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Dummy/Phantom Coils

#3
yep, it can go anywhere, as long as it's oriented the right way, i.e., at the same angle and direction as the coils it's supposed to hum-cancel. in fact, you want it to be well away from the strings so it won't produce any signal.

remember though, dummy coils kill a good bit of tone along with the hum.
 
#4
jallenbass said:
Do they have to be built into the pickup or can the be outside of it since they don't need to pickup string vibrations? Maybe in the wiring cavity?
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The idea is that the dummy coil picks up the hum field at the same strength as the pickups. So the assumption is that the hum at the coil is identical to that at the pickup. Usually this is pretty close to true. But it's still best to have it as close to the pickups as possible. They don't have to stacked into the same case, but you don't want it too far away either. One model of Alembic, for example places the dummy coil like a third pickup (but of course it only picks up hum) between the neck and bridge pickups. I'd be a bit reluctant to put it as far away as the wiring cavity.
 
#6
line6man said:
I experimented with a dummy coil the other week by hanging a pickup out the back of my control cavity.
As long as I had it facing the right way, it humcancelled fully.
Click to expand...

Of course it depends on how uniform the hum field is. So long as you are far enough away it's going to be pretty uniform. Yeah, you are right, the control cavity really isn't THAT far away!
 
#7
So other than the aforementioned tone change, why isn't this idea in use as a mod? It would seem that a dummy coil would be cheap to make and a Jazz bass has plenty of room under the pickguard to plow out a space for it.
 
#8
jallenbass said:
So other than the aforementioned tone change, why isn't this idea in use as a mod? It would seem that a dummy coil would be cheap to make and a Jazz bass has plenty of room under the pickguard to plow out a space for it.
Click to expand...

If you want both pickups to humcancel, they have to NOT be RWRP from each other, so adding a dummy coil usually means swapping pickups. The vast majority of pickup sets are RWRP to hum cancel together, so this generally means you have to resort to custom pickup sets.

Also, without active buffering, the impedance drop of placing the dummy coil parallel to the audible coil gives you a volume drop, and trying to blend two pickups with the dummy is going to defeat the humcancellation, because you have two coils adding constructively, versus only one coil adding destructively.
 
#9
The dummy coil can be anywhere. If you put it in the control cavity, makes sure the area where the coil is does NOT have shielding. Shielding will prevent the coil from picking up hum.

The coil can also be oriented in any direction as long as the axis is the same. So you can turn it 90°, but not have it on its side.

It will be wired out of phase with the pickup.

Keep in mind that the dummy coil will alter the tone of the pickup somewhat.
 
#11
line6man said:
Not necessarily.
You want the signals to be 180 degrees out of phase, of course, but the pickups may or may not be RWRP from each other already.

Pickups not RWRP should be wired out of phase, pickups that ARE RWRP should NOT be wired out of phase.
Click to expand...

A dummy coil has no magnetic polarity. When they say RWRP, it's Reverse Wound/Reverse (magnetic) Polarity. We don't have to worry about the RP part... on to the RW.

There's no reason to reverse wind a coil. You only need to reverse the electrical polarity. Winding it in reverse would do the same thing, but it's still wired out of phase since the coil is already reversed.

So if the two coils are wound in the same direction, you wire start to start, or finish to finish. Reversed coils would be start to finish, or finish to start.

If your coil is reverse wound, flip it over, and now it's not. ;)

Either way wire it up, if it's humming flip the coil over or reverse the wiring.
 
#13
SGD Lutherie said:
A dummy coil has no magnetic polarity. When they say RWRP, it's Reverse Wound/Reverse (magnetic) Polarity. We don't have to worry about the RP part... on to the RW.

There's no reason to reverse wind a coil. You only need to reverse the electrical polarity. Winding it in reverse would do the same thing, but it's still wired out of phase since the coil is already reversed.

So if the two coils are wound in the same direction, you wire start to start, or finish to finish. Reversed coils would be start to finish, or finish to start.

If your coil is reverse wound, flip it over, and now it's not. ;)

Either way wire it up, if it's humming flip the coil over or reverse the wiring.
Click to expand...

I used a regular pickup when I was experimenting with dummy coils, so the polarity and phase was important.
 
#14
line6man said:
I used a regular pickup when I was experimenting with dummy coils, so the polarity and phase was important.
Click to expand...

Doesn't matter. If the dummy has magnets and is picking up the strings, then it matters, but then it isn't a dummy pickup. Since you generally don't know which way it's wound, either it will buck the hum or it wont.

But it always has to be out of phase, whether it's wound that way or wired that way.

The only thing that makes a RWRP pickup in phase with the other pickup is the magnets, since the two coils are wound out of phase. The dummy has no magnets.

It's semantics, since you are pretty much saying the same thing, but they are never wired up in phase, it only looks that way.
 
#15
david's right, a proper dummy coil doesn't have any "polarity" to reverse, just the winding, and that's just a matter of orientation.

physically flip it one way and it hums, flip it over (or reverse the leads) and it cancels; that's all there is to it.
 

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