Perturbable Fields

[fieldwrangler]

Folks -

OK... time to take a leap here.

At present I have a need to build some very lightweight yet very efficient passive high-impedance pickups for guitar applications. I have a clue or two about how to get a bunch of wire crammed into a place in a magnetic circuit that will react fairly well to the relatively small disturbance caused by a teeny little string. But now I need to get more serious about the basic business of transducing string behavior into electrical signals.

My thoughts go to some figure of merit that takes into account not only the density of the flux around the wire (and the direction of its motion relative to the wire's length) but also the "instability" of the field with respect to a small disturbance such as a little string might cause.

And so, a couple of questions come to mind:

1. Is there a way to determine a benchmark for efficiency in guitar pickup designs?

2. Am I nuts to think that there might be a way to form a relatively unstable field that is more easily modulated by the very small motions of the very small things we call strings?

I need for the Big Boys to weigh in on this one; you know who you are.

Bob Palmieri

[fieldwrangler]

I should admit that this lightweight-efficient thing does beg the question about getting into the single-turn w/current transformer world.

However, I have zero experience in that area; at least with conventional hi-z stuff I feel like I have a small handle on some of the parameters.

Bob Palmieri

[big_teee]

Are you talking Humbuckers, when you say high output.
If you can you be more specific, I think you'll get more response.
Gud Luk,
T

[fieldwrangler]

Thanks, Tee!

I have no interest in selling anyone a pickup that hums.

[Joe Gwinn]

1. Is there a way to determine a benchmark for efficiency in guitar pickup designs?

The problem is not lack of ways, but a surfeit of ways. When you say "efficiency", what exactly are you trying to get more of?

2. Am I nuts to think that there might be a way to form a relatively unstable field that is more easily modulated by the very small motions of the very small things we call strings?

There is exactly one kind of electromagnetic field.

One can make magnetic circuits (versus fields) more efficient, under various definitions of efficient.

[fieldwrangler]

The problem is not lack of ways, but a surfeit of ways. When you say "efficiency", what exactly are you trying to get more of?

There is exactly one kind of electromagnetic field.

One can make magnetic circuits (versus fields) more efficient, under various definitions of efficient.

If there is a way to determine the amount of mechanical energy present in a vibrating string along a defined segment of its length, there should be a way to determine how much power a perfect pickup could deliver when positioned under that section.

[Mike Sulzer]

If there is a way to determine the amount of mechanical energy present in a vibrating string along a defined segment of its length, there should be a way to determine how much power a perfect pickup could deliver when positioned under that section.

The mechanical energy of the sgtring need not be computed. You need the velocity, or the string displacement and frequency, and the magnetic field produced by the magnetized string. Working with the mechanical energy of the string would be more complicated.

The voltage the pickup can deliver is what counts, not the power it could deliver. The pickup has a mostly inductive impedance over the useful frequency range. Matching it for maximum power transfer would not in general give the desired frequency response.

[Joe Gwinn]

If there is a way to determine the amount of mechanical energy present in a vibrating string along a defined segment of its length, there should be a way to determine how much power a perfect pickup could deliver when positioned under that section.

While one could envision doing such a thing, it isn't a good idea. If one does a perfect job of extracting the mechanical energy from a vibrating string, it will instantly stop vibrating. Because you stole its energy.

In musical terms, there would be zero sustain. It would be like playing the guitar immersed in heavy mineral oil.

[fieldwrangler]

Joe Gwinn wrote: "There is exactly one kind of electromagnetic field."

A comforting thought, this. Probably I was just indulging in my usual practice of throwing around terms too loosely. Do we have a good name for the shape and density of the "lines of force" (as they used to call them when I was in school) especially those that are external to the permanent magnet and pole pieces?

He also wrote: "If one does a perfect job of extracting the mechanical energy from a vibrating string, it will instantly stop vibrating."

And Mike Sulzer wrote: "The voltage the pickup can deliver is what counts, not the power it could deliver."

These statements do help me clarify my thinking. In a really efficient hand cranked generator, for instance, if we tried to drive a zero impedance load (short circuit) we wouldn't be able to turn the handle, right? And seeing as how such a generator depends on a known (circular) path for the moving element, and needs to fully surround said element in order to achieve its level of efficiency this ain't exactly the most relevant model for us.

So, let me try to put my query in another form: For a given vibrating string over a given length, what arrangement of wire & magnets will produce the most voltage into a high-impedance load with the least amount of wire, flux & string pull? And, let's assume that the pickup just sits under the string, no surrounding here.

I realize that once again I've put forth a messy question, and that forming the proper question gets one a larger distance toward the proper answer, but there you have it.

Bob Palmieri

[Joe Gwinn]

Joe Gwinn wrote: "There is exactly one kind of electromagnetic field."

A comforting thought, this. Probably I was just indulging in my usual practice of throwing around terms too loosely. Do we have a good name for the shape and density of the "lines of force" (as they used to call them when I was in school) especially those that are external to the permanent magnet and pole pieces?

Yes. We call them "lines of force" to this day. Not because such lines actually exist, but because it's a useful mental model.

He also wrote: "If one does a perfect job of extracting the mechanical energy from a vibrating string, it will instantly stop vibrating."

And Mike Sulzer wrote: "The voltage the pickup can deliver is what counts, not the power it could deliver."

These statements do help me clarify my thinking. In a really efficient hand cranked generator, for instance, if we tried to drive a zero impedance load (short circuit) we wouldn't be able to turn the handle, right?

Yes, the generator will become difficult to turn.

And seeing as how such a generator depends on a known (circular) path for the moving element, and needs to fully surround said element in order to achieve its level of efficiency this ain't exactly the most relevant model for us.

So, let me try to put my query in another form: For a given vibrating string over a given length, what arrangement of wire & magnets will produce the most voltage into a high-impedance load with the least amount of wire, flux & string pull? And, let's assume that the pickup just sits under the string, no surrounding here.

I realize that once again I've put forth a messy question, and that forming the proper question gets one a larger distance toward the proper answer, but there you have it.

Bob Palmieri

Given that surrounding the wire is excluded, you are pretty much restricted to the current designs. Probably the most efficient (in the sense of voltage output for a given string motion) is Kinsmans patented design with the laminated core and shield surrounding a single winding.

[fieldwrangler]

Probably the most efficient (in the sense of voltage output for a given string motion) is Kinsmans patented design with the laminated core and shield surrounding a single winding.

Haven't seen this one...

[David Schwab]

Haven't seen this one...

More Information for Telecaster

Joe, do we know he uses a single turn? It doesn't look like it in these photos, though he may be using extra string.

You can see the wire from the bottom coil in this image also:

http://www.fretdreamer.fr/parts/imag...sterBridge.jpg

[Mike Sulzer]

Let's examine the idea that it is reasonable to understand the optimization of a pickup output (as opposed to a vague idea of how it works) through the concept "perturbable field". That idea is based on the analogy between electrical circuits and so called magnetic circuits, an analogy associated with the term "variable reluctance". It is not possible to understand this analogy without some background that can be found here: Magnetic circuit - Wikipedia, the free encyclopedia. It is important to understand the section "Limitation of the analogy". It is also important to understand the last sentence of the "Circuit Laws" section: "More complex magnetic systems, where the flux is not confined to a simple loop, must be analysed from first principles by using Maxwell's equations."

The basic problem is that there are two levels to this analogy. The good case is when the magnetic circuit adheres very closely to the conditions that allow an analogy to an electrical circuit. This requires that the flux be confined to one or more simple loops. This does not describe a pickup, which has flux all over the place. There still is a more general analogy that applies:

electrical: the current density = the conductivity times the E field

magnetic: the flux density = the permeability times the H field.

Solving either one is equivalent and of equal difficulty. In general, neither provides any easy insight into the performance. But there could be cases where the electrical circuit is easier to understand than the magnetic. This requires working on the problem with a bit of physics, just some simple physics to make the problem simpler. I will do this in a future post.

[fieldwrangler]

This is a good prodding for me to at least get back up to speed on some concepts on which I do believe I once had something resembling a grasp. Or at least a grope.

'Meantime, my current thinking (so to speak) is that I'm gonna set up some arrangements of magnets and pole elements in position over a steadily vibrating string. Then I'm thinking I'll set up a dedicated rig with a Hall element which I'll move around the whole arrangement (with appropriate orientation of said Hall element) and see where I get the most activity. That's where I want to put the wire...

Bob Palmieri

[Joe Gwinn]

By single winding I meant a singlecoil (a bobbin with ~5000 turns of wire), not a one-turn coil.