Sampling the string: differences between single coil and humbuckers

[Mike Sulzer]

This discussion concerns the way in which a humbucker pickup samples the strings. What effect on the sound occurs when you have two coils rather than one?

When you take one of the two coils out of the circuit, several things happen:

1. The signal level drops approximately a factor of two.

2. The inductance drops a factor of two. This alters the resonant frequency resulting from the inductance and the cable capacitance, and has a significant effect on the tone.

3. The string is sampled differently.

Only number three is of interest here, and so we need to eliminate numbers one and two. This post describes how I did this, but briefly. (I will fill in later with additional posts.) Then there is an mp3 file to listen to. No music, just picking the open No. 6 E string changing between one and two coils.

In order to eliminate the effect of loading the changing inductance with the cable capacitance, my pickup test guitar has a preamp. It is switched from a unity gain follower to a gain of two when switching from two coils (in series, of course) to one coil. Each coil is loaded with a capacitor to set the resonance where I want it. The switch allows both coils or either one individually; they are not in quite the same place, and so could sound different.

The listening test uses the No. 6 string because it shows the most difference. The reason for this will be discussed below. The test alternates between one and two coils. Each change has an audible pop. It starts with one coil, then two, etc. One of the two coils is used in the beginning, the other towards the end, when in the one coil mode. The difference between one and two coils is large enough so that there is no need for fancy double blind AB testing in this case.

oneCoilTwoCoil.mp3

I need to do a lot of justification to convince you that this test has meaningful results. That will come in following posts.

[big_teee]

You don't have to convince me, and it will give us something new and different to discuss.
Please Leave Your Guns and Knives at the Door!
Terry

[Mark Hammer]

I'm not so sure the string IS sampled differently. I used to think otherwise, but have since changed my mind.

Ignoring what's magnet and what's slug for the moment, a PAF-style HB is essentially two stacked coils on a U-shaped magnetic structure. removing one of the coils from the circuit, doesn't change how the string is sampled any more than removing one coil from a stacked humbucker changes the sampling. The string is still being sensed within the same magnetic structure. What DOES change are the things Mike notes in #1 and #2. Personally, I would describe those not as "sampling" differences, but in sensitivity and resonant filtering differences.

Thanks to Mike's elegant use of level-matching, the tonal differences do not seem quite as robust as they might in the absence of level matching. They're still in evidence, just not as dramatic when one isn't distracted by drastic level changes.

It's a different topic, seemingly, but the same thing happens when people compare different diode types in distortion pedals. The radical differences that occur when the forward voltage is changed lead people to come up with all sorts of pet theories about qualitative differences in diode clipping when all we're dealing with is what happens when you raise or lower a clipping threshold. The big volume differences distract terribly.

[Mike Sulzer]

Ignoring what's magnet and what's slug for the moment, a PAF-style HB is essentially two stacked coils on a U-shaped magnetic structure. removing one of the coils from the circuit, doesn't change how the string is sampled any more than removing one coil from a stacked humbucker changes the sampling. The string is still being sensed within the same magnetic structure. What DOES change are the things Mike notes in #1 and #2. Personally, I would describe those not as "sampling" differences, but in sensitivity and resonant filtering differences.

You will see later that they have to be sampling differences because of the way the differences change from string to string. But we have a way to go before we get to that.

For now, I would like to comment on your magnetic model of a humbucker. The two sides are not tightly coupled together as you suggest, because the permeability of the alnico magnet is too low to achieve this function. You can see this from models: the flux leaks out. The truth of this is also demonstrated by the fact that humbuckers have little hum! If you do couple the coils in this way with high permeability material, this material responds to stray fields and couples them through the coils with opposite phase since the coils are on opposite ends of the coupler. But the coils are electrically out of phase, and so the signals add and you have horrible hum. How do I know? Well, I made one once before I figured out the problem.

[Mike Sulzer]

The pickup used in this experiment

It is best to have the coils as identical as possible. I took the screw coils from two Stew-Mac kits and used them. Each coil has 5,000 turns of number 43 poly. The resistance of each coil measures 4.72 Kohm in the evening. The first attachment shows the magnitude of the impedance of each coil up to 5 KHz. At most frequencies, it is hard to tell that there are two lines on the plot. The inductance of each coil is about 1.5 H, as determined from the magnitude of the impedance at the lower frequencies. The second attachment shows the phase of the impedances. They differ by as much a 0.6 degrees at 5 KHz, probably due to differences in the coil capacitance from not winding them exactly the same.

The third attachment shows responses from a coil excited by noise. These measurements are noisier than usual at the low frequencies because I use a very small coil suitable for direct coupling to a pole piece. For a humbucker, it is placed on its side between the two coils, and some field curves down through the pole pieces. The field through the coils is not as large as I would like. It is hard to place the coil so that the responses are equal to better than 0.5 db, and so I have adjusted the levels to be the same at low frequencies in order to see differences at high frequencies. The responses are within a fraction of a db at the higher frequencies, although the peaks do differ. Notice that the peak with both coils lies between the peaks for each of the singles. This is expected, and it also has significance for the listening test. If the audible differences are due to differences in the resonant frequencies, the audible differences should agree with the order in the measurements. They do not; in both cases, the single coil is brighter than the pair in series, even though one coil is higher than the series combination, and the other lower.

[David Schwab]

I'm not so sure the string IS sampled differently. I used to think otherwise, but have since changed my mind.

Ignoring what's magnet and what's slug for the moment, a PAF-style HB is essentially two stacked coils on a U-shaped magnetic structure. removing one of the coils from the circuit, doesn't change how the string is sampled any more than removing one coil from a stacked humbucker changes the sampling. The string is still being sensed within the same magnetic structure. What DOES change are the things Mike notes in #1 and #2. Personally, I would describe those not as "sampling" differences, but in sensitivity and resonant filtering differences.

Thanks to Mike's elegant use of level-matching, the tonal differences do not seem quite as robust as they might in the absence of level matching. They're still in evidence, just not as dramatic when one isn't distracted by drastic level changes.

It's a different topic, seemingly, but the same thing happens when people compare different diode types in distortion pedals. The radical differences that occur when the forward voltage is changed lead people to come up with all sorts of pet theories about qualitative differences in diode clipping when all we're dealing with is what happens when you raise or lower a clipping threshold. The big volume differences distract terribly.

If this is a comparison of a real single coil pickup, and a humbucker, then the issues are the magnet structure and the amount of wire on each of the pickups. A humbucker, even with one coil switched out, does not have the same shape magnetic field as a single coil, and of course half a humbucker is not wound the same. So you have a tonal shift that has nothing to do with string sampling when switching out a coil. You can this same tonal difference is you wind a tapped single coil and switch to half the windings, and then correct its output level.

Switching out a coil on a humbucker does eliminate some of the upper harmonic phase cancelation though, and lower frequency reinforcement.

So a better test would be a single coil wound the same as the two coils in the humbucker.

[Mike Sulzer]

If this is a comparison of a real single coil pickup, and a humbucker, then the issues are the magnet structure and the amount of wire on each of the pickups.

One coil of a humbucker is a real single coil pickup.

A humbucker, even with one coil switched out, does not have the same shape magnetic field as a single coil, and of course half a humbucker is not wound the same. So you have a tonal shift that has nothing to do with string sampling when switching out a coil.

If you think the shape of the permanent magnetic field has any effect other than at the string where it determines how strongly the string is magnetized at each location, you need to prove that. Such effects are not in the well understood behavior of E&M. I am not sure what you mean by the last sentence in this partial quote. In this test, you switch out one coil, leaving everything else the same as much as possible (circuit frequency response, level). This is what allows you to hear the differences from changes in string sampling.

You can this same tonal difference is you wind a tapped single coil and switch to half the windings, and then correct its output level.

That difference is almost entirely from the change in the resonant frequency and Q, and that is the effect that needs to be eliminated in order to make the true string sampling differences audible.

Switching out a coil on a humbucker does eliminate some of the upper harmonic phase cancelation though, and lower frequency reinforcement.

So a better test would be a single coil wound the same as the two coils in the humbucker.

In order to determine changes in sound and measured frequency response due to changes in how the string is sampled, we need to eliminate other differences as much as possible. What you are proposing would not do that. For example, a core of alnico magnets has a different permeability than one using steel screws. Not the same thing at all.

[David Schwab]

One coil of a humbucker is a real single coil pickup.

Right, but not a typical pickup that comes in guitars with single coil pickups. Half a humbucker is about 5,000 turns of wire, while a Strat pickup is about 7-8,000 turns. Even half a humbucker won't sound like both coils in a series humbucker. So how are you hearing the difference in string sampling? You are, but you are also hearing a very under wound pickup.

If you make a single coil wound to 8k, and then another wound to 4k, they will sound different, which is part of what you are hearing in this test.

One way to fix this is by stacking two humbucker coils on top of each other, and switching in the bottom coil (in phase) when going to the single coil mode. Or make a tapped humbucker coil.

If you think the shape of the permanent magnetic field has any effect other than at the string where it determines how strongly the string is magnetized at each location, you need to prove that. Such effects are not in the well understood behavior of E&M. I am not sure what you mean by the last sentence in this partial quote. In this test, you switch out one coil, leaving everything else the same as much as possible (circuit frequency response, level). This is what allows you to hear the differences from changes in string sampling.

A humbucker is sensing the string in two locations, and a single coil is not. The field over a Fender type single coil is also much stronger than over a humbucker and more vertical.

That difference is almost entirely from the change in the resonant frequency and Q, and that is the effect that needs to be eliminated in order to make the true string sampling differences audible.

Which is what I said.

In order to determine changes in sound and measured frequency response due to changes in how the string is sampled, we need to eliminate other differences as much as possible. What you are proposing would not do that. For example, a core of alnico magnets has a different permeability than one using steel screws. Not the same thing at all.

What I am proposing is that you make a single coil pickup as similar as you can to a humbucker. All your test is showing is that switching one coil out of a humbucker sounds thinner and brighter. You will get almost the exact same affect switching the humbucker into parallel.

Here's an example of that. You will hear series, parallel and then single coil. This is a narrow aperture dual blade bass humbucker. What you notice is that parallel and single coil sound almost exactly the same, with the single having a little more top end. Therefore the change in tone you are hearing is actually from the change in resonant frequency,combined with string sampling area and humbucker phase cancelation artifacts.

Series/Parallel/Single

A fairly easy way to show this is to make a sidewinder out of a humbucker. Use the same coils with the same amount of wire. Use the same magnet, between the coils, and loading in each coil. Now you will hear a very bright tone with extended high end that you don't hear in a humbucker due to high frequency cancelation.

Even if you use two coils next to each other, and wired in phase with the same polarity magnets, you will hear a change in tone. But is this from the change of sampling area, or notch filtering? Such an example is the 2 and 4 setting on a Strat 5 way switch.

So I'm sorry, but this does not seem like a valid way to test magnetic aperture. What you are hearing is different amounts of wire on the coils, and phase cancelation/reinforcement inherent in humbuckers.

You need to test two single coils with different size cores. You can make them both with ceramic magnet cores, as to eliminate adding more metal to one, and make one much wider than the other. Alembic had such pickups on the market for a while. Some used a wide ceramic magnet, and some had a narrow one.

It's pretty well established that the aperture of the coil/core makes a difference in tone.

[Mike Sulzer]

Even half a humbucker won't sound like both coils in a series humbucker.

Of course it won't. That is why I compensate for that effect as explained in two of my posts. Did you see the measurements showing how close the responses are of both individual coils and the series combination? Did you understand that the differences that remain, if responsible for the audible differences, would contradict those differences, and therefore cannot be responsible?

So I'm sorry, but this does not seem like a valid way to test magnetic aperture. What you are hearing is different amounts of wire on the coils, and phase cancelation/reinforcement inherent in humbuckers.

Again, are you aware that I have eliminated the differences in resonant frequency (inductance)? If so, then what do you mean by effects from different amounts of wire on the coils? "phase cancelation/reinforcement inherent in humbuckers": which happen from sampling the string in two places instead of one. I am isolating that effect from other effects.

You need to test two single coils with different size cores. You can make them both with ceramic magnet cores, as to eliminate adding more metal to one, and make one much wider than the other. Alembic had such pickups on the market for a while. Some used a wide ceramic magnet, and some had a narrow one.

Single coils with different core sizes would show an effect at a much higher frequency. That effect might be outside the range of the instrument/amp/speaker. Let's establish the effect for the wider spacing resulting from single versus double coil before considering that.

It's pretty well established that the aperture of the coil/core makes a difference in tone.

Why does the aperture of the coil matter? Who has shown this? (Certainly there is some effect. If I make the coil so big that the field lines from the string return through it, then you lose sensitivity. But I do not think that this is what you are talking about.)

[frankfalbo]

Having trouble understanding what conclusion you're trying to reach. Is it that string sampling length has little to do with why a wider aperature pickup sounds different? Other factors are the majority contributor?

The same pickup sounds different in the neck and bridge right? So the length of string sampled also means you're picking up sonic characteristics from a different location (more neckier, more bridgier is I believe the technical term)

Should you physically move the pickup's sensing coil (when listening to narrow aperature) to the center of the location of the wider aperature sample, rather than leaving it on one side or the other?

What about two single coils, arranged so the second single coil swivels out of the way and ceases sensing the string? Then replacing it with an empty bobbin that has magnetized poles so the field of the remaining coil is identical to what it was with the sensing coil in place (in other words still being tugged over by the opposing polarity in proximity)

Still, there will not ever be a test that can plot subtle differences in feel. There are shifts that literally are so small that they won't show up on an overlay graphic such as the ones you've posted, but make a difference in significant things like whether it's easier for me to get pinch harmonics, or the way one pickup interacts with another (passively of course)

[Mike Sulzer]

Having trouble understanding what conclusion you're trying to reach.

Drama, Frank, DRAMA. If you state the conclusions before looking at the evidence, it takes all the fun out of it. Also, there is this little matter of letting the evidence lead to the conclusions rather than the other way around.

Is it that string sampling length has little to do with why a wider aperature pickup sounds different? Other factors are the majority contributor?

Well, it should be clear that first I am trying to demonstrate the difference in the sound of two pickups with different apertures. Your statement implies that you already know the difference, but I have doubts about that since it is not that easy to eliminate other effects.

The same pickup sounds different in the neck and bridge right? So the length of string sampled also means you're picking up sonic characteristics from a different location (more neckier, more bridgier is I believe the technical term)

One should do the test in both (and possibly other) positions. As you noticed, this is the neck pickup. It was just a bit more convenient to start with it. I need to post some pictures of the test guitar. Should have done that already.

Should you physically move the pickup's sensing coil (when listening to narrow aperature) to the center of the location of the wider aperature sample, rather than leaving it on one side or the other?

What about two single coils, arranged so the second single coil swivels out of the way and ceases sensing the string? Then replacing it with an empty bobbin that has magnetized poles so the field of the remaining coil is identical to what it was with the sensing coil in place (in other words still being tugged over by the opposing polarity in proximity)

Those are complicated ideas. Let's see what we can learn from this simpler test first. That might help determine how much farther it is worth going.

Still, there will not ever be a test that can plot subtle differences in feel. There are shifts that literally are so small that they won't show up on an overlay graphic such as the ones you've posted, but make a difference in significant things like whether it's easier for me to get pinch harmonics, or the way one pickup interacts with another (passively of course)

Well, things you cannot understand through analysis and measurement, you just do. Not sure where the boundary between the two is in this case. Maybe we can learn something about that.

[Mark Hammer]

I guess what I'm probably having some difficulty with is understanding specifically what you mean to denote by "sampling". It is entiely possible we are thinking of two different, or at least non-overlapping, ideas.

A couple months back, I posted a musing I had about a pickup design in which instead of two slug-loaded coils with a bar magnet coupling them, there'd be 2 alnico polepiece-loaded coils, with a soft iron bar coupling them underneath. The premise was that there'd be a small mechanism for raising and lowering the bar, such that the two coils could be magnetically coupled...or not. Obviously, there are practical limits to how far away from the polepieces such a mechanism could move the bar, so it's not like the polepieces would have absolutely NOT coupling or influence through the bar, but at least it could be radically reduced. I figured I'd use different alnico rod types in each coil for the polepieces, just to lend even more variety.

the end result would be the following options:
both coils - coupled (series or parallel)
both coils - uncoupled (series or parallel)
coil A - coupled through other polepieces
coil B - coupled through other polepieces
coil A - uncoupled
coil B - uncoupled

That's a lotta stuff, there.

[Mike Sulzer]

I guess what I'm probably having some difficulty with is understanding specifically what you mean to denote by "sampling". It is entiely possible we are thinking of two different, or at least non-overlapping, ideas.

Consider a string on an electric guitar. It is magnetized to some magnitude in some direction in each small region along its length. Now consider a pickup coil. The magnetic field resulting from the magnetization of each small region along the string results in some flux passing through this coil. The degree to which a coil samples a particular region of the string is measured by the magnitude of the flux passing through the coil from that region compared to that from other regions. (Strong magnetization close to coil = strongly sampled)

Note that the concept of flux implies that when considering the field passing through the coil, it is the component of that field perpendicular to the loop of wire that counts.

[frankfalbo]

Drama, Frank, DRAMA.

Haha love it! Just don't have time for it right now, but I'll check the thread periodically.

...difference in the sound of two pickups with different apertures. Your statement implies that you already know the difference, but I have doubts about that since it is not that easy to eliminate other effects.

Not implying anything. I was trying to clarify whether that's what you were implying.

...Well, things you cannot understand through analysis and measurement, you just do. Not sure where the boundary between the two is in this case. Maybe we can learn something about that.

Let's just not forget that if a pickup sounds different the farther it gets from the bridge, then wider aperature includes audio from closer to the bridge and/or farther from the bridge. I realize that's logarithmic, in that moving a pickup 1/8" at the neck position is less different than the same 1/8" jog right near the bridge.

[David Schwab]

Consider a string on an electric guitar. It is magnetized to some magnitude in some direction in each small region along its length. Now consider a pickup coil. The magnetic field resulting from the magnetization of each small region along the string results in some flux passing through this coil. The degree to which a coil samples a particular region of the string is measured by the magnitude of the flux passing through the coil from that region compared to that from other regions. (Strong magnetization close to coil = strongly sampled)

Note that the concept of flux implies that when considering the field passing through the coil, it is the component of that field perpendicular to the loop of wire that counts.

Well Mike, some people, including myself, feel that it's the change in reluctance as the string disturbs the static field that causes the current to flow, and not so much the magnetized string. When everything is at rest the reluctance and the flux are constant.

Now for aperture, take a strat pickup and listen it to under the strings in the normal position. Now take that pickup and rotate it 90° to that it's parallel to the string, instead of perpendicular. Listen to it now.

Now its sensing a much longer partition of the string.

I make both wide and narrow aperture humbuckers, and a sidewinder. The apertures are like this:



Do you think they all sound the same, with the same windings and all? Now as you can see in the drawing, the wide aperture pickups also have thicker blades. But I have made them with the thin blades, and in fact just took the narrow aperture coils and spaced them father apart. Do you think they sounded the same? The sensing area has now changed, and all other factors have remained the same.. same magnets same cores, same coils.

The answer is no. They are very different sounding.

I'll be happy to dig up some sound clips, which I'm sure I have somewhere.