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Something Cool
I'm posting this here as a draft so that people can comment on readability and other issues. I always wanted to write this down because there is so much confusion out there.
The interference that your guitar or bass picks up comes in two variants:
It isn't black and white which one is which. As a basic rule of thumb you get electrostatic interference when there is any A/C voltage present (without shielding), just by the presence of the voltage
And you get electromagnetic interference only when that voltage is actually "doing anything". Examples of current actually doing anything include electric motors such as in the compressor of your fridge. But that also includes more obscure items such as fluorescent lights which actually work by flipping an electromagnet at very high frequency. So they move a physical object in a magnetic field and that is causing these lights to emit electromagnetic interference.
In practice much of the noise is both.
Shielding:
Shielding against electrostatic interference is easy: all you have to do is pick one of the two sides of your electric circuit (remember you always need a circuit for current to flow, two wires) and you turn that side into a huge conductive surface that you put all around the rest of the wiring. It doesn't matter which side as long as your shield is always the same side for everything, including your guitar, your guitar cable and your amp. The "hot" wire must be enclosed in the shield everywhere, or it picks up interference. Note that the hot wire isn't different in any way from the other side of the circuit, it was exactly the same until you picked which side you use for shielding. The shield works by not letting electrostatic interference reach much of the total circuit so it doesn't induce voltage in the circuit (remember the circle has to be complete for electricity to work).
[sidetrack] Strictly speaking this shield is not grounded, it works even if not connected to the house ground. That is easy to test by putting your amp on a ground lift or through a 2-prong power adapter. The shielding works the same way. It is unfortunate that Americans have gotten into the habit of calling all this stuff "ground", even when it isn't connected to house ground (or earth). I like the European/German term "mass" better for this huge surface. But I will continue to say "ground" and "grounded" in this writeup.
In practice guitar manufacturers are sloppy and they don't put a real conductor around all the hot wire. Instead they use the fact that your body is conducting and abuse you as a shield by connecting the strings via the bridge to ground. It is incomplete but it works OK because by the standard of signals a guitar pickup emits a very strong signal overpowering much noise, so this lousy shield works OK for most guitar playing. It is still life-threatening nonsense, though, which I might expand on later.
Now, for electromagnetic interference.
The bad news is that you cannot shield against electromagnetic interference with common methods. The conductor around the hot wire does nothing to electromagnetic interference. You would have to use MU metal (magnetically shielding material) which is very expensive and heavy and would interfere with your pickup's magnetic field. In practice the only MU metal found around guitars is a small shield between the coils in stacked hum-canceling single-coil sized pickups. They do that because these "Stacks" work by having a blind coil with no magnets and it would work a lot worse if the magnetic field from the other coil (with magnets) would extend into the lower coils which is supposed to have no field.
The good news is that in most environments electromagnetic interference is not nearly as bad as electrostatic interference and the short wires in a guitar or even the guitar cable do not pick up enough electromagnetic interference to be a real problem, so you just ignore the problem.
However, there is a place in a guitar that has enough wire length to pick up significant electromagnetic interference (and electrostatic, too) for this to be a problem, and that is the pickup coil.
You can fight electromagnetic interference only by cleverly collecting signals so that you have an opposite signal for your desired sound and the interference. In practice two methods are used:
Humbucking pickups work by collecting the opposite signal of your desired sound (the string movement relative to the pickup), then mix it back with the phase inserted and since the noise is not opposite the inverted phase kills the noise but doubles your sound. This kills both electromagnetic and electrostatic interference. But of course you pick up new noise (mainly electrostatic) in the wires if they are not shielded.
Note that electromagnetic interference works on the pickup coil without the help of the magnets. The pickup's magnetic field has nothing to do with electromagnetic interference. Even a pickup coil with no magnets picks it up. That is why blind coil systems such as in stacks work.
Some remarks:
Allright, trash this so that we can polish it
The interference that your guitar or bass picks up comes in two variants:
- Electrostatic interference
- (Electro-)magnetic interference
It isn't black and white which one is which. As a basic rule of thumb you get electrostatic interference when there is any A/C voltage present (without shielding), just by the presence of the voltage
And you get electromagnetic interference only when that voltage is actually "doing anything". Examples of current actually doing anything include electric motors such as in the compressor of your fridge. But that also includes more obscure items such as fluorescent lights which actually work by flipping an electromagnet at very high frequency. So they move a physical object in a magnetic field and that is causing these lights to emit electromagnetic interference.
In practice much of the noise is both.
Shielding:
Shielding against electrostatic interference is easy: all you have to do is pick one of the two sides of your electric circuit (remember you always need a circuit for current to flow, two wires) and you turn that side into a huge conductive surface that you put all around the rest of the wiring. It doesn't matter which side as long as your shield is always the same side for everything, including your guitar, your guitar cable and your amp. The "hot" wire must be enclosed in the shield everywhere, or it picks up interference. Note that the hot wire isn't different in any way from the other side of the circuit, it was exactly the same until you picked which side you use for shielding. The shield works by not letting electrostatic interference reach much of the total circuit so it doesn't induce voltage in the circuit (remember the circle has to be complete for electricity to work).
[sidetrack] Strictly speaking this shield is not grounded, it works even if not connected to the house ground. That is easy to test by putting your amp on a ground lift or through a 2-prong power adapter. The shielding works the same way. It is unfortunate that Americans have gotten into the habit of calling all this stuff "ground", even when it isn't connected to house ground (or earth). I like the European/German term "mass" better for this huge surface. But I will continue to say "ground" and "grounded" in this writeup.
In practice guitar manufacturers are sloppy and they don't put a real conductor around all the hot wire. Instead they use the fact that your body is conducting and abuse you as a shield by connecting the strings via the bridge to ground. It is incomplete but it works OK because by the standard of signals a guitar pickup emits a very strong signal overpowering much noise, so this lousy shield works OK for most guitar playing. It is still life-threatening nonsense, though, which I might expand on later.
Now, for electromagnetic interference.
The bad news is that you cannot shield against electromagnetic interference with common methods. The conductor around the hot wire does nothing to electromagnetic interference. You would have to use MU metal (magnetically shielding material) which is very expensive and heavy and would interfere with your pickup's magnetic field. In practice the only MU metal found around guitars is a small shield between the coils in stacked hum-canceling single-coil sized pickups. They do that because these "Stacks" work by having a blind coil with no magnets and it would work a lot worse if the magnetic field from the other coil (with magnets) would extend into the lower coils which is supposed to have no field.
The good news is that in most environments electromagnetic interference is not nearly as bad as electrostatic interference and the short wires in a guitar or even the guitar cable do not pick up enough electromagnetic interference to be a real problem, so you just ignore the problem.
However, there is a place in a guitar that has enough wire length to pick up significant electromagnetic interference (and electrostatic, too) for this to be a problem, and that is the pickup coil.
You can fight electromagnetic interference only by cleverly collecting signals so that you have an opposite signal for your desired sound and the interference. In practice two methods are used:
- Humbucking pickups.
- Twisted pair wiring. This usually isn't done in guitars because the signal from the pickups is strong enough to overpower electromagnetic interference picked up in the wires. But it is used in microphone connections. Microphones have a weaker original signal than guitar pickups and that is why microphone cables has two wires inside the shield and are symmetric. Even with twisted pair wiring you want shielding (such as in a XLR mic cable) because the twisted pair trick isn't doing a very good job. So you use TP to kill as much of the electromagnetic interference as you can but the much more common electrostatic interference is also kept back with shielding.
Humbucking pickups work by collecting the opposite signal of your desired sound (the string movement relative to the pickup), then mix it back with the phase inserted and since the noise is not opposite the inverted phase kills the noise but doubles your sound. This kills both electromagnetic and electrostatic interference. But of course you pick up new noise (mainly electrostatic) in the wires if they are not shielded.
Note that electromagnetic interference works on the pickup coil without the help of the magnets. The pickup's magnetic field has nothing to do with electromagnetic interference. Even a pickup coil with no magnets picks it up. That is why blind coil systems such as in stacks work.
Some remarks:
- If you get conductors, such as your shield, into the magnetic field of your pickups then you can kill some of the resonance peak via Eddy Currents. This is definitely a factor, it is why humbucker metal covers sound different.
- For this reason I do not recommend shielding the back of the pickup cavities with foil. If you have humbuckers it does nothing against noise except for the tiny bit of wire between coils and shielded wire. If you have single coils they will hum like mofos from the electromagnetic interference anyway and your shield only fights electrostatic interference. Your specific environment might have more static than magnetic interference and make it more effective, of course, but I recommend recording before/after so that you don't wonder why your Strat is now a lame duck.
- I do, however, recommend shielding pickup wire that is unshielded, namely between Strat pickups and with Fender basses between pickups and cavity. Some grounded foil around the length of the wire, but kept away from the actual pickups, does significant reduction in noise. If it is unshielded wire, shield it.
- Do not underestimate mechanical effects from shielding. Thick glue can always dampen resonance. Try putting that thick copper tape with self-adhesive glue on the top of an acoustic. Some old Gibsons have big brass boxes in the cavity. They echo when you tap them, heck you could use them in a drumset. It would be naive to think that these movements don't change the movements of the whole guitar.
- Capacitance from shielding on the other hand is definitely not a factor for passive guitars since there is no way that the cumulative capacitance of your guitar cavities (shielded or not) even remotely rivals the huge capacitance of your 15 feet shielded guitar cable to the amp, not to mention the natural capacitance of your pickup coils.
Allright, trash this so that we can polish it
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