, even if they are technically more interesting.
|
|
|
Post by stratotarts on Dec 11, 2016 19:00:50 GMT -5
Histerisis is not an issue because the currents are too small to move the medium along the curve Thanks, so you would say that the pickup generates a very tiny H field? The string generates a very tiny H field.
|
|
|
Post by stratotarts on Dec 11, 2016 18:58:40 GMT -5
I am creating this thread to continue the sub-discussion at other thread and give it its own subject, because I believe it will continue to be "on the radar". It concerns member ms's offsite posts Measuring pickup impedance with an A/D recording interface and Software for performing pickup analysis with a recording interface. There are good reasons to pursue this approach. Mainly, by capturing a complete black box representation of the pickups behaviour by recording complex impedance at all frequencies, it may be possible to produce accurate bode plots for an endless variety of load situations, without any curve fitting or analysis (as might be required to interpret data recorded only as a bode plot). It would still be possible to perform such curve fitting or other analyses, but it becomes optional. Member ms has suggested a simple test circuit that allows this to be done without any specialized amplifier circuits (such as the V5 integrator). There was some discussion about the effects of cable capacitance. I think that ms says it does not cause any problems of loading of the device under test (pickup). I do have my doubts about that, but I may be wrong. In any case, I have two suggestions to address that. One is to use the high impedance portion of the V5 as a buffer, omitting the integrator. Another is to introduce a compensating leg in the drive circuit. The idea is to allow the effects of capacitance in the two input channels to cancel, like this: Honestly, I am not sure about this, it's just an idea, and I have no way to test it right now. I agree that I do not have the necessary symmetry for high frequency gain difference cancelation due to cables and input capacitance since the output impedance of the line out on the 2X2 is 110 ohms. I need to add about 900 ohms. (For some reason I though it was 1K until I read the manual again.) However, I do not think it matters. With 1K as the highest impedance and the small capacitance from short cables, it does not seem to be an issue. I do not think this is a device loading issue. The voltage across the 1K resistor measures the current through the pickup very accurately, and the voltage across the series combination is used to derive the voltage across the pickup by subtracting the other sample from it. The 1K resistor is very slightly loaded, but you can compensate for this if you want. There is an over all gain matching issue, of course. This was easier with the Apogee Duet than the 2X2. Of course yo do not have to make the channel gains the same; you can introduce a compensating factor in the software. But you still have to get the gain right. Okay, then. For a lower output impedance you could use the headphone jack, if it matters. To deal with gain, I suggest some kind of calibration procedure. Edit - for example, you would insert some test load (resistor?) into the fixture, and run a calibration routine. It would adjust levels and save a gain constant in a file. The test routine would read the calibration constant stored in the file, and incorporate it in the calculations.
|
|
|
Post by stratotarts on Dec 11, 2016 18:56:07 GMT -5
You spoke of the phase in the other thread, is this the sort of data you're going for? Generally, yes. But this is the integrated signal. You need a different circuit to obtain Z. Basically you would use a polar to rectangular conversion (trig) to convert phase and magnitude to real and imaginary quantities. In electronics, the real term represents the resistance, and the imaginary term represents the reactance. Truthfully, people who are more familiar with the math can help better, especially because I'm kind of swamped with personal goings-on here so I don't have the time to hit the textbooks again.
|
|
|
Post by stratotarts on Dec 11, 2016 13:48:03 GMT -5
I am creating this thread to continue the sub-discussion at other thread and give it its own subject, because I believe it will continue to be "on the radar". It concerns member ms's offsite posts Measuring pickup impedance with an A/D recording interface and Software for performing pickup analysis with a recording interface. There are good reasons to pursue this approach. Mainly, by capturing a complete black box representation of the pickups behaviour by recording complex impedance at all frequencies, it may be possible to produce accurate bode plots for an endless variety of load situations, without any curve fitting or analysis (as might be required to interpret data recorded only as a bode plot). It would still be possible to perform such curve fitting or other analyses, but it becomes optional. Member ms has suggested a simple test circuit that allows this to be done without any specialized amplifier circuits (such as the V5 integrator). There was some discussion about the effects of cable capacitance. I think that ms says it does not cause any problems of loading of the device under test (pickup). I do have my doubts about that, but I may be wrong. In any case, I have two suggestions to address that. One is to use the high impedance portion of the V5 as a buffer, omitting the integrator. Another is to introduce a compensating leg in the drive circuit. The idea is to allow the effects of capacitance in the two input channels to cancel, like this: Honestly, I am not sure about this, it's just an idea, and I have no way to test it right now.
|
|
|
Post by stratotarts on Dec 10, 2016 17:52:37 GMT -5
The output of the SH-1n typical guitar circuit is -13dB at the self resonance frequency of 7650Hz. Any differences at that frequency are for all intents and purposes, inaudible. So I don't see the importance. It might be an overly simplistic model, but the effect of eddy current losses in all the experiments I have done, have been on the Q and never the frequency, except by extremely small numbers. I have performed many experiments where the components that produce eddies have been removed and compared - with almost no change in resonant frequency. The dominant eddy current loss is from the cores (if steel). Did you really replace the cores with something with the same permeability, but without the conductivity? That is not such an easy thing to do. How accurate is that self resonance measurement? How much capacitance does your set up have? I didn't replace any poles or cores. I'm referring to brass covers and the copper test loops that I used in my cover study. The input capacitance of the test box is 10 pF and the resistance is 11M. I'm not saying the resonant frequency is not affected by such things, I'm questioning the degree to which they are. Anyway, your approach is extremely interesting because it captures the phase response, therefore the complex impedance Z across the entire range of frequencies. Thus it is a complete black box model. However, attempting to interpret such plots directly seems to me unlikely to be very insightful. It would be better to allow software to transform the values into bode plots that are based on circuit models (either simple, as in the standard 200k/470pF load that we have adopted, or complex as John has done, where control circuit load models can be selected and tested. That is because those are the curves that directly define the tone. Essentially, one could proceed with practical application of the data and muse about the various effects of capacitance and inductance, rather than having to depend on them to obtain practical plots. I will have a good look at your stuff this week. Why not repost all your stuff in a new thread here? I'm sure it's going to generate a lot of focused discussion. Edit for an afterthought - you should test the idea of independence from cable/ADC capacitance by comparing results when adding a cap in parallel with the 1k load resistor. It's fine to theorize that it's okay, but it's a really easy practical test that would put to rest any concerns about that.
|
|
|
Post by stratotarts on Dec 10, 2016 14:01:41 GMT -5
The output of the SH-1n in a typical guitar circuit is -13dB at the self resonance frequency of 7650Hz. Any differences at that frequency are for all intents and purposes, inaudible. So I don't see the importance.
It might be an overly simplistic model, but the effect of eddy current losses in all the experiments I have done, have been on the Q and never the frequency, except by extremely small numbers. I have performed many experiments where the components that produce eddies have been physically removed and readings compared - with almost no change in resonant frequency.
|
|
|
Post by stratotarts on Dec 10, 2016 13:19:14 GMT -5
I'm still not sure what you mean, "you can see that the inductance does vary". I don't see anything like that on the chart. I have an SH-1 in front of me now, and I can confidently test the inductance using the swamping capacitor method. I did this using a 4.7nF and a 0.063uF cap, which produced peaks at 1130 and 315Hz, respectively. The results of the inductance calculations from that, are 4.14H and 4.03H. That is only a 3% difference. The imaginary part of the impedance with the effect of the capacitance removed is not a straight line, but rather is below it, increasingly so with increasing frequency. Therefore the effective inductance is less than the low frequency inductance. This is consistent with the apparent "anomaly" in resonant frequencies. I agree that there is not much drop between low frequencies and 1 KHz. Page 7, Least Squares Fit Parameter Results, upper left chart, "L-fit (henries)" vs. frequency. The inductance appears to peak at about 7H somewhere very low (the expanded scale makes it hard to interpret). It looks like it might be about 100 Hz. At 5kHz it has dropped to about 5H. Above that, there isn't much change. I don't have a P90 in particular to test, but assuming a self resonant frequency of about 8k and a loaded resonant frequency of about 3k (typical PAF) , from that chart the only difference would be about (5.2-4.7)H = 0.3H. I see how that would alter the resonant frequency, but I'm not grasping the importance. If the estimate for L at self-resonance is high, the only consequence that I can see, is that the capacitance might be underestimated. But in the application of that data, which normally is understanding how the pickup will perform in circuit, the self capacitance is usually a relatively minor factor. Edit - incidentally, this relates to my suggestion that it is better to measure inductance around 1kHz than at 120Hz (these are the Extech L meter settings).
|
|
|
Post by stratotarts on Dec 10, 2016 12:30:52 GMT -5
I'm still not sure what you mean, "you can see that the inductance does vary". I don't see anything like that on the chart. I have an SH-1 in front of me now, and I can confidently test the inductance using the swamping capacitor method. I did this using a 4.7nF and a 0.063uF cap, which produced peaks at 1130 and 315Hz, respectively. The results of the inductance calculations from that, are 4.14H and 4.03H. That is only a 3% difference.
|
|
|
Post by stratotarts on Dec 10, 2016 12:05:58 GMT -5
Thank you. That does look very interesting indeed. But, I can't find your system diagram. In the post that you linked to, there is only this: "The version used for these tests is shown here (p1010005.jpg). The jpg file is not accessible to me, although I admit it could be a browser issue. Have you considered consolidating the documentation in a stand alone document? Perhaps posting it here?
I didn't misread Carson's chart. I'm ignoring the slight rise between 10k and 1k.
It's doubtful that I have erred in estimating cable capacitance. It's an issue that we have been dealing with for almost a year.
|
|
|
Post by stratotarts on Dec 10, 2016 8:39:00 GMT -5
Welcome to the board. It is important to have as many different investigators bringing different approaches to the subject, in order to add a strong measure of verification to the studies. The online history of pickup investigation includes a number of "lone wolf" experimenters that sometimes got sidetracked because of the lack of peer review and support. I was one. This post is very interesting, but I think you will need to give more background and detail of your investigation, or nobody will be able to understand it. As I do not have any idea what your experimental setup is, I find it difficult to separate theoretical statements from actual observations. This is not intended as a criticism of your comments, as they are more than welcome. It's just that the context that makes them clear to you, may not be shared by many people (even) here. I measured the inductance of the SH1n at different frequencies here: thread link and it looked like the inductance does not vary much. Do my measurements not cover a wide enough frequency range to detect the effect you're talking about? The least-squares fitting that Dan Carson did ( link) found no increasing inductance above 1 kHz.
|
|