Shielding and dummy coils

[earthtonesaudio]

I have a (stock) Gibson Blueshawk that puts a dummy coil in series when either the bridge or neck pickup is selected by itself. When they're selected together, the dummy coil is bypassed and the pickups are RWRP with respect to each other.

With both pickups selected, there's no hum, but when I select one pickup (plus dummy coil) there's a little bit. Not as bad as without the dummy coil, but I'd like to reduce it if possible.


My question is, would improving the guitar's shielding help with the hum? Right now there's copper tape on the inside of the plastic coverings (one over the main control cavity and one over the dummy coil). The copper is not connected to ground or anything.

A similar question: would adding some metal slugs to the dummy coil (but installing a magnetic shield between it and the strings) help reject hum?

Schematic: http://www.blueshawk.info/circuit_diagram.htm


Thanks!

[David Schwab]

A similar question: would adding some metal slugs to the dummy coil (but installing a magnetic shield between it and the strings) help reject hum?

That's what I would do, and if you shield the guitar, do not shield the dummy coil. You need to induce hum into the coil to have it work effectively.

[Bruce Johnson]

Why would adding metal slugs to a dummy coil help it pick up hum? I don't understand that.

I'm right in the middle of experimenting with adding a dummy coil to my Scroll Basses. I've made up a fairly large oval coil form that fits crosswise under the "M-pickup", which is a cluster of 4 single coils. I wound the prototype dummy coil with 10,000 turns of #41, to about 6.4K. The M-pickup coils are wired in series, 7.7K total.

On my bench test mule, I set the dummy coil right beside the M-pickup, and tried hooking it up in series, parallel, forward and reverse. I'm kind of disappointed so far. It had a mild effect on the tone of the pickup, as expected, from the changes in resistance in the different combinations. But it didn't seem to do much of anything in reducing the hum from the florescent lights over the bench, regardless of how I wired it in. This was with the dummy coil sitting out in the open, with no shielding around it. I would think that it would have made a noticeable cut in the 60hz hum when it was wired to be canceling against the pickup coils. But I couldn't hear any difference between wiring it forward (same direction as the pickup) or reverse (opposite the pickup). Am I missing something about how dummy coils are supposed to work?

[earthtonesaudio]

I don't really understand it either. My feeling is that adding metal to the dummy coil won't actually make it pick up more hum, but the hum it does pick up will then generate a greater voltage.

Intuitively, it seems like adding metal to the dummy coil will make it more similar to the pickups, making their hum voltages more similar, and thus allowing better cancellation. What I don't understand is why you don't have to use a magnet for the dummy coil.

[David Schwab]

Why would adding metal slugs to a dummy coil help it pick up hum? I don't understand that.

Because you are increasing the inductance of the coil. And the metal slugs act like antennae.

This is the design on a dew humcancling pickups, such as the Kinmann and Dimarzio stacked pickups.

The bottom coil has a steel core, or extra steel slugs, and is magnetically shielded from the top coil. The bottom coil is being optimized to pick up hum. In the case of the DiMarzio design, the bottom coil also has less turns of wire and uses a larger gauge, to reduce the resistance.

A larger area coil would work as well.

[Bruce Johnson]

Okay, that makes sense. So, the idea is to increase the inductance and reduce the resistance. If I were to wind it with fewer turns, larger wire, and metal slugs, then I'd want to put it in series (wound opposite) with the pickup, right?

When you say the dummy coil should be magnetically shielded from the pickups above, do you mean that there should be a steel plate or plane?

I made the coil form as large as I could fit into the available chamber under the pickups. It's about 3" x 1.5" x 0.5" thick. It ends up being about an inch below the strings.

Overall, I like the way the M-pickup group sounds right now, and I don't want do anything to change it much. It's not really very noisy, but there is a little 60hz hum if you hold it close to an amp or a florescent lamp. One customer who lives locally brought his Scroll Bass in to see if I could do anything more. He was noticing it in loud rock club environments (which most of my customers don't play in). So, I thought this would be a good opportunity to try adding a dummy coil. The shielding is good, and the P-pickup under the bridge is very quiet.

[gregyancey]

Bruce, are your M-Pickups RWRP with each other? (i.e. E and A string pickups North-CW, D&G South-CCW for example) If this is the case then I think this is as hum canceling as you can get as each coil is matched for area, inductance, resistance, etc. The only way to get them more hum-canceling is to defy physics and have them all exist in the same space at once so they absolutely sample the same magnetic field.

[Mark Hammer]

The coils double as antennae. The polepieces will assure that the active pickup pulls in reception of channel W-HUM 60 loud and clear ("Classic hum from the 50's, 60's, 70's, and 80's, the way YOU want it, all day long, baby! W-HUM, sixty on your dial!"). You want the dummy to get equal "reception" of that channel, so they can cancel.

[David King]

Good point Greg. I can see a problem with the 5 strings but not the 4s unless they are all in magnetic and electrical phase for reasons of sound

Coil area and coil inductance are shortcuts but how do you calculate what you need in the way of turns to counter a specific coil's noise?

Mark you are sort of implying that the whole thing has to be tuned to the spectrum you want to address. Is that the case?

[Mark Hammer]

Nah, I'm just coyly (or was that coil-ly? ) reiterating David's point that hum-cancelling relies on the two coils sensing an equal amount of hum. If the string-sensing coil picks up way more hum than the dummy, you won't get the sort of cancellation and quiet you seek. So, even though it might seem counter-intuitive to want a dummy coil that picks up more hum, you want it to be as sensitive to the hum source as the string-sensing coil. And if the steel slug does that, go for it.

[Mike Sulzer]

Metal shielding is good for reducing electric fields. Cancellation with a coil is for reducing magnetic fields. These are two different things. One needs to do both.

Copper tape must be grounded to be effective.

If your dummy coil is not doing a good enough job, then its sensitivity to hum is different from the pickup. It could be greater, or it could be smaller. So cores will not necessarily help.

You can shield your dummy coil with copper tape to stop it from picking up electric fields, if that is the problem. This does not affect its sensitivity to magnetic fields very much. (It might sound strange that a coil designed to cancel magnetic fields can also pickup electric fields, but why not? Do not expect this to cancel electric field pickup from the pickup!)

[Bruce Johnson]

Greg;

No, the four M-pickup coils are wired together, same direction, same magnet pole direction. I tried wiring them alternately a while back to make the group as a humbucker. It did work to reduce the hum. The problem was that, when I blended it in with the other pickup, two of the coils would end up being out of phase. So, two strings would die out as you blend.

Yeah, my understanding is that the purpose of the dummy coil is to cancel the external magnetic field by matching the antenna-like properties of the pickups. That's why I made it the way I did, roughly matching the physical size and resistance of the group of pickups.

What Mike is saying makes sense; that shielding or not shielding the dummy coil shouldn't really have any effect on the magnetic field cancellation. However, if it's unshielded it could introduce more electric field noise. That's why I was planning on putting it into a shielded cavity under the pickups.

But, in my quick bench mule testing, it was unshielded. In any of the wiring combinations, it didn't seem to be adding any hum. It just wasn't doing much of anything.

Obviously, I'm going to have to do a lot more experimenting.

[David Schwab]

Bruce, you should be able to get the two pickups to mix. Look at a P/J bass. You have the split coil P pickup and a single coil J. The P is one coil North and the other South. You don't get one half out of phase with the Jazz pickup, but there are two ways to wire them up, and one works while the other doesn't.

Here's the DiMarzio Virtual Vintage Solo pro Strat pickup. This is stacked design, but essentially the bottom coil is a dummy. Now it's called the Virtual Solo, DP420.

The top coil (8.5K) has the magnets and a steel shield under it to redirect the magnetic field back up to the poles, and stopping it from effecting the bottom coil very much. The bottom coil (~2.67K) has the extra slugs, and is wound with larger diameter wire, and with less turns. The two coils are wired in series. By making the bottom coil more sensitive to hum, and making it lower resistance, it doesn't have much effect on the upper coil's tone, while still creating a very quiet pickup.

I have installed these pickups for customers. It sounds just like a single coil pickup, and is very quiet. This was the patent that was pretty much a copy of Kinman's patent. He used a laminated steel bottom bobbin on his Tele pickups. His strat pickups look a lot like the DiMarzio version, but have a shield that extends around the bottom coil.

Then Beller/Duncan got a similar patent for a stacked pickup. In that one, the shield under the top coil extends to be the core for the bottom coil. They claim this induces the noise into the bottom coil. The bottom coil is smaller than the top coil.

The various patents are:

Kinman; 6103966, 7189916 (he has more, but these are the main designs)

Dimarzio; 4442749, 5811710, 5908998 (Virtual Solo)

Kevin Beller (Duncan Classic Stack Plus STK-S4) 7166793

[Bruce Johnson]

David;

Thanks for the info on those various designs. That all seems to make sense. From that, I guess the direction I should go is to lower the resistance, raise the inductance, and add the slugs and steel shielding to increase the sensitivity. It may take some playing around to get the single dummy coil to match the sensitivity of the group of four pickups.

I'm thinking now that, in my quick tests on my bench mule, I simply wasn't hearing any reduction in the magnetic field hum because it was overwhelmed by electric field hum. In that test, both the bench mule and the dummy coil were completely unshielded. I was trying to listen for a small change in the middle of a steady roar of the same frequency. I really need to test it out in a fully shielded setup, like the instrument, to hear the magnetic field hum.

About the blending of the pickups:
The reason why the P-J setup works is because all three coils (2 P and 1 J) are wired so that they're going the same direction, right? The humbucking effect in the P-pair is because their magnets are opposite.

In my M-pickup group, the magnets of all four coils go the same way, and the coils were wired alternately. That's why the blending problem comes in.

Early on, I tried the group the other way, with the coils wired the same direction, but with alternating magnets; two coils North, two South. That was lousy; it completely screwed up the magnetic fields and the strings all sounded different. These coils are less than 3/4" apart, and these are strong magnets; three 1/4" x 3/4" Alnico 5's per coil. Alternating the magnets works on the P-bass pair because they are far enough apart.

[David Schwab]

About the blending of the pickups:
The reason why the P-J setup works is because all three coils (2 P and 1 J) are wired so that they're going the same direction, right? The humbucking effect in the P-pair is because their magnets are opposite.

The P is a humbucker. The coils have opposite magnetic polarity, and the coils are wired up out-of-phase (finish to finish). The opposite magnets makes it in phase again.

You can also wire a P pickup with the same magnets and out-of-phase. That will still hum cancel, but one half would be out of phase with a J.

The reverse magnets are not needed for a lone P pickup because no single string is sensed by both coils, unlike with a guitar humbucker.

So reverse the magnets on two of your coils and also wire those out of phase to the other two.