Moving a Coil Farther from the Magnets...?

[John Kolbeck]

I had a discussion related to this topic in this post: http://music-electronics-forum.com/t39633/#post386933 see number 42. Generally, the outer coil turns are farther from the stronger magnetic field and thus produce less voltage per coil turn than the inner coil turns. I outlined an experiment to observe and measure the inner and outer coil output differences.

Joseph J. Rogowski

It's true that a coil with more area has a higher inductance, and that the outer coil has slightly more area, so you're assuming that the slight increase in flux density for the inner coil will outweigh the slightly higher inductance of the outer coil. It sounds to me like it might be a wash, with respect to output voltage. Whichever configuration produces the higher inductance would have a slightly lower resonant peak, though.

[Richard]

A pickup's inductance is complicated by the fact that a pickup has core that is partially air and partially permeable metal, which is an unusual arrangement, but it seems to me that if the coil is wider, the flux will not link up with that permeable core as well. Are you saying that the geometric relationship between the coil and it's partially permeable core are irrelevant?

Another factor I forgot to mention is that the unused portion of coil will cause a strong eddy current if it's shunted, rather than disconnected. So it's like this:

- if the unused coil is full disconnected, it's almost but not entirely inert.
- if the unused coil is connected at only one end, it will capacitively couple with the active coil
- if the unused coil is grounded at both ends, current will flow, and this will cause eddy current resistance in the active coil

Would these effects be true of a humbucker or a stacked humbucker?

[John Kolbeck]

Would these effects be true of a humbucker or a stacked humbucker?

It's still true, but to a smaller extent. When the coil pickup is side-by-side with the primary, it will be mostly inert if it's fully disconnected or connected at one end. But if the second coil is shunted to ground, it will have continuity within itself, and so it will be generating a voltage internally that does not effect the rest of the circuit, but still creates an opposing magnetic field due to Lenz's law, and that opposing field can be though of as eddy current losses against the primary.

A stack would be the same; does nothing if fully disconnected, or connected at one end, but the lower coil would be further from the strings, so it would generate less eddy current losses when it's shunted.

If one end is connected in either case, there will be some capacitive coupling with the primary coil, but it's much a smaller amount than a tapped coil, where the two coils are in very close proximity to on another.

[Mike Sulzer]

Here we have a magnetic core which has 100 to 1000 times higher permeability (depending on purity) than air so it is the defining factor, period.

An open short core has much less effect than implied by its high permeability. Remember even a small air gap in the otherwise complete magnetic circuit has a significant effect on the operation of a transformer. A magnetic circuit that is mostly air gap cannot have a high effective permeability, maybe something like 10 at most.

[John Kolbeck]

I have a ceramic Strat pickup, I measured the inductance fully assembled, it was about 4 henries. Then I broken off the magnet and popped the steel slugs out and it read about 1.5 henry, so the steal pole pieces roughly triple the inductance in the context of a guitar pickup. The same Strat pickup with AlNiCo will usually read around 2.2 henries, because AlNiCo is less permeable than the steel.

[Mike Sulzer]

Yes. The ratio of inductances is significantly less than the ratio of permeabilities. The same effect holds true for guitar strings. As long as the permeability of the string is high, the exact value is not so important.

The steel core SC has a lower resonant frequency and higher eddy current losses than the Alnico SC. Not the same thing at all, more like a hum bucker. You might use fewer turns to get back part of the difference.

I have a ceramic Strat pickup, I measured the inductance fully assembled, it was about 4 henries. Then I broken off the magnet and popped the steel slugs out and it read about 1.5 henry, so the steal pole pieces roughly triple the inductance in the context of a guitar pickup. The same Strat pickup with AlNiCo will usually read around 2.2 henries, because AlNiCo is less permeable than the steel.

[Mike Sulzer]

One difference in using turns further from the core: The field lines induced in the core from the vibrating string go through the core and then loop back around to the string. If a loop of wire encloses the same field line coming from the string and going back to it, the output from that field line is lost since its signal cancels. This is because the field points in opposite directions in the two passes through the loop. The bigger the loop, the more field lines cancel.

Since the same effect holds true for the core by itself, a larger loop contributes less to the inductance.

This is not a really big effect for loops with the width of a guitar pickup, but if you used a loop several inches in diameter, you would get very little. But there is some loss with individual pole pieces because of the length of the coil.

[Richard]

It's still true, but to a smaller extent. When the coil pickup is side-by-side with the primary, it will be mostly inert if it's fully disconnected or connected at one end. But if the second coil is shunted to ground, it will have continuity within itself, and so it will be generating a voltage internally that does not effect the rest of the circuit, but still creates an opposing magnetic field due to Lenz's law, and that opposing field can be though of as eddy current losses against the primary.

A stack would be the same; does nothing if fully disconnected, or connected at one end, but the lower coil would be further from the strings, so it would generate less eddy current losses when it's shunted.

If one end is connected in either case, there will be some capacitive coupling with the primary coil, but it's much a smaller amount than a tapped coil, where the two coils are in very close proximity to on another.

I have a guitar with stacked single coils (Fender Vintage noiseless) that I am about to rewire, this and the thread about the single humbucker and the 5 way switch has me thinking about some other options. The pickups seem to have a falloff in response in the upper range cutting some chime out. When I ground the lower coil they are a bit over the top. When I first wired it up I had a bad pot so I believe my signal was passing through my treble bleed network which had a nice thin sound. Fortunately the guitar has plenty of room to work inside.

[Richard]

It's still true, but to a smaller extent. When the coil pickup is side-by-side with the primary, it will be mostly inert if it's fully disconnected or connected at one end. But if the second coil is shunted to ground, it will have continuity within itself, and so it will be generating a voltage internally that does not effect the rest of the circuit, but still creates an opposing magnetic field due to Lenz's law, and that opposing field can be though of as eddy current losses against the primary.

A stack would be the same; does nothing if fully disconnected, or connected at one end, but the lower coil would be further from the strings, so it would generate less eddy current losses when it's shunted.

If one end is connected in either case, there will be some capacitive coupling with the primary coil, but it's much a smaller amount than a tapped coil, where the two coils are in very close proximity to on another.

I have a guitar with stacked single coils (Fender Vintage noiseless) that I am about to rewire, this and the thread about the single humbucker and the 5 way switch has me thinking about some other options. The pickups seem to have a falloff in response in the upper range cutting some chime out. When I ground the lower coil they are a bit over the top. When I first wired it up I had a bad pot so I believe my signal was passing through my treble bleed network which had a nice thin sound. Fortunately the guitar has plenty of room to work inside.

[jvin248]

I found this online inductance calculator. I'm ignoring permeability of the core as the question is about the coil itself changing geometry keeping other variables constant. The basic inductance of the coil seems to change in a linear fashion (5000 turns, 42AWG wire) at various core diameters, I used quarter inch, half, three-quarters, one, and two inches.

So a '5000 turn' coil with a one inch core has 2.25 times the Inductance of a half inch core.

online calculator: https://www.eeweb.com/toolbox/coil-inductance (another is CalcTool: Solenoid properties calculator)

[Joe Gwinn]

Be aware that calculators for solenoids are not all that accurate for coils that don't resemble solenoids, which are far longer than their diameter.

Pickup coils are very short, typically about the length of the shortest thickness (end on) or a fifth the length of the widths (broadside).

There are formulas for short coils that are more accurate, but even these are approximate for something shaped like a pickup coil.