Shielding and dummy coils

[earthtonesaudio]

Finally got a chance to mess with this a little more. I noticed a couple interesting things. First, adding some iron in the vicinity of the dummy coil reduced hum, but at certain angles more than others. The iron came from the "bit" of a 4-in-one screwdriver (the kind you can take the phillips head out, turn it around for a different size phillips, that sort of thing). So about 1/4" diameter, maybe 2.5" long.

The other interesting thing I noticed is that the choke for the tone control circuit is located directly underneath the dummy coil. I haven't looked at the choke with a magnetic compass yet, but I have a hunch that its magnetic field is partially adding to the inductance of the dummy coil.

[Amit]

Metal shielding is good for reducing electric fields. Cancellation with a coil is for reducing magnetic fields. These are two different things. One needs to do both.

Copper tape must be grounded to be effective.

......

You can shield your dummy coil with copper tape to stop it from picking up electric fields..... This does not affect its sensitivity to magnetic fields very much.

.....

Do not expect this to cancel electric field pickup from the pickup!)

Since I'm no physicist, can anyone run down a list of what is considered electric field and what is magnetic field?
Every time I read about shielding it's stated that both types of interferences need to be addressed, but NO ONE is saying clearly what is what.

The two examples always given are "50Hz/60Hz mains hum" and "RF".
For mains? > it's magnetic > humbucking
For RF? > it's electric > shielding

But isn't it ALL just AC magnetic fields with different frequency?!
After all with enough thickness and hi permability you can even block mains hum just as good, if not better, then humbucking.
And are those the only things out there?


(help)

[Mike Sulzer]

Electric fields: These are produced by electric charge. Like charges repel, opposite charges attract. Interference from electric fields causes a voltage in the circuit that the amp then amplifies. The voltage that the input of the amp sees is a function of the applied field and the resulting field from charge moving in the circuit that tries to cancel it out. For example, a power cord usually "leaks" an electric field that is varying in time at 60 Hz and harmonics. A shield for electric fields, a piece of metal, has free charge which moves in such a way as to produce a field to cancel nearly all of the incident field.

Magnetic fields: produced by the motion of electric charge (currents). The pickup senses time varying magnetic fields by the law of magnetic induction, from the vibrating magnetized string or from interfering sources. For example, a power transformer may leak some magnetic field, especially because it has an iron core.

rf waves: These can propagate from long distances and have both an electric and magnetic field. They can be reflected by shielding for electric fields.

[Amit]

Thanks for the effort Mike, but I'm still in square one on this
Maybe it's a language barier but these lines sound the same to me:

Electric fields: These are produced by electric charge
Magnetic fields: produced by the motion of electric charge

So the "motion" is the key?

I (think I) undertand the magnetic field side of things, and what I know makes sence. meaning there is a magnetic field either from permanent magnet, electromagnet, earth's magnetosphere and so on. put a motion/vibration in these fields at one point and you can pick that "wave" somewhere else.
Cool, there is a "medium" that lets you work with it.
About RF, from what I understand it's also a magnetic wave in the magnetic field of the earth... at least that's how some texts explain it.

Now what is an electric field?
Here is where I'm lost...
At least from what I know, leaving Nicola Tesla and lightnings a side, electric currents needs conductive medium, like that power cord for example. it doesn't run thru air, so I don't understand what a pickup can sense as noise if the electric current in the power cord can't go air borne?
I mean, what is the medium here?
The magnetic field is taken, right?
I know Ether doesn't exist (even though in computer networks we still say Ethernet)
What's left? dark matter? dark energy??
It's not the Higgs boson because even particle coliders can't find it and they have way more wire then my Tele pickups!

I can understand that maybe the flow of current in the power cord creates a magnetic field that goes air borne and a guitar pickup can sense it... but that's magnetic... so whatwherewho is electric here?

I'm obviously missing something here but can't figure out "what"

Oh, and to top things off, I found this bit on wiki:
"In view of special relativity, the electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic field."

Huh?
Well I understand time/space and energy/matter, but this duo is still beyond me LOL

Help #2

[Mike Sulzer]

OK, let's just look at this question: what is an electric field?

It is related to electric charges. (Electrons and protons, two of the basic building blocks of atoms, have electric charge.) Electric charges attract or repel. How would you describe this in a quantitative way? If you had just two charges, there is a simple equation that describes the force as a function of the distance between the charges. But suppose you wanted to know the force at every point in space on a single charge due to many charges? For example, suppose you had two sheets of metal that have trillions of charges on them, resulting, for example, from connecting a voltage across them. It would be very complicated to write an equation with all those charges in it.

It turns out that you can use a single quantity to describe the effect of all those charges. This is the electric field. The electric field is a computational convenience, and also helps in understanding complicated situations, kind of a guide for intuitive understanding. In that sense, it takes on a reality of its own. But it actually relates to all those charges, and is just a simple way of allowing computation of the force resulting from them all.

[Bruce Johnson]

Here's how I've always thought of it:

Magnetic field interference primarily comes from transformers and transformer-like devices. In particular, the power transformers in guitar amps, the ballasts in flourescent lamps, and running electric motors. Magnetic field interference is very dependent on the distance to the transformer source. The closer you get the guitar to it, the louder it is. It's usually 60hz, because most transformers that we run into are working with common AC power. However you can hit other frequencies if the guitar is close to electronic equipment that has other types of internal transformers.

Magnetic field interference is reduced with humbucking; adding a matching but opposite coil to cancel it out.

Electrical field interference is primarily caused by AC electricity passing through long straight lengths of wire near you. Think of the wiring going through the walls and ceilings around you, and of big overhead power lines nearby. It's almost always 60hz in any place that you're likely to be playing a guitar. Electrical field interference is lower level and fairly constant. It doesn't vary much as you move around the room, and it's usually not as annoying as the magnetic interference. It's that small steady hum in the background.

Electrical field interference is reduced by shielding; surrounding the circuitry with a conductive metal shell that's connected to the ground side.

The reason for the confusion between them is obvious. Both kinds of interference are going on at the same time, and usually at the same frequency. That hum you hear is probably some of each. If you want to be quiet in all situations, you need to do both shielding and humbucking.

However, a non-humbucking guitar that is well shielded will be quiet as long as you keep it some distance away from those transformers.

I realize that this is a simplified explanation. I'm a mechanical engineer, not an electrical engineer

So, Mike, is this reasonably correct?

[cryptozoo]

David, thanks for posting the info and specs on that pickup. I had a customer asking me about that the other day...

[Amit]

> OK, let's just look at this question: what is an electric field?

I'm all ear.

> It is related to electric charges. (Electrons and protons, two of the basic building blocks of atoms, have electric charge.)

So far so good.

> Electric charges attract or repel.

I'm still here

> How would you describe this in a quantitative way?

Quantitive? as in numbers?
Doesn't really matter to us common people
I'm trying to understand the *mechanism*, from that what are the interferences a pickup picks (for lack of better word) and from that "how" to block them.

> If you had just two charges, there is a simple equation that describes the force as a function of the distance between the charges.

I think I see where your going

> For example, suppose you had two sheets of metal that have trillions of charges on them, resulting, for example, from connecting a voltage across them.

Did you just describe a capacitor???
I got it.

> It would be very complicated to write an equation with all those charges in it.

Guitar pickups can't sense equations

> The electric field is a computational convenience, and also helps in understanding complicated situations, kind of a guide for intuitive understanding. In that sense, it takes on a reality of its own. But it actually relates to all those charges, and is just a simple way of allowing computation of the force resulting from them all.[/QUOTE]

This last paragraph is almost saying "electric field is just a logical frame that helps engineers quantify force resolting from charge".
How can a single coil pick that?!

All kidding a side, the "capacitor" bit did it
Taking it from here, why humbucking (out of phase common mode cancellation) doesn't work for those noises?
I understand why a shield at ground potential does the job, but logically, if it's common mode noise, I should be able to cancel it with humbacking... right?


Anyway thanks, really appriciate your help on this Mike

[Berman]

A classical example for an electrical field is radio.
A pickup can only sensor changing electrical fields.

Ans electrical fields ar real ... go into a 10KVolt room, you will become an unconfortable feeling.

best regards Hermann

[Amit]

A classical example for an electrical field is radio.
A pickup can only sensor changing electrical fields.

Ans electrical fields ar real ... go into a 10KVolt room, you will become an unconfortable feeling.

best regards Hermann

Are you talking guitar pickup?
Shouldn't it be that a pickup sense changing "magnetic" fields and react by conducting current... hense electrical field?
This is all very confusing

Anyway, I just found the missing part I needed to visualize in my head how the electric and magnetic sides coinside in this electromagnetic stew

"You might wonder why, when a radio transmitter transmits something, radio waves want to propagate through space away from the antenna at the speed of light. Why can radio waves travel millions of miles? Why doesn't the antenna just have a magnetic field around it, close to the antenna, as you see with a wire attached to a battery? One simple way to think about it is this: When current enters the antenna, it does create a magnetic field around the antenna. We have also seen that the magnetic field will create an electric field (voltage and current) in another wire placed close to the transmitter. It turns out that, in space, the magnetic field created by the antenna induces an electric field in space. This electric field in turn induces another magnetic field in space, which induces another electric field, which induces another magnetic field, and so on. These electric and magnetic fields (electromagnetic fields) induce each other in space at the speed of light, traveling outward away from the antenna."

With that I understand now why Mike wrote earlier that RF "have both an electric and magnetic field" when I thought it was just magnetic.
I just couldn't understand how electric fields can propegate thru space by them selves. where/what was the conductive medium/mechanism?
Electric>Magnetic>Electric>Magnetic>Electric>yada>yada>


Cheers

[bigjimsullivan]

simple dummy

hey Thanx!! all, especially david for your patient in depth descriptions. but, it seems to me that
back in my 20s(the 1970s) that i achieved a quiet solution with just a standard 5-way blade swtich on a strat by simply screwing down the middle pickup to almost visual oblivian, leaving the factory wiring 'as is'. And having this work out very quietly in the
middle&bridge or middle& neck switch settings. granted those were normally my favorite "duck quack" and "funk" settings but the sacrifice of having the magnets much further away from the strings on the middle p.u. caused me less dismay over lost funk tone than it did joy at a quieter signal. and i've even heard of some guys going as far as to lower the mid to all the way under the pickguard with a thin plastic shield obtaining verey quet results. DOES THIS "RAP" HOLD WATER? OR WHAT DO YOU THINK WAS TAKING PLACE?
thanks all
Party -on

[David Schwab]

That's not a dummy coil, it's a pickup because it has magnets. The magnets have nothing to do with hum. The fact that you get the funk tones is because it is still sensing the strings. Lowering the pickup did nothing to reduce noise, only reduced the output of that pickup.

[Bruce Johnson]

As a follow-up to my posts from last year where I was experimenting with using a dummy coil to reduce noise from the M-pickups in my Scroll Basses:

I eventually gave up on the dummy coils. I tried maybe six different combinations of coil parameters, and none of them seemed to have much effect in hum cancellation at all. Each had varying effects on the tone, from little to mild depending on resistance and how it was wired, etc. The net result didn't seem to be worth the trouble.

Instead, I just switched my M-pickup design over to a full humbucker group. Now two of the four coil units have inverted magnets and reverse coil winding. At the same time, I added individual brass shielding shells around each coil and improved the shielding underneath. Only the top surfaces are unshielded. This combination is dead silent. I can hold the bass within a foot of a florescent lamp or an amp transformer and barely hear any 60 hz hum. To my surprise, switching over to full humbucker didn't change the tone output very much at all, as compared to my earlier design. That may be a result of the unusual coil geometry/configuration that I'm using.

So, that's results of my R & D. I'm not saying that dummy coils don't work or can't work. It's just that it may take a lot of experimenting to get the dummy coil to effectively match the pickup coil. The classic symmetrical humbucker is an easier path.

[David Schwab]

To my surprise, switching over to full humbucker didn't change the tone output very much at all, as compared to my earlier design. That may be a result of the unusual coil geometry/configuration that I'm using.

Because in essence they are single coils, since you only have one coil sensing each string. Humbuckers sound different because of the two coils sensing each string.

[Joe Gwinn]

As a follow-up to my posts from last year where I was experimenting with using a dummy coil to reduce noise from the M-pickups in my Scroll Basses:

I eventually gave up on the dummy coils. I tried maybe six different combinations of coil parameters, and none of them seemed to have much effect in hum cancellation at all. Each had varying effects on the tone, from little to mild depending on resistance and how it was wired, etc. The net result didn't seem to be worth the trouble.

I would have expected the dummy coil to work better than you experienced. I have two questions:

1. Were the geometric centers of pickup and of dummy coil more or less in the same place? In other words, were the coils nominally concentric?

2. How many turns did the pickup have and the dummy have?