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Humbuckers: Better sound covered or uncovered?

LarryOM

Guest
Messages
723
There can't be any significant current in the string since it's an open circuit. The tuning machine is attached to the head of your guitar which is made of wood (insulator). Maybe if you completed the circuit from the tuning machine to ground somehow, you'd have something.
 

2HBStrat

Senior Member
Messages
41,223
....... I have an old patent sticker Gibson pickup in an LP neck position and would like it to be just a tad more open and bright. It's never had the cover removed though so I'm looking for input into the difference- if there is any at all.
I could never tell any difference between pickups with covers or without, but I've read that you can get a cleaner sound from a neck humbucker by lowering the pickup but raising the individual screws in the pickup....you might try that first.
 

John Coloccia

Cold Supporting Member
Messages
9,584
There can't be any significant current in the string since it's an open circuit. The tuning machine is attached to the head of your guitar which is made of wood (insulator). Maybe if you completed the circuit from the tuning machine to ground somehow, you'd have something.
Well, since we're in the minutia anyway....

A capacitor doesn't support a closed path, but yet it can support AC currents. There's something called displacement current that essentially corrects for all the things you're concerned about, and does indeed allow electrons to jiggle around in a wire. If you don't believe that, stick a fork in the microwave and enjoy all the current! :D

(don't really do that :) )

But I agree, it's insignificant. I didn't want to say it doesn't exist because someone was bound to say that was wrong, so I thought I'd go the other way instead, but got in trouble anyway. :cry:
 

ngativ

Member
Messages
1,024
There is no field coming from the string. The field is coming from the magnet connected to the polepiece. The string lies within that field and when vibrating, causes the field to move within the coil.
No, that's not how it works . The whole purpose of the magnets is to magnetize the strings . When the string vibrates, its magnetic field also "vibrates", hence inducing the current and voltage in the coil . BTW... is precisely that variable magnetic field coming form the strings that produce the eddy currents in the cover

And there is also a induced force and current within the string. There is some power dissipation within the string since there is also electrical resistance, causing some loss of sustain . If the magnetization in string is intense enough, the loss of sustain could become important
 
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walterw

Platinum Supporting Member
Messages
39,206
If the magnetization in string is intense enough, the loss of sustain could become important
yeah, but by that point the magnet is pulling on the string so hard that it would sound terrible anyway.

(also, your auto-correct or something keeps coming up with "edgy" instead of "eddy" currents. there's certainly plenty of daring, avant-garde music being made from the currents produced by pickups and strings, but that's neither here nor there ;))
 

ngativ

Member
Messages
1,024
(also, your auto-correct or something keeps coming up with "edgy" instead of "eddy" currents. there's certainly plenty of daring, avant-garde music being made from the currents produced by pickups and strings, but that's neither here nor there ;))
haha! yeah , it was my fault this time . In fact, i make that same mistake rather often .

Indeed, maybe at that point the guitar is already unusable anyways...
 

walterw

Platinum Supporting Member
Messages
39,206
anyway yeah, good covers make a slight difference in treble, enough to where i usually recommend them or not in terms of appearance, shielding, and physical protection, rather than any significant sound difference.
A higher wind (thus higher DCR) coil will produce more signal than a lower wind coil..., therefor produce more eddy currents in the cover. A stronger magnet will also produce a stronger electrical signal in the coil...
interesting! it makes sense, and is more specific than just "hotter pickups have less treble, so there's less to spare".
Late sixties should be Alnico 5 with a brass cover. Removing a brass cover on an alnico 5 based pickup will usually open up the top end fairly noticeably.
really? that's not so cool :(

a good cover is one that's conductive enough to shield, but not so conductive that you get too much eddy current masking the tone; copper (for example) is a poor choice because it is so wonderfully conductive.

as i understand it, brass (being largely copper) is too conductive here, so brass covers have a significant tonal effect; traditional tele neck covers are brass just for this reason, to mellow them out.

"nickel silver" is the stuff to use, especially nice and thin, because it's enough to do some shielding without being so conductive it chokes off too much treble.
 

Keyser Soze

Member
Messages
1,472
...
If you remove the coil from the equation and just have a vibrating string, a magnet and a cover, you will not find any eddy currents...
A changing magnetic field will induce current in any conductor within that field. Granted, by itself, that current within the cover will be absurdly small.

Which is exactly why coils are a long strand of fine wire - that's the only way you can get string movement to create a useful electric signal.

But that sensitivity is also what makes the coil responsive to the subtle changes caused by the even smaller eddy currents within the cover.
 

Mreilander

Member
Messages
124
N The whole purpose of the magnets is to magnetize the strings.
This is a HUGE misconception.

The string is made of steel. It does not produce a magnetic field. When the string's position relative to the coil/core changes, the permeability changes so the magnetic flux enclosed by the coil changes slightly. The rate of change of the magnetic flux enclosed by the coil determines the induced emf in the coil .
Read more: http://www.physicsforums.com/showthread.php?t=649133

I have some thesis papers around here somewhere that I'm willing to share with you if you're interested, PM me if you are.

It basically comes down to this...

Becoming magnetized and producing a magnetic field are two different things.
When the string is within the pole pieces magnetic field, it becomes magnetized, but the field still belongs to the pole piece.
The string manipulates the magnetic field of the pole piece within the coil. If you take the magnets out of the coil and place them above the strings, the output will become very faint. The magnetized string on it's own does not have enough mass to produce a strong signal in the coil. The other issue is that the string is above the coil.... the field has to be moving through the coil to produce a signal.
 

ngativ

Member
Messages
1,024
This is a HUGE misconception.



Read more: http://www.physicsforums.com/showthread.php?t=649133

I have some thesis papers around here somewhere that I'm willing to share with you if you're interested, PM me if you are.

It basically comes down to this...

Becoming magnetized and producing a magnetic field are two different things.
When the string is within the pole pieces magnetic field, it becomes magnetized, but the field still belongs to the pole piece.
The string manipulates the magnetic field of the pole piece within the coil. If you take the magnets out of the coil and place them above the strings, the output will become very faint. The magnetized string on it's own does not have enough mass to produce a strong signal in the coil. The other issue is that the string is above the coil.... the field has to be moving through the coil to produce a signal.
NO! , the resulting magnetic field is a superposition of the induced magnetic field of the string plus the magnetic field of the pickup magnets , yes, obviously . But, the string is attracted to the magnets of the pickups , and that only means that the string becomes itself a magnet, albeit not a very permanent one .

So when the polarized string vibrates, there you have a vibrating magnet. That means there is a variable magnetic flux throughout the coil, hence an induced voltage . The magnetization of the poles don't change neither their relative position to the coil , so the pickup magnets are not the source of variable magnetic flux nor the signal .

The magnetic permeability is an intensive and intrinsic property of the material, just like density . Since electric guitar strings have a relatively high magnetic permeability , they will become magnets in the presence of an external magnetic field . From the string point of view, most of the lines of magnetic flux will pass through the string , going from one magnetic pole to the another inside the string . It is a magnet just like the poles of the pickup!.

But there is also a magnetic field coming out from the string. If you don't believe it, then try this simple experiment:

Cut two short segments of B string (13cm or 5 inches ). Take on of this segments and place it near the pickup. You'll see that the wire (B string segment ) will be attracted to the magnet. Take the other segment and place it near to the Low E string between the pickups. This wire will be also attracted to low E string, it will stick! and that means the string itself has become a magnet too .

Now, if you magnetize one of the wire segments using the magnets of the pickup, then you can use it to attract the other segment . Depending on how you polarize the segments they will repel or attract first, but at the end they will stick to each other . This is an irrefutable proof that the ferromagnetic strings can become magnets with their own magnetic poles and fields . And that's the vibrating magnetic field coming from the vibrating strings that the guitar pickup "picks up"

This is basically an application of faraday's law
 

Dandolin

Member
Messages
846
This is all fascinating, and I say that admitting it is beyond my experience--I have to squint to follow it. I'd be interested to see the discussion keep going.

Also--can anyone comment on what, if any, impact a cover like what you see on old P-Basses--not enclosing the coil, but coming up and over the strings and coil, would have on tone, sustain, output, etc.? Would it make a difference if that cover were ferromagnetic?
 

ngativ

Member
Messages
1,024
This is all fascinating, and I say that admitting it is beyond my experience--I have to squint to follow it. I'd be interested to see the discussion keep going.

Also--can anyone comment on what, if any, impact a cover like what you see on old P-Basses--not enclosing the coil, but coming up and over the strings and coil, would have on tone, sustain, output, etc.? Would it make a difference if that cover were ferromagnetic?
What is supposed to be the purpose of that kind of covers in the first place? I am sure that it has less effect on the timbre since that kind of cover is not on the way between the string and the pickup . But in my opinion, putting more metallic and ferromagnetic things near the pickup that are not supposed to vibrate in the first place, is not in general a good idea to begin with . Unless, there is some special purpose that worths it
 

Mreilander

Member
Messages
124
and that only means that the string becomes itself a magnet, albeit not a very permanent one .
The string segment becomes magnetized. It does not become a magnet. It does not form its own lines of flux. The lines of flux from the magnetic pole are extended and follow the string.
http://www.slideshare.net/tininmartinez/fem-of-the-magnetic-field-of-guitar-pickupspdf

See page section 1.2.1 in linked document. Specifically the part that says:
"Ferromagnetic materials have no inherent magnetic field, but greatly enhance any field in which they are placed."

See Section 5.3 for 2D models showing the lines of flux and how they are manipulated by a guitar string.


So when the polarized string vibrates, there you have a vibrating magnet. That means there is a variable magnetic flux throughout the coil, hence an induced voltage . The magnetization of the poles don't change neither their relative position to the coil , so the pickup magnets are not the source of variable magnetic flux nor the signal .
Changing the lines of flux with the string does offset the pole of the pole piece slightly, and it is this that the coil is registering.

The magnetic permeability is an intensive and intrinsic property of the material, just like density . Since electric guitar strings have a relatively high magnetic permeability , they will become magnets in the presence of an external magnetic field . From the string point of view, most of the lines of magnetic flux will pass through the string , going from one magnetic pole to the another inside the string . It is a magnet just like the poles of the pickup!.
Again, being magnetized is different than being a magnet. The steel is not the source. If you look at the 2d models in the linked article, the string does not have it's own lines of flux. The lines from the pole piece get re-routed through the string for a short distance and then work their way to the opposing pole on the bottom of the pole piece. This means the string does not have its own field, but is drastically manipulating the field of the pole piece.

Cut two short segments of B string (13cm or 5 inches ). Take on of this segments and place it near the pickup. You'll see that the wire (B string segment ) will be attracted to the magnet. Take the other segment and place it near to the Low E string between the pickups. This wire will be also attracted to low E string, it will stick! and that means the string itself has become a magnet too .
This is just extending the original field further. If you continue to attach string segments in a chain, they will eventually not be able to stick to each other because there is a limit as to how far that field can be extended. All the lines of flux need to work their way back to the bottom of the pole piece somehow.
 
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Dandolin

Member
Messages
846
I don't think it is normally for tonal benefit--although some old lap steel pickup (Rickenbacker, Oahu) seem to include something like that in the design concept. You don't really see the P-Bass type covers on newer instruments--I think they were for cosmetics, protection of the exposed coil, and maybe a little bit to provide a resting place for the hand for certain players (pick players?). I have also seen them on electric guitars from the 50s and 60s....
 

Mreilander

Member
Messages
124
I don't think it is normally for tonal benefit--although some old lap steel pickup (Rickenbacker, Oahu) seem to include something like that in the design concept. You don't really see the P-Bass type covers on newer instruments--I think they were for cosmetics, protection of the exposed coil, and maybe a little bit to provide a resting place for the hand for certain players (pick players?). I have also seen them on electric guitars from the 50s and 60s....
The supro pickups in lap steels need that steel cover to distribute the magnetic field. The magnet for that particular pickup are actually the risers on the outside that hold the steel plate up, and are attached to the lower steel plate, magnetizing that as well. The plates then magnetize the steel screws in the coils below the strings. they are not cosmetic, but essential for that design.
 

ngativ

Member
Messages
1,024
The string segment becomes magnetized. It does not become a magnet. It does not form its own lines of flux. The lines of flux from the magnetic pole are extended and follow the string.
http://www.slideshare.net/tininmartinez/fem-of-the-magnetic-field-of-guitar-pickupspdf
No, both are misinterpreting what is going on . That guy is basically describing what the software is drawing in the screen .

See page section 1.2.1 in linked document. Specifically the part that says:
"Ferromagnetic materials have no inherent magnetic field, but greatly enhance any field in which they are placed."
Ok, but they can be magnetized so they produce a magnetic field. A wire has no "inherent" magnetic field neither, but if you introduce a current in a wire loop you will create a magnetic field, magnetic poles and a magnet . Something similar happens with ferromagnetic materials, but you magnetize them with an external magnetic field , so they become magnets with an inherent and lasting magnetic field . In some cases long lasting magnetic fields.

See Section 5.3 for 2D models showing the lines of flux and how they are manipulated by a guitar string.
Yes, the magnetic field changes because the string is vibrating, so?

Changing the lines of flux with the string does offset the pole of the pole piece slightly, and it is this that the coil is registering.
Nope

Again, being magnetized is different than being a magnet. The steel is not the source. If you look at the 2d models in the linked article, the string does not have it's own lines of flux. The lines from the pole piece get re-routed through the string for a short distance and then work their way to the opposing pole on the bottom of the pole piece. This means the string does not have its own field, but is drastically manipulating the field of the pole piece.
Wherever you have magnetic south poles north poles, there you have a magnet . Those poles produce a magnetic field similar to how opposing electric charges produce an electric field . (+) <---------> (-)

If you magnetize something, that means you are creating magnetic poles there, hence a magnet.

And that's why you have to magnetize the poles of the pickups, just like alnico bars alloys have to be magnetized first by some electromagnet (magnets that produce a magnetic field via current ) or other kind of strong magnets .

Alnico keeps its magnetic field (or magnetized state) stronger and longer than most materials .That's why they are called "permanent" magnets . But because of the inevitable increase of entropy, their magnetization decreases with time . So , at some point, you will have to re-magnetize the bar (or pole or whatever )

The same thing happens with the electric guitar strings . They are permeable to external magnetic fields . So they are more susceptible to create magnetic poles because of the influence of the external magnetic field. Similar to conductors that polarize their net charge in the presence of a external electric field.

These magnetic dipoles , quadrupoles, or whatever also create their own magnetic lines. If you draw the line forces with a compass, you will see that the magnetic lines travel from one magnetic pole to another . And that's what you see in some visual representations of with the finite element method in ANSYS

.Since you are magnetizing the string, you are creating a magnet of the string. The same thing you did before with the magnets of the pickups

This is just extending the original field further. If you continue to attach string segments in a chain, they will eventually not be able to stick to each other because there is a limit as to how far that field can be extended. All the lines of flux need to work their way back to the bottom of the pole piece somehow.
What on earth that means? :D LOL Take those magnetized segments of wire to china and they still will be magnets , far away from the pickup. The thing with electric guitar strings is that they are poor permanent magnets . However, the magnets of the pickups are quite stable and they don't care about the magnetization of the strings , so they don't change their state at all ... at least for the next years .
 

Mreilander

Member
Messages
124
Take a Supro lapsteel pup that Dandolin described.



The strings are completely surrounded by a magnetic field. If the magnetism of the string is all that induces a voltage in the coil, then there would be no need for the steel polepieces in the coil, if what you are saying is true.


If you remove these pole piece screws and reassemble the pickup, it will not work. Yet if what you say is true, it should since the strings are still magnetized, and there is a magnetized plate above the strings and below the coil, in very close proximity to the strings. The field has to be manipulated through the coil.

If you don't want to take my word for it, try it yourself. Take a fender single coil with a plastic bobbin, pull out the pole pieces and set it under the strings. hold a magnet above the string. That magnet is now magnetizing the string, yet you will not have any output from the coil.
 
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John Coloccia

Cold Supporting Member
Messages
9,584
MReilander is right. The coil sits in a magnetic field created by the pickup's magnet. When something like steel or nickel passes through the field, the field changes because the magnet's field lines simply prefer to travel through the metal than through the air. You don't get something for nothing so when field lines over here wiggle around, the entire field wiggles around, putting the coil in a changing field and inducing a current. Simple as that.

For a dramatic example of steering field lines, consider what happens when you put a soft iron bar across a horseshoe magnet. The external field basically goes poof because the iron bar effectively steers the field into a what can be loosely considered to be closed circuit....and they're referred to as magnetic circuits, in fact.
 






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