Low Impedance Over and Under Pickup Design

[Mike Sulzer]

Look at the initial attack of a string using the neck pickup where the string movement is the greatest, then look at the bridge pickup. You will see the greatest asymmetry with the neck pickup because the string has the greatest mechanical swing for the first few cycles farther away from the bridge. It is still present at the bridge pickup but to a lesser extent.

Joseph,

The bridge pickup sees much less fundamental than the neck pickup. The amount of difference decreases as the harmonic number increases. Therefore you naturally see more harmonics from the bridge pickup. When I conduct the test described above with the bridge pickup, I see more harmonics than with the neck pickup. This is a clear indication that the harmonics are from the string vibration, not from pickup non-linearity.

In order to see how big the effect of the distortion is, I ran a FEMM model for a steel core with an alnico magnet. I measured the field above and below a point centered .1 inch above the pole piece. 0.01 inches above the reference point, the field has decreased about 6%. 0.01 inches below it has increased 7%. I then computed the distorted wave form resulting from this, shown here: http://www.naic.edu/~sulzer/effectOfChangingFeild.png. Both the undistorted sine wave and the distorted wave form are shown. The effect is small enough so that I do not think that it can be distinguished from the harmonics in the string vibration.

[David Schwab]

There's a reprint of an old Bartolini catalog that talks about strings... unfortunately most of the image links are broken...

STRINGS and MAGNETS and COILS

[Mike Sulzer]

There's a reprint of an old Bartolini catalog that talks about strings... unfortunately most of the image links are broken...

STRINGS and MAGNETS and COILS

That is a pretty good description considering that it is from a manufacturer's catalog! The idea of the ripples moving in both directions is a nice way to look at it, but it is not complete. If you look at the string just before the pick releases it, the string position as a function of location can be thought of as a sum fundamental and harmonic displacements that evolve when the pick releases the string. The higher harmonics as a group can be thought of as moving ripples, but the fundamental certainly cannot. Also, the elliptical motion is something that evolves, but is not present at the very start. (The stretched string wants to return straight to its non-stretched state, but it overshoots, of course, and asymmetries at the terminations cause transfer of energy into the other plane.)

Where it really falls down is in pushing a company product. You do not need a special pickup to favor the up and down motion (like a bridge) because pickups are already sensitive to just that motion.

The discussion of aperture is good. Note that a humbucker is described as sensing at each pole tip rather than along the whole width of the coil. However, figure 4 is misleading and disagrees with the text. It shows the correct aperture for a single coil, covering just the pole piece. For the humbucker it should show just the two pole pieces, not the whole length. The figure is inconsistent; it cannot be right for both.

[bbsailor]

Joseph,

The bridge pickup sees much less fundamental than the neck pickup. The amount of difference decreases as the harmonic number increases. Therefore you naturally see more harmonics from the bridge pickup. When I conduct the test described above with the bridge pickup, I see more harmonics than with the neck pickup. This is a clear indication that the harmonics are from the string vibration, not from pickup non-linearity.

In order to see how big the effect of the distortion is, I ran a FEMM model for a steel core with an alnico magnet. I measured the field above and below a point centered .1 inch above the pole piece. 0.01 inches above the reference point, the field has decreased about 6%. 0.01 inches below it has increased 7%. I then computed the distorted wave form resulting from this, shown here: http://www.naic.edu/~sulzer/effectOfChangingFeild.png. Both the undistorted sine wave and the distorted wave form are shown. The effect is small enough so that I do not think that it can be distinguished from the harmonics in the string vibration.

Mike,

Nice graph. The .01" string peak vibration above and below the .1" string resting location above the magnet represents a very lightly plucked string possibly at the bridge pickup. Try the same graph but with a plus and minus .02 and again with a plus and minus .03" string peak vibration. I think you will see numbers that represent about a 25% to 30% amplitude difference between the 0 volt reference point. Your example now shows a 13% (6% + 7%) difference so a string picked slightly harder will show a greater difference.

Lighter strings typically show a wider peak vibration range.

This thread has evolved into an informative technical discussion.

Joseph Rogowski

[Rick Turner]

I would include coil dimensions in aperture descriptions, after all, that's where the current is induced. The coil is bathed in flux, and it is the modulation of the flux which induces current in the coil, not a pole piece or magnet in the center of the coil.

I started playing with aperture in some of my earliest (yes) hand wound pickups in 1969, and some of my now-vintage Alembic pickups were trapezoidal with wider aperture under the bass strings and narrower under the trebles.

Agreed that the Bartolini literature is self-contradictory. However, Bill and Pat had and have every right to flog their products in their own literature. Read some early 20th century Gibson literature if you really want to see it laid on thick!

The place at which a pick stretches and releases the string does indeed set up it's own set of reciprocal and usually enharmonic set of partials which die down pretty quickly. That's part of the essentially enharmonic percussive noise that is the head of a plucked string note, and it's one of the issues that confounds pitch to voltage or MIDI converters and makes for difficult tracking.

[David Schwab]

That is a pretty good description considering that it is from a manufacturer's catalog!
...
Where it really falls down is in pushing a company product. You do not need a special pickup to favor the up and down motion (like a bridge) because pickups are already sensitive to just that motion.

I think you have to look at the fact that Bill Bartolini was involved in a lot of research into acoustics and sound where it concerned guitars. He published several papers on such topics. I remember reading some years ago that he had an electric guitar that could simulate the tone of a nylon string acoustic.

Bartolini Company History

I think as with a lot of things, you have an idea or sound in mind and you try to explore ways of getting to that end. In his case it seems he wanted to get acoustic guitar tones from electrics. He was one of the first replacement pickup makers, and has been doing things quite differently since the beginning. A lot of other pickup makers just wind wire on a coil and stick a magnet in. His early designs were quite elaborate. No one goes thorough so much effort to make something if they feel it doesn't work the way they wanted.

So he may be pushing a product, but his pickups sound quite different from other makers. A lot of people seem to like them too, especially bass players.

[Rick Turner]

The bridge pickup sees less fundamental than the neck pickup as long as you're not playing way up the neck, as so many do!

On of the fundamental weirdnesses of under string magnetic pickups is that the ratio by which their aperture window divides the strings is constantly changing as you play up and down the neck. This is why the whole thing of putting a pickup in a "sweet spot" is such utter nonsense. In fact, a neck pickup may very well be the more trebly of two pickups if it's at the very end of the fingerboard, and one is playing right up there in 19th to 24th fret territory.

The only pickups that transduce a harmonically "perfect" signal are those buried in the bridge.

[David Schwab]

On of the fundamental weirdnesses of under string magnetic pickups is that the ratio by which their aperture window divides the strings is constantly changing as you play up and down the neck. This is why the whole thing of putting a pickup in a "sweet spot" is such utter nonsense.

Exactly my thoughts. Sure, neck pickups sound nice at the 24th fret location, but as soon as you fret a note the harmonics move. I have been known to put a pickup where I think it looks good on the body. One location is as good as another, just different.

Here's an interesting thing to try. Take a solid body instrument and listen to it un-amplified. This works best with basses. First stick your ear on the upper body horn, in the front surface, the face of the guitar. Pluck a few notes and listen.

Now stick your ear in the same location, but on the side of the body, so you are now 90° from where you were. Pluck some notes and listen again. It sounds very different. I guess this could be from standing waves, but could it also be from the way the movement of the strings excites the body?

Likewise I hear something quite different if I pluck a string from the top instead of the side. So I think there may be something to the idea that conventional pickups are more sensitive to up/down and not sideways movement, or vice versa, and that the two sound very different from each other.

So pickups between the strings could be very interesting.

[Mike Sulzer]

I would include coil dimensions in aperture descriptions, after all, that's where the current is induced. The coil is bathed in flux, and it is the modulation of the flux which induces current in the coil, not a pole piece or magnet in the center of the coil.

No, the pole piece magnetizes the string right over it; the magnetization falls off very quickly outside of that. You cannot have output from a part of the string that is not magnetized.

Furthermore, the pole piece tends concentrate the changing flux from the vibrating string, increasing the output. It only does this very near to right over it.

So the aperture is very much set by the pole piece, not the coil. Remember, it is the total changing flux through the coil that matters. The fact that the flux is mostly in the middle of the area of a loop of wire is not a problem as long as the loop is not so large that the return lines of flux cancel out the ones passing through the pole piece.

[Rick Turner]

Mike, if your "model" of how a pickup works is accurate, then how does a non-magnetized section of string induce a current in a coil? If the string's effect is only over a pole or pole piece, then we should get very little out of a pickup. That is obviously not true, therefore I think you may need to rethink your mental image of what's going on. I think the "magnetized string" model is only one mode in which pickups operate.

[Mike Sulzer]

Mike,

Nice graph. The .01" string peak vibration above and below the .1" string resting location above the magnet represents a very lightly plucked string possibly at the bridge pickup. Try the same graph but with a plus and minus .02 and again with a plus and minus .03" string peak vibration. I think you will see numbers that represent about a 25% to 30% amplitude difference between the 0 volt reference point. Your example now shows a 13% (6% + 7%) difference so a string picked slightly harder will show a greater difference.

Lighter strings typically show a wider peak vibration range.

This thread has evolved into an informative technical discussion.

Joseph Rogowski

Joseph,

You are right, the larger the amplitude of string motion, the greater the effect of the non-linearity. Do you have some information on how far the string moves? I made an attempt to measure it once, and got about .01 inches, but that could be wrong.

Mike

[Mike Sulzer]

Mike, if your "model" of how a pickup works is accurate, then how does a non-magnetized section of string induce a current in a coil?

It doesn't.

If the string's effect is only over a pole or pole piece, then we should get very little out of a pickup.

Why?

That is obviously not true, therefore I think you may need to rethink your mental image of what's going on. I think the "magnetized string" model is only one mode in which pickups operate.

Sorry, I am not basing this on a mental model of what is going on, but on an actual understanding of how it works. This is physics. I do physics and engineering for a living and understand how E&M works.

If you really think that there is some other mode that pickups work in, show me with facts and understanding.

[Rick Turner]

Kind of makes me wonder about a DC biased coil... Pump some DC into a coil and capacitively couple to a preamp stage to block the DC. Eliminate permanent magnets entirely. This was done on a brand of lap steels in the '30s and '40s. You had a switch on the amp that would send DC up to the guitar, and charge the strings from a single coil. Then you'd flip the switch and it would turn off the DC and route the pickup signal to the input of the amp. You had to do a string magnet recharge every so often...probably quite often. But you could just use the coil as an always on electromagnet as well as pickup coil. Hmmm...modulate the DC bias with the string signal as a "linearizer"... Talk about a hot pickup...

[Mike Sulzer]

Kind of makes me wonder about a DC biased coil... Pump some DC into a coil and capacitively couple to a preamp stage to block the DC. Eliminate permanent magnets entirely. This was done on a brand of lap steels in the '30s and '40s. You had a switch on the amp that would send DC up to the guitar, and charge the strings from a single coil. Then you'd flip the switch and it would turn off the DC and route the pickup signal to the input of the amp. You had to do a string magnet recharge every so often...probably quite often. But you could just use the coil as an always on electromagnet as well as pickup coil. Hmmm...modulate the DC bias with the string signal as a "linearizer"... Talk about a hot pickup...

That is a good example, Rick. Magnetize the strings, and the pickup works. Otherwise it does not.

[Rick Turner]

Mike, the classic theory of pickup operation is that the ferrous metal strings disturb a static magnetic field inducing a current in local conductors...the coil. So that would be mode 1

Another mode could be "moving shunt" with the string being a magnetic shunt between two or more poles...for example the North/South of a humbucker. I do think this is one of the more important modes for the micro-humbuckers that make up the Roland GK series of synth controller pickups. They suggest very, very close to string mounting, and clearly when mounted this way, the string is acting as a dynamic magnetic shunt in the magnetic circuit.

And, of course, we have the moving magnet theory whereby either permanent or electromagnet is used to charge the string...but is it a monopole? Highly unlikely! Or does a single coil pickup charge the string as a tri-pole? North or South directly over the magnets and weaker opposite pole on either side of that? Or is the bottom of the string one polarity and the top the other? Hmmmmm....

I've got the questions if you have the answers...