How to calculate the size of a dummy coil?

[belwar]

Probably going to need Joe's help with this one.

Im looking at creating a guitar for myself as im bored. Something like a bizarre les paul special with dual P-90's and I think im going to mess around with dummy coils. I'm trying to learn the variables so that I can build the design of the dummy coil into the guitar (thus teaching myself the concept). My current thought is that Im going to laminate a 1/8"-1/4" thick top on the guitar, so I could possibly put the dummy coils in between the neck and bridge pickup inside the guitar and invisible to the outside. I could see this having several advantages as the dummy coils are out of reach of the magnetic field of the other pickups, but also are in the same hum generating area as the P-90's.

So far from reading here i've picked up a few things...

1) Larger diameter wire is better for the dummy coil as it will less the impact on tone.
2) Voltage Generation between both the active and dummy coils need to be equal.
3) Inductance does not have to match between the two coils

So I believe I understand WHY the above is true, but it leaves me with several questions

1) How can a coil with a non magnetic core be capable of creating voltage? How does it do it?
2) Does the dummy coil need a ferrous core? Adding the core is going to increase the inductance, but is it going to affect the voltage creation potential? Also what effect would it have increasing the size of the core on the dummy (maybe less winds required and less loss of tone?)
3) How can one create a consistant hum field in order to measure the voltage created by hum in each coil?
4) How would you determin the size of the dummy coil? I have a spool of high quality 36 AWG and thought that this would be a good choice.
5) I've read about "area-turns", but dont understand the concept - can anyone explain it to me?

Thanks for listening to the ramblings of a madman on christmas night.

b.

[Chuck H]

So far from reading here i've picked up a few things...

1) Larger diameter wire is better for the dummy coil as it will less the impact on tone.

Why? The coil could be in parallel so the resistance would have no effect on the signal path.

2) Voltage Generation between both the active and dummy coils need to be equal.

And

1) How can a coil with a non magnetic core be capable of creating voltage? How does it do it?1)

The only signal that needs to be "equal" is the hum from outside magnetic fields. Since this is mostly caused from the antenna effect of the long winding, and that voltage should be very low, there should be no need for an iron core.

3) Inductance does not have to match between the two coils

I would make the two coils as close to identical as possible, sans iron core (or pole magnets or whatever). What you want is two identical antennea in parallel / out of phase. It's actually important not to have an iron core so that the dummy coil will not generate any voltage from string pickup that could be phase cancelled.

3) How can one create a consistant hum field in order to measure the voltage created by hum in each coil?

Not sure. Proximity and all, getting a consistant measurement as well as knowing it's potential effect re: magnetic fields is a job for equipement that most amp and guitar techs aren't even aware of. But if you follow other correct criteria it shouldn't be needed.

4) How would you determin the size of the dummy coil? I have a spool of high quality 36 AWG and thought that this would be a good choice.

For the method I'm describing I would make a coil as identical to a P90 as I could determine.

5) I've read about "area-turns", but dont understand the concept - can anyone explain it to me?

No idea.

FWIW I know little about dummy coil systems in use on current production instruments. I usually hang at the "guitar amps" forum. But some of the "dummy coil" theory you posted looked suspect. So I just rambled whatever tech I know that makes some sense. If I'm missing anything I'm sure someone with "dummy coil" experience will chime in to correct me.

Chuck

[Possum]

....

Get hold of a Guild Bluesbird, dual P90s and switchable dummy coil....

[Dave Kerr]

Belwar, Joe does the math for calculating the appropriate number of turns of large dummy coils here. There's also some good discussion in that thread about the advantage of having alarge diameter, low turn dummy coil.

I think the best reason for using thicker wire (say, 38 AWG) for so few turns is that it's easier to work with.

The Chiliachki patent can be downloaded here, it can give you some ideas for fine tuning the wiring of the dummy coil.

[Mike Sulzer]

Two very different ways to make a dummy coil are:

1. make it identical to the pickup coil, including the cores. Then you will get an easy match in hum sensitivity, but have the unpleasant effect of doubling the inductance.

2. Make it larger (and air core, since the larger size can compensate for the lack of steel). Then you can use fewer turns to get the same hum sensitivity, but the inductance is lower, and thus affects the tone of the pickup less. The reason for this is that sensitivity to hum is proportional to the number of turns, while inductance is proportional to the square of the number of turns (for constant coil shape). (And larger area increases the sensitivity.)

I prefer to use identical coils but add actively; this avoids the change in tone due to the increase in inductance. Actually, the coils need not be identical, since you can adjust the gain in an amplifier.

Be prepared to adjust the number of turns on a large dummy. It is very difficult to compute exactly.

[David Schwab]

Why? The coil could be in parallel so the resistance would have no effect on the signal path.

A lot of dummy coil systems have them in series. By using a low resistance dummy coil you wont get the big tonal change you get with a coil that matches the pickup. And putting the two coils in parallel does effect the signal path. Try it and see. You will also get a drop in volume.

If you look at the Chiliachki/Suhr system, you see just this. They use a large area coil of larger diameter wire.

Some of the newer generation stacked single coils also work with a bottom coil with larger wire and with more metal added and shielded from the string sensing coil's magnets. The DiMarzio virtual vintage pickups work very well and use this idea.

[Joe Gwinn]

So far from reading here i've picked up a few things...

1) Larger diameter wire is better for the dummy coil as it will less the impact on tone.

Not necessarily, although thicker wire is easier to handle. Voltage depends on turns count and area and the nature of the core (if any), but does not depend at all on resistance.

2) Voltage Generation between both the active and dummy coils need to be equal.

True, as cancellation is at base a subtraction yielding zero. This applies to hum voltages; one wants music voltages to differ, so they do not cancel.

3) Inductance does not have to match between the two coils

True.

So I believe I understand WHY the above is true, but it leaves me with several questions

1) How can a coil with a non magnetic core be capable of creating voltage? How does it do it?

All coils generate voltage in a changing magnetic field. At low levels of magnetic field, presence of an iron core causes the field through the coil to increase by a factor. perhaps a large factor. By "low levels of magnetic field" I mean less than that sufficient to saturate the iron. Pickups operate in the low-field regime.

2) Does the dummy coil need a ferrous core? Adding the core is going to increase the inductance, but is it going to affect the voltage creation potential? Also what effect would it have increasing the size of the core on the dummy (maybe less winds required and less loss of tone?)

No iron core is required, although having a core allows a coil to generate a far larger voltage than without the core. But one can increase the voltage in other ways, such as making the coil larger. This is discussed under area-turns product.

3) How can one create a consistant hum field in order to measure the voltage created by hum in each coil?

Make a Helmholtz coil and drive it with 50 Hz or 60 Hz as appropriate. The classic approach is to drive the Helmholtz coil with a filament transformer in turn driven by a small variac. Make sure the Helmholtz coil can handle the power without overheating, even if left on for an hour. Also make the Helmholtz coil fairly large, so it will generate a uniform field over a large enough volume to encompass the pickups and their nearby dummy coils.

http://en.wikipedia.org/wiki/Helmholtz_coil

4) How would you determine the size of the dummy coil? I have a spool of high quality 36 AWG and thought that this would be a good choice.

See area-turns product discussion, next. AWG 36 should work just fine.

5) I've read about "area-turns", but don't understand the concept - can anyone explain it to me?

For simplicity, the following discussion first assumes the absence of all ferromagnetic substances.

The area-turns product is based on Faraday's Law of Induction: http://en.wikipedia.org/wiki/Electromagnetic_induction

The above URL takes one into a mass of math, complete with vector fields and partial differentials. Some translation is in order, focused on the first equation

.

Consider the lowly loop, immersed in a time-varying but uniform (over space) magnetic field. The EMF (Electro Motive Force, that is the voltage, represented by the script letter E in the first equation) induced in the loop is proportional to the time rate of change of the total magnetic flux going through (linking) the loop. [The d/dt part means time rate of change, and the capital Phi sub B is the total flux through the loop.]

What's this capital Phi sub B? It's the total magnetic flux(~flow) through the loop. In a uniform magnetic field perpendicular to the plane of the loop, the flux through a loop is simply the product of the field strength and the physical area of the loop.

The time rate of change is proportional to both the amplitude of the change in magnetic field, and also to its frequency. In the case of hum, the frequency is set by the local power system, so only the amplitude varies. The larger the amplitude, the larger the rate of change.

So, for a single loop, the induced hum voltage is the product of the loop area and the strength of the hum field.

Now, what if there are multiple turns? Well, each turn acts as if it were a lowly loop isolated from all others, and generates its voltage as described above. Each loop generates the same EMF, in every detail. If we connect these loops in series to make a multi-turn coil, the loop EMFs simply add, so for N turns we get N times the EMF of a single loop.

So, for a N-turn coil, the voltage is the product of the loop area, A, the number of turns, N; and the strength of the hum field.

Rearranging, the voltage is the product of loop area and turns count, NA; and the strength of the hum field.

In a practical coil, not all turns are exactly the same size, but we can still determine the overall area-turns product of a coil. In effect, this is the sum of the areas of all the turns.

This assumes a non-ferrous core. If there is a ferromagnetic core, it will cause more flux than expected from the area alone to link the coil, causing the voltage to increase by a factor. However, in pickups, it's hard to predict this factor from first principles, so one resorts to empirical methods.

In practice, I would set up the Helmholtz coil and measure the hum voltage from the pickup and from a nearby trial dummy coil, and use the ratio of the voltages to estimate how many turns to add or remove from the dummy coil to achieve equal voltages.

[David King]

Very useful and timely stuff.
Thanks.

[Possum]

....

I believe the Suhr system has some kind of tuning screw to adjust the loop to your pickups, maybe the patent shows this?

[belwar]

Make a Helmholtz coil and drive it with 50 Hz or 60 Hz as appropriate. The classic approach is to drive the Helmholtz coil with a filament transformer in turn driven by a small variac. Make sure the Helmholtz coil can handle the power without overheating, even if left on for an hour. Also make the Helmholtz coil fairly large, so it will generate a uniform field over a large enough volume to encompass the pickups and their nearby dummy coils.

So if I understand this device (and you) correctly this device driven with a 60hz signal from a transformer would create an area of maximum potential hum between the two coils. I could then use a DMM to measure the Voltage created by the active coil, and the voltage created by the test dummy coil. Boy if you could build a big enough one you could really test the shielding on a guitar I suppose!

Have you ever come across plans for building one of these? I wouldnt know where to begin. Im sure I could get the transformer easily, but designing the coil without a schematic would be excedingly difficult and potentially dangerous.

Thanks again for you post, i'm studying your response and will surely have questions soon!

[Joe Gwinn]

So if I understand this device (and you) correctly this device driven with a 60hz signal from a transformer would create an area of maximum potential hum between the two coils. I could then use a DMM to measure the Voltage created by the active coil, and the voltage created by the test dummy coil. Boy if you could build a big enough one you could really test the shielding on a guitar I suppose!

Correct.

Have you ever come across plans for building one of these? I wouldn't know where to begin. I'm sure I could get the transformer easily, but designing the coil without a schematic would be exceedingly difficult and potentially dangerous.

I have not seen plans, but I have not looked either. For testing pickups (and whole guitars) the Helmholtz coil need not be precise or pretty. A pair of wooden coil forms wound with ordinary hookup wire and held apart by wooden dowels would work.

My main question is how strong the max field must be, the issue being to generate large enough hum voltages that one can easily measure them with sufficient accuracy using ordinary voltmeters hooked to a pickup coil. The stronger the max field, the greater the coil power, the harder the cooling problem. The bigger the coil, the more heat-dissipation area.

Making the coil big enough to accept a standard guitar will lead to high power levels and very large coils, needed to establish the field over the necessary volume, so that would be a separate design.

I'll think about it.

[belwar]

the actual construction of the coil sounds simple enough. two coils of f equal size placed in parallel to each other. the area in between the two coils ends up with a stable magnetic field. I think the big question that one could potentially mess up is how many turns of what gauge wire? im thinking that a medium awg wire like 24-26 would work and could probably contain the heat, but I just don't know what electrical properties the coil need to have to be effective. does it require high resistance or low - does it require a lot of turns or just a few?

[Joe Gwinn]

the actual construction of the coil sounds simple enough. two coils of equal size placed in parallel to each other. the area in between the two coils ends up with a stable magnetic field. I think the big question that one could potentially mess up is how many turns of what gauge wire? I'm thinking that a medium awg wire like 24-26 would work and could probably contain the heat, but I just don't know what electrical properties the coil need to have to be effective. does it require high resistance or low - does it require a lot of turns or just a few?

You are right that physical construction won't be difficult. Although I'm leaning away from two sheets attached in parallel with dowels to a big U made of three plywood sheets attached at right angles, with coils on the two parallel sheets, to allow access to the uniform-field region from three sides.

It's the electrical issues that require the thought.

[Bruce Johnson]

I'm partly joking, but why not just hang a cheap 4' flourescent lamp down low over your pickup testing rig? And mount a couple of various bad transformers nearby, wired up so they're under load. I mean, the real purpose of developing out dummy coils and humbuckers is to make your guitar quiet in the environment that it's likely to be used in. Why not create a close little environment of the noisy things that you'd typically find around a studio, and then tune your guitar's system to cancel them?

[David Schwab]

Or a lamp dimmer! Or a CRT monitor.

I was quite happy when I was able to get my latest pickups several inches from my CRT monitor with much less buzz than I would get sitting in my chair in front of the thing.

The coil idea is interesting.