How to explain Hum-cancelling

[JohnH]

I think we could do with a reference thread about hum-cancelling. So as a GM (Genetically Modified, Globule Modulator), I’m proposing that in this discussion thread, we draft what it might say, then, after it has rambled on for a while, followed some blind alleys and amusing diversions, it is distilled into a short version that can be posted in the reference section.

So I suggest we start with a first attempt at it, then all who want to can take it, and add delete and amend until we agree.

Lets keep it as simple as possible with minimal electro-theory, since those who understand the theory probably already understand this stuff anyway..

Here goes....


Hum-cancelling Pickups and Pickup Combinations

Signal Induction in a Coil

When a coil of wire exists in a changing magnetic field, an electrical signal will be induced in it. In a guitar pickup, this is caused mainly by two effects:

The moving strings interact with the pickups magnet to cause variations in the magnets magnetic field. This results in a signal related to the strings motion, which is what we want from a guitar pickup.

But additional unwanted signals are also induced, comprising unwanted hum and noise. These are not related to the strings motion but are caused by electromagnetic fields from other sources, such as mains wiring and electrical equipment. Also, they are not directly related to the pickups magnet but only to its coil.


Changing Phase

Due to the two effects described above, the coil produces signals that vary with time, positive then negative etc. Reversing the electrical connections to the ends of the coil will invert the phase of both sound and noise signals eg, positive, negative , positive becomes negative, positive, negative etc.

What is important here is whether, when following a path through the coil say from hot to ground, the route is clockwise or anticlockwise around the axis of the magnetic field. Anything that reverses this will reverse the phase of the output (sound and noise). So, reversing the wires, turning the coil upside down, or physically rewinding it in the opposite direction are all equivalent, since they all reverse this coil direction.

Reversing the polarity of the magnet will reverse the phase of the sound signal, but has no effect on the noise signal.


Cancelling Noise

When we combine two coils together in a guitar, we would usually like the noise and hum signals to cancel out (ie be out of phase), but the two sound signals to reinforce each other (ie be in phase).

So if the coils are wired in reverse, then the hum will be cancelled. If they had the same magnetic polarity, the sound would also be out of phase and largely cancelled. But if one has the opposite magnetic polarity, the sound signal is reversed again, Two reverses put it back in phase and the pickup sounds correctly in phase, but with minimal hum.

The conclusion is that for two coils to be hum cancelling and in-phase, they must have different magnetic polarity, and one coil must also adopt one of the three methods to reverse the coil direction.

In a normal type of hum-bucker, each coil is sensing a different pole of the same magnet, so they naturally have different magnetic polarity. The coils are internally connected in series, start to start, or finish to finish, ie, they are wired in opposite directions, so combined with their opposite magnetic poles, sound is in phase.

In single coil guitars, some pickups are described as ‘reverse wound, reverse polarity’ so that they are hum cancelling with the other normal pickups . They don’t actually have to be physically wound any differently, just connected or placed differently (I suspect that no one builds a winding machine that runs backwards just to do this?).


More to follow, but please copy and edit, comment, add or delete...

Cheers

John

[darkavenger]

Very nice, this is a great resource!

While it's a great start, I'd like to mention that humbuckers aren't the only way of removing noise. LACE sensors do quite a good job, so maybe a small section on shielding?

[newey]

+1, John, for a clear and concise explanation for the non-technical among us.

I might add some diagrams showing (as if on an oscilloscope) 2 signals in and out of phase, as a visual aid. You might also make the point numerically, using 180 degrees OOP vs. 0.

More broadly, then, a discussion of phase as well as hum-cancelling- something to which newcomer inquiries can be linked for both topics- would be helpful.

[reTrEaD]

Apr 23, 2012 6:56:50 GMT -5 JohnH said:

In a normal type of hum-bucker, each coil is sensing a different pole of the same magnet, so they naturally have different magnetic polarity. The coils are internally connected in series, start to start, or finish to finish, ie, they are wired in opposite directions, so combined with their opposite magnetic poles, sound is in phase.

This is correct. I'd suggest bolding the word wired. To the best of my knowledge, the convention for HB coils seems to be:
Both coils for HB pickups are wound clockwise.

Apr 23, 2012 6:56:50 GMT -5 JohnH said:

In single coil guitars, some pickups are described as �reverse wound, reverse polarity� so that they are hum cancelling with the other normal pickups . They don�t actually have to be physically wound any differently, just connected or placed differently (I suspect that no one builds a winding machine that runs backwards just to do this?).

Strange as it may seem, "normal" and "RWRP" SC pickups are wound in opposite directions. I don't know if they run the spindle in reverse or if they have a platter that can accommodate the coil form/polepiece assembly being mounted "upside down".

The convention regarding SC coils is:
Finish is always "hot".
"Normal" pickups are CW with South "up" (nearest the strings).
"RWRP" pickups are CCW with North "up".

These are not the greatest photos, but if you look very carefully, you can see the fine wire of the winding. The "start" emerges at the bottom of the winding, closer to the middle (left/right) of the form.

[reTrEaD]

Below is a comparison of rotating a pickup (no effect) and flipping it (effectively changing the winding direction).

[reTrEaD]

Below is a comparison between Reverse Winding (or flipped coil) and Reverse Connection.

[reTrEaD]

What does North mean?

When we speak of magnetic polarity, North is short for "North seeking pole". If we suspend a magnet freely, the end, edge, or face of the magnet that faces north is the "North seeking pole" or simply "North".

In this drawing North is toward the upper left:



Opposites Attract
The indicator needle on a compass is a magnet. If we put two magnets in close proximity, they will align North to South. Note that the North end of a magnet will repel the North end of the compass needle and attract the South end of the needle.

Attachments:

[jmartyg]

You guys rock. i can't +1 both you within an hour.

please delete this post to save on clutter.

[newey]

jmg:

please delete this post to save on clutter.

We don't delete posts here. We'll take the clutter over taking the chance of destroying continuity. It's easier to keep it all in sequence than to try to decide what can safely be deleted.

What does North mean?

When we speak of magnetic polarity, North is short for "North seeking pole". If we suspend a magnet freely, the end, edge, or face of the magnet that faces north is the "North seeking pole" or simply "North".

This probably isn't the place to point out that "North" (magnetic north, that is) is only a temporary condition, until the Earth undergoes its next geomagnetic reversal, or "chron". I know, "too much information".

But, since some people learn better by analogy, we might point out that humbucking pickups aren't the only place this sort of thing is done. If you have a pair of those noise-cancelling headphones, they operate in an analogous fashion.

You're listening to your ipod or whatever, and at the same time, you're getting the external noise of a GE fanjet turning many thousands of rpm right outside your window in Seat 19F. So, the noise-cancelling headphones have a small microphone on the outside to pick up the external noise. The mic's signal is then phase-reversed and fed into the earpiece, where it cancels out (to some extent) the in-phase noise intruding from outside the phones. Just as in a humbucker, the 2 noise signals partially cancel each other out, leaving you rocking out with just your tunes.

[JohnH]

Thanks for the comments and additions - I like the curly coil diagrams. Ill add some more at the weekend, on phase, out of phase, how to sort out hum cancelling combos of unknown or different pickups, and some thoughts on different configurations of hum cancelling coil groups.

J

[reTrEaD]

Apr 26, 2012 14:05:03 GMT -5 newey said:

We don't delete posts here. We'll take the clutter over taking the chance of destroying continuity. It's easier to keep it all in sequence than to try to decide what can safely be deleted.

Right. This thread is all about discussion. The more the merrier. Regardless of the clutter, if there is one tiny bit of useful information that is mentioned in the process, John will glean that for use in the thread he posts in the Reference Section. Bring it on, and bring it all.

Apr 26, 2012 14:05:03 GMT -5 newey said:

This probably isn't the place to point out that "North" (magnetic north, that is) is only a temporary condition, until the Earth undergoes its next geomagnetic reversal, or "chron". I know, "too much information".

This is as good a place as any, Newey. But I think the more interesting bit of trivia is the fact that the "Earth's North Magnetic Pole" is actually a South (seeking) pole.

[darkavenger]

Apr 26, 2012 15:46:06 GMT -5 reTrEaD said:

Apr 26, 2012 14:05:03 GMT -5 newey said:

This probably isn't the place to point out that "North" (magnetic north, that is) is only a temporary condition, until the Earth undergoes its next geomagnetic reversal, or "chron". I know, "too much information".

This is as good a place as any, Newey. But I think the more interesting bit of trivia is the fact that the "Earth's North Magnetic Pole" is actually a South (seeking) pole.

So which way is north or south while the poles are in the middle switching!?!?

Do you think that some basic info on Faraday cages and shielding should be included, maybe with a link to a more in depth explanation?



I'm not much of a firm believer in star grounding since there is only one path to ground, but the shielding is a great resource and can have dramatic effect.

[reTrEaD]

Apr 26, 2012 20:08:28 GMT -5 darkavenger said:

So which way is north or south while the poles are in the middle switching!?!?

The Earth will continue to rotate around the same axis, so "North" will still be north and the Sun will still rise in the East and set in the West.

But the indication of compasses will change. After the pole switching is completed, the end of the needle that traditionally pointed north will now point south.

During the switching, compasses will point in various directions, depending where they are. Multiple weak north and south poles will be present around the globe. GPS using satellites will become extremely valuable since magnetic compasses will be mostly useless.

Apr 26, 2012 20:08:28 GMT -5 darkavenger said:

Do you think that some basic info on Faraday cages and shielding should be included, maybe with a link to a more in depth explanation?



I'm not much of a firm believer in star grounding since there is only one path to ground, but the shielding is a great resource and can have dramatic effect.

Imho, shielding is a very worthy topic but doesn't really fit this particular discussion. Both shielding and hum-cancelling have the same ultimate goal - reduced hum and noise in the guitar signal.

But shielding is about preventing hum from entering the system. Hum-canceling is about adding an "equal but opposite" amount of hum into the system.

[reTrEaD]

I'd like to submit the following list of terms for consideration


Terminology:

Start - The end of a coil where the winding began. Windings nearest the start are buried under the later windings. This is an "absolute" term.

Finish - The end of a coil where the winding is completed. Windings nearest the finish are in the outer layers of the windings. This is an "absolute" term.

Clockwise (aka CW) Coil - A coil where the finish end is more clockwise relative to the start coil (as viewed looking down, from the strings). This is a relative term. Placing a coil "face up" will maintain the original rotation. Placing a coil "face down" will change the direction of the rotation.

Counter-clockwise (aka CCW) or Anti-clockwise (aka ACW) Coil - A coil where the finish end is more clockwise relative to the start coil (as viewed looking down, from the strings).

Clockwise End - The end of the winding that is clockwise relative to the other end. Finish is the clockwise end of CW coil. Start is the clockwise end of a CCW coil. Naming the ends CW or CCW is seldom used, but can be very useful. While still a "relative" term, it's closer to being absolute. With a South up magnet, the polarity of the output at the CW end will be positve when the strings move closer to the pickup and negative when the strings move away.

Counter-clockwise End - The end of the winding that is counter-clockwise relative to the other end. Finish is the counter-clockwise end of CCW coil. Start is the counter-clockwise end of a CW coil.

Connectivity - Refers to which end of a coil (Start or Finish) is connected to ground and which end is connected to "hot".

Normal Connectivity - Start = Ground, Finish = Hot

Reverse Connectivity - Start = Hot, Finish = Ground

Normal Wound Normal Polarity - A coil/magnet assembly that has the coil oriented so the finish end is more clockwise than the start, and the magnet is South up (South end closest to the strings). The finish end is connected to hot. The Neck and Bridge SCs of most Stratocasters are NWNP. The Screw Coil of most HBs are NWNP.

Reverse Wound Reverse Polarity - A coil/magnet assembly that has the coil oriented so the finish end is more counter-clockwise than the start, and the magnet is North up (North end closest to the strings). The finish end is connected to hot. The Middle SCs of most Stratocasters are RWRP.

Reverse Connected Reverse Polarity - A coil/magnet assembly that has the coil oriented so the finish end is more clockwise than the start, and the magnet is North up (North end closest to the strings). The START end is connected to hot. The Slug Coil of most HBs are RCRP. A RCRP functions the same as a RWRP.

[JohnH]

A bit more...

Hum-cancelling with Out of-phase Coils

Sometimes we want to hear the bright honky sound of two coils or pickups out of phase. The fundamental note may then be more or less cancelled out, but due to differences in the coil or pickup position along the strings, there will be relatively more high harmonics present, because the phase of the strings motion varies with length and the coils will be picking the strings motion from positions that are often moving out of phase.

To get the out of phase sound, one coil has its connections wired in reverse. This also reverses the phase of the hum/noise signal. So in this arrangement, we don’t want to also reverse magnetic polarity, we want two similar magnetic polarities.

Sorting out Hum-cancelling Arrangements

A common scenario is a wish to combine outputs from coils of various partly unknown pickups in a switching scheme. They may not all be from a consistent source and it may not be clear how to configure the coils for optimum hum reduction.

Based on the above, to get in-phase hum cancelling, the first step is to work out the relative magnetic polarities – ie which are north and south. This can be done with a magnetic compass, or just relatively by testing which pickup poles attract and repel (ie like poles repel, opposites attract). Working out absolutely which is north and south is not strictly necessary, it is just required that the two groups of like polarity are identified.

Having done that, the pickup and coil combinations can be assigned. No matter what wire connections, or winding directions, or coil flippage applies, in-phase humcancelling combos will always be made with coils of opposite magnetic polarity. So if you connect such coils together, the result will either be in-phase hum cancelling (correct), or out of phase with added hum (no good).

If it turns out wrong, swap wires on one coil to fix it. Better, is to work out the in phase wiring first, using the Screw-driver Pull-off Test (see reference section).

[reTrEaD]

Apr 27, 2012 22:49:16 GMT -5 JohnH said:

A bit more...

I like it, John. But there is one section that might benefit from clarification.

No matter what wire connections, or winding directions, or coil flippage applies, in-phase humcancelling combos will always be made with coils of opposite magnetic polarity. So if you connect such coils together, the result will either be in-phase hum cancelling (correct), or out of phase with added hum (no good).

You and I both know exactly what you mean. Two coil/magnet combinations where the magnets are of opposite polarity. But since you say "magnetic polarity of the coils", it might be misinterpreted as having something to do with the coil itself, rather than the actual magnet?

Then again, I might be too focused on using a "For Dummies" language. Maybe it's safe to assume that the reader will recognize that you are referring to the polarity of the magnet itself.






I thought about what darkavenger had to say about shielding. Maybe it would make for a good introduction. I wrote this last night. Feel free to use all or excerpts from it in the final thread.


Introduction

In the fight against hum and noise in our guitars, we have two powerful weapons. Shielding and Hum-canceling. The objective is the same, but the strategies are radically different.

With shielding, the strategy is to intercept the energy from an external magnetic field (AC hum from power lines, transformers, stray fields generated by motors, etc) and "drain it" harmlessly to ground, before it can enter the system. Shielding techniques deserve a discussion of their own. But for the moment, suffice to say they alone are not the complete answer. Since magnetic pickups must be receptive to the magnetic energy generated by the string moving in a magnetic field, they will be vulnerable to external magnetic fields. At the very least, we will have hum entering from the same direction as the signal created by the strings. Then we turn to the second weapon (hum-canceling) to mitigate the hum that has entered the system.


Hum-canceling - basic concept

If we take two signal sources that are equal (amplitude) but opposite (polarity) and add them together, our net result is no signal. This is great in the fight against hum, but would also kill the signal generated by the string if it too, was equal and opposite. So we need the signal generated from the string to be the same on both sources. OR we need the signal generated by the strings to be present on one source but absent on the other.

Dummy Coil

Let's take a moment to briefly examine the strategy of having the string signal absent on the second source. We can do that by locating the second source away from the strings and/or by removing the string signal generating component (magnet) from the second source. In either case, we will have hum present on both sources, but string signal present only on the primary source. The two sources are then connected in opposite polarity. The hum is canceled but the string signal is not canceled. Success! But there is a price to be paid...

The second source will affect the frequency response (tone) and amplitude of the string signal that is present on the first source. If we put the two sources in parallel, the loading of the second source will cut the amplitude of the original signal in half. And the inductance of this combination will be half of what one source would have by itself. So the tone will be somewhat brighter. If we put the two sources in series, most of the string signal will be present at the output (as long as the external load is a relatively high impedance) but the inductance of the two series-connected sources will be double that of one source. So the tone will be somewhat darker.

There are ways to minimize these adverse tonal affects while maintaining (nearly) complete hum-canceling. But that would be best served by a separate discussion. So let's move on to examining two sources with string signal and hum present on both sources.


Humbucking pickups and hum-canceling pairs of single coil pickups

In both cases, we have two sources that have hum and string signal present. But the hum has opposite polarity on the second source and the string signal has like polarity on the second source. So the hum cancels and the string signal is reinforced, when we add the two sources together. Great! But how did we get here?

We can (and will) digress into how two coils can be configured so they have opposite polarity. But for now, let's focus on separating the pickup into two separate functions. Generating a signal from a vibrating string and sensing the signal we generated.

Sensing the signal - the coil
An electromagnetic wave will induce a current in a coil. The winding direction, orientation (face up, or face down), and connection (which end is hot and which end is ground) affect the polarity of both the hum from an external source and the signal generated by the string.

Generating the signal - the magnet and string
The string moving within the stationary field provided by the magnet generates an electromagnetic signal that is sensed by (induced in) the coil. If we change the direction of this stationary field (flip the magnet) this changes the polarity of the signal generated by the string. But flipping the magnet has virtually no effect on the hum. That was generated by an external source.

Therefore, the orientation of the magnet (South up or North up) will be the key to the relationship of hum polarity and string sensing polarity, on a given coil. Since we want the hum signal to be of opposite polarity on both sources and the string signal to be of like polarity of both sources, we will use this key to achieve the desired goal.

Reverse and DOUBLE-reverse
(putting it all together)
If we reverse the polarity of the signal (both hum and string signal) on the second coil, the hum will be canceled. But to get a string signal that has the same polarity as the first coil, we also need to change the polarity of the string signal the second coil is sensing. We accomplish this by flipping the magnet. By inverting the string signal that is being generated AND inverting the sensing of the signal, we have accomplished a "double-reverse" for the string signal on the second coil. The string signal is now the same polarity on the second coil as it is on the first coil. Success! But again, there is a price to be paid...

We now have the advantage of having string signal on both sources. So when these are added together they reinforce each other. But like the dummy coil strategy, connecting the two coils in parallel will decrease the inductance (brighter tone. Connecting the two coils in series will increase the inductance (darker tone). And the two string signals will not be identical. Since the signals were generated by different regions of the string, they won't have exactly the same amplitude and ratio of fundamental to harmonics. This will result in a more complex tone than a single coil. Not necessarily a bad thing. In some cases this is actually desirable. But it won't be the same as a the tone from a single coil pickup.

Whenever we DO want the more complex tone of two or more coils together, we should use the advantage that hum-canceling can provide, whenever this is possible and practical.

It's time to digress into examining the relationship of coil winding, coil orientation, and coil connection.


Edited to address the deficiencies noted in Newey's reply, below.

[newey]

RT-

Excellent. But I had trouble digesting:

So we need the signal generated from the string to be the same equal and the same on both sources

That's a bit awkwardly worded I think.

Also:

The string moving within the stationary field provided by the magnet generates an electro magnetic signal that is sensed by the coil.

I think we need to say something about the signal being induced in the windings of the coil. When you say "sensed by the coil" it brings to mind the Lace Sensor type of pickups, which operate somewhat differently from a regular pickup.

I know, it's a fine line to write something understandable for the layman and yet still sufficiently exact- tough to do. I don't mean to be picayune on these points, just some things to consider.

Finally,

With shielding, the strategy is to intecept the energy from an external magnetic field and "drain it" harmlessly to ground,

The layperson is going to have difficulty equating an "external magnetic field" to RF noise or 60 Hz hum, at least not without a more than passing familiarity with Electromagnetic Theory.

[reTrEaD]

Apr 28, 2012 15:01:14 GMT -5 newey said:

I know, it's a fine line to write something understandable for the layman and yet still sufficiently exact- tough to do. I don't mean to be picayune on these points, just some things to consider.

This is exactly the sort of thing that's needed at this point in the discussion. I'll put some thought into rewriting those parts and make the appropriate edits.

Thanks!

[dna9656]

Did you know Laurens Hammond (the Hammond Organ guy) came up with the "twisted pair" to cancel line hum? I'm NOT an engineer but it seems to me to be somewhat analogous to how HB pickups work.

[reTrEaD]

Apr 29, 2012 0:00:45 GMT -5 dna9656 said:

Did you know Laurens Hammond (the Hammond Organ guy) came up with the "twisted pair" to cancel line hum?

Pretty sure that was invented much earlier, by Bell.

Apr 29, 2012 0:00:45 GMT -5 dna9656 said:

I'm NOT an engineer but it seems to me to be somewhat analogous to how HB pickups work.

There are some differences, but the basic concept is the same.

[newey]

Analogies, if they're close enough, can be a good way to facilitate understanding. When I read about noise-cancelling headphones for the first time, I immediately said "Aha! Like a humbucker!"

If I hadn't known about Humbuckers beforehand, it wouldn't have made much sense.

As I was driving yesterday, another analous situation came to mind, although not as close an analogy, since it involves optics. But I'm sure we've all had the experience of wearing polarized-lense sunglasses and looking at the rear window of a car in front (the glass in most modern cars is polarized, except for the windshield).

(The following is an oversimplified explanation; the optical physics actually get pretty complex here- and I'm no kind of physicist anyways . . . )

Polarized glasses reduce glare because ordinary white light is randomly polarized. The polarized lenses pass light through only if it's polarized in one direction (similar to a magnet being either "N" or "S").

When ordinary light reflects off a surface (what we call "glare"), its polarization changes, so the reflected light is blocked by the lens.

But now, back to the rear window example. When you look through polarized glasses at a second polarized piece of glass, you see a "checkerboard" pattern in the glass of the car in front's rear window. The effect is an interference pattern generated because the polarity of your sunglass lenses is aligned differently than that of the window glass you're looking at/through.

This "checkerboard" pattern has a technical name, an "extinction cross"

If you pull the glasses off your face and hold them out in front of you so that you can rotate the lenses while still looking through them, you will notice that, as you rotate the lenses, you'll find a spot where the checkerboard effect disappears. At that point, the polarization of your sunglasses matches up with the window, and all light of the same polarity can pass through both polarizers.

Now, rotate the sunglass lenses 90°. You'll see the window become totally opaque. It looks like a mirror, you'll see a reflection in it, but won't be able to see through the car window in front at all.

This happens when the polarity of the two pieces of glass are exactly opposite- light waves polarized one way by the first lens then are blocked by the second lens (or window).

This is, optically, pretty much the same as what happens electromagnetically with a humbucker. However, with a HB our only choices are 0 or 180°, there's no "in-between" states as there were when the two lenses were rotated.

[ashcatlt]

That's an interesting effect, but not really the same thing at all.  Polarized glasses (or windows) actually block the light which doesn't fit through the slots.  If the slots run horizontally, then light waves which wiggle up and down can't get through and are reflected back away from your eyes.  If the slots are vertical, then waves that wiggle side to side bounce away, and you don't see them.  

When you line up two pieces of polarized optics at 90 degree angle nothing gets through.  It's not a matter of destructive wave interference like in humbuckers, just selective reflectivity. 

Here's a couple of more closely analogous examples:

A figure-8 microphone usually has one diaphram open on both sides.  Sound waves striking from one side cause the output to have one polarity, and hitting from the other has the opposite polarity.  If the sound comes from somewhere between the two sides, it pushes on both sides of the diaphram equally, but in opposite directions, and the thing just physically doesn't move, so there's no output.  You can fake a figure-8 by putting two cardiod mics back to back (pointing opposite directions) and inverting the polarity of one.

Remember that pic sumgai posted of Ozzy (wasn't it?) singing into two mics?  Well, he was actually only singing into one of those.  The other was slightly off axis from his mouth, and picked up much less of his voice.  The two mics did, though, pick up pretty close to equal amounts of stage noise and bleed from the PA.  With one polarity-flipped that noise and bleed is cancelled, allowing them to crank the vocal mic to compete with Tony and Geezer and Bill with less worry about feedback.  Not that you could tell Ozzy's voice from feedback!  

Recording a vocal in the control room:  put the mic in the "sweet spot" - equidistant from the monitor speakers - and flip the polarity on one of the speakers.  Now you can blast the track at the vocalist, and he hears it with maybe a little bit of weirdness, but the mic hears only his voice, because the "noise" signals from the speakers are canceling where the mic is.

Then there's the stereo-widening/karaoke thing.  Take a stereo mix and flip polarity of one side.  If you leave it stereo you get a dramatic widening effect.  Elements panned to the middle (coming out both speakers equally) are attenuated, and elements panned to either side seem to be even further out.  The vast majority of pop music has the vocals panned dead center, so if you sum these two signals to mono the vocals cancel out, and you have a karaoke mix of the song.  Course, you'll also usually lose the bass, the kick and snare drum, so you might do this only to a certain frequency range.

[dannyhill]

Hi guys,

I have a question.

If I have two pickups of a similar design but quite different DC resistance and Alnico material but one is RWRP wrt to the other, will I get humcancelling in series but less hum cancelled in parallel configs?
Cheers,

D

[reTrEaD]

Hard to predict, Danny. Magnet strength has little if anything to do with hum, but the magnetic "permeability" might. And the difference in DC resistance could be due to a different number of turns or different gauge wire (or both).

If the higher DC resistance is because of more turns, I would expect the hum output to be greater on that pickup (all other things being equal). I reckon (no proof) that the better hum-cancellation would occur when the dissimilar pickups are in parallel.

[col]

Might a mathematical explanation help (for some, at least)? How about something like this:

Output phase direction is dependent upon the winding direction of the pickup coil wire and the magnet polarity. Change one, you change the output direction; change both, the output phase direction is unaltered.

Mathematically:

winding direction x magnetic polarity = output phase direction

1 x 1 = 1

change winding direction:

-1 x 1 = -1

change magnetic polarity:

1 x -1 = -1

change winding direction and magnetic polarity:

-1 x -1 = 1

[JohnH]

Sept 3, 2012 10:39:16 GMT -5 reTrEaD said:

If the higher DC resistance is because of more turns, I would expect the hum output to be greater on that pickup (all other things being equal). I reckon (no proof) that the better hum-cancellation would occur when the dissimilar pickups are in parallel.

Agreed, and I have found that to be the case in practice. As an interesting illustration , my Shergold demonstrates it. On that guitar, each pickup can be series, out of phase series or single. The OOp series are very thin sounding, and also hum significantly since theres no cancellation. But when that is combined in parallel with a single coil from the other pickup (for which hum phase is opposite), its silent, and much less hum than the single coil on its own.

That led me to think more about it, and i wrote it up on my old geocities page:
www.geocities.ws/guitarcircuits/cancelhum/cancelhum.html

The overall hypothesis for two pickups would be that if you have two similar pickups, one with more turns than the other, the extra hum created by more turns is compensated in a parallel combo, by the extra impedance. This benefit would not occur in series though.

John

[dannyhill]

[jerry]

Apr 23, 2012 6:56:50 GMT -5 JohnH said:

.... So I suggest we start with a first attempt at it, then all who want to can take it, and add delete and amend until we agree.

Lets keep it as simple as possible with minimal electro-theory, since those who understand the theory probably already understand this stuff anyway...

I'm new here, so please forgive me if I accidently step on someone's toes. I don't do it intentionally, (well, not usually), but it seems I sometimes focus too much on the topic and not enough on the delivery -- your patience is appreciated.

I like what you've started with, but wonder if we might be making this more complicated than we need to do for the neophyte or non-technical reader. I normally explain it to the non-engineer or techy as something like this:

"All wires, but especially coils, act as antennas to electro-magnetic waves and magnetic fields. Vibrating guitar strings disrupt the magnetic field of the pole magnets and that generates an electrical signal in the coils that very closely represents the frequency and level of the vibrating string(s). We then amplify that signal and reproduce it acoustically with a speaker.

Unfortunately, the pickup coils also pickup and increase the level of other such disruptions, such as the waves from the power lines, from the ballasts of fluorescent lamps, from noisy universal motors (their commutators), etc., that add noise to the desired guitar signal. The pole pieces are not really part of picking up these unwanted waves, they primarily pickup the guitar strings vibrations.

Humbucking pickups have two coils instead of one. They are manufactured to be identical, but are intentionally wired out-of-phase. That cancels both the noise and the guitar signal. So, the pole pieces are reversed and the guitar signal's phase is no longer cancelled, but the hum and noise picked up by the coils alone remain cancelled. The result is a functioning guitar pickup that is quite impervious to unrelated noise signals induced from outside sources."

I realize that the foregoing leaves a lot out. But my experience with non-technical people is that they really just want to get a basic understanding of the process and definitely don't want to learn anything about inductance node equations, loop equations, or such.

I hope this helps and will not be offended if this post is discarded :-)

Thanks,

Jerry

[sumgai]

Dec 2, 2012 17:37:12 GMT -5 jerry said:

Hear, hear, what he said! ;D

Dec 2, 2012 17:37:12 GMT -5 jerry said:

I hope this helps and will not be offended if this post is discarded :-)

Discarded? Discarded, he says?

Why brother, what I'm about to tell you is, you will, repeat, WILL contribute more postings of this nature!!   Nice and simple, to the point without any gobbledygook, and yet 100% accurate - just what we need here in The NutzHouse, no doubt about it.

Don't make us come out there and hunt you down, you just sit right there and dream up more little bon mots that us Nutz can digest in nugget-sized chunks.

Errr, please?!


And FWIW, to the NutzHouse!





sumgai

[gumbo]

...and I'm just glad to find that my sunglasses aren't magnetic after all!

....welcome, all the way from Oz, too... ;D