Measuring pickups Capacitance?

[bbsailor]

Is anybody going to wind a bifilar (two wires in parallel) pickup coil and measure its impedance curve in series configuration?

Here is a thought experiment to consider and some may want to make this a real experiment.

Consider a pickup with four independent sections with an equal number of turns.

Upper section: Inner coil 1; Outer coil 2
Lower section: Inner coil 3; Outer coil 4

If you consider the inner coils being similar to a Strat type pickup, tall and narrow. Then you consider the Upper or Lower section coils 1 and 2 or 3 and 4 to being a short and flat pickup like a Jazzmaster or P90 type.

If you stimulate the pickup configurations with a drive coil or a vibrating string, would you have the same voltage output with either stimulation way or would the outer coil have less output being farther from the magnets?

If you consider the output of the total upper section compared to the lower section it is like the upper section is closer to the strings than the lower section and the upper section would have a higher output.

Creating a good mental model of a pickup shape supported by some measurements being done with a more realistic vibrating string or a stimulation coil should lead to a more deep understanding of the issues involved in this discussion.

How would you rank the output level from each coil in this experiment?

Joseph J. Rogowski

[Mike Sulzer]

This statement is correct. Mike Sulzer is/was right and I apologize. I was on the wrong track.

The total induced voltage/EMF would show at the inductor's terminals only in an open loop situation without any current. But any real world inductor like a PU is terminated at least by its own distributed capacitance. Just as you said.

I am glad we have agreement!

[Helmholtz]

I need to correct at least this previous statement of mine:

I never denied that the ac field generates a voltage. In fact, the voltage induced across the inductance of the PU images the voltage across the inductance part of the exciter coil (which is not directly accessible, though), namely a voltage rising linearly with frequency.

Instead it should read: ...., the voltage induced in series with the inductance...

Thanks again to Mike Sulzer for the "private lesson".

My alternative explanation with the current transformer isn't as simple as I thought. To make it work, the transformed primary circuit + leakage inductances taking care of loose coupling would have to be included in the complete equivalent circuit, leading to branched currents. This makes it quite complex and unattractive.

[Helmholtz]

Here is a picture of my integrator coil(s):

[ATTACH=CONFIG]48710[/ATTACH

And here is a low pass PU transfer measurement using the coil:

coil integrator.pdf

A second pair of these coils is used in my magnetizer:

[Antigua]

If you stimulate the pickup configurations with a drive coil or a vibrating string, would you have the same voltage output with either stimulation way or would the outer coil have less output being farther from the magnets?

Here's a plot I created with my home made tapped coil, comparing the inner and outer coils with a small ficoil above the pickup:



(note that the secondary peaks after the primary resonance are irrelevant, but I included them because they're sort of interesting anyway. Thanks again to Helmholtz for the auto voltage scaling tip that allowed these to be seen.)

The outer coil shows about +2dB over the inner coil. Both are 4,000 turns, but the outer coil involves more area and wire length, of course. Ultimately the deciding factor for the sum voltage comes down to which coil catches a greater overall flux change. I'm not sure of the exact geometry or integrations that are at play, but it intuitively stands to reason that bigger net catches more fish.

If you consider the output of the total upper section compared to the lower section it is like the upper section is closer to the strings than the lower section and the upper section would have a higher output.

That's also very similar to simply raising or lowering a pickup, or measuring the two halves of a stacked humbucker. I've done that before, and documented it on another forum. Needless to say, the lower coil generates quite a bit less voltage.

Creating a good mental model of a pickup shape supported by some measurements being done with a more realistic vibrating string or a stimulation coil should lead to a more deep understanding of the issues involved in this discussion.

I'd be interested to talk about the difference between an external field coil and a guitar string as far as how similar they are and aren't, and whether there is something peculiar about a guitar string that makes it requisite for thorough pickup testing. The major difference IMO would relate to the transient, because when you first pluck the string it displaces a wide area, and then it settles down to a rather small amount of movement, at which point the geometry is nearly static, where as an external coil's geometry is literally static. I think overall they're more alike than they are different, both put out an EMF from from an nearly fixed point in space.

[Antigua]

Is anybody going to wind a bifilar (two wires in parallel) pickup coil and measure its impedance curve in series configuration?

Do you have a theory as to what could be expected? Wouldn't this greatly increase the voltage potential across the self-capacitance?

[Antigua]

Here is a picture of my integrator coil(s):

[ATTACH=CONFIG]48710[/ATTACH

And here is a low pass PU transfer measurement using the coil:

[ATTACH]48711[/ATTACH]

A second pair of these coils is used in my magnetizer:

[ATTACH=CONFIG]48712[/ATTACH]

Thanks for posting the picks. Those coils look pretty large. I'd be interested in trying this method, do you know what the minimum inductance value and voltage rating would be for a suitable coil to be purchased on, say, Mouser? Is your exciter coil in the picture somewhere as well?

[Helmholtz]

Do you have a theory as to what could be expected? Wouldn't this greatly increase the voltage potential across the self-capacitance?

I'd expect a strong anomaly.

BTW, could you measure the interwinding capacitance (i.e. capacitance between the separate windings) of your 4-wire tapped coil PU, as this was the OP's question?

[Antigua]

I'd expect a strong anomaly.

I'd expect that also, but from a time management perspective, it's a fair amount of work to create the pickup to demonstrate something that has no apparent practical value.

BTW, could you measure the interwinding capacitance (i.e. capacitance between the separate windings) of your 4-wire tapped coil PU, as this was the OP's question?

Given the issue of reflected capacitance, I'm not exactly sure how this is best accomplished. Which testing procedure would you recommend to get the most accurate capacitance between them?

[Helmholtz]

Thanks for posting the picks. Those coils look pretty large. I'd be interested in trying this method, do you know what the minimum inductance value and voltage rating would be for a suitable coil to be purchased on, say, Mouser? Is your exciter coil in the picture somewhere as well?

I recommend a minimum inductance of 10mH and a max. DCR of 2.5Ohms. What matters is the L/R ratio, where R includes the exciter coil's DCR. If this is too low, bass response will suffer. Also important is that the self resonance of the coil lies well above the highest frequency of interest. My coil has a resonant frequency of 45kHz. A too low coil resonance causes the output signal to rise at high frequencies. I am not familiar with the inductors'/chokes' range of any distributor, but I think that a suitable choke doesn't need to be bigger than 3x3x3 cubic inches. I would look for speaker crossover parts.

The exciter coil I used for the measurement consisted of 20 turns wound around a strat PU cover.

[Helmholtz]

I'd expect that also, but from a time management perspective, it's a fair amount of work to create the pickup to demonstrate something that has no apparent practical value.

The reason why I am interested in such an experimental PU is that up to now there is no direct proof that the build of the coil can actually produce anomalies. This would be an extreme situation with maximum coupling between the two sections wired in series. If such kind of PU does not show anomalies, I have to assume a different reason, namely partial shorts in the winding. And this should be quite alarming from a QC point of view.

Given the issue of reflected capacitance, I'm not exactly sure how this is best accomplished. Which testing procedure would you recommend to get the most accurate capacitance between them?

The reflected capacitances appear in parallel with each winding but not between separate windings. With a 4 wire PU there are 4 possibilities to connect the LCR meter between the 2 windings. I would measure all 4 of them, the most meaningful being the C value between end of inner coil and start of outer coil.

[Joe Gwinn]

The reason why I am interested in such an experimental PU is that up to now there is no direct proof that the build of the coil can actually produce anomalies. This would be an extreme situation with maximum coupling between the two sections wired in series. If such kind of PU does not show anomalies, I have to assume a different reason, namely partial shorts in the winding. And this should be quite alarming from a QC point of view.

One can detect partial shorts by looking at the excess of AC resistance (measured with a DE5000 at 1 KHz) over DC resistance, this excess reflection the sum of all power losses. A shorted turn will cause a large increase in loss.

One compares the loss in known good units to units under suspicion to detect the outliers.

[Helmholtz]

One can detect partial shorts by looking at the excess of AC resistance (measured with a DE5000 at 1 KHz) over DC resistance, this excess reflection the sum of all power losses. A shorted turn will cause a large increase in loss.

One compares the loss in known good units to units under suspicion to detect the outliers.

Not necessarily. A single shorted turn won't show at all. Even 100 hard-shorted outer turns change Rs@1kHz by less than 10%. A more sensitive method is to compare the Q values of the resonance in the impedance plots. But verified reference PUs are rarely available.

You may want to read this article by Prof. Manfred Zollner:
https://gitec-forum.de/wp/wp-content...he-winding.pdf

There are several variables that influence the results: number of turns shorted, position of the shorted section within the coil influencing coupling - and contact resistance of the "short". The article only covers perfect coupling.

One may argue that only shorts that noticeably effect the loaded transfer response will be audible. But I would be alarmed by other evidence as well, because this indicates that something is wrong with the wire quality or the winding technique.

[Antigua]

I agree that Q factor would reveal internal short circuits well. A very extreme example is the loss in Q (and everything else) when you short one of the windings of a tapped single coil.

I'm very suspicious that shorts exist, though, because it would require some mechanism to have stripped the insulation off, not just a segment of wire, but also another segment of wire that happens to be in physical contact with the first. Suppose insulation was missing from a few feet of magnet wire, through some sort of manufacturing defect which I've never heard of, the total size of that continuous short would be very small compared to the rest of the coil. I can appreciate that the enamel used in the old days might have been prone to failure, I bet the modern polyurethane insulations are probably much less prone to failure.

Based on the accumulation of plots of home made and commercially available pickups, that secondary resonances come from non uniformities across the network of internal capacitances within the coil, due to uneven turn layering. I was able to partially model this concept with LTSpice. The very loose home made coil that was wound with minimal tension showed the most dramatic secondary resonances, and was least likely to have an internal short.

[Helmholtz]

Based on the accumulation of plots of home made and commercially available pickups, that secondary resonances come from non uniformities across the network of internal capacitances within the coil, due to uneven turn layering. I was able to partially model this concept with LTSpice. The very loose home made coil that was wound with minimal tension showed the most dramatic secondary resonances, and was least likely to have an internal short.

I agree that the uneven layering of turns is the more probable explanation of secondary resonances, but there is no solid proof yet - only some probability. I have found new strat PUs that show both an unusual low Q as well as a very strong antiresonance-resonance pair. And I am convinced that such secondary resonances could be modelled from the equivalent circuit of partial shorts shown by Zollner if distributed capacitances are included. Fact is that winding shorts do happen but are only likely to be detected if output is noticeably reduced and/or the sound is really dull. So I suspect some dark figure.

Hence my wish for a bifilar wound PU. There were requests from other posters also.