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Shielding, why would it help mag hum?

Discussion in 'Pickups & Electronics [BG]' started by peakdesign, Sep 28, 2008.

  1. peakdesign


    Aug 25, 2008
    Forgive me if this has been raised before. Most of the hum in a passive bass with mag pickups seems to come from the pickups themselves. You can see this by moving the instrument around in the interference and by the fact that hum from two single coil pickups cancels out to a large degree by being added out of phase when the balance is centered. So 1) how much improvement can you expect out of (electrostatic) foil shielding that has obviously NO effect on magnetic fields? and 2) what prevents a redesign of the circuit using lower resistance pots (and a bigger cap) to lower the whole impedance of the circuit and reduce electrostatic hum pickup that way? 3) where can I get data on typical Jazz pickups, the resistance, the frequency response, impedance at different frequencies, etc?
  2. Stealth


    Feb 5, 2008
    Zagreb, Croatia
    I'll just explain #1 and let others handle the rest. Most types of metallic foil (including copper tape) dampen or eliminate the magnetic fields extending across the surface, where they'd normally pass through if the shielding was, say, plastic or glass. As you said, it's not an electrostatic effect, but it's closely linked - effectively, the shielding works as a Faraday cage which blocks off EMI from all directions it's applied on. Obviously, that makes the open surface (that is, the one where the pickup's exposed, outward pole ends are) still susceptible to EMI, but it's still better than no shielding.

    To expand on it - where there's electrons, there's electrostatic forces. Where the electrons are moving due to external EMI, there's a magnetic effect and the corresponding counter-force. That's why Faraday shielding works.
  3. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Interference isn't just from magnetic fields, it's also from electrical fields. Yes, they are two parts of the same thing, but effect pickups in different ways. Humbuckers don't block out magnetic interference, just electrical interference.

    Electrostatic shielding helps with the magnetic field noise, even though it doesn't block magnetic fields.

    Lowering the value of pots and caps will give you a very low output and a dark tone because it's not lowering the impedance of the pickups, just the load they are seeing.

    The problem with jazz bass pickups is they are single coils, so unless you have both of them on you have no hum cancelation at all.
  4. peakdesign


    Aug 25, 2008
    This isn't a simple subject by any means, and I'm not trying to be a smartbutt. I think the answer is simply because typically the noise we care about is electrostatically coupled into the bass.

    A compass doesn't stop working in a faraday cage, as far as I know. Does it? Let's say there's a transformer nearby. It will create a field, and the magnetic component, I believe, passes unimpeded through any faraday cage. In fact, some transformers are built with shielding between the windings, and yet they still pass current. In that case, the shielding on the bass is only blocking the electrostatic component. There's the thing about electrostatic noise in practice being higher frequency and traveling farther than magnetic noise, likely it's the effect of the series capacitor that is the air and environment making a high pass filter, so the higher pitch buzz made up of higher harmonics of 60 Hz is more electrostatically coupled, and the fundamental at 60 Hz and the lowest harmonics like 120 Hz are more magnetically coupled? So the high impedance lightweight bass volume/tone circuit picks it up electrostatically in the unshielded wiring, a capacitor is formed between the wiring and the radiating object, and as we know, a capacitor passes high frequencies better. The copper shielding will help that by shorting out the electrostatic wave to ground. Put the fully shielded bass near a transformer, turn up one pickup, and I bet it will still be there. Can anybody experimentally confirm this with a superbly shielded bass? I think if you put copper shielding on the face of the pickup it would not prevent the strings from sounding. Massive shielding made of Mu metal has to be used to block magnetic waves, I believe.

    It seems like most of the noise we are dealing with is in any case high harmonics of the 60 Hz field, electrostatically coupled into the bass, and that's why the Faraday cage of shielding works. Anybody totally sure of how it all works? It's funny how our understanding of the most basic principles gets shaky as we examine them closely. Let's see, the attraction between opposite charges is what makes an electrostatic field, and the coupling of spins is what makes a magnetic field? The magnetic coupling is like a big transformer, the electrostatic like a big capacitor. So the definition of the field lies in how it's coupled, but basically the two are the same thing? Whoa! The electrostatic and magnetic are present together, but not necessarily. The coupling between two circuits can use both. It's that the electrons in the source move, so the potential to make a field is there to move the electrons at the target, but unless it couples there's no field because no forces are expended? Or is the potential enough to be called a field? You can't measure a field without sucking on it. :cool: Heavy!

    Then there's the effect of the player and bass as antenna relative to the amp's ground, but a hardwired good ground mostly shorts that out if the interference is low frequency. Radio interference might still get coupled in that way.

    A lower impedance stiffer circuit would not pick up interference as much, because it would "short it out" more, but it's true you can't lower the impedance of the output circuit without also lowering the impedance of the pickups, which I guess would mean fatter wire, a more massive magnet, and then it damps the strings more? Does anybody explain the underlying physics of the different pickups, why different pickups are made the way they are, and act the way the do?

    p.s. Humbuckers as far as I know don't improve noise immunity mainly by blocking electrostatic interference. As an engineer, I just don't see how, unless they are well-shielded. Is that part of the construction? I thought they had counteracting coils that pick up the magnetic interference in two phases and add the two to try to null them out, much like the two parts that make up a precision pickup, and the two jazz pickups if they are relatively balanced?
  5. peakdesign:
    Most of your post above is correct. There are a few things that you may want to reconsider.
    1. A compass works in a Faraday cage because the magnetic field is static. As are most of the magnetic fields we encounter.
    2. The definition of fields doesn't have to do with coupling. Electric fields aren't generated due to positive and negative charges - a point charge will generate an electric field. See Coulomb's law. Electric fields and magnetic fields are not completely separate phenomena. Current (moving charge) will generate a magnetic field. So both are present.
    3. Remember what Einstein said - a magnetic field to one observer will appear as an electric field to another observer in a different frame of referece.

    This is probably getting a little too theoretical for talkbass. ;)
  6. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Very well put! I didn't feel like doing all that typing... lol

    That's why I said electrostatic shielding doesn't block magnetic fields.

    That's correct. Humbuckers only cancel common mode noise induced into both coils. They don't cancel the strings because the magnets in each coil are reverse polarity, and since the two coils are wired electrically out-of-phase, the string's signal is in phase, while the electrical noise is out. And that's always low frequency noise (hum). The electrostatic shielding handles the higher frequency stuff (buzz) which is the harder of the two to get rid of since the wavelength is probably too small and slips past the humbucking coils. Most of the posts here about noise is always that high frequency buzz that's coupled to the metal parts on the bass.
  7. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Yeah, I touched on that too... magnetic fields and electric fields are really the same thing, just different.

    If you really want to freak people out with theoretical stuff just point out that it's photons that propagate both electrical fields and magnetic fields! So photons are sticking your magnets to your fridge.

    After all it's all part of the electromagnetic spectrum, aka, light.
  8. ehque


    Jan 8, 2006
    Erk. I'm a physics major and i don't need this.

    Static magnetic fields may propagate through non-ferrous faraday cages, but i think that static magnetic fields induce no current in the electronics, so its a moot point.
  9. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Physics is fun! Except the math part. Math is never fun. :rollno:

    You are correct.
  10. RyreInc


    May 11, 2006
    Kalamazoo, MI
    I love threads like this! :hyper:
  11. JFace


    Apr 17, 2008
    Columbus, OH
    I had a physics prof once say "You never fully understand math, you just get used to it."
  12. That's a good one. But without math physics is impossible. Even with math, some physics doesn't make a whole lot of sense (i.e., QM).
  13. SGD Lutherie

    SGD Lutherie Banned Commercial User

    Aug 21, 2008
    Bloomfield, NJ
    Owner, SGD Music Products
    Well the physics is happening anyway without us knowing the math, but we use the math to understand it.

    That's my favorite kind!

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