Switchable additional winds

[Helmholtz]

Attack response would generally correspond to transient signals. You know a lot about transformer issues, is there any analogous consideration is transformer design, especially when you choose to use steel core versus ferrite? Modeling the guitar pickup and string as a loosely coupled transformer seems to draw an endless number of parallels otherwise.

Requirements for transformer and PU core materials are vastly different. In PUs technically inferior materials often give preferable sound results. A PU has more and different functions. I don't consider PUs as transformers. Both use induction, but so do other technical devices.

[catalin gramada]

Are the coils stacked or winded with two wires in the same time ?

[Antigua]

As mentioned earlier, calculating the capacitance from the resonance peak of a PU's impedance is not very precise.

1)There are additional parasitic capacitances from the setup: Wiring, input capacitances of scope/probe/meter etc.
2)The peak frequency of the impedance of a parallel resonant circuit is always somewhat lower than the theoretical resonant frequency given by 2*pi*fres = (L*C)^(- 0.5), depending on damping. If you use the peak of the transfer response instead of the impedance response, the error will be even greater.
3)The inductance drops with increasing frequency and is always lower at resonance than at 100Hz. This effect depends on core materials and eddy current paths. It is typically small for strat type PUs with alnico cores not having metal covers.

I've been measuring the capacitance of more Strat pickups with the DE-5000, and for some pickups, I'm getting very different results than I had based on the measured inductance and the resonant peak, like twice as little, or twice as much. Do you believe the DE-5000's measurements are more likely to be accurate? You mentioned that the inductance measure at 100Hz might not be accurate at the resonant frequency, though especially in the case of Strat pickup where eddy currents are low, the inductance tends to read the same value whether testing at 100Hz or 1kHz.

One example, I have a Fat 50 for which I measured 148pF from 2.4H inductance and a 7.9kHz resonance. The DE-5000 in Cp @ 100Hz is giving me back 66pF. This disparity is alarming because it means one method or the other is liable to be rather off.

One fun thing I was able to do with the DE-5000 was, while measuring capacitance, I pressed in on the coil with my fingers and I could see the capacitance climb by about 2pF when squeezed.

[Antigua]

Requirements for transformer and PU core materials are vastly different. In PUs technically inferior materials often give preferable sound results. A PU has more and different functions. I don't consider PUs as transformers. Both use induction, but so do other technical devices.

I'm not suggesting the pickup is a transformer by itself, but some others who I've talked to about this describe the pickup as a primary and the guitar string as being like a secondary, with poor coupling between them. In fact I think the idea has been discussed on this forum once or twice. Where has a real transformer's coils would have a mutual inductance near unity, the pickup and the string would have a linkage that is a small fraction of unity, due to large air gap separation. Based on your broader understanding of transformers, do you think this analogy is useful, or not similar enough to draw a comparison?

[Helmholtz]

I'm not suggesting the pickup is a transformer by itself, but some others who I've talked to about this describe the pickup as a primary and the guitar string as being like a secondary, with poor coupling between them. In fact I think the idea has been discussed on this forum once or twice. Where has a real transformer's coils would have a mutual inductance near unity, the pickup and the string would have a linkage that is a small fraction of unity, due to large air gap separation. Based on your broader understanding of transformers, do you think this analogy is useful, or not similar enough to draw a comparison?

No, a magnetic PU is a transducer. It is actually an electrical machine (including the strings), an electrical power generator followed by an integrated lowpass filter.

[Helmholtz]

One example, I have a Fat 50 for which I measured 148pF from 2.4H inductance and a 7.9kHz resonance. The DE-5000 in Cp @ 100Hz (??) is giving me back 66pF. This disparity is alarming because it means one method or the other is liable to be rather off.

I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

One fun thing I was able to do with the DE-5000 was, while measuring capacitance, I pressed in on the coil with my fingers and I could see the capacitance climb by about 2pF when squeezed.

Two reasons:
1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.

I feel that this topic better belongs to the thread "Measuring capacitance..."

[big_teee]

Maybe you guys could consider a Private Message, PM.
Since You're going over, & over, the same ground?

[Chuck H]

Maybe you guys could consider a Private Message, PM.
Since You're going over, & over, the same ground?

Thank you Terry! I thought the same thing and even unsubscribed to the thread since it was looping for so long and I got tired of it popping up in my box. Not that some great stuff isn't being covered. What doesn't go over my head has been hashed and rehashed.

[Antigua]

No, a magnetic PU is a transducer. It is actually an electrical machine (including the strings), an electrical power generator followed by an integrated lowpass filter.

The thing is you can't model mechanics with a modeler like LTSpice, and since the interaction between string and pickup is electromagnetic, I think there is value is being able to abstract the relationship in terms that can be electrically modeled.

[Antigua]

Maybe you guys could consider a Private Message, PM.
Since You're going over, & over, the same ground?

I've found a ton of useful info on MEF, and I mean lots, through Google searches. If this is a PM, the info will not be accessible to anyone else who pursues this subject later.

[Joe Gwinn]

I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

Two reasons:
1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.

This in a nutshell is why it is not considered a good idea to attempt to measure coil capacitance a at ten times the self-resonant frequency. Too fiddly. Also, all manner of effects that are irrelevant at the normal operating frequency range can become significant, causing errors. The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.

[big_teee]

I see very little pickup winding relevance!
I'll go back to my yard mowing!
T

[Antigua]

This in a nutshell is why it is not considered a good idea to attempt to measure coil capacitance a at ten times the self-resonant frequency. Too fiddly. Also, all manner of effects that are irrelevant at the normal operating frequency range can become significant, causing errors. The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.

I have seen this effect in strat PUs. The reason was that the second condition for use of the LCR meter at 100kHz not beeing fulfilled, namely a smooth -6dB slope at and in the vicinity of 100kHz. You may verify by measuring the impedance plot up to 100kHz or better 1MHz. This measurement requires using "Automatic Voltage Scale". If you see additional peaks and dips, this means that the PU shows multiresonant behavior, which indicates that something is wrong with it. This effect can be caused by partial winding shorts.

Supposing even 100kHz is prone to error due to secondary resonances in that range, what if a meter was able to test at 1 MHz? It looks like such meters cost well over a thousand dollars, but maybe a hand held that can test 1MHz will hit the market eventually.

Here are plots using the auto voltage scaling, and yes, they fix the noise floor issue amazingly well. Thanks again!

These first two are without the integrator amp, just the pickup into the Velleman PSU200:


closer detail


This plot is with the integrator amp designed by Ken Willmott in between the pickup and the Velleman



It appears that there are three impedance anamolies past the resonance, a small one at 48kHz, and a taller one at 93kHz, and another at 308kHz. I'm guessing that second resonance at 93kHz is that of the 100 turn exciter coil connected to the integrated function generator, due to it's prominence. I'm not so sure what accounts for the other smaller peaks beyond the resonance.

Two reasons:
1) Pressing on the coil compresses the winding and thus increases distributed capacitance.
2) You body is a big capacitor to ground. Touching the winding or hot leads creates a second capacitor wired in series with your body, the touched part beeing the second "plate" of the capacitor. In result total capitance (to ground) increases.

I feel that this topic better belongs to the thread "Measuring capacitance..."

I believe it's #1, because if it were #2, simply touching the pickup should induce the difference, but the increase corresponded to pressure. It demonstrates in real time the relationship between tension and capacitance, which is really cool.

[Helmholtz]

The standard approach since the 1920s (and documented in Terman) has been to measure the self-resonant frequencies with a number of film capacitors connected across the coil, and solve for the fixed capacitance.

"Terman's method" is fine, at least as long as inductance does not change with frequency, one takes care of parasitic capacitances of the measuring setup and the system (PU) has only one resonance. Maybe a little time-consuming. The theory behind is simple. Always nice to have different options. But each method has its drawbacks. I am not going to revive an old, fruitless discussion, though. Prefer being pragmatical instead, as PU capacitance is not a primary influencer of sound.

[Helmholtz]

The easiest and more accurate way to plot a PU's impedance response is to directly drive the PU from the generator with a series resistor of 100k to 1M ( the higher, the better). The idea is measuring the voltage across an impedance driven by a constant current source.