Dead Magnet

Discussion in 'Just Pickups' started by 61fury, Mar 23, 2016.

  1. Antigua Tele

    Antigua Tele Friend of Leo's

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    I actually put this theory to the test. I took a pickup with no magnet in it, measured the Gauss, and it was like 3 to 5 due to extremely low residual magnetism in the slugs. I hooked the pickup up to an amp, measured again, and the Guass didn't change at all. Any small quantity of voltage coming from the amp only produced microscopic amounts of electromagnetism, if any at all. I now have a test pickup that has not slugs or anything, I'll test it out tonight and see if I find the same result in that case as well.

    OTOH, I tried my best to demagnetize an AlNiCo bar by exposing it to the opposite polarity of a neodymium, and IIRC I couldn't get the bar to read any lower than 10 or 20 Gauss, where it would typically read around 500. So my belief is that the magnet in OP's pickup is not truly dead, and that it's producing some magnetism, even if it can't be directly felt.

    Another thing I discovered is that the audible output won't be linearly proportionate to the magnet strength. For example, if you take a 500 Guass AlNiCo and take it down to 250 Guass, the audible output doesn't drop in half, in only becomes a tad quieter. It's more of a change in tone than audible output, so the magnet can get pretty weak and you will still hear something.

    I've tested the individual strength of all six pole pieces in various Strat pickups in my guitars and much to my surprise the pickups retain good string to string audible output, even when the amount of Guass varies widely from magnet to magnet. I found that a lower charged magnet will give slightly a softer, slightly less harsh top end, and that when there were under charged magnets among fully charged ones, it was impossible to perceive any string to string variances when strumming full chords. If the whole pickup is made up of undercharged magnets, THEN you will hear that low-Gauss difference when strumming a full chord.
     
    Last edited: Apr 12, 2016
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  2. Zepfan

    Zepfan Poster Extraordinaire

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    From the article: Pickups can be either passive or active, passive being more common and cheaper. Passive pickups receive their power through the amp via the input cable. When the amp is turned on, the electricity flows through the pickup to create the magnetic field. However, active pickups require a battery to power the pickup. They have a lower output so it uses the battery as a preamp to boost the guitar's output signal. When you plug the cable into the guitar, the battery automatically turns on the preamp, pickups and all. To shut the battery off, you simply unplug the cable from your guitar's input jack.
     
  3. JustMike

    JustMike TDPRI Member

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    Just to keep beating that horse. In that link it says that pickups recieve power from the amp. If that were true you could measure power from the amp input with no guitar attached. You also would measure no AC signal at the output jack of an unplugged guitar with an oscilloscope or voltmeter. The only voltage/current from a guitar with passive pickups is from the magnetized strings vibrating back and forth across the pickup coil. I'm just a random guy on the internet but you can find an interview of Seth Lover by Seymour Duncan where Seth discusses the same thing.
     
  4. Zepfan

    Zepfan Poster Extraordinaire

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    Oh like this.
    The idea that the strings movement is what generates the needed current to make the pup work would require a lot more movement from the string than actually happens and the magnet would have to be a lot stronger for that to work.
     
  5. Antigua Tele

    Antigua Tele Friend of Leo's

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    This is not true, and you can prove as much at home with a volt meter. As an example, if you go to DiMarzio's site http://www.dimarzio.com/pickups/humbuckers/paf-36th-anniversary it will say "Output mV: 250" , or whatever the value happens to be, for every pickup they offer. They get that value by plucking a string in some uniform manner and measuring what comes out of the guitar, voltage wise.

    If it were true that the amp fed voltage to a pickup, by way of a DC bias, and somehow created and electromagnetic field that would interact with the strings to create a line AC, for one thing, you need several volts of DC at a minimum, but also you'd have no need for the permanent magnets, all you would need is perhaps the permeable core. It's interesting to ponder if it could be done, but obviously the existing passive pickup scheme is a lot more economical.
     
    Last edited: Apr 12, 2016
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  6. teleaddicted

    teleaddicted Tele-Holic

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    Seems that in the early '80s String Vision used to make an acoustic guitar soundhole pickup that worked like that.
    The pickup was a simple coil with no magnet, but you had a magnetic "magic wand" to magnetize the strings.
    You obviously had to have steel strings on the guitar and preamplify the weak signal.
    Never had the chance to try one though.
    (It was described on the "Guitar Electronics for Musicians" by Donald Brosnac).
     
  7. JustMike

    JustMike TDPRI Member

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    The string vision pickup is a perfect example. The strings need to be ferromagntic (steel, nickel, etc.). When you magnetize strings ...or other metal you are aligning the atoms in the same direction, the more atoms you align the more magnetic field up to the point where they are all aligned, after which you can't get a stronger field. You can demagnetize the metal by adding energy like a physical shock or heating to randomize the atoms. If you want a bigger magnetic field just use thicker strings to add more atoms to be aligned. Electricity and magnetism are two side of the same coin. One of the four forces of physics. Move a wire thru a magnetic field, electricity is induced in the wire, the energy comes from the movement. Move an electric current through a wire and a magnetic field is produced. The field comes from the movement of electrons (current). Adjustable pole pieces simply change the shape of the magnetic field to more fully magnetize the strings. Strings stay magnetized until some outside energy, (heat, vibration, external magnetic field) randomizes the atoms. The permanant magnet in the pickup keeps the atoms aligned.
     
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  8. Antigua Tele

    Antigua Tele Friend of Leo's

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    For a typical guitar pickup not requiring a magnetic "wand", the strings only have to be permeable. It's not necessary for them to retain magnetism since the they are always in the presence of a magnetic field anyway.

    Although it brings up an interesting point: as the string moves away from the coil, the drop in flux density in the coil will be higher if the string does not retain a charge, versus if it does. Therefore, for a given permeability, a string with a higher remanence (retains more magentism) should produce a bit less output than one with less, and maybe even alter the harmonic structure of the output signal ever so slightly.

    I wonder how long a typical guitar string holds it's charge, and if it discharges fast enough to lose a significant amount of remnant magnetism when it's at its furthest point in travel away from the string.
     
  9. jefrs

    jefrs Doctor of Teleocity

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    Pickups 'work' by the vibrating string interfering with the magnetic field from the magnet thus inducing a current in the coil: they do not need to be strong magnets, indeed it is usually better if they are not (or they suck the strings down).

    Try testing the magnet not as a fridge magnet but with something small like a cut off end of a thin guitar string (nickel and iron are both magnetic)
     
    Last edited: Apr 12, 2016
  10. tpaul

    tpaul Poster Extraordinaire

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  11. Antigua Tele

    Antigua Tele Friend of Leo's

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    That's just what many intuitively believe, and it's often repeated, but it's not true.

    I'll try to explain why. First, per Faraday's law, voltage is induced on a conductor when the amount of flux changes (gaining or losing), and the voltage is higher when the change is more rapid. So we need flux to change, and it needs to change a lot. Where do you think most of the change is deriving from a) a magnetic field that is being "interfered with", or b) a magnetized string that is moving widely and rapidly?

    But it goes beyond that. Not only is the string and it's associated magnetic field moving more than the coil or pole pieces and it's associated magnet fields, but they are not even moving or changing at all. The magnetic lines of flux in the magnet and/or pole pieces does not change, because the geometric shape of those objects, and the amount of magnetic charge they possess, do not change, they are static.

    As the string moves through the permanent magnetic field, the "change" you intuitively expect in the shape of the magnetic fields is really just the vector sum of the immobile permanent magnet (or pole pieces) and the moving string combined together. When you put a permeable material near a magnet, it does not "suck away" flux from the permanent magnet, but rather it's own atoms align, and it becomes it's own magnet. The portion of the vector sum that represents change in the flux density in the pickup coil, thereby inducing a current, is 100% from the magnetized string, and it's moving magnetic field. The permanent magnet's flux is all around, yes, but as it does not change, it does not cause current.

    In summary, the purpose of the permanent magnet in the pickup is to charge the string, and nothing more. The amount of attention that is placed upon where they are relative to the coil is misguided. Their significance to the pickup's operation becomes secondary, such as "does the AlNiCo alloy produce eddy currents in response to the coil's time varying magnetic field?"

    As cause and effect causes these various magnetic fields to form in and around the pickup, you can think of them as all being isolated and existing by themselves. You don't have to ask, "what does this magnetic field do since there is a stronger magnetic field near by?" because they don't interact that way. For example, nobody ever talks about how a more powerful ceramic magnet will interact with the magnetic fields of eddy currents elsewhere in the pickup, because it simply won't. Here's an analogy, you have a 10K candle power flashlight. Then you shine a 1 million candle power flashlight at the 10K flashlight. Does the brighter 1 million candle power flashlight change the amount of light coming out of the weaker 10K flashlight, or otherwise effect it's operation in any way? Now, if you make the 10K flashlight blink Morse code, the amount of change in light as it turns on and off will still be exactly 10K candlepower. The presence of the brighter 1 million candle power light does not cause any more or less light to be added and subtracted. Such is the case of a stronger, static magnetic field being in the presence of the weaker, but moving, magnetized string.
     
    Last edited: Apr 12, 2016
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  12. Zepfan

    Zepfan Poster Extraordinaire

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    Okay answer this one. When building a guitar or putting in pickups, I always plug in a cord to the amp and tap on the pickup with my finger and tap the body to make sure the pickup works before putting the strings on.
    So how does the pickup work and put sound through to the amp without the strings being on the guitar?
    The pickups poles had no contact with any metal, there wasn't any strings vibrating over the poles to initiate any field.
     
  13. Antigua Tele

    Antigua Tele Friend of Leo's

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    Microphonics from the coil.

    When it comes to dampening, there are two relevant types of forces, stronger impact forces (such as tapping on the pickup) and weaker sustained forces that are able to induce resonant movement. Wax potting is able to absorb enough movement so as to prevent a weaker force from inducing sustained resonance, which is what feed back is, and why it takes a moment to become loud, but the wax doesn't dampen the coil well enough to suppress transient impact energy.
     
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  14. Zepfan

    Zepfan Poster Extraordinaire

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    When it comes to strings making the pickup work by movement and there's no strings on the guitar, then the pickup had to get a field from somewhere to be able to do what you just stated. And the movement of magnetized strings over the magnetized pole would be needed to generate the field. Also have to consider some guitars that have these kind of pickups, but use Nylon strings. No metal to be magnetized there.

    I found a write up on the net about electrons from tubes or capacitors flowing through the cord to the guitar's electronics on their way to ground. I asked Robrob about this since he has experience in this area and he said, "There is grid voltage that goes to the guitar through the cord to ground". This is a small amount of voltage/current that would be present at all times. Think about the "Death Cap" on old tube amps, the current flowed through the input channel on it's way to ground, but in a deadly amount of current instead.

    This flow of small current would give the guitars electronics and pickups an "energizing" from flowing electrons to ground, which is why current shows up on the input channel through the voltmeter. This is why the pickup in my guitar without strings to moved transmitted sound and guitars with Nylon strings with magnetic pickups will work.

    So actually we are both right in many ways.
     
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  15. Antigua Tele

    Antigua Tele Friend of Leo's

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    The magnetic field comes from the permanent magnet. When you tap on the pickup, the coil windings very briefly changes position within the magnetic field. The flux changes, a voltage is induced. The reason a wax potted pickup is less microphionic is because it restricts the wire's ability to move, but they aren't completely immobile, for when you tap on the pickup, it sends a shock through the windings which registers as movement, and so a voltage is induced.

    It's is not necessary for the amp to provide voltage to the pickup. You could plug your guitar into a volt meter, pluck the string, and you will see voltage.

    Piezo transducer; it's a different thing.

    Just read up on Faraday's law, it says nothing about needing an external power source in order for magnetic induction to take place. The whole premise of an electric generator is that it produces electric current where you had none before.
     
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  16. telemnemonics

    telemnemonics Doctor of Teleocity Ad Free Member

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    The moving magnet inducing current in a coil principle applies to many electric devices like electric motors and generators or alternators.
    A generator like the gas powered unit for power outages induces current via a moving magnet and coil, with no electric current applied to the generator (alternator) to make it function.
    If you move a magnet next to a coil, a current is generated in the coil.
    This works the other way around as well.
    If you apply current to a coil wrapped around a piece of iron, it generates magnetism in the iron.
    This may be what gets many of us mixed up, like the writer for Premier Guitar.
    But a lot of Premier Guitar writers seem a little mixed up.

    A vinyl record stylus has the same moving magnet and coil design, and the amount of movement of the stylus (magnet) is much smaller than a guitar string, yet the current it generates in the coil is enough for the preamp in your stereo to amplify.
    Then there are moving coil phono cartridges that generate so little current you need a preamp before the preamp in your stereo.

    I would guess that the foil pickup magnet was weak enough to not lift its own weight and hold to metal, but had enough gauss to magnetize a string a little bit.
     
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  17. Zepfan

    Zepfan Poster Extraordinaire

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    If the coil winding's of said pickup were that loose I'd have a real pickup problem. The coil is tight around the bobbin I assure you. How do you know if said pickup is wax potted or not? Have you ever talked through a pickup before? Can be done with the volume turned all the way up and there's not any shock to the pickup in that scenario or strings moving to cause the result. I'm just going by the link that was provided earlier that mentioned that metal string that are magnetized and move is what makes the pickup work. Kind of a reach to say it worked because of loose windings. Said pickup has been in a hollow body guitar before without microphonic issues.

    I didn't mention a Piezo. Have you ever seen a electric bass with nylon strings? I have and the nylon strings work without being magnetized.


    We know that pickups convert vibration to signal, but is the magnet and coil strong enough to create sufficient electro-magnetic field on it's own or does it need help? That's the real puzzle. It's hard to tell because of the presence of grid voltage being put into the guitar from the amp to begin with. Testing a guitar separate from an amp would still have a field present and take awhile to fade away.
    A electro-magnetic coil that can produce it's own field has to be very strong like a magneto, and a pickup that strong would most likely have the strings stuck to the magnets all the time, assuming they are metal strings that is.
    All Generators, alternators and electric motors have electronics and a power source that energizes them. Automotive applications have a battery source while home/industrial have wall voltage. The only motors or generators without a battery/wall voltage are old types with magneto's that have to be turned with the force of a gas powered motor. I should know since I was a auto/motorcycle/small engine mechanic, rebuilder of generators/alternators/electric motors and automotive electrician for 20 years.
     
  18. Tony Done

    Tony Done Friend of Leo's

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    ^^^heh? The standard magnetic pickup is a generator, nada mas. The electric current/voltage it generates in the coil comes for the changing magnetic field resulting from the string moving in the magnetic field generated by the magnet, and is sufficient to drive an amplifier preamp. Or, the strings could be magnetised and wouldn't need the magnet in the pickup. Either way, it is you picking that provides the energy, not some external electrical source.

    Your bass string might have had ferromagnetic cores or windings.
     
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  19. Zepfan

    Zepfan Poster Extraordinaire

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    I think that the only way to be certain that the pickup doesn't need to be energized by the amp's grid voltage, would be checking a guitar that has spent the last year in a case under someone's bed or in a closet.
     
  20. Zepfan

    Zepfan Poster Extraordinaire

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    As far as I know, nylon strings don't have any magnetic properties.
     
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