How to calculate the size of a dummy coil?

[Joe Gwinn]

I found a good PDF on the basic construction of a Helmhotlz Coil (see attached)... They are more concerned with testing magnets then they are with creating a field, but it does handle the basic structure. The rings be twice as far apart as the radius of the helmholtz coil. The coil should be at least 3x the size of the item being tested.

Their helmholz coil is used only as a receiver (the integrating fluxmeter), and may not be suited for use as a generator, the issues being physical size and power dissipation. But something like that could work.

I'm new to this uniform-nonuniform hum field stuff. Can anyone here describe the difference between the two so that I can get a handle on it?

If the hum field in a room is uniform, the strength and direction of the hum field does not vary with location.

The strength and direction of a non-uniform field does vary with location in the room.

[Joe Gwinn]

Mike, I have a cheap VFD you can try on that circuit. Without a filter it can knock out TV reception in a three block area (according to my neighbors).

Even an expensive VFD can do this. I would also run the three phase wires from VFD to motor in thinwall steel conduit and greenfield (where flexibility is needed). Steel makes a very good shield, and avoids the need for expensive shielded power cables.

[Mark Hammer]

You guys are making it more complicated, and in some ways more simple, than it really is.

EVERY pickup coil functions like a radio antenna. The "station" it picks up consists of a nearby broadcast from the strings, and a more distant "broadcast" from 60hz sources. As you know from cheap radios, reception can often vary simply by turning the radio in a different direction. As *I* know from having a workshop with fluorescent lighting, turning your guitar around can affect how much hum you pick up. When a dummy coil is inserted into the equation, the degree of 60hz cancellation created is a function of the extent to which the two antennae in use are matched with respect to the signal picked up. Obviously, if one coil picks up a strong signal and the other is always much weaker, or so separated in space that they never pick up the same source to the same degree, hum-cancellation will be minimal. As the two coils become better matched with respect to what they pick up, cancellation becomes more complete. The ideal is that the two coils are matched for turns/sensitivity, and so close together that they pick up the exact same amount of hum, no matter whether you are turned this way or that (Hey, didn't Seth Lover come up with something sorta like that? ). In between those two extremes are degrees of cancellation and noise-immunity.

It is possible to stick a dummy coil of some shot-in-the-dark dimensions and number of turns in whatever guitar cavity is available for use and get some hum-rejection. It may not be as much as you'd like, but as long as it does not suck tone, something is better than nothing. Seems to me that one could make a coil-form with a backplate/bottom made of etched PC board and arrange for multiple taps in the coil, each with their own solder contact point on the bottom plate, and simply try out taps until you find the one that provided best hum-reduction. Once, on a long drive, it occurred to me that thismight be a viable product: a muilti-tap "universal" dummy coil. Only problem is that the cavity it would need to fit into isn't quite as "universal" as the coil could be.

The John Suhr approach to the problem - winding a wide perimeter coil along the inside of the vibrato backplate on a Strat - is an elegant one because it: a) uses many fewer turns, thereby reducing any potential change in inductance and tone resonances, and b) covering an area that provides equivalent sensitivity for all 3 Strat pickups (i.e., as an antenna, it senses the same amount of hum in comparison to any of the 3 pickups, no matter where you're standing). Of course, if you're playing anything other than a Strat, it doesn't work so well.

[Mike Sulzer]

You guys are making it more complicated, and in some ways more simple, than it really is.

EVERY pickup coil functions like a radio antenna.

Mark,

It is complicated.
1. Dummy coils, like humbuckers, are intended to cancel low frequency nearby magnetic fields.
2. Most antennas designed to pickup radio waves are sensitive to the electric field. Some pickup magnetic fields (the ferrite core coil in an AM radio). Straight wire antennas pickup electric fields; the field along the wire causes a current to flow under the right conditions (generally a resonance).
3. The electric and magnetic fields of a radio wave are propagating fields; they are capable of carrying energy to a great distance, or even to infinity. They are described by a wave solution of Maxwell's equation. You can also view them as a stream of particles (photons) each carrying a certain amount of energy and momentum.
4. You can generate 60Hz radio waves, but it takes a huge antenna to do it efficiently. Some folks modulate electric currents in the ionosphere in order to accomplish this. Pickup coils are generally not very efficient at picking up these waves. Larger coils in an appropriate circuit are better.
5. The local magnetic fields from hum sources die off quickly with distance, although these days there are so many sources you can detect hum almost anywhere.
6. These local fields do not radiate energy to infinity, but they do, obviously, couple energy to detectors, such as pickup coils, that are nearby.

[Mark Hammer]

Miscommunication. I didn't mean antenna in the literal sense. rather, it was intended as an analogy.

The local magnetic fields from hum sources die off quickly with distance

...and that is why where one places a dummy coil has some bearing on how much hum-cancellation it can produce. Work out the perfect coil parameters based on the math alone, and you'll find that, if the pickup and coil have any appreciable distance between them, that your cancellation will go out the window if you so much as turn a few degrees to the left or right, simply because one of either the dummy or the pickup is detecting more local field than the other by virtue of varying proximity to the source.

What matters most in cancellation is whether the amplitude of hum detected by either the pickup or dummy is a match for the other source. That amplitude is a functionof both coil properties AND distance. This is why something like a multi-tapped dummy struck me as potentially practical: it might be that my hum source is coming from this side or that, and either my dummy or my real pickup may detect more hum from that source because of it. Being able to adjust the sensitivity of the dummy seemed like a reasonable thing to do, although again, I think John Suhr's approach was much smarter.

[Mike Sulzer]

Miscommunication. I didn't mean antenna in the literal sense. rather, it was intended as an analogy.


What matters most in cancellation is whether the amplitude of hum detected by either the pickup or dummy is a match for the other source. That amplitude is a functionof both coil properties AND distance. This is why something like a multi-tapped dummy struck me as potentially practical:

Understood; please excuse my over-reaction.

Distance: it is a vector field so direction (pickup and dummy in the same direction) matters as well. But I think you are right, the amplitude effect is dominant. I like to do the summation electronicly and use a pot to adjust the level.

[santellan]

What if you added an RF ID antenna concept to a Printed Circuit board that was intergrated into the pickguard and was located directly around each pickup.

It would give you the closest distance to each pickup but a fare amount of design work would be needed to determine the one generic overall value (in mH) to determine the size and number of layers for the board.

Any ideas what that value may be?

[David Schwab]

What if you added an RF ID antenna concept to a Printed Circuit board that was intergrated into the pickguard and was located directly around each pickup.

It might also infringe on the Chiliachki/Suhr patent. Although the most familiar iteration of that invention is the coil on the trem spring cover, it's really meant to encircle the pickups under the pickguard.

Of course if the implementation was different, that's all you'd need, and claim theirs as prior art. (or not... I'm certainly not a patent attorney!)

[santellan]

Refer to patent 6800800 without the active notch filter that was released prior to Ilitch's patent.

[LeonWhite]

David, I'm brand new here but have read the thread (and some related ones) to find an answer but can't. You make reference to putting the coil in parallel and the change in volume etc. I have tried that as part of getting my dummy coil to effect a single coil in any way. I realize there is something amiss with my test rig, but other then volume drop, are there any other impacts? I ask because parallel may be a better solution for me. will it still buck hum (I think it should)? will its hum cancelling be changed in anyway? Other then the common changes in volume and tone related to series vs parallel are there any other changes I should be on the look out for? Any help would be appreciated.
This forum is fabulous! thanks to all who have made it so in the past and now.
Leon White

[shane1b4u]

I experimented with the dummy coil thing and use it in all of my recording strats now, i wind my dummy coils with the same size wire as my pickups, I wind roughly half as much wire as the pickup and shorted my dummy coil across the leads of the pickup, it cut out almost every bit of noise but was really ice pick sounding, some ferrite slugs out of an old transistor radio in side of my dummy coil warmed that sound back up, mighta been luck, I dunno, but I like the results enough that I use it on at least 3 strats now, i use carpet tape to stick the coil under the pots. I read somewhere on this forum that Stevie Ray had a setup like this in his guitar using a coil from a P bass. Experiment and have fun with it.

[captcoolaid]

Dummy coils

I wind with 43 for my dummy I have found that a 1k reading is non invasive per tone however does result in hum canceling.

[LeonWhite]

dummy coil question

thanks. You're using the coil in parallel if I understand correctly. Are the strats all single coil?
Leon

[LeonWhite]

I wind with 43 for my dummy I have found that a 1k reading is non invasive per tone however does result in hum canceling.

Is your guitar a single coil? (strat perhaps?)
thnks
Leon

[shane1b4u]

yep

and if you dont use a reversed coil in the mid position, you only need 1 dummy coil.