How to calculate the size of a dummy coil?

[David King]

Don't we still need a very uniform field to be able to equalize the two coil's outputs? I suppose we could measure them one at a time in the exact same spot.. Is a 50-60 Hz sine wave the ideal frequency and wave form to test shielding in a guitar? I'd think white noise or pink noise would give us a better idea of what our shielding is capable of at the various frequencies we need to worry about.

As far as designing a helmholtz coil it should be a simple matter of ohms law once you look up the resistance per foot of the wire you want to use. You just need to decide how much power/heat you can dissipate to air.

[Joe Gwinn]

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?

It's a valid question, even if asked in jest.

The problem with that conglomeration of noisemakers it can be very difficult to figure out who did what to who, leaving one unclear as to what to do next. So while the real world has all those things in it, our test setups must be far simpler, allowing us to deal with one problem at a time.

The problem with nearby transformers et al is that their fields are very non-uniform, which many perfectly useful humbuckers cannot fully cancel. So, we start with uniform fields.

One can make humbuckers to handle nonuniform fields, but it's harder than for uniform fields, and the practical need is unclear.

[belwar]

The problem with that conglomeration of noisemakers it can be very difficult to figure out who did what to who, leaving one unclear as to what to do next. So while the real world has all those things in it, our test setups must be far simpler, allowing us to deal with one problem at a time.

The problem with nearby transformers et al is that their fields are very non-uniform, which many perfectly useful humbuckers cannot fully cancel. So, we start with uniform fields.

One can make humbuckers to handle nonuniform fields, but it's harder than for uniform fields, and the practical need is unclear.

So correct me if I am wrong but what you are saying is all 60 cycle hum is not created equal? I was under the impression that it was, it was only the stength of the hum (amplitude?) that changes.. i.e. 12 db worth when you are 20 ft away from a source or 18 db when you are 10 ft away (I just made those numbers up FYI :> )

What troubles me about this is that the more strength (amplitude?) the hum signal has, the more voltage it produces in the active coil. How then, can you design a dummy coil to ONE specific matching voltage? based on the goal that you want to match the voltage output of the active and dummy coils .. Plus if the active core has a ferromagnetic core and the other coil is an air coil, the ammount of voltage created by hum (as it increases in intensity) will change at different rates in each coil...

THIS is where I get lost!

[David King]

A dummy coil will only work with a uniform field. It shouldn't need to work with fields that aren't uniform since it's easy enough to back away from the source until you minimize the parallax between your pu coil and the dummy coil.
Not many guitar players insist on sitting on top of their amps anyway.

Again as I understand it we are canceling magnetic hum so the core doesn't figure into the calculation?

[Bruce Johnson]

The test rig I was picturing would have several known 60hz generating devices, located at distances that are similar to what the instrument would encounter in the real world. Each could be switched on and off. I sort of have that now. My electronics bench has a 4' flourescent hanging about 3' above the bench top, and I have an amp directly under the bench top. When I'm finishing up and checking out a new instrument, I listen to it with the lamp on and off to compare. And I run the output over to another amp about 6' away, and listen for the difference with the under-bench amp on and off. It may be crude, but it's empirical! Or something like that...

[Joe Gwinn]

A dummy coil will only work with a uniform field.

It isn't quite true, as one can arrange things to cancel nonuniuform fields. The simplest example is a pickup centered in a larger circular dummy coil. This can fully cancel a field that varies linearly with position. Fields with more complex variations will not be fully canceled. There are more complex dummy-coil designs that will cancel these more complex fields, and so on.

What is true is that most humbucking approaches used on guitars assume constant field, and are none the worse for it, so the practical need to cancel non-uniform fields is not strong.

It shouldn't need to work with fields that aren't uniform since it's easy enough to back away from the source until you minimize the parallax between your pu coil and the dummy coil.
Not many guitar players insist on sitting on top of their amps anyway.

Exactly.

Again as I understand it we are canceling magnetic hum so the core doesn't figure into the calculation?

Well, cores do figure into the practical use. I ignored cores in the description of area-turns product for simplicity, but most pickups do have cores.

[Joe Gwinn]

So correct me if I am wrong but what you are saying is all 60 cycle hum is not created equal? I was under the impression that it was, it was only the strength of the hum (amplitude?) that changes.. i.e. 12 db worth when you are 20 ft away from a source or 18 db when you are 10 ft away (I just made those numbers up FYI.)

Each hum source generates its own hum field. While all are at 60 Hz (or 50 Hz) and harmonics, the shapes of the fields vary greatly. A pickup sees the sum of all these fields, and one can treat what the pickup sees as if it were a single combined hum field. If the pickup is well away from the closest hum source, the hum field around the pickup will be almost uniform, and any reasonable humbucking design will work just fine.

What troubles me about this is that the more strength (amplitude?) the hum signal has, the more voltage it produces in the active coil. How then, can you design a dummy coil to ONE specific matching voltage? based on the goal that you want to match the voltage output of the active and dummy coils .. Plus if the active core has a ferromagnetic core and the other coil is an air coil, the amount of voltage created by hum (as it increases in intensity) will change at different rates in each coil.

What saves us is that both pickup and dummy coil generate a voltage proportional to the strength to the local hum field, so if the voltages are equal at one hum field strength, the voltages will be equal at all field strengths.

[Mike Sulzer]

The electrician that wired my house provided me with a fine hum generator for testing pickups (when used with a load such a large old Sony CRT TV). I have not checked the wiring to see exactly what he did, but since it is the standard circuit with two switches, one at each end of the room, it is likely that the result is a room size loop of wire. Normally I do not plug anything into this circuit, but if I want a hum generator, the TV does fine. It provides a signal rich in harmonics, better than a transformer driven coil. And it is a big signal. Joe's 20 db rule does not apply here. You need much more than 20 db of cancelation if you want to play with any real gain with the tv plugged into that circuit.

[David King]

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).

[Joe Gwinn]

The electrician that wired my house provided me with a fine hum generator for testing pickups (when used with a load such a large old Sony CRT TV). I have not checked the wiring to see exactly what he did, but since it is the standard circuit with two switches, one at each end of the room, it is likely that the result is a room size loop of wire. Normally I do not plug anything into this circuit, but if I want a hum generator, the TV does fine. It provides a signal rich in harmonics, better than a transformer driven coil. And it is a big signal.

I would figure out what he did. It could be a significant wiring error, the details depending on where in the world you are.

In the US, loops are not generally allowed in the power system, so the White (neutral) and Black (hot) conductors travel side by side, so their magnetic fields cancel. In Europe, ring connections are allowed, but the hot and neutral conductors still travel side-by-side, and the magnetic field also cancels.

Your experience with the TV implies that something is miswired in your house. In the US, the most likely cause would be interchange of white (neutral) and green (safety ground) conductors somewhere. In Europe, a break in one or more ring conductors and/or interchange of neutral and safety conductors is a possible cause.


War Story 1: In the early 1970s, my then boss complained of weird interactions between various lights in his new house. I allowed that it sounded like a problem with power grounds. This led to a dinner invitation, but bring your tools. It turned out that none of the screw connections (of solid #14 wire to switch or outlet) in the house were tight, and after a few years of atmospheric corrosion the contact resistance had grown enough to be significant. When the house was built, the electrician formed the wires around the correct terminals, and his assistant was supposed to come around and tighten all the screws. But the assistant never came, and the inspector didn't notice.

War Story 2: In the late 1970s, some friends complained that they got shocks off the plumbing fixtures in the bathrooms. I didn't really believe them, but came over anyway, voltmeter in hand. It turned out that if one rattled the drain control rod in the sink in the little bathroom just off the kitchen, sparks fell from under the sink. Huh? This was a very old house, with knob-and-tube wiring feeding the ceiling light of the kitchen, and the plumbers had broken the return wire while installing brand new copper drains for the 2nd floor bathroom. The net effect was that the return for the kitchen light was through the new drains, which connected to the copper cold water pipes through the sink hardware. I have no idea why nobody was electrocuted.

Joe's 20 db rule does not apply here. You need much more than 20 db of cancelation if you want to play with any real gain with the tv plugged into that circuit.

I assume you mean my estimate that 20 dB of cancellation is enough. That's quite the field. How much current does the TV draw? It will say on the nameplate in the back.

[Mike Sulzer]

Wow! The second story in particular does make you think. In my case I can find no "unexpected" voltages or bad grounds. This is why I think it must be like shown here:http://www.naic.edu/~sulzer/powerLoop.png. I think the guys just did not pull enough wires through the conduits. So they used two for the end contacts of the switch by one path, and then found a different way to get neutral path.

This is Puerto Rico, US electrical code, but sometimes a bit of local flavor.

[Joe Gwinn]

Wow! The second story in particular does make you think. In my case I can find no "unexpected" voltages or bad grounds. This is why I think it must be like shown here:http://www.naic.edu/~sulzer/powerLoop.png. I think the guys just did not pull enough wires through the conduits. So they used two for the end contacts of the switch by one path, and then found a different way to get neutral path.

It may be OK, even if it isn't quite up to code, but I would inspect it very closely.

What are the dimensions of the room in question?

This is Puerto Rico, US electrical code, but sometimes a bit of local flavor.

There is a reason that electrical codes are very belt-and-suspenders. In my 2nd story, the electrician did nothing wrong.

I would add that knob-and-tube wiring does yield large loops, and so would be able to test humbuckers.


War Story 3: In the late 1970s, a fire started in a friend's apartment while she was at work. As it happened, a retired fireman lived nearby and saw the smoke, and called the fire departmet, who were able to save the apartment. The direct fire damage was limited to one room. I had the opportunity to make a detailed inspection, including being allowed to take things apart to inspect them. The fire started in an electrical outlet that was feeding a fan. This was summer in Baltimore, and it was pretty hot. The fan, never having been oiled, had a bearing seize up, so the fan drew a large current, enough to cause the terminal where wiring in wall connected to screw terminal on outlet to become incandescent. The screw terminal had loosened over the years.

Afterwards, whenever I got a new apartment, I turned the power off and went around the apartment tightening all terminal screws and replacing any lick-and-promise wiring, and replacing any worn outlets and switches.

[David Schwab]

Almost all of the outlets in this house are upside-down, and most of them are wired backwards as well! Grounds aren't connected either. We only rent, but I had to fix a few of them so my audio equipment would work without outlet testers showing problems.

Also all the faucets are backwards, which took some getting used to when we moved in. Nothing worse than trying to adjust the water when you are taking a shower, and you turn up the cold instead of the hot!

Makes you wonder how many homes were not put together right, and were never really inspected.

[Joe Gwinn]

Almost all of the outlets in this house are upside-down, and most of them are wired backwards as well! Grounds aren't connected either. We only rent, but I had to fix a few of them so my audio equipment would work without outlet testers showing problems.

Also all the faucets are backwards, which took some getting used to when we moved in. Nothing worse than trying to adjust the water when you are taking a shower, and you turn up the cold instead of the hot!

Makes you wonder how many homes were not put together right, and were never really inspected.

This sounds like a homeowner doing his own work. Another 1970s Baltimore friend had a house that was full of such problems. The prior owner's son fancied himself an expert <everything>. We called him Clyde Bumfuck, as we spent some years fixing one thing after another.

[belwar]

Well here we go back to topic...

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..

Im 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?