prototype testing of hum canceling P-90 design

[Mike Sulzer]

... then the bottom coil has extra steel slugs to increase the inductance and is would with less turns, and I think heavier gage wire.

I don't think that is quite right.

1. Add slugs to the bottom coil. This increases the inductance and the sensitivity to magnetic fields by the same amount.

2. Now take turns off. This decreases both the inductance and the sensitivity to magnetic fields. However, it decreases the inductance by more because the inductance depends approximately on the square of the number of turns while the sensitivity to fields dpends linearly on the number of turns.

The goal is to make a bottom coil that is just as sensitive to magnetic fields as the top coil while not increasing the inductance of the two coils in series any more than necessary over the inductance of the top coil alone.

[David King]

I should have scanned the thing with my gauss meter to see if and how the bottom field was re-directed by the laminated steel, wrap-around comb thingy.

My guess would be that the bottom coil was larger in area than the top, with finer wire and no core so that it would be less audible when wired in parallel?

[freefrog]

The reason for that is the lower steel bobbin has higher inductance. He uses laminated steel to reduce eddy currents.

DiMarzio did a copy and makes them with the steel shield under the top coil, which has the magnets, and then the bottom coil has extra steel slugs to increase the inductance and is would with less turns, and I think heavier gage wire.

Here's a DiMarzio Virtual Vintage Solo Pro. The top coil reads 8.53k, while the bottom is 2.45k.

Thx for your answer, David. Your posts are always interesting to me - and are even one of the reasons why I've registered in this forum, as the old hobbyist that I am :-))

I don't think that is quite right.

1. Add slugs to the bottom coil. This increases the inductance and the sensitivity to magnetic fields by the same amount.

2. Now take turns off. This decreases both the inductance and the sensitivity to magnetic fields. However, it decreases the inductance by more because the inductance depends approximately on the square of the number of turns while the sensitivity to fields dpends linearly on the number of turns.

The goal is to make a bottom coil that is just as sensitive to magnetic fields as the top coil while not increasing the inductance of the two coils in series any more than necessary over the inductance of the top coil alone.

I should have scanned the thing with my gauss meter to see if and how the bottom field was re-directed by the laminated steel, wrap-around comb thingy.

My guess would be that the bottom coil was larger in area than the top, with finer wire and no core so that it would be less audible when wired in parallel?

I had carefully read the Kinman patent and I have inspected the P90Hx before to mount it in my guitar but my test were too superficial and my memories are not fresh enough to give a useful answer to your posts, gentlemen.

All I can say is that the last Kinman noiseless designs have a really low DC resistance noise sensor: for example, it's just a bit over 1k in the set of Impersonator 54's that I have here (while the noise coil was a 1500ohm thing in the last "second generation" DiMarzio Area that I've tested).

[Mike Sulzer]

I should have scanned the thing with my gauss meter to see if and how the bottom field was re-directed by the laminated steel, wrap-around comb thingy.

My guess would be that the bottom coil was larger in area than the top, with finer wire and no core so that it would be less audible when wired in parallel?

The field that you want to direct away from the bottom coil is the time varying field produced by the vibrating string.

I cannot see any benefit to making the bottom coil air core. It would need to be quite a bit larger to do its job of canceling the hum field.

[David Schwab]

I don't think that is quite right.

1. Add slugs to the bottom coil. This increases the inductance and the sensitivity to magnetic fields by the same amount.

2. Now take turns off. This decreases both the inductance and the sensitivity to magnetic fields. However, it decreases the inductance by more because the inductance depends approximately on the square of the number of turns while the sensitivity to fields dpends linearly on the number of turns.

The goal is to make a bottom coil that is just as sensitive to magnetic fields as the top coil while not increasing the inductance of the two coils in series any more than necessary over the inductance of the top coil alone.

I based that on both examining the pickup, and reading DiMarzio's patent. They want the bottom coil to be lower in resistance so that it doesn't mess with the tone of the top coil. The extra inductance in the bottom coil, along with the top coil's shield directing the noise to the bottom coil makes it more sensitive to hum, even though it has less wire, and/or larger gauge wire on it. In a side view of that pickup, the lower coil is not as tall as the top coil. I did not to see what the wire gauge was, but it reads only 2.45K.

In patent 5908998 they state the following:

Another preferred embodiment of the present invention provides an electromagnetic pickup device for a stringed musical instrument having an upper bobbin positioned above a lower bobbin, each of the bobbins having a body and a coil of wire wrapped around the body. In this configuration, the bobbins are mountable on the instrument below the strings. The coils have axes perpendicular to the strings and at least one of the bobbin bodies has two or more holes therethrough which may be positioned below the strings when the device is mounted to the instrument. Also included are a magnetic device for generating a magnetic field around the bobbins and one or more magnets (pole pieces) extended through the holes. Again, a crucial part of the invention is ferromagnetic material which is positioned within or beneath the lower bobbin body to increase the device's inductance as described above.

Preferably, the upper bobbin coil has more turns of wire than the lower bobbin coil and the wire of the lower bobbin coil has a heavier gauge than the wire of the upper bobbin coil. It is also preferable to include an integral plate of magnetic material having a base disposed between the bobbins perpendicular to the coil axes and two side walls extending upwardly and perpendicularly from the base about the bobbin.

One advantage of increasing the inductance of the lower passive hum-cancelling bobbin is that it permits the number of wire turns of the upper bobbin to be higher than the number of turns of the lower bobbin. Substantially reducing the number of turns of the lower bobbin while maintaining or increasing the turns of the upper bobbin has the positive effect of lowering the impedance of the pickup at higher frequencies, which will enhance the amplitude and quality of the pickup's treble response. By employing a heavier gauge of wire in the lower bobbin than in the upper bobbin, DC resistance of the pickup is further reduced.

The patents for the Virtual Vintage are 4442749 for the stacked design, and 5908998 for the increased inductance design.

I've installed several of these and they really do sound like a single coil, and they don't hum even at high gain.

Duncan has a similar design, patent number 7166793, and Kinman's patents are 7189916 and 7022909 for the pickup with the laminated steel lower bobbin.



More Information for Telecaster

On Kinman's Strat pickups, he also adds the extra slugs to the bottom coil:

[Mike Sulzer]

We agree that keeping the resistance down will reduce the effect on the tone. But is it not then even more important to keep the inductance down? But this is no surprise: the purpose of a patent is to protect your idea without actually revealing how it works.

[David Schwab]

But this is no surprise: the purpose of a patent is to protect your idea without actually revealing how it works.

Except it does work, and it's not hard to see what they did when looking at an actual pickup, and not the patent. The patent might be vague, but the pickup isn't. I did some experimenting on a similar setup and it was easy to get it to work. The higher inductance is in the bottom coil which doesn't sense the strings, just the noise.

The Kevin Beller version is interesting.

[Mike Sulzer]

The higher inductance is in the bottom coil which doesn't sense the strings, just the noise.

So you measured the inducances?. How did you do it and what are the values?

[David Schwab]

So you measured the inducances?. How did you do it and what are the values?

I did not, but the fact that the bottom coil has more steel in it would imply a higher inductance than the top coil, plus, that's what is noted in the various patents from Kinman, DiMarzio and Beller. Kinman's bottom Tele bobbin is made from steel.

But if someone has an LCR meter and a bobbin, it's easy enough to drill some extra holes, and take a measurement with and without the extra slugs.

[Mike Sulzer]

I did not, but the fact that the bottom coil has more steel in it would imply a higher inductance than the top coil, plus, that's what is noted in the various patents from Kinman, DiMarzio and Beller. Kinman's bottom Tele bobbin is made from steel.

But if someone has an LCR meter and a bobbin, it's easy enough to drill some extra holes, and take a measurement with and without the extra slugs.

Really? You add steel and the inductance goes up? Only that idiot on MIMF who got me thrown off was so stupid that he considered a statement like that a hanging offense.

Go back and look at your statement I originally responded to. You said add steel and take turns off. Inductance goes down fast when you take turns off. There is no way you can say inductance goes up when you do both of those things unless you measure it or do a careful calculation.

Also remember what you are trying to do. You want to get high enough sensitivity in the bottom coil to cancel the hum of the upper coil while keeping the inductance low enough so that you do not affect the highs too much. Increasing the amount of steel allows you to reduce the number of turns. Sensitivity goes approximately linearly with turns, inductance approximately with the square. Think about it; draw a diagram. Do whatever you need to explain it to yourself. Figure it out; do not rely on dumb ass patent applications.

[Steve Conner]

Sensitivity and inductance are the same thing in this respect, I think. The two degrees of freedom for this problem space are: the sensitivity of the bucking coil to magnetic fields in general, compared to the signal coil: and the coupling between the bucking coil and the string's magnetic field. You could think of the latter as a sort of mutual inductance.

The goal is of course to get both coils with equal and opposite sensitivities to magnetic fields in general, and then maximise the difference in coupling to the strings. A humbucker is the logical conclusion of that: the coupling is also equal and opposite, for double the signal output. The second best is a dummy coil embedded in the guitar far from the magnets, giving zero coupling for that coil. All other noise cancelling designs are third best.

[Mike Sulzer]

Sensitivity and inductance are the same thing in this respect, I think. The two degrees of freedom for this problem space are: the sensitivity of the bucking coil to magnetic fields in general, compared to the signal coil: and the coupling between the bucking coil and the string's magnetic field. You could think of the latter as a sort of mutual inductance.

The goal is of course to get both coils with equal and opposite sensitivities to magnetic fields in general, and then maximise the difference in coupling to the strings. A humbucker is the logical conclusion of that: the coupling is also equal and opposite, for double the signal output. The second best is a dummy coil embedded in the guitar far from the magnets, giving zero coupling for that coil. All other noise cancelling designs are third best.

Sensitivity and inductance are related, but they are not the same thing. Sensitivity depends on the number of turns: straight from the law of magnetic induction, a voltage appears around each turn of the coil, and the coils are all in series. Inductance also depends on the law of magnetic induction (surprise!), but the flux produced by a current in one loop intersects all loops to a greater or lesser degree. Therefore, for constant geometry, the inductance depends on the square of the number of turns.

The vibrating temporary magnet (the string) causes changing flux through both coils. The idea is to minimize it through the bottom coil. This happens naturally due to the greater distance, but a shield to redirect the flux helps as well.

[David Schwab]

Inductance goes down fast when you take turns off. There is no way you can say inductance goes up when you do both of those things unless you measure it or do a careful calculation.

The point is, you remove turns, and then increase the inductance with more steel to make it better match the coil with less steel and more windings. I'd assume DiMarzio did some calculations, because the pickups work as advertised. They sound like single coils, and they don't hum. They also sound very different from older stacked coil designs which had a lot of low end cancelation and then were wound hotter to compensate.

Think about it; draw a diagram. Do whatever you need to explain it to yourself. Figure it out; do not rely on dumb ass patent applications.

No, you need to figure out why these patents work, while you keep saying they don't. I looked at the pickup, and it matches the patent, and it works as it should. It's that attitude that got you banned at MIMF. Everyone is wrong but you. That would include DiMarzio and Bartolini, since you say their patents are bogus.

[Mike Sulzer]

No, you need to figure out why these patents work, while you keep saying they don't.

Where did I say the pickup does not work? What I said was that the explanation (especially as interpreted by you) does not work. Now you are saying what I said back in the beginning of this discussion.

No, what got me banned from MIMF was stating the truth. At the time, you were willing to accept what that guy said, that an air core coil identical to another coil except for steel cores cancels hum perfectly. However, I was not the only one to say that you can hear the hum rise in a humbucker as you remove each core from one of the coils. I was just more persistent.

If you meant what you are now saying back in the beginning of this conversation, why did you not say so instead of arguing?

[David Schwab]

Where did I say the pickup does not work? What I said was that the explanation (especially as interpreted by you) does not work.

Why does the explanation not work? I'm just saying what's in the patents. I've tried variations of the patents and it worked. I haven't pursued the idea further because I'm not making any stacked pickups. But I did play around with it.

I think the Kinman and Beller versions sum up the idea pretty well. DiMarzio took Kinman's original idea and used two of their previous patents combined to arrive at the same thing.

Now you are saying what I said back in the beginning of this discussion.

No, I said:

The reason for that is the lower steel bobbin has higher inductance. He uses laminated steel to reduce eddy currents.

DiMarzio did a copy and makes them with the steel shield under the top coil, which has the magnets, and then the bottom coil has extra steel slugs to increase the inductance and is would with less turns, and I think heavier gage wire.

Here's a DiMarzio Virtual Vintage Solo Pro. The top coil reads 8.53k, while the bottom is 2.45k.

You replied:

I don't think that is quite right.

1. Add slugs to the bottom coil. This increases the inductance and the sensitivity to magnetic fields by the same amount.

2. Now take turns off. This decreases both the inductance and the sensitivity to magnetic fields. However, it decreases the inductance by more because the inductance depends approximately on the square of the number of turns while the sensitivity to fields dpends linearly on the number of turns.

The goal is to make a bottom coil that is just as sensitive to magnetic fields as the top coil while not increasing the inductance of the two coils in series any more than necessary over the inductance of the top coil alone.

Now notice that you didn't say anything in the thread until I posted how the Kinman and DiMarzio pickups work.

You have to remember that the top coil is shielded. It's a magnetic shield, and it's also grounded. The bottom coil is not grounded or shielded. Therefore the top coil is already less sensitive to noise. It's all a balancing act, and it works very well.

In the case of the DiMarzio pickup, the bottom coil is only 2.45k, while the string sensing coil is 8.53K. The bottom coil is noticeably shallower, so even with the same gauge wire, you would never get the same amount of turns. If they are indeed using larger diameter wire, they end up with even less.

So either way, there is less wire on the bottom coil than the top coil. But it still cancels hum from the top coil amazingly well. In my experiments, removing the top shield, or the bottom slugs reduces the hum canceling ability of the pickup. I also tried this with non stacked arrangements, as with a regular humbucker, so the two coils were just as close to the strings.

so if their explanation is wrong, please give yours on why these designs work.