Measuring the capacitance of an inductor or Inductors

[Mike Sulzer]

I was not talking about analyzing a pickup with VR. I was saying VR is pretty much how pickups work. It was in reference to the "disturbing the field" thing.

Describing "how something works" is an analysis. As Joe has said, you can analyze pickups in different ways. You have no right to say that your preferred method of analysis is "how it works" or "what it is". Nor do I, but I have good reasons why another method leads to an intuitive understanding that is less confusing.

Transformers are not transducers. They aren't made to sense moving metal objects in the gap between the poles. They don't have permanent magnets. But sure, they have cores and coils, and all the inductance rules apply, but I feel if you are using a transformer as a model, you are missing things that go on in pickups. Like VR.

I should have said that many transformers do have a gap. It has to do with avoiding saturation. In that case the transformer core looks very much like the core used in a high efficiency VR sensor where a piece of magnetic material rotates in and out of the gap of a core that is otherwise completely closed. And in either case (sensor or transformer) the magnetic circuit can be analyzed very easily because you have field lines confined to the core and passing pretty much straight through the gap. The core part is like a low value resistor. The gap is like a higher value resistor. Because the gap has a simple pattern of field lines, you can use a simple formula to compute what the resistance (or reluctance) is. In the case of the sensor, when the rotating piece moves in, the reluctance goes way down, allowing you to easily compute the change in flux.

Computing the reluctance in the guitar pickup case requires computing the field pattern, such as with FEMM. The vibrating string changes the reluctance only a small amount. How do you compute this without using FEMM or some similar method?

Someone recently referred to a paper that did some calculations. I believe it made some useful simplifications. Do you remember how the authors did it?

[David Schwab]

Computing the reluctance in the guitar pickup case requires computing the field pattern, such as with FEMM. The vibrating string changes the reluctance only a small amount. How do you compute this without using FEMM or some similar method?

Yes, by all means do that. But thats one little part, and then the rest of the things going on all seem to interact with each other. Can we isolate them and get an accurate picture of what's going on? Some effects are probably caused my multiple things. Tweeze them apart, and it's gone. So how do we know if the test alter the outcome?

I read everything you and Joe write, because I'm interested in it. But I also see changes I can make and what they do. I think I know why it's happening, but in the end it sometimes doesn't matter.

It reminds me of the Kasha guitar. Dr. Michael Kasha looked inside a classical guitar and thought it was all wrong and designed one using physics. Do they sound better? They sound different. It's not a huge difference though. So even without science behind classical guitar design, luthiers figured out how to improve things by trail and error, and sometimes just having a hunch or two. So Kasha & Schneider made improvements, but it wasn't as radical a change in tone as the changes in the instrument's construction would imply.

[Mike Sulzer]

Yes, by all means do that.

No, I prefer not to use VR at all for pickups.

Wasn't one of Kasha's goals to make a louder guitar with more sustain? Did he succeed?

[David Schwab]

No, I prefer not to use VR at all for pickups.

And that's what I'm talking about. You seem to make assumptions right from the start. Like with the eddy current thing. Joe gives you tests to do so you can see. Have you tried them?

I know it's the norm to have an opinion and then test for it, but it seems obvious that pickups use VR, since there are many industrial sensors based on that.

Wasn't one of Kasha's goals to make a louder guitar with more sustain? Did he succeed?

He wanted to make a more efficient guitar, and with more lows and highs.

The neck is stiffer and the sides thicker for more sustain, and the sound hole is moved so there is more of the top to vibrate.

Did he succeed? He made some improvements, but there seems to be a lot more effort put into the guitar for the return you get. And some people don't like the way they sound.

So he didn't make a normal classical guitar louder, he made a new kind of classical guitar tone.

I think they sound nice... the little I played any of them. I met Richard Schneider back in '95. He made very nice guitars.

[Mike Sulzer]

And that's what I'm talking about. You seem to make assumptions right from the start. Like with the eddy current thing. Joe gives you tests to do so you can see. Have you tried them?

I know it's the norm to have an opinion and then test for it, but it seems obvious that pickups use VR, since there are many industrial sensors based on that.



He wanted to make a more efficient guitar, and with more lows and highs.

The neck is stiffer and the sides thicker for more sustain, and the sound hole is moved so there is more of the top to vibrate.

Did he succeed? He made some improvements, but there seems to be a lot more effort put into the guitar for the return you get. And some people don't like the way they sound.

So he didn't make a normal classical guitar louder, he made a new kind of classical guitar tone.

I think they sound nice... the little I played any of them. I met Richard Schneider back in '95. He made very nice guitars.

Pickups do not use VR; people do when they analyze them. Or not. I do not. What did that article I referred to use?

I am not the one making assumptions right from the start. I am questioning such assumptions. (Are eddy currents really responsible for reducing the inductance of a pickup coil with increasing frequency, explaining the decrease in inductance seen with Extech meters from 100 Hz to 1000 Hz? I say no, and I have shown why the meter is wrong, and why the inductance does not decrease due to decreases in the permeability from eddy currents.) So by not going along with what everyone else says, then I am making assumptions? Baloney! Why do you have such unthinking loyalty to the status quo? You brought up MIMF earlier, and say this is the same. Yes, but more so. That guy was really wrong, and you know it. Yet you think I am wrong for trying to understand what is correct. Go think about why you are doing this.

Kasha: You are dodging the issue. You claim that Kasha did not succeed in making a better guitar using science. Yet you agree that one his goals was a louder (therefore more efficient) guitar. If he succeeded in making a more efficient (louder) guitar he was successful in applying science to improve the guitar in the way he intended. Was he?

[David Schwab]

Pickups do not use VR; people do when they analyze them. Or not. I do not.

Whatever you say Mike.

Are eddy currents really responsible for reducing the inductance of a pickup coil with increasing frequency, explaining the decrease in inductance seen with Extech meters from 100 Hz to 1000 Hz? I say no, and I have shown why the meter is wrong, and why the inductance does not decrease due to decreases in the permeability from eddy currents.

Here's a better question... why does placing conductive metal in or near the pickup reduce the reproduction of high end? Put a cover on a pickup. Listen the tonal change. Now make a slit in it. Why does making a slit in the same cover decrease the loss of high end?

Kasha: You are dodging the issue. You claim that Kasha did not succeed in making a better guitar using science. Yet you agree that one his goals was a louder (therefore more efficient) guitar. If he succeeded in making a more efficient (louder) guitar he was successful in applying science to improve the guitar in the way he intended. Was he?

I claimed nothing. I said they sound different. I did not play one and think "wow, this is the best guitar I ever played!" Some people claim it sounds like a piano. Others don't care for them, or think they make exaggerated claims. I haven't played one enough to have an opinion and I'm not a big acoustic guitar player anyway. I think they look cool though.

And does louder equate to more efficient? That makes the assumption that it's louder at all frequencies. Or maybe just louder at some frequencies... like the bass. It might be more efficient at some frequencies. But so would a larger guitar. They also have an upper side mounted sound hole, so they would sound louder to the player. And making the guitar louder was not the single goal of the design. Banjos are louder, but do they sound good for classical guitar music?

I like the idea of the Kasha/Schneider design. I have no experience building one. But my point was that for all the work that goes into them, and it is an awful lot of work, the returns are not as much as you would expect. So was that a success? I don't know.

Also, it could very well be that the increase in efficiency is due to the stiffer neck and sides. My experience with basses shows that a very stiff neck produces a clearer tone, especially in the low end. And of course more sustain. Louder acoustically and more sustain are kind of opposing forces. The more energy to put into making sound, the less sustain you often have. Like a banjo.

I've also played a Rainsong graphic acoustic, a Martin acoustic with an aluminum top and plastic sides, and one of those acoustic guitars with the strings anchored in a fan shape on the rim of the top to remove stress. None of them sounded like anything but an acoustic guitar. And the Rainsong was very quiet.

I love new ideas, but some of them seem like too much effort for zero returns. But it's good to try new ideas.

[Mike Sulzer]

Whatever you say Mike.
Here's a better question... why does placing conductive metal in or near the pickup reduce the reproduction of high end? Put a cover on a pickup. Listen the tonal change. Now make a slit in it. Why does making a slit in the same cover decrease the loss of high end?

As I said above when up asked the same question: Eddy currents. As I said above, I am not saying that eddy currents do not do that. You must know this by now.

You cannot be asking this question in good faith. I will not communicate with you again.

[David Schwab]

As I said above when up asked the same question: Eddy currents. As I said above, I am not saying that eddy currents do not do that. You must know this by now.

You cannot be asking this question in good faith.

Are eddy currents really responsible for reducing the inductance of a pickup coil with increasing frequency, explaining the decrease in inductance seen with Extech meters from 100 Hz to 1000 Hz? I say no, and I have shown why the meter is wrong, and why the inductance does not decrease due to decreases in the permeability from eddy currents.

OK you answered that question.

I will not communicate with you again.

Promise?

[RedHouse]

He sucked you in David!


(in the voice of Red from That 70's Show)...

"I'd show him some eddy currents with my foot up his ass"

[Possum]

....

Say how we get back to the original intent of this thread? How do you measure capacitance of a pickup in a way that is useful and that doesn't use a college lab full of super expensive gear? If it CAN be measured is it of any actual use?

[Mike Sulzer]

Say how we get back to the original intent of this thread? How do you measure capacitance of a pickup in a way that is useful and that doesn't use a college lab full of super expensive gear? If it CAN be measured is it of any actual use?

As I wrote yesterday use the method that Joe suggested. If that is too much trouble, you could do what I suggested earlier with one external capacitor.

I think the most important information is the location and width of the resonance, but it is good to know the actual values of the L and C. They determine how high or low the impedance is as well, and thus indicate how much the pickup is influenced by external circuit components.

[RedHouse]

Say how we get back to the original intent of this thread? How do you measure capacitance of a pickup in a way that is useful and that doesn't use a college lab full of super expensive gear? If it CAN be measured is it of any actual use?

Possum, really good question 'ol man, and something in common for most of us. I would be concerned with the capacitance if determinable, not as a critical parameter but more like a build-check.

...As I wrote yesterday use the method that Joe suggested. If that is too much trouble, you could do what I suggested earlier with one external capacitor.....

Now you're on the same team as Joe? c'mon Mike.

[Mike Sulzer]

More on the method Joe suggested

Now you're on the same team as Joe? c'mon Mike.

Not quite. I do not agree with this: "Plot added capacitance versus the square of resonant frequency in radians per second." We have this relationship"

(2*pi*f)^2 = 1/(LC)

You can denote 2*pi*f by "omega" if you wish. Then in words this says: "The square of omega varies as one over the product of the inductance and the capacitance."

We can rearrange this to give:

C = 1/((2*pi*f)^2).

This implies that there is a linear (as in draw a line) relationship between one over f squared and C. Joe has left out the "one over", and you cannot get a straight line if you do this. Note that you can use omega if you, or any other constant multiplier. The linear nature of the relationship is unaffected.

I did some measurements with capacitor values varyin between 94.3 pf and 1010 pf. Resonant frequencies varied between 9371 Hz and 3550 Hz. When plotted acording to what I said above the result is this:

http://www.naic.edu/~sulzer/coilCap1.png

The red points are the data; the blue line is drawn through them. I used my computer to plot it instead of paper and pen, and I used a polynomial fitting program to draw the line, but you could do that on paper, too. The line is actually a quadratic (parabola); it is very nearly a straight line. Since it is a quadratic, it can curve if it needs to. It curved only a little bit, maybe not even significantly. This means that the inductance of this humbucker coil, with cores in place, does not vary very much between 3.5 KHz and 9 KHz.

The measured capacitance is 74 pf. You can get this from the plot or from the zeroth order fitted coefficient.

I will discuss the lack of much variation in the inductance in the discussion about varying inductance sometime later.

[RedHouse]

....I will discuss the lack of much variation in the inductance in the discussion about varying inductance sometime later.

Oh good, now we can all sleep at night.

[Possum]

OK for the retards among us, me being chief retard, I dug this back out, Joe's method. I tried it again and now it seems to work, maybe I used the wrong resistor. So, I plodded through the whole equation to get to C. I had an inductance of 3.957 Henrie. A peak res frequency of 6.2khz, and a coil resistance of 7.721K. Calculated Q was 19.91. That seems awfullly high unless its Q at the resonant frequency? At a 1khz on the Extech Q is about 2.5.

Joe's example is great but not a real pickup. This is a real pickup, so I ended up with a number something like .0000000066, there may have been more zeroes in there. I don't know what 10 with the small 11 Farads next to it in the final part on the bottom really means, but it just looks like it means x 10 according to the answer he got. if thats true then my answer was still infinitesmally small, and I don't know what it means. I tried some online calculators and end up with huge numbers, so that was no use. The pickup is a '63 Patent.

I did try the resonant frequency hookup and happened to have a 1meg ohm pot laying around that I used, so I got readable peaks from buckers and strats. Now if I hook up my computer oscilloscope programs across the 1megohm pot it won't read any peaks at all. Using a driver coil and hooking the pickup straight into the Mac's line in I don't get any readable peaks either. So how DO you hook this up to an oscilloscope program with line in on an oscilloscope or audio input?

And what IS the capacitance of my '63, my results don't make sense.....