Measuring pickups Capacitance?

[bbsailor]

In the past I had a similar view, that there is something special about pickups, and that my goal is to find it, but some other people pointed out to me that I haven't established with certainty that anything special really exists to find. It might be how I feel about the pickups, some good psychological connotation, that makes me believe there is something special. That's how it remains to this day, I'm not sure that I wasn't imagining the thing I set out to look for.

The easiest way to marry specs with subjective experience would be to, not only have a blind fold and a friend help you conduct an test free of extrinsic factors, but to also have as many specifications as possible about those pickups. Unfortunately, only the DC resistance is readily available, so I'm working towards providing other guitarists more extensive specs for popular pickups, including capacitance measures.

When measuring pickups put a 200K Ohm resistor for single coils or 350K Ohm resistor in parallel for humbucker pickups with about 350pf capacitor (resistor and cap in parallel) across the pickup output to simulate what the pickup looks like loaded by the typical volume pot and the typical coax cable capacitance. These value represent the full load on the pickup when either a 250K Ohm pot or 500K Ohm pot is in parallel to the typical amp input impedance of 1 Meg Ohm. Doing the same measurements with this added load will probably change the upper frequency peaks that you are seeing. The main audio effect will be a slight reduction of the peak resonance near 3KHz to 5KHz due to the fact that the coil impedance is highest at pickup resonance and the pot loading will reduce the peak somewhat. The capacitance loading of the coax cable (350pf simulated load) will also lower the resonant frequency.

Bottom line: Try to do all tests in the same environment that the pickup sees when mounted in the guitar and used in typical situations about 10 ft from the amplifier. This will allow your ears to be in tune better with what you see on the graphs.

Joseph J. Rogowski

[Helmholtz]

This sounds like what we've been doing in this thread, using the VElleman bode plot and function generator, similar to what is described here http://www.syscompdesign.com/assets/...ar-pickups.pdf but using a 1meg resistance instead of 56k.

I don't quite agree with some details in this papers. As a consequence the f-responses in figure 5 are wrong. Will elaborate if someone cares.

[Antigua]

I don't quite agree with some details in this papers. As a consequence the f-responses in figure 5 are wrong. Will elaborate if someone cares.

This PDF is a prominent search result when searching for information on how to measure the response of guitar pickups. Any critique you have would be valuable to anyone who finds both that PDF and this thread.

[Mike Sulzer]

This sounds very interesting. But my expertise in signal and system theory is rather limited, so I have no feeling for the power and benefit of such method. What kind of signal do you use? A kind of noise?
But, being a pragmatic, my main question is: How do your results differ from those of standard single frequency point measurements with a current source? Can you show some?

The current version uses Golay complementary sequences, that is, a pair of codes that together weight all frequencies equally, or have no side lobes, as a radar person might say. This gives faster more accurate measurements than the noise like signals I have used before. (Since the measurement is a ratio, taken in the frequency domain, equal weighting in the code is not strictly necessary for accuracy, but it does give more uniform signal to noise ratio.)

There is no need for a very high input impedance amplifier to drive the sampler, nor for a good current source. It can be done with a two channel recording interface, cheap these days, something many people already have.

I use an audio package in Python, incorporated into custom software for the signal processing.

Here is an example taken with an earlier version, but it is interesting because it shows two measurements with the same coil, alnico cores and steel.

[Antigua]

When measuring pickups put a 200K Ohm resistor for single coils or 350K Ohm resistor in parallel for humbucker pickups with about 350pf capacitor (resistor and cap in parallel) across the pickup output to simulate what the pickup looks like loaded by the typical volume pot and the typical coax cable capacitance. These value represent the full load on the pickup when either a 250K Ohm pot or 500K Ohm pot is in parallel to the typical amp input impedance of 1 Meg Ohm. Doing the same measurements with this added load will probably change the upper frequency peaks that you are seeing. The main audio effect will be a slight reduction of the peak resonance near 3KHz to 5KHz due to the fact that the coil impedance is highest at pickup resonance and the pot loading will reduce the peak somewhat. The capacitance loading of the coax cable (350pf simulated load) will also lower the resonant frequency.

Bottom line: Try to do all tests in the same environment that the pickup sees when mounted in the guitar and used in typical situations about 10 ft from the amplifier. This will allow your ears to be in tune better with what you see on the graphs.

Joseph J. Rogowski

I use 200k and 470pF as fixed "loaded" values. I use the same values for humbuckers and single coils for the sake of consistency. Myself and someone else had settled on these values, and then I found out later Helmuth Lemme used the exact same values here http://www.planetz.com/wp-content/up..._Technique.pdf , so it seems to be a reasonable in-between standard.

[Helmholtz]

I use 200k and 470pF as fixed "loaded" values. I use the same values for humbuckers and single coils for the sake of consistency. Myself and someone else had settled on these values, and then I found out later Helmuth Lemme used the exact same values here http://www.planetz.com/wp-content/up..._Technique.pdf , so it seems to be a reasonable in-between standard.

After extensive testing I have settled with guitar cables having 1000pF to 1200pF, both for my Strats and my Les Pauls. The wiring harness in a typical vintage LP adds 300 to 500pF. A tube amplifier input adds another 150pF typically. I use vintage style PUs.
A realistic load resistance for strats is 100k to 200K(bridge PU) and 200k for LPs. This includes typical amplifier/pedals input resistance.

I don't think any pro player would be comfortable with a stage cable of 10ft or less.

[Antigua]

As a guitar player I have settled (after extensive testing) with guitar cables having 1000pF to 1200pF, for my Strats as well as my Les Pauls. The wiring harness in a typical LP adds 400+pF.

400pF for the pots and hookups? I measured 70pF per foot for the braided wire used with PAF humbuckers, but 400pF overall seems high.

Fender guitars usually feature non shielded hookups and wiring, so the capacitance there is really low, well under 50pF I'd recon.

A tube amplifier input adds another 150pF typically. I use vintage style PUs.
A realistic load resistance for strats is 100K..200K and 200k for LPs. This includes typical amplifier/pedals input resistance.

Since peak freq. will vary from rig to rig, I think it's best to settle on a "center", and then let people shift the frequency up or down mentally, depending on their own rig. So if you know you like 1000pF cables, you know that you will have a peak that is somewhat lower than the standard loaded data points. If the intrinsic L and C are known, then any loaded peak freq. can be solved for, and because inductance factors more prominently, L is is the more valuable metric to have on hand.

I don't think any pro player would be comfortable with a stage cable of 10ft or less.

Pros often use wireless units too, which sometimes have selectable capacitance, or a fixed values. I analyzed a Line 6 G10 and found that it imparted about 120pF capacitance http://www.strat-talk.com/threads/th...ay-g10.467237/

[Helmholtz]

400pF for the pots and hookups? I measured 70pF per foot for the braided wire used with PAF humbuckers, but 400pF overall seems high.

The neck PU signal in a LP runs through around 3ft of coax wire between PU and jack. My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.
The self-capacitance of humbuckers with cloth-insulated coax cable is often dominated by the cable attached .
The capacitance of the guitar cable is the strongest influencer of PU frequency response besides inductivity.

[Helmholtz]

The current version uses Golay complementary sequences, that is, a pair of codes that together weight all frequencies equally, or have no side lobes, as a radar person might say. This gives faster more accurate measurements than the noise like signals I have used before. (Since the measurement is a ratio, taken in the frequency domain, equal weighting in the code is not strictly necessary for accuracy, but it does give more uniform signal to noise ratio.)

There is no need for a very high input impedance amplifier to drive the sampler, nor for a good current source. It can be done with a two channel recording interface, cheap these days, something many people already have.

I use an audio package in Python, incorporated into custom software for the signal processing.

Here is an example taken with an earlier version, but it is interesting because it shows two measurements with the same coil, alnico cores and steel.

alandsteelcores.png

Does this alternative method reveal any additional information compared to standard methods? Is there any direct comparison? Of course the effect of a finite source impedance can always be compensated via calculation.

[Mike Sulzer]

Does this alternative method reveal any additional information compared to standard methods? Is there any direct comparison? Of course the effect of a finite source impedance can always be compensated via calculation.

Well, I suppose there could be no more standard way of measuring impedance than by forming a ratio, as a function of frequency, of the voltage across and the current through the device, using superior inexpensive technology. But let's leave this aside for now. But yes, there is more to do. Consider the capacitance: it is of little importance itself because it is so small compared to other capacitances in the guitar circuit. But it does get in the way because it causes a resonance that makes it hard to see the effects of the metal on the impedance. So the capacitance is found, just for the purpose of removing it from the impedance, by a non-linear least squares fit to a set of samples taken well above the resonance where the capacitance is dominant. The low frequency inductance and the resistance are in the model of the impedance, and the parameters fitted to are the capacitance, and two parameters associated with the coupling to the metal, k, the coupling coefficient, and Rse, the effective resistance of the "secondary" reflected back to the primary. Once the C is found it is taken out of the impedance, and you can see in the plots that the real part increases above the pickup resistance as frequency increases, and the imaginary part decreases below the reactance of the coil inductance.

[Antigua]

My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.

I first read about the cloth shield hookup varying in capacitance by humidity in Manfred Zollner's book Physik der Elektrogitarre, and it's a rather significant claim, because people make hay over the littlest things, but never talk about how their pickups sound different between dry and wet climates. I tried a little experiment where I left shielded cloth hookup wire outside, measured the capacitance, the put it in the oven to dry it out, then measured again, but the capacitance didn't change much. I might give it another try, though, ensuring that it's very damp and then very dry.

[Antigua]

I think this model jives somewhat closely with the notion that having the winding fall into the edges puts a capacitance across two distant sections of the coil:



In this model, the series inductance and resistance are broken into three sets, with the middle "RL" set having a capacitance in parallel with it, as would happen if, say, the 3,000th winding somehow managed to come side by side with the 2,500th winding, by falling into a gap at the edge of the coil, or something or that sort. The result of the model is a side by side impedance dip and spike, which isn't 100% like what is seen in the practical plots, which appears to have only an impedance spike, but it has the similar characteristic of a second high frequency resonance. Maybe with a little more refinement the model can duplicate the practical plot completely.

[freefrog]

The neck PU signal in a LP runs through around 3ft of coax wire between PU and jack. My Stew Mac and Allparts coax wires measure over 120pF/foot. But this value strongly increases with ambient humidity in summer months. The values I specified are quite realistic.

Even humidity aside, my experience agrees with your statements.

I've measured 268pF per meter in normal conditions on the braided shielded that I use.

And there's a serious lenght of cable in a LP... This site recommends to have 5ft at disposal for such a wiring: Six String Supplies ? How to Wire a Les Paul (50s Wiring)

Let's add to it the cables coming from the pickups themselves and the stray capacitance of other components: it forms a highly capacitive inner wiring. Its tonal effect is especially obvious IME when both pickups are selected.

I don't think any pro player would be comfortable with a stage cable of 10ft or less.

BTW, Helmoltz, you have an outstanding brand of cable in Germany: Sommer... Their LLX coax. wire measures 52pF per meter (published value that I've checked with a lab meter). And IME, it's a sturdy cable, whose only relative flaw is its limited flexibility.
Of course, this observation hasn't much interest for you since you use high capacitance cables. But it would be a pity not to share with all potential readers a possibly useful info, while Sommer cables are so unequally known by musicians around the World... :-)

[Helmholtz]

BTW, Helmoltz, you have an outstanding brand of cable in Germany: Sommer... Their LLX coax. wire measures 52pF per meter (published value that I've checked with a lab meter). And IME, it's a sturdy cable, whose only relative flaw is its limited flexibility.

Yes, this Sommer cable is fine, as is e.g. Klotz GY 107 ("La Grange"). I use both types with different lenghts. But generally I have no need for extra low specific capacitance, as this forces me to buy and use cables measuring 15m or more in length.

[Helmholtz]

Well, I suppose there could be no more standard way of measuring impedance than by forming a ratio, as a function of frequency, of the voltage across and the current through the device, using superior inexpensive technology. But let's leave this aside for now. But yes, there is more to do. Consider the capacitance: it is of little importance itself because it is so small compared to other capacitances in the guitar circuit. But it does get in the way because it causes a resonance that makes it hard to see the effects of the metal on the impedance. So the capacitance is found, just for the purpose of removing it from the impedance, by a non-linear least squares fit to a set of samples taken well above the resonance where the capacitance is dominant. The low frequency inductance and the resistance are in the model of the impedance, and the parameters fitted to are the capacitance, and two parameters associated with the coupling to the metal, k, the coupling coefficient, and Rse, the effective resistance of the "secondary" reflected back to the primary. Once the C is found it is taken out of the impedance, and you can see in the plots that the real part increases above the pickup resistance as frequency increases, and the imaginary part decreases below the reactance of the coil inductance.

Thanks. Seems like a powerful and useful tool. Unfortunately I don't have the time to dig any deeper and make myself familiar with your method at present.