Measuring pickups Capacitance?

[Antigua]

It is not a frequency-dependance that is influenced by the shape of the coil.

It sounds like you're defining the coil as the aperture. I suppose there is a discussion to have as to what constitutes the aperture as a whole, but I'd rule out considering only the coil alone.

The returning flux has the same frequency-dependance as the primary flux.

Exactly, any size or shape of coil is going to receive the same information, it's just a question of amplitude.

[Mike Sulzer]

Two rectangular bar type magnets with the same magnetic pole facing the metal bar which holds the string pole pieces spreads the magnetic field differently than the magnetic field in a Jazzmaster with short round magnets.

I think the pole pieces (near the strings) affect the shape of the field at the strings more than the magnets below the coil. What do you get if you take out the pole pieces while holding the magnets in place?

[bbsailor]

I think the pole pieces (near the strings) affect the shape of the field at the strings more than the magnets below the coil. What do you get if you take out the pole pieces while holding the magnets in place?

The pole pieces simply put the magnetic field up over the top of the bobbin and closer to the string than if they were removed. I would suspect that even with the pole pieces removed there would be less output from the pickup since the lower bar in the center of like pole magnets would still project some magnetic field up to the strings. I would guess that observing the FEMM models of the magnetic field of various style pickups:

Tall cylinder magnets and types
Short cylinder magnets and types
Metal plates under the pickup
P90 with center focused like poles under the bobbin

would provide a visual indication of what coil shape would produce more voltage from either the inner winding, outer winding or the upper winding compared to the lower winding. This is from a string rather than a stimulation coil that may not accurately represent the voltage distribution in various sections of a pickup coil (using a stimulation coil).

Joseph J. Rogowski

[Helmholtz]

It sounds like you're defining the coil as the aperture. I suppose there is a discussion to have as to what constitutes the aperture as a whole, but I'd rule out considering only the coil alone.

Definitely not. Please read my posts carefully. The shape of the coil has no influence on aperture. But I thought we were talking about the effects of wide and/or fat coils. I only mentioned aperture because it kind of determines the width of the source of the returning ac flux.

[Mike Sulzer]

The pole pieces simply put the magnetic field up over the top of the bobbin and closer to the string than if they were removed. I would suspect that even with the pole pieces removed there would be less output from the pickup since the lower bar in the center of like pole magnets would still project some magnetic field up to the strings. I would guess that observing the FEMM models of the magnetic field of various style pickups:

Tall cylinder magnets and types
Short cylinder magnets and types
Metal plates under the pickup
P90 with center focused like poles under the bobbin

would provide a visual indication of what coil shape would produce more voltage from either the inner winding, outer winding or the upper winding compared to the lower winding. This is from a string rather than a stimulation coil that may not accurately represent the voltage distribution in various sections of a pickup coil (using a stimulation coil).

Joseph J. Rogowski

If you remove the pole pieces, the source of the magnetic field is farther from the strings. This makes the field at the strings weaker, but also less focused so that the field that remains is more spread out along the strings. Thus the aperture is wider.

[Antigua]

The pole pieces simply put the magnetic field up over the top of the bobbin and closer to the string than if they were removed. I would suspect that even with the pole pieces removed there would be less output from the pickup since the lower bar in the center of like pole magnets would still project some magnetic field up to the strings. I would guess that observing the FEMM models of the magnetic field of various style pickups:

Regarding the contribution of the AlNiCo bars in a fully assembled P-90, I'm still in the process of learning, but my understanding is that the voltage generated through the loop of wire depends on a change in flux density through the area of the loop, and the more more perpendicular the lines of flux are through the area, the more voltage you get. It seems to me that if you "stretch" the magnetic field so that it fans out over a wider area, with AlNiCo bars or whatever, you decrease the tendency for lines of flux to be perpendicular with the loops of wire in the coil, and you cause them to become more sideways. My feeling is that at best you get a net zero change, if not a loss.

The big caveat is that AlNiCo has a rather low permeability, and so it will not draw the magnetic field out as well as two steel bars would in place of the AlNiCo bars.

[Antigua]

If you remove the pole pieces, the source of the magnetic field is farther from the strings. This makes the field at the strings weaker, but also less focused so that the field that remains is more spread out along the strings. Thus the aperture is wider.

This could be a means of creating a very wide aperture, for the sake of demonstration: take guitar with a P-90 pickup, remove the pole piece from under a string and it's immediate neighbors. Then maybe see how that plucked string sounds as is, probably very weak. Then, hold a bar magnet over the pickup, perpendicular to the strings, which should create a very narrow aperture. After that, hold the bar parallel to the string, which, assuming the magnetism along the length of the bar magnet is relatively uniform, should create an aperture with a width of about 2 1/2 inches. The fact that the magnetized segment of string exceeds the boundaries of the coil would cause some cancellation, but not total cancellation. Harmonics lengths that receive uniform magnetization (or close to it) should cancel, and they should be of a low enough frequency to be audible. I'll give this a try later and see what happens.

[Helmholtz]

There is another component to the aperture, more important for steel pole pieces than alnico. A high permeability pole piece increases the flux from the vibrating string most effectively when the source of the flux is right over it. You can see this by using a very small exciter coil and making measurements in various positions. This is not a big effect, but because of it, the aperture from the two effects is somewhat smaller than from the magnetization alone.

Lacking a better definition, I tend to see the aperture as the effective length of string the PU can read and over which string motion is averaged. While high µ pole pieces better focus ac flux and thus influence flux distribution in the coil, I don't see how this could change the string length being sensed, even if the returning flux gets somewhat squeezed. After all the aperture of the P-90 was measured with these influences taking effect.

[bbsailor]

Regarding the contribution of the AlNiCo bars in a fully assembled P-90, I'm still in the process of learning, but my understanding is that the voltage generated through the loop of wire depends on a change in flux density through the area of the loop, and the more more perpendicular the lines of flux are through the area, the more voltage you get. It seems to me that if you "stretch" the magnetic field so that it fans out over a wider area, with AlNiCo bars or whatever, you decrease the tendency for lines of flux to be perpendicular with the loops of wire in the coil, and you cause them to become more sideways. My feeling is that at best you get a net zero change, if not a loss.

The big caveat is that AlNiCo has a rather low permeability, and so it will not draw the magnetic field out as well as two steel bars would in place of the AlNiCo bars.

Antigua,

To further your research, look for a MEF thread labeled “Moving coil pickup for the technically curious”. All you need is a small audio transformer with an 8 ohm or 4 ohm low side to anywhere from 10K to 20K on the high side. Simply alligator clip the transformer low side from behind the nut to behind the bridge on an acoustic guitar or an electric guitar where the strings are not electrically connected in parallel with the test string. Attach the high impedance side to the amp input. Take a long rectangular magnet and place it perpendicular under the string and measure or listen to the plucked string output. Now, rotate the magnet to be parallel with the string so a longer magnet window is stimulating the string and listen or measure a higher output.

With this setup you want the wires feeding the transformer to be one tenth the resistance of the string under test to minimize resistive losses. The 8 ohm winding acts as a bridging input about 7 to 10 times higher than the impedance/resistance of the string to produce the highest output. If you measure the raw voltage with a scope probe across the test string you will see a raw voltage in the low millivolt range. The turns ratio of the audio transformer will boost the output from 25 to 50 times depending on its turns ratio. To find the approximate turns ratio of any transformer, divide the impedance of the low primary side into the value of the high secondary side and take the square root of that number. Example 20000/8 equals 2500. The square root of 2500 is 50 for a turns ratio of 1 to 50.

Tinkering with this stuff is a great way to learn things that give you insight into pickup design and performance.


Joseph J. Rogowski

[Mike Sulzer]

Lacking a better definition, I tend to see the aperture as the effective length of string the PU can read and over which string motion is averaged. While high µ pole pieces better focus ac flux and thus influence flux distribution in the coil, I don't see how this could change the string length being sensed, even if the returning flux gets somewhat squeezed. After all the aperture of the P-90 was measured with these influences taking effect.

Yes, but it is this statement that needs some modification: "Aperture corresponds to the magnetized string length." Magnetize a short section of the string half a meter from the pickup, and it contributes very little to the signal. Move the magnetized section closer, and it contributes more. The coil and the pole piece determine how much, and so they are involved in this component of the aperture, although this is not a big effect for magnetization over the coil.

[Helmholtz]

Yes, but it is this statement that needs some modification: "Aperture corresponds to the magnetized string length." Magnetize a short section of the string half a meter from the pickup, and it contributes very little to the signal. Move the magnetized section closer, and it contributes more. The coil and the pole piece determine how much, and so they are involved in this component of the aperture, although this is not a big effect for magnetization over the coil.

Do you mean that PUs with different cores differently weight the point contributions along the aperture length and thus produce differently shaped aperture functions?
While such effect is conceivable, it would show especially in different slopes of the aperture functions. However the measured aperture functions of the JM pu and the P-90 are extremely similar in shape.
From this I conclude that such aperture shaping effects must be very small in typical PU applications. My description (not definition) of the aperture as "magnetized string length" may not be completely precise but is quite useful in practice.

[Mike Sulzer]

Do you mean that PUs with different cores differently weight the point contributions along the aperture length and thus produce differently shaped aperture functions?
While such effect is conceivable, it would show especially in different slopes of the aperture functions. However the measured aperture functions of the JM pu and the P-90 are extremely similar in shape.
From this I conclude that such aperture shaping effects must be very small in typical PU applications. My description (not definition) of the aperture as "magnetized string length" may not be completely precise but is quite useful in practice.

The effect is more than conceivable, it is significant, if not large. Since the aperture function is not rectangular, it is subject to a definition, such the distance between half power points in the direction along the string. Therefore, the aperture width is a function of everything that affects the aperture function.

[David King]

Like the "Unknown unknowables"?

[Helmholtz]

The effect is more than conceivable, it is significant, if not large.

This is an unsubstantiated claim.
I gave numbers and arguments that support my evaluation of the effect. You are free to do your own aperture measurements and demonstrate the influence of different cores. I am willing to give this another thought when there is valid evidence in realistic situations.
For reference, the shape of the aperture function of SCs is similar to a Gaussian (no matter if alnico or low carbon steel cores) and I used the -16dB points for the aperture lengths I specified.

[Mike Sulzer]

This is an unsubstantiated claim.
I gave numbers and arguments that support my evaluation of the effect. You are free to do your own aperture measurements and demonstrate the influence of different cores. I am willing to give this another thought when there is valid evidence in realistic situations.
For reference, the shape of the aperture function of SCs is similar to a Gaussian (no matter if alnico or low carbon steel cores) and I used the -16dB points for the aperture lengths I specified.

Lots of things give approximately Gaussian shapes. Two factors that have Gaussian shapes multiply together to give another Gaussian. So I do not think that you have shown that the string magnetization alone sets the aperture. The measurements that I have made with a small exciter coil show that the sensitivity drops off as the coil is moved away from an initially centered location.