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Covers On or Off Humbuckers?

Discussion in 'Just Pickups' started by texmck54, Nov 18, 2015.

  1. jvin248

    jvin248 Poster Extraordinaire

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    A guitar pickup that close to a running laptop is much more than a trivial amount of noise. It is surprisingly noisy.

    Easy test for others that want to evaluate the noise issue is use shielding tape (leave the backing on) to create a 'cover' and hook a ground wire to it. This can be done under strings at tension so you can see 'with and without' A/B comparison.

    As for the comment/question on Strat shielding under the pickups ... that's what Lace Pickups pretty much do:
    [​IMG]
     
  2. Antigua Tele

    Antigua Tele Friend of Leo's

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    Eddy currents are caused by the current they opposed, which is why they are somewhat uniform in how they cause an added impedance. Are you referring to the time dependent effects of hysteresis? Do you know for a fact that the hystersis effects of a pickup result in audible distortions, such as the cancellation of particular frequencies, or are you talking in terms of theoretical differences? The bode plot " amplitude by frequency" analysis for the '59, with the cover in place, has a rather smooth curve, I didn't observe any unusual nicks or notches, as you'd expect if there were to be audible distortions at give frequencies or frequency ranges. For example, suppose eddy currents were to cause an especially high impedance at 10k, we should see a dip in the bode plot at 10k, but nothing like that is apparent.
     
  3. Derek Kiernan

    Derek Kiernan Friend of Leo's

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    It's magnetic and affects harmonics, so you can't measure it as a frequency response or impedance. Even if you could isolate it down to frequency where the harmonics are canceled, it makes no sense to graph as a general response, as it would be different across strings.
     
  4. Telepathist

    Telepathist Tele-Afflicted

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    Off = hotter
    ImageUploadedByTDPRI1448238338.348213.jpg


    ( ''""-.-"""-.
    !!!!=I===I=======(==) :
    ( ,,,,-..___.'
     
  5. Antigua Tele

    Antigua Tele Friend of Leo's

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    OK, so you're not talking about eddy currents induced electromagnetically by the guitar pickups coil, you're talking about eddy currents that the moving magnetized string itself incites in the nickel cover, which would then counteract the magnetic field of string, sort of like an "eddy current brake" that works against the string itself. Is this what you're saying? That's a novel idea, I've never heard this stated as a draw back to pickup covers.

    What mechanism would make this effect harmonic dependent? What would cause the roll-off to not be uniform? Whey would specific harmonics be subject to cancellation by eddy currents?

    How would you determine what percentage of losses are due to the moving string creating counter-active eddy currents, versus the coil creating counter active eddy currents?
     
  6. JD0x0

    JD0x0 Poster Extraordinaire

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    My covered buckers are just as noisy as the uncovered ones and my singles (excluding the 60hz hum) as well, in that situation (facing a running laptop)

    I recognize the cover adds shielding, but doesn't seem to make too much of a difference when the pickup is pointed at a laptop, IMO.
     
  7. Derek Kiernan

    Derek Kiernan Friend of Leo's

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    http://www.billlawrence.com/Pages/Pickupology/eddy_currents.htm

    Eddy currents alter sound and output of a pickup and play an important role in pickup design.

    Eddy currents are induced in metals in the vicinity of an AC magnetic field, creating a secondary magnetic field which opposes the inducing magnetic field of the coil. The dimensions, conductivity and permeability of the metal, along with the frequency of the current in the coil, determine the magnitude and phase relation of the eddy currents. An internal short in a pickup coil forms a conductive loop which, also, becomes the source for internal eddy current interference.




    The movement of the magnetic field is excited by the movement of the string, so you don't need to have the eddy currents acting "on" the string to manipulate the harmonic content translated to the signal output by the pickup.The magnetic fields generated by eddy currents are insignificant to the string movement itself.

    The partial and variable harmonic cancellations are way some pickups have strange tonal shifts. If it were simply an issue of resonance, then Bill couldn't make an Alnico poled pickup any clearer or less edgy than anyone else, just one that has sharper coloration (if it was only about the resonance) or a bit sweeter (if we're focusing on actual highend losses outside the resonance). If the partial cancellations weren't happening in this instance due to eddys induced by shorts in the coil, then you have no ability to explain the tonal shifts that occur that make a pickup harsh or edgy. Some pickups are clearer throughout the entire range of the tone control, while some pickups can basically be muddy even while sounding harsh, no matter what setting you're on. If all eddy currents did were lower the Q and attenuate the resonance, wouldn't you universally expect a pickup to be less harsh or less edgy, not more?



    You can use the Q at 1 kHz to calculate the impedance change due to eddy currents by isolating it as part of the effective electrical resistance, but you potentially have to account for inductance changes in some instances with covers when interpreting the Q.

    Since it's the relation between the inductive reactance/effective electrical resistance, you can measure the inductance and use a quick inductive reactance calculator like here: http://www.electronics2000.co.uk/calc/reactance-calculator.php

    If you get a 2.2 for Q and you know the inductive reactance, you can work backwards and get the effective electrical resistance at the frequency. If you get a different Q with a change, the relationship between the two effective electrical resistances gives you a percent difference. [this is perhaps more for everyone to understand how the testing works]


    Beyond the losses you can test due to impedance, to calculate out what the actual voltage output losses to a given note on a string would require actually figuring out how to do a comparison across all the entire harmonic spectrum in a particular setup and consistently reproduce the same note with the instrument, but it's unlikely that you're going to be testing two pickups with the same design, one perfectly free of eddy currents, the other plagued by them. It's understandable that very few have approached it in a rigorous way, especially considering the industry doesn't talk about eddy currents at all.
     
  8. Derek Kiernan

    Derek Kiernan Friend of Leo's

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    PAF-style humbuckers are not the best at noise canceling partially due to asymmetries in the design. Metal covers definitely can help in that department.
     
  9. guitarmikey

    guitarmikey Tele-Afflicted

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    On my natural Sheraton Epi changed the pups to Gibsons Classic '57 with golden covers on. On my Tele, added a nickel cover to the SH-1 I mounted in neck, it sounded too bright without that cover.
     
  10. 24 track

    24 track Doctor of Teleocity Silver Supporter

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    I have 2 epi's a sheridan with and a 335 dot studio with out covers there is a marked difference in the A/B between the 2 guitars the dot has a blusier tone the sheridan has a jazzier tone, I like them both for that reason
     
  11. Antigua Tele

    Antigua Tele Friend of Leo's

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    Citing Bill Lawrence is of no use, because he's no longer alive, we can't contact him for clarification, and he spoke in generalities. To prove that something is actual and real (for your own benefit as well as everyone else's) you have to be able to prove is back to the fundamental laws of physics. It's a low quality reference because it leaves more questions than it answers. I think by "phase relation" he must be referring to hysteresis or B/H curves, as described here http://www.allaboutcircuits.com/tex...chpt-9/mutual-inductance-and-basic-operation/ , but who knows.

    How can you say with any certainty that eddy interaction with the string itself is insignificant? Are you just taking a guess, or is that a physical basis for this?

    I'm confused as to why you mention harmonics at all in relation to a pickup's function, since their frequncy dependent transfer characteristic are all that is relevant. For example, the 2nd harmonic of an open E is a 12th fretted E. In either case, the pickup is seeing induced voltage at a frequency of 165 Hz, why would the pickup care if the 165Hz came from an open E harmonic, or a 12th fret fundamental?


    You can't say, "If not for ABC, we would not hear XYZ", because XYZ is subjective. The fact that you believe you hear XYZ is not proof of XYZ's existence. IMO, what you describe as being "harsh", "clear" or "edgy" is merely the result of how you interpret difference resonances and Q factors, and not an indication that something more complex is occurring.
     
  12. GCKelloch

    GCKelloch Tele-Afflicted

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    Ah, I found you. You’ve been doing a lot of research, lately. Bravo. Don’t worry about the detractors, but do consider that most of what you are doing has already been done by several others before you, and as far back as the 50’s by Willi L Stich (Bill Lawrence). I hope you have learned some history of Bill, and perhaps studied his findings since the 50’s at Framus, and his 60’s era designs used by several jazz greats, then mentoring Larry DiMarzio, through being chief engineer at Gibson throughout the 70’s, his innovative L500, L45 & L90 blade HB and noiseless NF SC designs since, to his highly efficient, sensitive, versatile, low Q, low noise MicroCoils shorty before his passing. I and many others here and in other forums knew him. You can do research on him yourself, and you can do worse than try to understand his writings and patents without imposing your own perspective on it all, as I had done many times, and been repeatedly “scolded” for. I’d say it’s ground zero for understanding guitar pickups.

    Opposing magnetic eddy-currents is a well-established phenomenon in diamagnetic materials. There are also paramagnetic materials which create a field in the same direction as that which is applied. It’s not the same as hysteresis, which I had trouble grasping at first. It’s similar, though. Hysteresis is how the lingering magnetic charge in a material affects the transfer function. I never learned how it affects the sound in a pickup, but there could be an minor interaction between the magnetized string and the core material. If you accept the “fact” of diamagnetism, you can deduce from it that a diamagnetic metal cover will create at least minor opposing fields below the strings, which can somewhat cancel the field in the string. Bill did mention that eddy-currents can affect string harmonics. It may not affect a strong fundamental fretted high on the neck much, but that same opposing force can then affect the higher weaker harmonics to partially cancel them in complicated ways. That can create note phasiness that may sound harsh or not.

    I think there is some confusion about coil vs cover vs core eddy-currents that may have been lost in translation to those who spoke with him on the subject. Bill was a pioneer in the field for sure, but there are other very smart people working out the related science. We had at least one discussion with a few of them over at the ME forums. Bill's writings and patents are very concise, often taking several readings to fully grasp. The literal nature of it can be hard to accept in a way, but he did mean exactly what he wrote...and no more or less. That’s the part I struggled with for years.
     
    Last edited: Jun 27, 2016
  13. rigatele

    rigatele Tele-Afflicted

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    I have the greatest respect for Bill Lawrence. He really "got it". I'm collaborating with Antigua Tele in his measurement efforts. I do not believe that "it's all been done before". A lot of pickup research has been done, but the field lacks scientific and standardized approaches to measurement. Also there is not enough quality published material, and it lacks generality, often focusing only on specific theoretical aspects.

    Many theories are expounded about effects that, while having a sound scientific basis, have not been evaluated quantitatively with sufficient confidence to assert that they have an audible outcome. Furthermore, much of the research remains proprietary, thus self interested and biased. A patent is not a scientific claim, it is a claim on the value in the marketplace that an innovation has. In that case, the scientific validity is not necessarily of interest to the inventor, and often can not be fully validated by the examiner.

    Additionally, the advent of new technologies and component materials has left a long gap in time since when a lot of the original work was done. Concrete and complete measurements are not currently available for more than a small handful of actual product types.

    So, I say the work has barely begun.
     
  14. Antigua Tele

    Antigua Tele Friend of Leo's

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    Thanks for chiming in. IMO, the biggest problem is whole issue of psycho acoustics. Pickups are very simple, but people believe the differences they hear from one to the next are very complex, and this means the pickup itself must be complex. What they don't realize is that what makes it complex is psycho acoustic phenomena, not the pickup. I'm not a professional physicist, but I've done enough research to determine that there isn't enough physical content in a guitar pickup to justify that level of complexity that is attributed to them.

    Take scatter winding for instance. The physics says that at best, it can only marginally impact parasitic capacitance by putting additional space between the wire, and marginally impact inductance by reducing inductive coupling, but only to a very tiny degree, in no way that makes a pickup special and distinctive. People believe pickups are complex, because their ears tell them it is so, and they believe the pickup winder has something to do with this because they become brand loyal to certain winders, and they know that the layering pattern is specific to a winder to some extent, and so they make the logical leap that the "scatter wind" is the hear and soul of the pickup, a unique fingerprint that nobody else can duplicate, and so a reputable winder can charge a lot of money for a pickup and somehow people think they're getting a deal. If people can be informed and convinced of the power of psycho acoustic phenomena, then hopefully the bottom will fall out from under these myths.
     
  15. rigatele

    rigatele Tele-Afflicted

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    That is one problem with the approach I'm currently taking. I assume that a really thorough technical measurement would be accepted as a bottom line by most people. But that may not be the case. First of all, they have to understand what it means. Secondly, they may claim that it isn't valid because it isn't what their ears tell them. They may argue about the testing methodology, but at least that discussion takes place (or should take place) in a technical context. So I see the myth busting in two phases. Phase one is to determine the actualities by technical means. That may or may not support various theories. Phase two deals with the unsupported theories. These can only be dealt with by double blind testing.

    The logistics of technical testing are much easier (at least, for me). The fact that it can be easily achieved is what is motivating me to put a lot of time into it now. The second part, blind tests, require organization and involvement of many people, and in order to be credible, really require a lot of money and work by many people. So I don't expect to get involved in that, at least now. It certainly would be fun! I feel that I should mainly limit my assertions to what is technically provable. I believe that planting the seeds of doubt is enough.

    Furthermore, an important aspect of my efforts now is to generalize and popularize what I am doing, and by making it more of a global effort, to promote an ego-free body of information that everyday guitar players could use to make rational and economical buying decisions. I am trying to say, my goal is not just to bust myths and leave a horrible sucking vacuum. I'm hoping to entice people with the prospect of having more real control over the options that they choose.

    So I hope that people who have been working with pickups for a very long time don't see me as a self-proclaimed expert. I'm deliberately going slow and verifying everything I want to know. I hope that they would cooperate, for example by perhaps providing access to samples, so I don't want to alienate them. But, if a fact based approach is not palatable to them, that may not be possible.
     
  16. whiteop

    whiteop Tele-Afflicted

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    On too. Only thing I hate is that EVERY Seymour Duncan pickup cover or SD pickup with a cover that I buy the finish always goes bad. Every single one and you have to buy their pickup covers because the standard ones won't fit.
     
  17. jvin248

    jvin248 Poster Extraordinaire

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    Psycho acoustic effects become a non-issue with blind and double-blind testing, which is not used for most of the 'comparison' testing and hence all the near-religious fervor on both sides of any comparison. At the end of the day it's really difficult to know if the 'blue' sky is the same shade of 'blue' between several people - it's what 'blue' the sky has always looked like to each individual. Lacking solid comparison metrics people fall into using general metaphors for what they experience.

    Loaded statements can lead to people digging in on their belief system and not listening to any alternative views "scatter winding for instance. The physics says that at best, it can only marginally impact parasitic capacitance by putting additional space between the wire, and marginally impact inductance by reducing inductive coupling, but only to a very tiny degree, in no way that makes a pickup special and distinctive"

    A repeatable measurement system is necessary, one that anyone can duplicate and successfully produce the experimental results themselves. Best yet if the measurement system is common, simple, and inexpensive to use (like ohm and volt meters).

    Keep the passion on uncovering the truths and falsehoods of the old myths. Focus on the measurements with numbers and we'll all benefit.
     
  18. rigatele

    rigatele Tele-Afflicted

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    The type of metal that is used for the cover makes a huge difference. Brass has the most influence, while some of the alloys ("nickel silver", I think) are more transparent. All of them will reduce the output somewhat, but the latter type do so very uniformly, so a slight adjustment up of volume will leave the sound mostly unaffected. The brass ones create a dip in the mids and highs, while not affecting the lows so much. This is clearly visible in testing.

    This is what I have on two guitars now, but I'm thinking of going back to solid covers eventually if I can get some really good ones:
    zebra_opencover.png
    One problem with that is that they should really be wax potted after the cover is put on, not before. I've done it but not with vacuum, and I'm thinking it's important to do it that way. I'm not set up to do that.
     
    Last edited: Jun 30, 2016
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  19. Antigua Tele

    Antigua Tele Friend of Leo's

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    It seems you think those underlined portions are subjective, but they're really not.

    It's very improbable that scatter winding reduces parasitic capacitance by more than 10 picofarads, just from having compared and computed the parasitic capacitance from a variety of "scatter wound" and machine wound pickups. Guitar cables increase the parasitic capacitance by up to 30 picofarads per foot, so just using a longer or shorter guitar cable negates the gains or losses.

    It's unlikely the scatter winding would reduce the inductance by more an 100 millihenries, and that's being very generous. You can decrease the millihenries by 100 by simply putting fewer turns of wire on the coil. The permeability of the materials surrounding the coil will also throw the inductance off by a millihenries. A ceramic Strat pickup for example is about one 1 henry (that's a lot) greater than a classic AlNiCo pickup due to the steel in the pole pieces alone.

    Even if the values are not precise, the relationships between these factors is precise. There's just no room for anything resembling a "fingerprint", or a pickup winder's magic touch, to squeeze in to what is a matter of well established physics.

    The inductance and parasitic capacitance don't impart sound qualities on their own, they combine together to determine the resonant peak of the pickup, which is it's most distinctive and audible quality. A longer cable adds parasitic capacitance and lowers the resonant peak, an unmistakable difference in sound one you know what you're listening for. A higher inductance by itself will also mean more output voltage, but raising or lowering the pickup, or using larger or smaller or more permeable guitar strings also increases or decreases output voltage, so that too is a moving target, and not characteristic of the pickup.

    If you learn all there is to know about inductor and transformer design, most of these issues come up in those scenarios, and many hundreds of millions more inductors and transformers are produced than guitar pickups, so that is where the wealth of knowledge is, and you will see that issues like scatter winding don't exist in that realm, but certainly would if there were any practical value in it. If an inductor designer is serious about reducing parasitic C, they don't "scatter wind", they'll use what is called a "basket weave" or a "spiderweb" coil design. They would probably laugh at the idea of "scatter winding" to achieve those ends. Issues such as wire insulation thickness, magnetic coupling and eddy current losses are very much known and discussed, all of which relates directly to the performance of guitar pickups just as much as it relates to inductors and transformers.

    It's telling that pickup makers don't say "our pickups sound best with 500k pots with a 5% +/-, a 10 foot guitar cable, set 4mm from 10 gauge D'Addario strings, into a guitar amp with an input impedance of 500K ohms", because if you diverge from a fixed set of values, the characteristics of the pickup will also diverge. Most of them seem to realize their products aren't magic, but nor are they quick to correct customers who believe they are.

    You see a lot of people say "this pickup has a flat response" or a "mid scoop" or "tight low end", and the truth is, from a technical standpoint, neither is usually true since the bass and midrange of nearly all pickups is fairly flat, with the differences occurring in the treble range. That's just the way they perceive the outcome in their particular setup, and people should know that distinction "this pickup has a flat response to my ear, in my particular guitar, through my amp with these particular settings". I've also been seeing people talk a lot about pickups "compressing" - doesn't happen, in fact I observed an excess of 10 coming out of my test pickup the other day, I'm probably lucky I didn't fry it.
     
    Last edited: Jun 30, 2016
  20. Antigua Tele

    Antigua Tele Friend of Leo's

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    I think if you just create a step by step "how to" guide for setting up a testing rig, and then put that on your blog, then more people will be inclined to try it out. Make it clear that the integrator is optional, but explain why someone would want one, since that might be a high hurdle for many people (unless you decide to sell them, of course). The USB oscilloscope and driver test coil can be had for $180, some wire and a suitable bobbin, and the integrator was about $30 in parts, give or take, so it's an educational, budget friendly activity. I'd like to try the Rightmark software also, it would just be nice to make it all work with an actual USB oscilloscope.

    The hardest part for me has just been scraping together information from disparate resources. Your first blog entry http://kenwillmott.com/blog/archives/152 was very helpful, but it contains some statements that are non obvious to non electrical engineers. Just one example, you had said about your exciter coil "The ferrite E cores that surrounded it were easily removed", presumably that was desirable both to decrease the inductance and keep the core from increasing the inductance of the pickup being tested, but that's not immediately obvious to most curious guitarists. It's not even apparent to many people how an exciter probe can use used as a stand-in guitar string in the context, and even that is very interesting in it's own right.

    The 'smart guys' over at the MEF forum take the view that if you don't have an education in electrical engineering, then you'd have no business pursuing this endeavor, and they lack much patience for the layman, but that bar eliminates 99.5% of us, so if that's the position one wants to take, this stuff will just remain a mystery. There's a saying "if you can't explain it to a six year old, you don't really understand it", and I don't know if that's true, but I think it says that it's certainly worth attempting. The payoff is huge if you succeed.
     
    Last edited: Jun 30, 2016
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