Mike Sulzer

John reply from August has it exactly right. The 250K pot makes very little difference at the lower frequencies. It just lowers the Q of the resonance a bit and so takes out some highs. Leaving the tone control out of the circuit means that it no longer loads the pickup. Remember, the .02 microf cap is like a direct connection at the higher frequencies near the resonance, so the tone circuit loads the pickup just like the volume circuit.

"I know that polepieces must be directly below the strings,if not the magnetic field will stop the string faster." That is certainly not true. I would use that pickup as long as it sounds OK.

Thank you, David. Here is a bit more on the subject that Eric brought up near the end of previous page. It is useful to consider what these things have in common: power or audio transformers, electric motors, electric generators, and speakers. First, they all use magnetic fields; but more important, they all use magnetic field to transfer energy as efficiently as practical. They are what I would call "tightly coupled" systems. When you put power on the primary of the transformer it is transferred to the circuit in the secondary and the circuit in the primary feels the effect of the "load". Motors and speakers turn electrical energy into mechanical; generators do the reverse. Generators are hard to turn because of the electrical energy produced and going into the circuit. Motors and speakers draw energy from the source so that it can be turned into mechanical energy,and some into sound in the case of the speaker. All these devices use strong magnetic fields and confine the fields to the device as much as is reasonable. They use ferromagnetic material to do this, in continuous enclosed structures as much as possible and small "air gaps" where necessary. A pickup is very different. The permanent field is kept pretty weak so that it does not disturb the vibration of the string. The poles are short rods, or similar, that do not enclose the field lines. As a result, the pickup couples weakly to the string, and so the string does not transfer a lot of energy to the coil and attached circuit, and the tiny current flowing in the coil does not have a significant on the magnetic field or the string. You can think of the coil as just "along for the ride". You can analyze magnets, pole pieces and strings without considering what happens in the coil. Then you need to analyze what happens in the coil and circuit.

"Mike, I left that discussion as I have a policy of not taking part of discussions were people (not you Mike, not you) start to call each other BS" OK, Peter, I understand. I am stubborn, and figured the best thing to do was to ignore it and keep going. "My problem with the last P90 thread we had was your way of using science in a way that isn’t contributing to the discussion. " I do not see any way to understand this stuff without science. I will try to keep it simple. "...the pickup is a complex system, involving a lot of different disciplines of science and we need to understand all of them in depth to be able to draw conclusions." Very true. That is what makes pickups interesting. "3 The science gives us very few *practical* leads to how we can enhance the existing pickup designs. In real life there are only a few factors that we can change; the shape of the coil, the shape and strength of the magnetic field, the amount of turns and the resistance (via different wires). " In addition to those things, you can use different types of magnetic materials to good advantage. But you really do need to understand the science. "Frequency Response": I mean the effects of the things in the circuit. The law of induction says the sensitivity continues to increase with frequency. But that is not what gets to the amplfier. The circuit elements make a resonant low pass filter. One can get an idea of the importance of this by going to the Seymour Duncan web page and looking up the resonant frequencies of various pickups. Do these frequencies correspond with how the various types of pickups sound in general? I think so, at least approximately. So it is important to study the pickup circuit and understand how it works. I will try to think of better ways to explain these things. Mike

Thank you for responding. This is interesting. Current flowing in the coil could indeed cause a magnetic field which could influence the strings, but it would have to be large to do so. The effect would be to damp the string vibration because energy would be dissipated in the electric circuit. But this is not a significant effect. If it were, you would notice the sustain dropping as you raised the pickup. (But a strong permanent field can do this: string pull, or stratitis.)

You want paint intended to shield against electric fields, not magnetic fields. Magnetic paint need not be conductive, but it could be. Too bad yours is not. Magnetic paint is not effective at shielding magnetic hum because it is the pickups that are sensitive to magnetic fields, not the wiring, and if you completely shielded them, they would not work.

Peter, I have attempted to answer your questions on that other P-90 thread as best as I can. Remember, you introduced Faraday's law of induction, and you asked me to explain why the component through the coil is what matters. (And you have not yet commented on that explanation.) The question is why does a P-90 sound very different from, for example, a Fender type SC? Multiple reasons, but I think the most important difference is the frequency response. The P-90 has a lower resonant frequency than a strat type pickup. Different resonant frequencies mean that they sound different. The coil inductance is one very important factor in determining the resonant frequency, but it is not the only one. And the dimensions of the coil are a factor in determining the inductance, but not the only ones. But what I was really referring to in my post that you quoted is the belief that a wide coil samples the string over a greater part of its length than a narrow coil. The FEMM plots do not support that claim. Is it still not clear why?

The magnetic field falls off with 1/D^3 not (^2) except near the magnet. That is, the field is a very good approximation of a dipole (having + and - nearby) once you are "far" from it in terms of its size. Close up is complicated. (This is from, for example, page 186 of the latest version of Jackson.) Are you sure you want to stating that a lot of the sound has to do with the dimensions of the coil when the FEMM plots show that the string is affected by the permanent magnet only near the pole piece?

No, I have the same problem. But I have a new enough mac (intel chip) so that I can use a beta version of Apple's "boot camp" on it, whihc allows booting in either 10 or windows. But you have to get windows. I am working on this now.

This is not that easy to calculate; if you are an expert with FEMM, you could model it. Those magnets are about the right size. I have made plenty of pickups with neos, and one nice thiing is that you can get several different sizes of magnets and try them all. No need to glue them on for testing.

But the law of induction is the complete story as far as the voltage around the loop. We just have to determine the component of the fluctuating field through the coil: 1. The permanent magnetic field from the core temporarily magnetizes the string near the core. We know from the FEMM plots that the vertical component of the string magnetization falls off rapidly with distance along the string away from the core. (Details could be provided with additional modeling.) 2. The field from any small magnet is like that of a magnetic dipole except very close to it. Thus the field falls off as the cube of distance except very near the "string magnet". 3. How is the fluctuating component generated? When the string moves up and down the field moves with it. Since the field falls off with distance. The field through the coil changes. If the field did not fall off with distance, the field through the coil would not change and the pickup would not work. 4. The core guides the field lines through the coil, amplifying the field and slowing the fall off. But it mostly affects the field lines that enter right from above it. The result is that the vertical component of the fluctuating component of the magnetic field through the core comes from the area not too far from right over the core. So the pickup does not sample over the full part of the string over the coil, only near the core.

Peter and Pete, Why is it the vertical component that matters? Remember the equation that Peter posted near the beginning of this thread? It contained the time rate of change of the flux, phi. To understand what this flux is, go to "faraday's law of induction" in Wikipedia, and look at the first equation. The left side says "the voltage around a loop". The right side says "the negative of the time rate of change of the result of adding up some stuff". The result of adding up that stuff is the flux, phi. The stuff is the "dot" product of B at each point (on the surface enclosed by the loop) and "dA". B is a vector, meaning it has a direction and a magnitude. "dA" is also a vector. It can be thought of as a very small piece of the surface where we are looking at B. The direction of dA is defined as perpendicular to the surface at that point. The dot product says take the two vectors that surround the dot, and if their directions are perpendicular, the result is zero. If they are parallel, the result is just the product of the magnitudes of the vectors. For directions in between, the result is less than the product of the magnitudes, depending on how much their directions differ. In our case if we think of vertical as away from the surface of the guitar, that is, the direction of the long axis of the pole piece, dA is in this direction. So the vertical part of the magnetic field is what matters.

But the wide sampling as related to the width of the coil does not stand up to analysis. Look at the FEMM plots on page one. For both the P-90 and the Fender type, the magnetic field has a large vertical component only very nearly over the pole piece. Away from the pole piece, the field becomes weaker and more horizontal. Remember, it is only the vertical component (that is, perpendicular to the plane of a loop of wire) that matters.

If you take some time to learn about the physics, you would see that the only way something put in a magnetic field can make a a big difference is by becoming magnetized. Magnetic field are produced by currents. All materials have atomic currents. In ferromagnetic materials these small current domains can line up to reinforce each other and have a big effect.

Joe, there is no belief involved here. I am describing how ferromagnetic materials work. If you want to understand this topic, do some reading or take a course. I repeat: when you put a ferromagnetic material such as steel in a magnetic field, the small domains of atomic currents, if they are initially disorganized or randomly oriented, tend to line up making a magnet. I think I gave you a clue as to how to learn about this above. An electromagnet is a coil of wire with a core, often soft iron or steel. When you run current through the coil, the resulting magnetic field is much stronger than if the core were not there. Look up electromagnet on Wikipedia. If you do not believe that, find some other references. I am not sure if you are purposely arguing with me or just ignorant. This is the last time I wil respond to one of your posts until you have something worthwhile to say.

We are talking about a temporary magnetization. Materials vary as to how much they retain. In fact, a permanent magnet could be just thought of as a ferromagnetic material where the retention time once the exterior magnetization is removed is essentially forever. I am not surprised you cannot measure any retained magnetism in a string, but that was a good measurement to make! I thought it would retain very little, and there is not a lot of material there to start with in a string, so it is hard to detect.

"analysis" I am just referring to the discussion in this thread. The thing that needs to be determined is that the fluctuating field from the magnetized string is small compared to the steady field. Then you can ignore the effect of the fluctuating field on the internal state of the permanent magnet as far as significantly altering its strength. Consider: 1. The string is a small weak magnet.* 2. The string moves just a little bit so the fluctuating part is even smaller. *How weak is a good question. I think the permeability of the steel used in strings is not as high as that of the soft steel used for the pole pieces. It is hard to find a material that does everything that you want! But the field that the string is in from the permanent magnet is quite a bit weaker than the field inside the pole piece. You can see this from the line spacing in the FEMM plots shown earlier in this thread. Then the field from the "string magnet" falls of with distance also. But the core stops it from falling off too fast. Could someone who knows FEMM do the case of a very small magnet (the string) over a steel pole piece? That would show how fast the field falls off.

" Anything that disturbs that magnetic field...." A way of analyzing the disturbance to the total field (that is, making that general statement more specific) is to represent the total field as the sum of two or several fields, and to examine the individual behavior of each. "The sensitivity of the coil, the shape of the field and strength of the coil and how much of the string is open to disturbing the field, all influence what an how much and to what degree these things are transfered into electrical energy." Yes, and what the analysis shows is that the shape of the fluctuating part of the field (from the vibrating string "magnet") through the coil matters, while the shape of the permanent field from the pickup's magnet only matters at the string because the purpose of this field is to magnetize the string.

Consider this: is an electromagnet a magnet? Of course, but only when the current in the coil is flowing. The magnetic field from the current causes the small domains of atomic current in the steel to line up, producing a much stronger field than from the current alone. So is the part of the guitar string over the pole piece a magnet? Of course, but only if the magnetic field from the pole piece causes the atomic currents in the steel string to line up. Just like the electromagnet, but the field that causes the alignment is from the aligned atomic currents in the pole piece rather than from current in the coil of wire. But your comment quoted above indicates that you do not understand how a pickup works. If the string were magnetized permanently, the permanent magnet in the pickup would be unnecessary, and even if present, the relative polarities would be irrrelevant. When the string vibrates in this case, the field fluctuatons through the coil would be essentially unchanged if you changed the polarity of the pickup magnet. I think your sarcasm is getting in the way of clear thinking.

Joe, you must be thinking that when you move a wire loop through a magnetic field in such a way as to change the flux cutting the loop, you induce a voltage around the loop. In a guitar, the string is not acting as a conductor, it is the magnet.

Peter, I apologize for not yet responing to this: "Everybody: Try this: Lay down two magnets or one magnet and a magnetic (not magnetized) object on your desk pretty close to each other. Put a piece of paper on top. Sprinkle carbon steel (or other magnetic, not magnetized material) file dust/shavings (or what ever it might be called). The metal dust will lie down, following the magnetic field lines, pretty much like Steven Kerting’s FEMM simulations. Watch the pattern close to magnet 1. Move magnet 2. What happens with the pattern close to magnet 1? The pattern close to magnet 2? The pattern changes around both magnets/magnetic objects." The field from each magnet fills all space, but is largest close to the magnet. Two magnets far apart appear to have their own individual fields because the field from the other one is small. As they brought together, what you see is not the pattern of individual magnets, but a new pattern. Remember, the magnetic field is a vector field. This means that the value at any point is the vector sum of the two fields. This field can have a new direction and magnitude, or even cancel out in some places, depending on the relative orientation of the two magnets. It is also true that the magnets can affect each others internal state. Whether this is a big effect or not depends on the type of magnet and how close they are.

"There does seem to be some misunderstanding here. A pickup's coil detects disruptions to it's permanent magnetic field" I would say that the pickup's coil detects fluctuations (in time) of the magnetic field (through the coil). These fluctuations come from the vibrating magnetized string. If you change the permanent magnetic field inside the coil somewhere without changing the permanent field at the string, you have not changed the fluctuations at all, and so the pickup output is the same. The permanent field inside the coil and the fluctuating part do not interact*; the total field is just the sum of the two. When you take the derivative in Peter's equation you get the same thing if you consider the total field or just the fluctuating part because the derivative of a constant (the permanent field) is zero. That is: d(phif)/dt and d(phif + phip)/dt are the same, where phif is the fluctuating flux and phip is the permanent or unchanging flux. *Now this is very slightly approximate. You can alter the internal state of a magnet with a strong magnetic field. This happens with large currents as in a speaker with alnico magnets. This might be what you are thinking of. But the fluctuating field from the vibrating string is weak. That is why it takes 5000 turns of wire to do a good job of picking it up, and then you still have to amplify. So you can ignore the effect of the vibrating string affecting the strength of the magnet at the string frequency. The permanent field of the magnet is just huge compared to the fluctuating part.

This appears to be about both intination and temperment. The idea comes in at least four forms for different temperments. One of them is the standard equal temperment. If you check the pictures, you see that the frets for this are not very much perturbed from straight. But I do not see any reason to do this; if you are not happy with normal bridge compensation, you can do nut compensation as well. There are some articles on that somewhere. As for the other temperments, if that is what you want for a particular kind of music, then do it.

Here is an illustration of how the flux decreases further from the string. Remember that funny kind of humbucker (stacked), that was never too popular? Where you had two coils wound on the core, one above the other? Those coils are connected out of phase to cancel the hum. But you still get signal, some signal, but not as much as you would like. If the flux did not decrease, the signal would cancel completely. The more modern stacked humbuckers mentioned above use a shield between the two coils, so you get less signal cancelation, and so more signal.

"Mike, have you measured where the flux changes the most?" No I have not, but one certainly expects magnetic fields to decrease with distance from the source. Maxwell's equations (which describe all of E&M) are really well proved. Understanding them was one of the things leading up to special relativity, and so on. In any case, one could make such a measurement approximately by partioning the coil into two or more sub coils along the core. "Meaning we get a lesser and lesser output when the frequency goes down?" Yes, that is right. I would look at it this way: The voltage is proportioinal to d(phi)/dt. (derivative of phi with with respect to time). The equation for a sine wave of frequency f is sin(2(pi)ft). The derivative of this with respect to time is (2(pi)f)cos(2(pi)ft). This is proportinal to frequency, and so you get less at lower frequencies, and more at higher frequencies. The frequency response of the pickup is also modified by the properties of the circuit it is in, as I described above.