Reduce Wiring Noise: Avoid Magnets

[soulsonic]

I found an interesting phenomenon in a guitar I rewired recently. The cavity is shielded extensively with copper foil, but I was still getting a hum that seemed much louder than it should have been. I knew the problem was likely because of long wires: this is temp install of a new pickup to test it out - it goes into a different guitar later - so I left the leads long. What I had done was stuff the excess wire underneath the pickup. I had hoped that since it has copper everywhere, it would have less chance of picking up hum (was already worried about this wire being a problem). Despite the shielding, it was still picking up some bad hum.

Then I realized what the problem likely was: because the bundled wires were directly under the pickup, they were within the pickup's magnetic field, which made them much more sensitive to picking up noise! So, I moved the bundled wires away from the pickup to an area near the volume control; also made the bundle more "messy" because a neat coil of wire is the best antenna. The hum is very much reduced from what it was before. Of course, when I do the final installation, I will cut the wires short and give them additional shielding with copper tape (they are unshielded wires; which seems unusual for a modern humbucker, but no big deal). I will definitely make a point to avoid routing wires directly under pickups whenever possible now.

Anyone else experience anything similar? Is this something that's already common knowledge, but I just never heard it?

[plush]

What type of guitar and pickups are we talking about? Could a lack of of shielding under the pickups be the cause?

I can't really tell how fixed magnetic field can make wires under the pickup "more sensitive". If the field were altternating, then - yes, it'd induce some current.

[soulsonic]

What type of guitar and pickups are we talking about? Could a lack of of shielding under the pickups be the cause?

The entire pickup cavity is shielded with copper. The pickup is a Wilde L500L humbucker with chrome plated casing. Its lead wires are unshielded.

I can't really tell how fixed magnetic field can make wires under the pickup "more sensitive". If the field were altternating, then - yes, it'd induce some current

I believe it works the same as the pickup does generally: a coil of wire in a static magnetic field. Various other magnetic fields, AC noise, etc... interact with that static field and induce the noise into the coil of wire. What I'm saying is that I think the leads behaved as a secondary pickup that was getting this noise because of being within the magnetic field.

[Intripped]

If I'm not wrong the silent SC backplate from Suhr works in a similar manner, putting an extra coil in the PU's magnetic field.
So, in theory, if you wind up the wires in the right direction (anti-phase with the PU's coil), you could actually decrease the hum. The same principle of humbuckers.

[soulsonic]

If I'm not wrong the silent SC backplate from Suhr works in a similar manner, putting an extra coil in the PU's magnetic field.
So, in theory, if you wind up the wires in the right direction (anti-phase with the PU's coil), you could actually decrease the hum. The same principle of humbuckers.

Gibson Blueshawk did this too.

[plush]

I believe it works the same as the pickup does generally: a coil of wire in a static magnetic field. Various other magnetic fields, AC noise, etc... interact with that static field and induce the noise into the coil of wire. What I'm saying is that I think the leads behaved as a secondary pickup that was getting this noise because of being within the magnetic field.

Afaik, the pickup works with alternating magnetic filed, that is created by string vibration - string is made of a metal with magnetic properties, magnetic field is induced into it through pickup poles, when the string is moving, the magnetic field is alternating, creating the current, which is transfered to the winding, creating a signal.
If there's a strong alternating magnetic field nearby (huge transformer), than it's magnetic field will be coupled into both the coil and the rest of the guitar wires including the cable, potentiometer and the amp itself (since your typical shielding does not provide any protection from magnetic field).

If there's a stray electric field nearby (which is almost 100% of all causes of signal/hum interference), it can also be coupled into the winding and unshielded cables.

So, let's say, your pickup cavity is electrically shielded, but there is no shield on top of it, then all the cable mess inside the cavity will pick all the electrical noise. In most cases, pickup cover helps, Also, the pickup plate should be grounded.

EMI = electric + magnetic fields.
Your typical shielding protects only from electric field only.
Large magnetic field will require, lets say, a couple of millimeters of annealed steel to reroute. Magnetic field produced by the pickup is relatively weak and is not the case here, IMO.

I'm no advocate for long cable mess though. I may be wrong somewhere.

If I'm not wrong the silent SC backplate from Suhr works in a similar manner, putting an extra coil in the PU's magnetic field.

Nah, it's made to cancel the interference coming from stray electric fields (mains hum, fluorescent lamp drivers, etc.). Not the magnetic field of pickup.
It works similar to humcancelling pickups with their dummy coils.

[JiM]

Its lead wires are unshielded.

Yep, that would be the main culprit.

Afaik, the pickup works with alternating magnetic filed, that is created by string vibration - string is made of a metal with magnetic properties, magnetic field is induced into it through pickup poles, when the string is moving, the magnetic field is alternating, creating the current, which is transfered to the winding, creating a signal.

There will be alternating eddy currents in the string, creating tiny magnetic fields, but i think that the main reason a current is induced in the coil is the perturbation of the magnetic field of the magnet by the moving string.

[plush]

the main reason a current is induced in the coil is the perturbation of the magnetic field of the magnet by the moving string.

This. I was a kinda wrong with my representation, thanks.
Still, I can't find any significant magnet field capable of inducing parasitic current into unshielded wire mess under the pickup. I believe it's an electric interference, that is picked up by wire mess.

[JiM]

It would act exactly as the coil wire : unshielded wire bathed in the magnetic field of the pickup.
The baseplate is usually non-magnetic (with the notable exception of many Telecaster bridge pickups), so the mangnetic field passes through.
Simplified view :


More detailed modeling : https://www.moore.org.au/pick/06/06_gobd.htm

Agreed, there would be just a couple of loose turns, and the magnetic field shape may be strange depending on pickup construction, but the effect can be significant. Microphonic as well as the turns can move.
The direction of winding will also have an impact, some humbucking "single-coils" use such a low-turn-count reverse-wound secondary coil below the main coil.

If those wires were shielded, the current would have been induced in the shield and dumped to ground.

[plush]

If those wires were shielded, the current would have been induced in the shield and dumped to ground.

But why? Magnetic field would penetrate the copper shield and induce current in both signal and ground wires.

[phatt]

Still got me stumped as to why makers still use unshielded cable for PU's.
Every time I open a fender and see the cloth wire I shake my head.
Phil.

[george giblet]

There's two types of fields that affect guitars: Electric fields and Magnetic fields.

Shields only fix Electric fields.

For a shield to eliminate magnetic fields the shield material has to be made of high permeability magnetic material and must surround the object - like mu metal shields on the old oscilloscope CROs. That's not going to happen on a guitar.

Copper shielding doesn't do much for 50Hz magnetic fields. unless it is very thick.
It shields via eddy currents induced in the conducting material.

See,
https://chemandy.com/calculators/skin-e ... ulator.htm

For 50Hz, 50Hz = 5e-5 MHz, skin depth = 9220 um = 9.2mm
9mm shielding isn't going to happen.

but for radio frequencies say 1MHz, skin depth 65um = 0.065mm
which is in the realm of copper tape.

Twisted pair and coax work on a different scheme altogether. They rely on cancelling voltages induced in the wires due to (AC) magnetic fields. If you have a gradient in the magnetic field the voltages don't cancel 100%.

DC fields due to magnets have no place in the discussion at all. They don't cause any interference.
Permeable materials like alnico in pickup magnets can make the pickups pickup more hum. Lumps of iron etc can do similar things. This works like a ferrite rod in an AM radio. It makes the field near the coils stronger. It's the material that does it not the fact it is magnetic.

[soulsonic]

I get really confused when you guys keep talking about electric and magnetic fields as if they were separate things, because every flow of electric current creates a magnetic field. They're the same. You can talk about a static or flowing current of a field as being different, but they're all magnetic. And magnets and ferrous metals affect them all. And non-ferrous metals (copper wire) become a part of that system, do they not? And guess what, I'm not a stone statue, I don't stand perfectly still as I play, so even a static field within my guitar becomes a flowing field relative to the outside fields it interacts with.

[plush]

I get really confused when you guys keep talking about electric and magnetic fields as if they were separate things, because every flow of electric current creates a magnetic field. They're the same. You can talk about a static or flowing current of a field as being different, but they're all magnetic. And magnets and ferrous metals affect them all. And non-ferrous metals (copper wire) become a part of that system, do they not? And guess what, I'm not a stone statue, I don't stand perfectly still as I play, so even a static field within my guitar becomes a flowing field relative to the outside fields it interacts with.

Okay, so we know that even though EMI term is always described as a combination of both magnetic and electric fields (since they are both interrelated), both fields can exist withouth each other in a simplified system
- permanent magnets do not have and do not produce any electric field unless their moment/flux is altered
- static charges, having an electric field, do not produce any magnetic field unless their charge is changed

But...

Then I realized what the problem likely was: because the bundled wires were directly under the pickup, they were within the pickup's magnetic field, which made them much more sensitive to picking up noise!

But... there's no way how static magnetic field of a guitar pickup will make local wires more or less sensitive. It can induce current w/ frequency of it's changing magnetical flux. But there is no way stray fields will make pickup magnet change it's flux.

All the noise is induced via lack of shielding and pesence of stray aelectric fields. Pickup magnets have nothing to do with this.

If not, then how?

[george giblet]

I get really confused when you guys keep talking about electric and magnetic fields as if they were separate things, because every flow of electric current creates a magnetic field. They're the same. You can talk about a static or flowing current of a field as being different, but they're all magnetic. And magnets and ferrous metals affect them all. And non-ferrous metals (copper wire) become a part of that system, do they not? And guess what, I'm not a stone statue, I don't stand perfectly still as I play, so even a static field within my guitar becomes a flowing field relative to the outside fields it interacts with.

In all of those cases the effects are tiny to the point that they would be lost in the fields that have real effect. They are all misapplication of science examples.

DC/constant fields don't cause interference.

As far as moving goes, you would need to move at beyond human speeds to induce enough voltage. At best you would get a thud by waving a coil by a magnet but definitely not a buzz.

If you have 240V AC at 10kHz across some capacitor plates the current is very small. The high voltage means the electric field is large but the small current means the magnetic field is small. If you have 200A AC at 10kHz flowing through a wire the current is high making the magnetic field high and the voltage drop is low making the electric field small. When you are relatively close to objects one type of field can be higher and cause more interference than the other field. These are called near fields and they are the ones that cause problems at low frequencies. If you moved 30km away the ratio of the fields might balance out to EM wave ratios but at that distance the strength of the fields would no longer be measurable.