Post by ms on Feb 28, 2020 10:16:48 GMT -5
There appear to be two types of hum bucking pickups. The most common uses two side by side coils along the string connected (usually) in in series with opposite electrical polarity. The two pole pieces magnetize the two locations along the string with opposite magnetic polarity. The other kind samples the string in one place. It uses symmetry in the magnetic circuit to cancel the effect of fields from distant sources, fields that change little in intensity over the size of the pickup. A nearby source has a field that changes significantly over the size of the pickup, and so it breaks the symmetry.
The attachment shows the result of a FEMM calculation, modeling the field resulting from a small magnet above a ferrite pole piece structure with a permeability of 500. The small magnet is a simple model for a magnetized string. The calculation uses the planar mode and so the pole piece extends indefinitely into and out of the page, and thus does not show the end effects that would be present with a structure of finite length, sufficient to cover the six strings. Also, in the model the string is really a flat sheet; this would alter the "retention" of flux in the ferrite structure somewhat. In the plot, flux lines have been selected to make it clear that the the flux lines from the magnet spread out, that is, the field intensity weakens with increasing distance from the magnet. But once the lines encounter the ferrite, the spreading is greatly reduced.
The hum B field is a vector, and so for cancelation we consider three independent components:
1. Along the vertical axis. The upper and lower vertical arms of the structure are in fields of equal intensity; the resulting fields in the horizontal arms cancel.
2. In and out of the page. Perpendicular to the coil axis, and so nothing is picked up.
3. Left to right. The coils have opposite polarity, and so this component cancels.
B Field lines must return to their source, and the horizontal arms of the ferrite structure guide the split of the flux to the left and right. Some flux escapes into the lower part of the structure. So we add coils to the horizontal arms to get a sidewinder. The correct polarity adds the signal and subtracts hum. About half the flux from the small magnet passes through each coil. The flux escaping out the bottom of the structure is lost.
The hum B field is a vector, and so for cancelation we consider three independent components:
1. Along the vertical axis. The upper and lower vertical arms of the structure are in fields of equal intensity; the resulting fields in the horizontal arms cancel.
2. In and out of the page. Perpendicular to the coil axis, and so nothing is picked up.
3. Left to right. The coils have opposite polarity, and so this component cancels.
The vertical arms tend to shield the two horizontal coils from each other, but nonetheless, there is some coupling, and as a result of the opposite polarity connection, the resulting mutual inductance reduces the total inductance somewhat.
There is another way to make use of this ferrite structure: put coils on the upper and lower vertical arms instead. Now we have a stacked humbucker where the horizontal arms tend to shield the two coils from each other. We expect about the same signal level because we also lose the flux through the lower vertical arm because it subtracts from the total. We also have a similar amount of negative mutual inductance to reduce the total inductance.
Of course we can stretch the vertical or horizontal arms to fit the available space. With a long space along the strings (standard humbucker, P-90) we would probable choose to stretch in the horizontal direction, and make a sidewinder. With a narrow space (strat, etc.) we would probably choose to stretch in the vertical direction and use a stack. We must use ferrite in this case; steel would lower the Q too much for a strat pickup.
What happens if the permeability is low? The second attachment is the same as the first but the permeability is three. This would not be nearly as effective. (Actually the second attachment is the same as the first. The third is with a permeability of three. I have not found a way to eliminate an image after a mistake.)
The attachment shows the result of a FEMM calculation, modeling the field resulting from a small magnet above a ferrite pole piece structure with a permeability of 500. The small magnet is a simple model for a magnetized string. The calculation uses the planar mode and so the pole piece extends indefinitely into and out of the page, and thus does not show the end effects that would be present with a structure of finite length, sufficient to cover the six strings. Also, in the model the string is really a flat sheet; this would alter the "retention" of flux in the ferrite structure somewhat. In the plot, flux lines have been selected to make it clear that the the flux lines from the magnet spread out, that is, the field intensity weakens with increasing distance from the magnet. But once the lines encounter the ferrite, the spreading is greatly reduced.
The hum B field is a vector, and so for cancelation we consider three independent components:
1. Along the vertical axis. The upper and lower vertical arms of the structure are in fields of equal intensity; the resulting fields in the horizontal arms cancel.
2. In and out of the page. Perpendicular to the coil axis, and so nothing is picked up.
3. Left to right. The coils have opposite polarity, and so this component cancels.
B Field lines must return to their source, and the horizontal arms of the ferrite structure guide the split of the flux to the left and right. Some flux escapes into the lower part of the structure. So we add coils to the horizontal arms to get a sidewinder. The correct polarity adds the signal and subtracts hum. About half the flux from the small magnet passes through each coil. The flux escaping out the bottom of the structure is lost.
The hum B field is a vector, and so for cancelation we consider three independent components:
1. Along the vertical axis. The upper and lower vertical arms of the structure are in fields of equal intensity; the resulting fields in the horizontal arms cancel.
2. In and out of the page. Perpendicular to the coil axis, and so nothing is picked up.
3. Left to right. The coils have opposite polarity, and so this component cancels.
The vertical arms tend to shield the two horizontal coils from each other, but nonetheless, there is some coupling, and as a result of the opposite polarity connection, the resulting mutual inductance reduces the total inductance somewhat.
There is another way to make use of this ferrite structure: put coils on the upper and lower vertical arms instead. Now we have a stacked humbucker where the horizontal arms tend to shield the two coils from each other. We expect about the same signal level because we also lose the flux through the lower vertical arm because it subtracts from the total. We also have a similar amount of negative mutual inductance to reduce the total inductance.
Of course we can stretch the vertical or horizontal arms to fit the available space. With a long space along the strings (standard humbucker, P-90) we would probable choose to stretch in the horizontal direction, and make a sidewinder. With a narrow space (strat, etc.) we would probably choose to stretch in the vertical direction and use a stack. We must use ferrite in this case; steel would lower the Q too much for a strat pickup.
What happens if the permeability is low? The second attachment is the same as the first but the permeability is three. This would not be nearly as effective. (Actually the second attachment is the same as the first. The third is with a permeability of three. I have not found a way to eliminate an image after a mistake.)