hum-buckers - how they function

[James Collins]

Anyone ever use a spectrum analyzer on their bass output signal? It would be interesting to look at the frequency sweep under various conditions.

Good read everyone, thanks for the physics refresher.

I've been doing a lot of reading on this lately. This is the best article I have read that does what you're asking about. The lower resistance pots reduce all frequency volumes. They just reduce the resonant peak frequency more because there is so much more volume to lose. The tone pot shifts the resonant peak frequency to lower frequencies. At least that is what my take is. Most other articles seem to indicate that your brain cannot comprehend what is going on and gloss over parts they don't seem to understand.

Link Removed

 

[zontar]

So what about Series/parallel?
I have a guitar with hum buckers that are wired for series/parallel for each humbucker.
You can also wire separate pickup series or parallel--whcih would work whether hum buckers or not.
But within a hum bucker they can also be series/parallel.

the Seymour Duncan site explains it better than I can-

Series and parallel wiring usually refers to two separated but related issues. The most common usage refers to how two coils in a humbucking pick up are connected to each other. With series wiring the individual coils are connected end to end. Current flows first through one coil and then the other. This is the way most humbucking pickups are wired. With parallel wiring the individual coils are connected to each other at both ends and current flows through both coils at the same time. Pickups wired in parallel are brighter sounding and have considerably less output than an identical pickup wired in series.

The terms series and parallel are also used to describe the way in which separate pickups are connected to each other in the guitar by the pick up selector switch. In the vast majority of guitars the pickups are connected to each other in parallel. The same rules apply to pick ups wired in series and parallel as a humbuckers coils wired in series and parallel. Two pickups wired in series will have higher output and a fuller tone.

What are series and parallel wiring?

 

[Amalgam]

How hard is it to coil tap a two coil humbucker into a split coil humbucker, like a P bass?

I'm looking at a Nordstrand Big Rig which is similar to a humbucking '51 P bass pickup, and I'm not sure what I want to do with it as far as series/parallel/single, or coil tap.

I'm planning on placing a Nordstrand Dual Coil flush also with parallel/series/single internal wiring. Kinda wondering how that'd work if they were both in single coil mode. Could I wire these two pickups together like a Jazz bass when they're in single coil mode? Or since they're so different is it a waste to try?

 

[BlueTalon]

Is the comb filter based on nodes?
and would it be possible to limit the amount of filtering with a strategic spacing?

Yes, and kinda. If you pick up a 51 style P bass and play the 5th harmonic (not the musical 5th, but the harmonic of an open string that has a wavelength of 15th of the open string length, you can hear it acoustically, but through the pickup (which is located on a node for that note), you'll get pretty much nothing out - that's one of the dips in that comb filter.

With more than one pickup, you can play a bit with the spacings of the pickups to try to minimize the worst part of the comb filtering, but it's kinda like a balloon - make it better here, it pops out more there. You pretty much have to decide what you think "better" means before going in to do that exercise (I've actually done that), because there is no magic place where everything gets better at once. OK, if you move the pickup under the bridge, then there is no comb filtering. But there is also no output.

Nodes move every time you fret your bass. That 1/5th of a string length is still 1/5th of a string length when you play a G on the E string, but the location of that node has changed relative to the pickup. Today's magnetic pickups have no way of tracking that change in position. If/when someone develops a pickup that uses lasers to detect string vibration rather than magnetic fields, and can change the point at which it is sensing the string based on what fret (if any) is being used, then the concept of using nodes to determine the optimum place to sense string vibration becomes feasible.

But I'm guessing that at that point, hum would no longer be an issue on such a bass, and so hum canceling wouldn't be necessary.

 

[Amalgam]

@BlueTalon I've looked into these, but it was a while back. The midi capabilities seem really interesting.

LightWave Optical Pickup System - Willcox Guitars

 

[James Collins]

How hard is it to coil tap a two coil humbucker into a split coil humbucker, like a P bass?

I'm looking at a Nordstrand Big Rig which is similar to a humbucking '51 P bass pickup, and I'm not sure what I want to do with it as far as series/parallel/single, or coil tap.

I'm planning on placing a Nordstrand Dual Coil flush also with parallel/series/single internal wiring. Kinda wondering how that'd work if they were both in single coil mode. Could I wire these two pickups together like a Jazz bass when they're in single coil mode? Or since they're so different is it a waste to try?

I don't think it would be a waste necessarily. What I end up finding is I like a couple of the settings most. Then you go from having 15 don't different settings to needing only 3 of them.

I would think the maximum choices would be to wire each pickup to a 3 way switch (parallel humbucker, series humbucker, single coil) then combine those into a 5 way switch that would do neck, neck and bridge parallel, bridge, neck and bridge series, neck and bridge out of phase. By my count that would give over 30 tone options. At some point it is like a Moser circuit that you need a diagram to operate.

Having a humbucker with a split coil p bass option are quad coils. Nordstrand makes one.
MM4.4 Quad Coil

 

[Doc Blue]

I've been doing a lot of reading on this lately. This is the best article I have read that does what you're asking about. The lower resistance pots reduce all frequency volumes. They just reduce the resonant peak frequency more because there is so much more volume to lose. The tone pot shifts the resonant peak frequency to lower frequencies. At least that is what my take is. Most other articles seem to indicate that your brain cannot comprehend what is going on and gloss over parts they don't seem to understand.

Link Removed

I'll take a look at the link, thanks.

If my hearing was any good I'd tune by ear. Having "moderate to severe" loss doesn't help (left ear is good at 500 hz, 1k and up is -50 to -60 db, right ear is good to 1k, same loss at higher freqs.). For all I know the tone I like could sound like **** to everyone else.

So here I am, trying to use science to tune art.

Edit. Good article. Gives me more insight into system performance, and reminded me of the stuff I learned 40 years ago.

 

[micguy]

Nodes move every time you fret your bass.

Yes, that's true, but your rationale past that misses one point: The comb filtering is a wavelength filter, and since the pickups don't move relative to the bridge, most of the comb filtering does not change when you fret a note. If the pickup is under a node for the 5th harmonic of an open string note, yes, when you fret a note, it's not under a node for the 5th harmonic of that note, but a few frets up the neck , it is under (or close to) the 4th harmonic of that note, and a few frets further up, it's under (or close to) the 3rd harmonic of that note, etc.....

The comb filter for a given string stays pretty well fixed as you move along the string, and thus its interactions with the note being played vary according to the position of the fret. As you point out, where the nodes are on an open string isn't where the nodes are all the time, but the comb filtering that results from pickup positioning and the string tuning can't be dismissed because you're fretting a note. It's always there.

 

[micguy]

I've been doing a lot of reading on this lately. This is the best article I have read that does what you're asking about. The lower resistance pots reduce all frequency volumes. They just reduce the resonant peak frequency more because there is so much more volume to lose. The tone pot shifts the resonant peak frequency to lower frequencies. At least that is what my take is. Most other articles seem to indicate that your brain cannot comprehend what is going on and gloss over parts they don't seem to understand.

Link Removed

Lower resistance pots do reduce the volume of everything by a bit - but, generally not much in the ranges we're talking about. They reduce the resonant peak more because, at resonance, the impedance of the pickup/capacitor combination goes up considerably - the higher impedance of that circuit at that frequency means it's easier to steal energy away from the system, so the little energy that the pot steals means the peak comes down.

The tone pot isn't what shifts thing frequency wise. The tone cap, when the tone control is set at minimum, loads the pickup with more capacitance (the cable and tone cap are in parallel at that point), and the resonant frequency goes down. The tone pot is a damping element in the circuit - it modifies things in a different way than the capacitors do.

 

[James Collins]

I think this is mincing words. Here is the diagram I was referring to from the linked website.

This is with constant capacitance. The vertical axis is output voltage. The horizontal axis is frequency. The peak is resonant peak frequency. The different lines are different resistance loads. Decreasing the load decreases all output voltages of all frequencies. These are actual measured output voltages. If it was some interplay of inductance or capacitance only certain frequencies would be affected...

...Like in this case where the capacitance is changed while the resistance is left constant.

The impedance should be lowest at the resonant peak.

 

[BlueTalon]

Yes, that's true, but your rationale past that misses one point: The comb filtering is a wavelength filter, and since the pickups don't move relative to the bridge, most of the comb filtering does not change when you fret a note. If the pickup is under a node for the 5th harmonic of an open string note, yes, when you fret a note, it's not under a node for the 5th harmonic of that note, but a few frets up the neck , it is under (or close to) the 4th harmonic of that note, and a few frets further up, it's under (or close to) the 3rd harmonic of that note, etc.....

The comb filter for a given string stays pretty well fixed as you move along the string, and thus its interactions with the note being played vary according to the position of the fret. As you point out, where the nodes are on an open string isn't where the nodes are all the time, but the comb filtering that results from pickup positioning and the string tuning can't be dismissed because you're fretting a note. It's always there.

How does a comb filter stay "pretty well fixed" as the bass is fretted up and down the neck? As a string is fretted, that string effectively gets shorter -- which means the comb filter based on stationary pickups becomes coarser in relation. (I'm not sure if "coarse" is the right word to describe it, but it works for hair combs.) How much coarser depends of course on which fret is being used.

But ultimately, I don't think it matters in anything more than a theoretical sense. With a new set of strings, there isn't any real tone/timbre change from one fret to the next. With the exception of the pitch change, playing a C or D or E on the A string sounds essentially the same as playing the open A string.

Consequently, I'm of the opinion that pickup position really doesn't matter as much as many here seem to think. For example, I think a P bass could have it's pickup located 1/4"-1/2" either way and nobody would ever notice. I understand I'm probably in a distinct minority, and that's OK. My main bass is a 3-pickup jazz, and I intentionally placed the pickups without regard to anything other than visual aesthetics (2" edge-to-edge), and it sounds great.

 

[micguy]

How does a comb filter stay "pretty well fixed" as the bass is fretted up and down the neck? As a string is fretted, that string effectively gets shorter -- which means the comb filter based on stationary pickups becomes coarser in relation. (I'm not sure if "coarse" is the right word to describe it, but it works for hair combs.) How much coarser depends of course on which fret is being used.

But ultimately, I don't think it matters in anything more than a theoretical sense.

When you pluck a string, you send 2 little pulses up and down the string - one goes toward the bridge, the other towards the nut, or if you've fretted a note, towards that fret. Sorry if this is hard to visualize - in Physics class we used to use high speed cameras or stroboscopes to see these things, so to me they're relatively easily to visualize. Anyway, whenever one of those pulses crosses the pickup on the way down the string (towards the bridge), it also (a specific time later) comes back from the bridge (in inverted polarity) and crosses the pickup again. No matter what fret you're using, given that the string is at a fixed tension, the time between those pulses is fixed. The two pulses form an impulse response, which is... a filter. A comb filter to be precise, because its a filter who's dips (and peaks) repeat every so many Hz. That filter depends on the propagation velocity of the string (which depends on its tension and linear mass density) and the position of the pickup(s). That comb filter does not depend on what fret is chosen, so it stays "pretty well fixed". Choosing a different fret changes the overall round trip time of the pulses, and therefore the pitch you hear, but the comb filter that determines the content of the harmonics that you hear does not change when you change to a different fret. It changes when you change strings, as the propagation velocity on different strings is different (It has to be if you want them to give you different notes).

Does this matter in anything more than a theoretical sense? Well, if I put a second pickup on the bass,and combine the output of both, I add impulses to the impulse response, which changes the comb filter. Does changing the resultant comb filter make a difference? People discuss this all the time here - a P has one pickup, and therefore a specific impulse response/comb filter. A J has, 2, so the impulse response/comb filter is different. If you play a P and J (both pickups on) and don't hear a difference, then.... it doesn't matter. If you do hear a difference, then yes, it matters. People her get excited about the change of position of the bridge pickup on a J - what that does is change the impulse response a bit (the impulses from the bridge pickup are closer together in time), and the comb filter is different as a result.

 

[byacey]

As well, with a pickup located close to the bridge, the pickup never "sees" the full excursion of the fundamental or second harmonic, which makes for a very bright, top end heavy signal, comprised mostly of 3rd harmonics and above.

 

[micguy]

As well, with a pickup located close to the bridge, the pickup never "sees" the full excursion of the fundamental or second harmonic, which makes for a very bright, top end heavy signal, comprised mostly of 3rd harmonics and above.

To “see” full excursion of the fundamental, you’d need to locate the pickup at the twelfth fret.

 

[BlueTalon]

When you pluck a string, you send 2 little pulses up and down the string - one goes toward the bridge, the other towards the nut, or if you've fretted a note, towards that fret. Sorry if this is hard to visualize - in Physics class we used to use high speed cameras or stroboscopes to see these things, so to me they're relatively easily to visualize. Anyway, whenever one of those pulses crosses the pickup on the way down the string (towards the bridge), it also (a specific time later) comes back from the bridge (in inverted polarity) and crosses the pickup again. No matter what fret you're using, given that the string is at a fixed tension, the time between those pulses is fixed. The two pulses form an impulse response, which is... a filter. A comb filter to be precise, because its a filter who's dips (and peaks) repeat every so many Hz. That filter depends on the propagation velocity of the string (which depends on its tension and linear mass density) and the position of the pickup(s). That comb filter does not depend on what fret is chosen, so it stays "pretty well fixed". Choosing a different fret changes the overall round trip time of the pulses, and therefore the pitch you hear, but the comb filter that determines the content of the harmonics that you hear does not change when you change to a different fret. It changes when you change strings, as the propagation velocity on different strings is different (It has to be if you want them to give you different notes).

Does this matter in anything more than a theoretical sense? Well, if I put a second pickup on the bass,and combine the output of both, I add impulses to the impulse response, which changes the comb filter. Does changing the resultant comb filter make a difference? People discuss this all the time here - a P has one pickup, and therefore a specific impulse response/comb filter. A J has, 2, so the impulse response/comb filter is different. If you play a P and J (both pickups on) and don't hear a difference, then.... it doesn't matter. If you do hear a difference, then yes, it matters. People her get excited about the change of position of the bridge pickup on a J - what that does is change the impulse response a bit (the impulses from the bridge pickup are closer together in time), and the comb filter is different as a result.

First things first, thank you for that response! That was actually very easy for me to visualize, because as you were describing it, the first thing I thought of was a garden hose. When you stretch it tight, you can jiggle it on one end and watch the resulting pulse travel up the length of the hose to the other end, and then watch the inverted pulse travel back. And depending on how firmly the two ends are held, the pulse can travel back and forth a few times before dying.

Sooo... If I'm understanding you correctly, then every magnetic pickup will always act as a comb filter, even a single-coil pickup on a one-pickup bass, because of the inverted pulse echo coming from the bridge? (In that specific instance, what if the string is plucked directly above the pickup? Would that reduce the effect?) So then, additional coils/pickups do not create the effect, they add complexity to the effect that's already there. That's the part I didn't get before, and it explains why the sound of a string sounds essentially the same as you travel up the fretboard.

So the concept of comb filtering is disassociated from nodes? My understanding still is that nodes move as you fret in different places. Also, is there any aspect of comb filtering that does change with where the bass is fretted? That pulse travels in both directions, and the time it takes to bounce of the fret end is variable depending on the fret. Or does it only matter on the end closest to the pickup?

Is it possible to have a perfect storm of comb filtering in which certain frequencies get canceled altogether?

 

[James Collins]

Is it possible to have a perfect storm of comb filtering in which certain frequencies get canceled altogether?

Yes. You can play one frequency out of phase with another and cancel the sound entirely. That is what happens with noise canceling.

 

[BlueTalon]

Yes. You can play one frequency out of phase with another and cancel the sound entirely. That is what happens with noise canceling.

I guess my question should have been, has anyone had that happen in a real world situation that they've noticed?

 

[byacey]

I never said or asked this:
byacey said: ↑
Is it possible to have a perfect storm of comb filtering in which certain frequencies get canceled altogether?

Yes. You can play one frequency out of phase with another and cancel the sound entirely. That is what happens with noise canceling.

 

[BlueTalon]

I never said or asked this:
byacey said: ↑
Is it possible to have a perfect storm of comb filtering in which certain frequencies get canceled altogether?

Yeah, I said it. He probably multi-quoted, and then made the mistake when he was shortening things.

 

[megafiddle]

When you pluck a string, you send 2 little pulses up and down the string - one goes toward the bridge, the other towards the nut, or if you've fretted a note, towards that fret. Sorry if this is hard to visualize - in Physics class we used to use high speed cameras or stroboscopes to see these things, so to me they're relatively easily to visualize. Anyway, whenever one of those pulses crosses the pickup on the way down the string (towards the bridge), it also (a specific time later) comes back from the bridge (in inverted polarity) and crosses the pickup again. No matter what fret you're using, given that the string is at a fixed tension, the time between those pulses is fixed. The two pulses form an impulse response, which is... a filter. A comb filter to be precise, because its a filter who's dips (and peaks) repeat every so many Hz. That filter depends on the propagation velocity of the string (which depends on its tension and linear mass density) and the position of the pickup(s). That comb filter does not depend on what fret is chosen, so it stays "pretty well fixed". Choosing a different fret changes the overall round trip time of the pulses, and therefore the pitch you hear, but the comb filter that determines the content of the harmonics that you hear does not change when you change to a different fret. It changes when you change strings, as the propagation velocity on different strings is different (It has to be if you want them to give you different notes).
...

The same results can also be arrived at by simply looking at the node locations above the pickup for different notes.

The A string is convenient for it's integral value of 55 Hz. With a pickup at one fourth the open string length, the open string will have harmonic nodes over the pickup at the 4th, 8th, and 12th harmonics (to exemplify just the first three). The respective frequencies are 220, 440, 660 Hz. The output is minimum at these frequencies, and represent notch frequencies in the comb filter. These notches continue upward at 220 Hz intervals.

The A at the 12th fret is also convenient for the same reason. This note, at half the speaking length, will have harmonic nodes above the pickup at the 2nd, 4th, and 6th harmonics. But the respective frequencies are the same as for the open string, 220, 440, 660, and represent the same comb filter response.

To look at one more example, a just intoned D (73.333... Hz) on the A string will have harmonic nodes above the pickup at the 3rd and 6th harmonics. Once again, the respective frequencies at 220 and 440, are two of the comb filter notch frequencies.

So the comb filter response remains fixed for a given open string length, open string pitch, and pickup location. What changes with different fretted notes, is which harmonic nodes become aligned with the pickup aperture, and therefore which harmonics are rejected for each individual note. For the pickup location above, the open string will retain its 2nd harmonic, while the 12th fret note will lose its 2nd harmonic.

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