The difference between RWRP and non RWRP

[David Schwab]

In the RWRP mode the pickup was situated normally. In the non RWRP mode the pickup was basically rotated 180 degrees such that the bottom flatwork was then above the pickguard, with the lead wire connections pointing towards where the neck would be and the staggered magnets pointing down into where the pickup cavity would be. The pickup was centered with respect to the height of the other two pickups in both positions.

You are missing the point. There is no "RWRP mode". A pickup is only reverse polarity when combined with another pickup of opposite polarity. And the only thing that is changing is how the two signals from the pickups interact. With reversed magnetic polarity, and reversed coil polarity (it does not have to be wound in reverse), you cancel hum. But inductance has nothing to do with that. The coil and the magnets has not changed.

But a single pickup can not be reverse polarity by itself because the polarity is arbitrary without combining it with other pickups.

Even if it were interacting with the other pickups, it would have been interacting before you turned it over the exact same way.

[Mike Sulzer]

You are missing the point. There is no "RWRP mode". A pickup is only reverse polarity when combined with another pickup of opposite polarity. And the only thing that is changing is how the two signals from the pickups interact. With reversed magnetic polarity, and reversed coil polarity (it does not have to be wound in reverse), you cancel hum. But inductance has nothing to do with that. The coil and the magnets has not changed.

But a single pickup can not be reverse polarity by itself because the polarity is arbitrary without combining it with other pickups.

Even if it were interacting with the other pickups, it would have been interacting before you turned it over the exact same way.

David, you are missing the point. He is saying the difference in inductance is due to the interaction. The pickups are wired in parallel. When one is flipped over, there is a phase change in the addition of the signal that is induced from one pickup to the other. This changes the inductance by a small amount. It shows that you will not get the same tone when you use RWRP or not, but the difference is very small.

[Mike Sulzer]

I just posted this over at TheGearPage. I thought you guys might find it interesting.

There is a slight, but measurable, difference between RWRP and non RWRP.

Here's how I determined it.

I mounted a set of pickups in a pickguard, the middle pickup being RWRP. To convert the middle pickup to non RWRP, I removed it from the pickguard and flipped it over, effectively flipping both the polarity and winding direction relative to the other two pickups with everything else held exactly constant. I did not hard mount the middle pickup so I could flip it quickly, but took care to ensure that it was at the right height relative to the other two (although slight differences in height don't affect the measurement anyway). I also maintained the integrity of the pole piece location (i.e. the low E magnet stayed in the low E slot).

I measured the inductance of the bridge/middle pair wired in parallel at 1 kHz with a Extech model 380193 LCR meter. I took ten measurements in a row at each orientation in the space of about 5 minutes, changing the RWRP orientation between each measurement. With each measurement, I toggled the measurement frequency between 120 Hz and 1 kHz to reset the meter.

The RWRP inductance is 1591.97 +/- 0.63 mH
The non RWRP inductance is 1579.13 +/- 0.60 mH

This is only a difference of about 0.8% but it is statistically significant (by a pretty wide margin).

This tests suggests that the mutual coupling between the two coils of a humbucker is significant since they are a lot closer. This coupling might explain the effect of using two coils wound with different numbers of turns some people exploit to get a different sound.

[ScottA]

You are missing the point. There is no "RWRP mode". A pickup is only reverse polarity when combined with another pickup of opposite polarity. And the only thing that is changing is how the two signals from the pickups interact. With reversed magnetic polarity, and reversed coil polarity (it does not have to be wound in reverse), you cancel hum. But inductance has nothing to do with that. The coil and the magnets has not changed.

But a single pickup can not be reverse polarity by itself because the polarity is arbitrary without combining it with other pickups.

Even if it were interacting with the other pickups, it would have been interacting before you turned it over the exact same way.

I think more correctly I should say, a RWRP and non RWRP "environment", although in the case of the 1591.97 mH result it is a true RWRP "mode", i.e. two pickups wired in parallel with opposing winding directions and opposing magnetic fields.

And you are correct (not that I am missing the point), in this experiment the intrinsic properties of the pickups have not changed. That is the whole point, in fact. In order to do this experiment properly you have to have it set up so that the intrinsic properties are constant.

What I am measuring, I believe, is the effect of the magnetic fields of the adjacent pickups. When you flip the middle pickup over, the relative magnetic field compared to the other pickups does change, and the effects of this change are measurable and significant.

Play with this kind of stuff if you can measure it (you have to be looking at AC parameters, I don't think you will see this stuff in DC measurements). Take three pickups (with charged magnets) and line them up side by side and connect your meter to the middle one. Now move the outer two away from the middle one. You can see the change in inductance in real time as you do this (with the Extech, you may have to toggle the meter to get it to respond faster depending on the direction and magnitude of the change) and the change is reversible. Or connect your meter to a pickup and measure it. Take another pickup of the same type and polarity, and place it on top of the first pickup such that the magnetic fields are aligned and the magnets are in contact. Again, you will see, in real time, significant and reversible changes in inductance.

[ScottA]

This tests suggests that the mutual coupling between the two coils of a humbucker is significant since they are a lot closer.

I would agree with that. Read my response to David, above. In that single coil stacking experiment I see changes closer to 2.5%.

[ScottA]

I think the primary physics active here is the effect of the magnetic field strength on the inductive behavior of the AlNiCo pole piece material.

[Steve Conner]

When messing around with pickups, I found there was a slight difference in frequency response depending which end of the coil you used as ground, and which end you took the output from. The reason, as far as I know, is that the two ends have different self-capacitances, and that affects the resonant peak.

By reversing the polarity you presumably changed which end of the coil you took the output from.

The self-capacitance can influence the value of "inductance" a LCR meter would measure, for reasons that have been analyzed ad nauseam by Joe Gwinn, Mike Sulzer et al. Almost anything can influence it, in fact, because the response of a pickup is too complex to be summed up by a single number like "inductance". The LCR meter does its best, but that isn't usually too good.

[Mike Sulzer]

I think the primary physics active here is the effect of the magnetic field strength on the inductive behavior of the AlNiCo pole piece material.

I do not think it has anything to do with the permanent magnetic fields. (You could retry the experiment you described just above with a non-permanent magnetic material with the same permeability as charged alnico. I think you will get essentially the same results.)

If you believe in inductance, you have to believe in mutual inductance as well. That is what you expect the explanation to be.

[ScottA]

I do not think it has anything to do with the permanent magnetic fields. (You could retry the experiment you described just above with a non-permanent magnetic material with the same permeability as charged alnico. I think you will get essentially the same results.)

If you believe in inductance, you have to believe in mutual inductance as well. That is what you expect the explanation to be.

Mutual inductance is proably at work here as well but the effect of the magnetic field on the inductive response of the material is key. Substituing another material of similar inductance would not necessarily show the same effect, unless the material had exactly the same response as a function of magnetic field as AlNiCo. For instance, the steels commonly used in pickups exhibit very little variability as a function of magnetic field in the range typically seen in guitar pickups.

I did another test this morning. I took a brand new, uncharged AlNiCo magnet and measured its inductance in a test rig. I measured the field strength at the top of the magnet with a little RB Annis guage (all I have at this point).

Magnetic state: Normalized Inductance
Al5 un magnetized 1
Al5 10G (external magnet added) 1.01
Al5 22G (fully charged) 1.18
Al5 32G (external magnet added) 1.10
Al5 22G #2 (mag removed, remeasured) 1.17

Al5 15G (degaussed) 1.10
Al5 22G #3 (recharged) 1.20

The measurements were made in that order. Note that with the external magnet attached the first time (10 G) the pole piece was not significantly charged. The charging is done with a much larger magnet.

I also measured a 1018 piece of the same size as the AlNiCo

1018 (no field) 1.000
1018 15G (ext mag added) 1.003

[ScottA]

When messing around with pickups, I found there was a slight difference in frequency response depending which end of the coil you used as ground, and which end you took the output from. The reason, as far as I know, is that the two ends have different self-capacitances, and that affects the resonant peak.

By reversing the polarity you presumably changed which end of the coil you took the output from.

The self-capacitance can influence the value of "inductance" a LCR meter would measure, for reasons that have been analyzed ad nauseam by Joe Gwinn, Mike Sulzer et al. Almost anything can influence it, in fact, because the response of a pickup is too complex to be summed up by a single number like "inductance". The LCR meter does its best, but that isn't usually too good.

I did not have to reverse the leads. Flipping the pickup over changes the winding direction relative to the other two pickups. This is not a self-capacitance or contact resistance effect.

And, for the record, I'm not trying to say these measurements reflect the whole story, but they do highlight an important and significant part of it.

[Mike Sulzer]

Mutual inductance is proably at work here as well but the effect of the magnetic field on the inductive response of the material is key. Substituing another material of similar inductance would not necessarily show the same effect, unless the material had exactly the same response as a function of magnetic field as AlNiCo. For instance, the steels commonly used in pickups exhibit very little variability as a function of magnetic field in the range typically seen in guitar pickups.

I did another test this morning. I took a brand new, uncharged AlNiCo magnet and measured its inductance in a test rig. I measured the field strength at the top of the magnet with a little RB Annis guage (all I have at this point).

Magnetic state: Normalized Inductance
Al5 un magnetized 1
Al5 10G (external magnet added) 1.01
Al5 22G (fully charged) 1.18
Al5 32G (external magnet added) 1.10
Al5 22G #2 (mag removed, remeasured) 1.17

Al5 15G (degaussed) 1.10
Al5 22G #3 (recharged) 1.20

The measurements were made in that order. Note that with the external magnet attached the first time (10 G) the pole piece was not significantly charged. The charging is done with a much larger magnet.

I also measured a 1018 piece of the same size as the AlNiCo

1018 (no field) 1.000
1018 15G (ext mag added) 1.003

You have shown that the permeability of alnico is a function of its magnetic state. This is well known and is shown by the usual "loop" B-H curve. But you had to make large changes in the state to see 10 or 20 percent difference in inductance.

I do not believe that the permanent magnetic field of one pickup could alter that of another enough to show the effect you saw with them spaced as in your pick guard experiment. I believe that you had the pickups in parallel in that experiment. You can eliminate nearly all of the mutual inductance effect by repeating the experiment with one pickup alone (with the others left open).

[ScottA]

You have shown that the permeability of alnico is a function of its magnetic state. This is well known and is shown by the usual "loop" B-H curve. But you had to make large changes in the state to see 10 or 20 percent difference in inductance.

But in the RWRP experiment the measured changes in inductance were very small (a fraction of a percent vs. on the order of 10% in the table directly above), as the relative changes in field strength would be expected to be.

I do not believe that the permanent magnetic field of one pickup could alter that of another enough to show the effect you saw with them spaced as in your pick guard experiment. I believe that you had the pickups in parallel in that experiment. You can eliminate nearly all of the mutual inductance effect by repeating the experiment with one pickup alone (with the others left open).

Read the second section on the control experiment (post #3). I believe this is what you are asking for. Also, try the three pickups on a table thing I talked about in post #18. You can see how the effect falls off with distance pretty directly.

[Mike Sulzer]

But in the RWRP experiment the measured changes in inductance were very small (a fraction of a percent vs. on the order of 10% in the table directly above), as the relative changes in field strength would be expected to be.



Read the second section on the control experiment (post #3). I believe this is what you are asking for. Also, try the three pickups on a table thing I talked about in post #18. You can see how the effect falls off with distance pretty directly.

OK, the one in post three seems to imply that about half the effect is due mutual coupling and half to something else. Putting any highly permeable magnetic material, permanent or not, near a coil affects the inductance. So I am not yet convinced that the pick guard result in post one is an effect due to permanent magnets.

[ScottA]

OK, the one in post three seems to imply that about half the effect is due mutual coupling and half to something else. Putting any highly permeable magnetic material, permanent or not, near a coil affects the inductance. So I am not yet convinced that the pick guard result in post one is an effect due to permanent magnets.

I appreciate your healthy level of skepticism!

One thing to consider, though, is that AlNiCo is really not that permeable as magnetic materials go. Pretty close to 1.

Something like a highly permeable base plate attached directly to the bottom of a pickup will certainly cause changes in inductance higher than what I measured in the RWRP test, but put that plate an inch away from the pickup and the effect is negligible. I just tried it in fact, and this is the case.

Pickup alone: 2715 mH (this is the same pickup from yesterday, it was actually jumping around between 2715 and 2716, but wanted to settle in at 2715).

Pickup with a big chunk of sheet metal attached to the bottom (very scientific): 2855 mH
Pickup with same chunk touching the cover: 2725 mH
Pickup with chunk 1 pickup spacing away: 2715 mH

[Mike Sulzer]

That's convincing!