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Help me understand dummy coils in MM style pickups.

Benjamin Strange

Benjamin Strange

Commercial User
Dec 25, 2002
9,737
310
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New Orleans, LA
www.ufodeathcult.com
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Owner / Tech: Strange Guitarworks
#1
I've been scouring the forums and internets at large for the answer to this, and I'm coming up short. I want to understand exactly how dummy coils are wired into MM style pickups.

What I'm trying to sort out is exactly where the dummy coil is wired in to the pickup. On a standard Seymour Duncan 4 conductor humbucker you'd have a wiring scheme like so:

Black=North start
White=North end
Red=South finish
Green=South start

Where is the dummy coil connected? Is it wired in series, between the two coils (possibly sharing the Red and White wires) or is it wired in parallel to both coils (possibly sharing a common ground)? I ask because I want to understand how the pickup works when hooked up to the standard series/parallel/split switch that many MM style basses employ. The dummy coil is only active in the split position, and I'm just not following how it's getting deactivated in the series and parallel modes. Surely somebody here knows what's going on inside these things - help me out!
 
#2
It's been ages since I've had a stingray 3-band pre on the workbench, but from memory, you are right. The dummy coil is only switched in while the switch is in the split coil setting, when it's in parallel with the bridge coil. This is essentially now a stacked humbucker, vaguely similar to a SD hot stack, or a kinman guitar pickup. In series and parallel, the dummy is not connected, or both ends of it are earthed or some such wiring. It all happens at the switch. Again, it's been ages, but I recall the switch is a 4P3T. One set of poles is used to change the preamp's gain slightly to give the same output at all three settings of the pickup. The other three poles do the pickup coil switching.
 
#3
Benjamin Strange said:
I've been scouring the forums and internets at large for the answer to this, and I'm coming up short. I want to understand exactly how dummy coils are wired into MM style pickups.

What I'm trying to sort out is exactly where the dummy coil is wired in to the pickup. On a standard Seymour Duncan 4 conductor humbucker you'd have a wiring scheme like so:

Black=North start
White=North end
Red=South finish
Green=South start

Where is the dummy coil connected? Is it wired in series, between the two coils (possibly sharing the Red and White wires) or is it wired in parallel to both coils (possibly sharing a common ground)? I ask because I want to understand how the pickup works when hooked up to the standard series/parallel/split switch that many MM style basses employ. The dummy coil is only active in the split position, and I'm just not following how it's getting deactivated in the series and parallel modes. Surely somebody here knows what's going on inside these things - help me out!
Click to expand...

I didn't know that MM style basses had a dummy coil. Usually a dummy coil needs to be active to work effectively. (like Alembic) A dummy coil is only used when you select a single coil. A normal MM style pickup is a humbucker. This means it has two coils each wound in the opposite direction. This causes any magnetic hum field to be cancelled whether the two coils are in series or parallel. In order to keep string signals in phase the magnets in one of the coils are reversed. It then cancels hum but still both coils pick up the strings in phase.

A passive dummy coil works the same way but with a single lone coil. And to try to keep the single coil tone you don't want the second coil to pick up the strings so it has no magnets. That way it's like a humbucker only the second coil does not pickup the strings, only hum. Then second coil is then called a "dummy" coil because it does not pickup strings.

So you have to have a switching arrangement that puts the dummy coil say in series when chosing a single coil. The bad news is that the added coil adds inductance to your single coil pickup and can change the tone to quite an extent at times. And that is why Alembic mixes the hum signal in an active circuit so the pickup tone is not changed.

That's the basic story.
 
#4
bassbenj said:
I didn't know that MM style basses had a dummy coil.
Click to expand...
the sterlings and the 5-strings do, the ones you see with 3-way blade switches.

you get normal stingray parallel, a "single coil" setting with the dummy underneath engaged, and a fatter series sound.
 
#5
In order to keep string signals in phase the magnets in one of the coils are reversed.... A passive dummy coil works the same way
Click to expand...

True, but actually in this case the dummy coil has the same magnetic polarity. As I mentioned, when switched to "split coil" it's essentially like a hot stack, not a traditional humbucker. If memory serves, the stingray dummy coil has metal slugs, but not actual magnets. Therefore its magnetism is induced and much lower than the main two coils. It is actually wired out of phase to cancel hum. The reason it doesn't cancel too much of the string sound is because of its low magnetism and also because it's so much further from the strings.
 
#7
It's actually called the "phantom coil". It's in the middle of the body under the pickguard on the HS basses. Separate from the other pickups. It looks like the neck pickup in an HS configured Sterling.

This is a google image search showing it: https://www.google.com/search?q=sti...OrsQTy3oCwCQ&ved=0CAkQ_AUoAQ&biw=1920&bih=955

A long time ago, someone posted about the fact that it has magnets and it can actually be used if it's raised up. This works.
 
#8
Oops, I think we've got our coils crossed. My posts have been about this stingray pickup, with the dummy coil underneath:

post-12-1216738980.jpg


As you can see, one end of the dummy coil is hard wired to earth via the purple wire, and the other to the switch via the black wire.

SR5%20Wiring.jpg


The colour scheme form the pickup wires to the board is simply repeated with the wires to the switch. The grey, brown, orange and blue wires have a pole of the switch to themselves (bottom left) and as you can see the grey wire is terminated at the board right next to the opamps. The others here go to different resistors to control the gain of the preamp so that all three settings of the pickup have the same output.

The violet wire is earth. You can see that the green wire is earthed in pos 1 and connected to the white wire in pos 3. This is essentially the series-parallel switching.

Getting back to the dummy coil, you can see that its black wire is connected to the yellow wire when the switch is in the middle (split) setting, but not connected to anything at any other time. That yellow wire goes back to the board, and without the preamp in my hand or at least a schematic, it's hard to say exactly how this is added to the circuit, but as I mentioned, it's in my memory that it's essentially in parallel with the bridge coil, and probably with some filtering to reduce its tonal impact.

Again, it's been several years since I've had one of these in bits on my workbench, so I'm happy to be corrected on any of this...
 
#9
walterw said:
the sterlings and the 5-strings do, the ones you see with 3-way blade switches.

you get normal stingray parallel, a "single coil" setting with the dummy underneath engaged, and a fatter series sound.
Click to expand...
I always thought the Stingray 5 had a filtered Series mode like the Ray35 from SBMM.. I looked up the diagrams on the music-man.com website and found out I was wrong.. damn..
 
#12
Ok, upon further research I've discovered that the dummy coil is usually wired in parallel with the neck coil, as that's the coil which usually is soloed when the pickup is split. In keeping with the Seymour Duncan color coding, it would look like this, yes?:

Black=neck coil start
White=dummy coil start
Red=bridge coil start
Green=bridge coil end
Bare=neck coil and dummy coil end

This is starting to make sense, except for how the dummy coil figures in when the pickup is wired in series. Here's what I understand so far:

In single coil mode, the Neck coil start and the Dummy coil start are joined and sent to hot, which puts them in a parallel configuration with each other. Since the Bridge coil isn't hooked up, it's deactivated.

In parallel humbucking mode, the Neck coil start and Bridge coil start are joined and sent to hot, the Dummy coil start and Bridge coil end are sent to ground, which deactivates the Dummy coil, since both the start and end of the Dummy coil are sent to ground.

Now here's the part I don't understand:

In series humbucking mode, wouldn't the entire signal flow need to be flipped? Since the Neck coil end is permanently grounded, wouldn't it need to be: Bridge coil end to hot, Bridge coil start and Neck coil start wired together, and leaving the Dummy coil start out? Otherwise, if the Neck coil start were still used as the hot, and the Dummy coil start and Bridge coil start were connected, and the Bridge coil end to ground, would that not ground out both the Dummy coil and the Bridge coil? Or would they somehow both be running in parallel with each other and then in series with the Neck coil? I'm not really following how this could possible work - yet it seems to work just fine in Stingray 5s, which can switch between the three modes. Does this make sense?

EDIT: Aha! I just realized that the Stingrays use a special switch which allows for flipping the entire pickup signal flow to achieve series wiring the way I described it. Ok, I get it now. Phew!
 
#15
Hi Benjamin, I reply after 10 years from the publication of your post. There is no special switch to carry out the insertion of the phantom coil. You have to use a 3-blade Ernie Ball switch position. So you get the typical insertion of the 5-string MM, that is: parallel (bridge coil with neck coil) - series (bridge coil with phantom coil) and series (bridge coil with neck coil). From a forum I read the ghost coil must be placed in opposition to the bridge coil. As soon as I have done the necessary tests in the field if it works well I will divulge the scheme that I have prepared but if someone wants to do experiments then ask me that I turn the drawing. The scheme I derived would allow you to connect any non-original preamplifier using the typical insertion without sacrificing coils. Greetings
 
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