Achieving a given pickup inductance via parallel vs series

[sumitagarwal]

I've been looking at a lot of stack and rail Strat-sized pickups and notably they almost all have higher winds/inductance than vintage single coils for a darker and higher output. Many guitarists prefer this result, but for those that want a brighter lower output a common recommendation is to wire them parallel instead of in series.

It made me wonder: what is the difference in result between achieving a given inductance via coils in series versus coils in parallel?

For example, two 3H coils in parallel should be around 1.5H,or two 0.75H coils could be out in series for about the same 1.5H. How would these two configurations sound or behave differently?

[stratotarts]

The inductance is the primary factor influencing the response. However, the factors that determine inductance do not only change the inductance. Because for example, if there are more wire turns, the physical geometry is changed, parts of the coil intersect different parts of the string field and so on. Capacitance will increase with more wire turns, and so on. But those side effects are very subtle compared with the main factor, inductance. DC resistance will increase with the wire turns.

The problem with your example, the ratio of the wind counts needed to differentiate the two models are so extreme, that you are kind of black box testing the minor factors but in an aggregate way. To understand those factors, you need to design experiments that will separate them out and test them individually. Because, for a "pure" implementation, i.e. "perfect" pickups, there would be NO difference between the two models.

By the way, a stack will always have more wire turns simply because there are two coils, and limited space to wind on. Since the bottom coil doesn't contribute to the output, the overall output level will be less, and so designers must add more turns (usually with finer wire) to compensate so the pickups aren't perceived as weak sounding.

[sumitagarwal]

Mar 6, 2024 9:32:08 GMT -5 stratotarts said:

The inductance is the primary factor influencing the response. However, the factors that determine inductance do not only change the inductance. Because for example, if there are more wire turns, the physical geometry is changed, parts of the coil intersect different parts of the string field and so on. Capacitance will increase with more wire turns, and so on. But those side effects are very subtle compared with the main factor, inductance. DC resistance will increase with the wire turns.

The problem with your example, the ratio of the wind counts needed to differentiate the two models are so extreme, that you are kind of black box testing the minor factors but in an aggregate way. To understand those factors, you need to design experiments that will separate them out and test them individually. Because, for a "pure" implementation, i.e. "perfect" pickups, there would be NO difference between the two models.

Thanks, yes that makes sense. Based on what you're saying I think some of the implications would be:

• Higher wind would sense a larger string area
• Higher wind would be a larger target for interference and pick up more noise
• Higher wind would have more total capacitance - this should have a dulling effect on the sound, right?

[stratotarts]

No, a higher wind count does not in itself, mean that it will be more prone to interference. That has more to do with size of the coil's internal area. However in a noise cancelling design, that may not matter very much if the noise cancelling method is well engineered.

You need to look at parameters in the context of the entire guitar circuit including controls. Yes, the capacitance makes a difference. But the self capacitance of a typical pickup is much less than that of the circuit it's in. Thinking again - for your example, the change in capacitance would be the biggest difference between them, because parallel wiring has about 4 times the capacitance as series.

The part about sensing a larger string area is controversial, or at least subtle. It does not automatically follow that it would affect the sound audibly. It has been proven that the sensing area does make a difference, but I think not on the scale of a few millimeters such as would be the case with comparing a low wind and high wind on the same pickup frame.

[ms]

The string is sensed  over the pole piece; the location of the sensing changes little with changes in the winding.

[antigua]

Mar 6, 2024 8:03:44 GMT -5 sumitagarwal said:

I've been looking at a lot of stack and rail Strat-sized pickups and notably they almost all have higher winds/inductance than vintage single coils for a darker and higher output. Many guitarists prefer this result, but for those that want a brighter lower output a common recommendation is to wire them parallel instead of in series.

It made me wonder: what is the difference in result between achieving a given inductance via coils in series versus coils in parallel?

For example, two 3H coils in parallel should be around 1.5H,or two 0.75H coils could be out in series for about the same 1.5H. How would these two configurations sound or behave differently?

I think coil tapping is a good way to get two choices of inductances. The most popular tapped coil on the market are Seymour Duncan's offering, tapped SSL-3, SSL-4 or SSL-5 but they split the coil 50% and 100% which IMO are far from idea choices. The 50% would be weak and the 100% would be dark and dull, no middle ground.  I bought a Chinese Tele neck pickup with tapping of 3.1H or 2.3H  www.tdpri.com/threads/waaah-music-stores-red-colored-tele-pickups.1093066/ , and the split is about 75% and 100%

This gives me an idea though, a Rail style humbucker could be designed more around the idea of being a two tone single coil, where you have one blade/coil with an inductance of 2 henries, and another blade/coil with 1 henry, which could be split to 2h or series to 3h, and then you'd get a choice of vintage or "hot" single coil tones. In series it would humbuck partially also. The Fender Fireball pickup in the Meteora model works on this basis, but in a full sized PAF form factor. I think in a single coil sized rail humbucker it could be a killer single coil stand-in product. 

Regarding the coil size and sensing area, my understanding is that the coil has almost nothing to do with the sensing window. It's acting as an antenna, which if placed far away, pickup up little sound, and if placed close, it picks up more sound. Having the coil be wrapped around the pole pieces is just the most idea place that it could be, but in fact there are pickups that place the coil outside, such as the TV Jones / Gretch HiLoTron.

Suppose that you magnetize just the bridge pickup, but you activate the neck pickup, and also suppose that the neck pickup has no magnetization at all. Very faintly, you would hear the sound of a bridge sound from the neck pickup, because that's where the string that is being magnetized. The magnetized portion of string is broadcasting a signal.

Although, if a coil is arranged in such a way that it receive both positive and negative flux at the same time, which must happen to some degree, since the coil and the guitar string are not in the same exact place, then you get some proportionate cancelation. That cancellation shouldn't be especially frequency dependent, unless the part of the signal that cancels and the parts that does not are of difference frequency profiles. To some small extent that would happen, because the guitar string's harmonic output varies along its length. If more content from further down the string cancels as opposed to content at the bridge, then more high end treble will have been preserved and lower harmonics will have cancelled out. 

[sumitagarwal]

Mar 7, 2024 0:46:46 GMT -5 antigua said:

Mar 6, 2024 8:03:44 GMT -5 sumitagarwal said:

I've been looking at a lot of stack and rail Strat-sized pickups and notably they almost all have higher winds/inductance than vintage single coils for a darker and higher output. Many guitarists prefer this result, but for those that want a brighter lower output a common recommendation is to wire them parallel instead of in series.

It made me wonder: what is the difference in result between achieving a given inductance via coils in series versus coils in parallel?

For example, two 3H coils in parallel should be around 1.5H,or two 0.75H coils could be out in series for about the same 1.5H. How would these two configurations sound or behave differently?

I think coil tapping is a good way to get two choices of inductances. The most popular tapped coil on the market are Seymour Duncan's offering, tapped SSL-3, SSL-4 or SSL-5 but they split the coil 50% and 100% which IMO are far from idea choices. The 50% would be weak and the 100% would be dark and dull, no middle ground.  I bought a Chinese Tele neck pickup with tapping of 3.1H or 2.3H  www.tdpri.com/threads/waaah-music-stores-red-colored-tele-pickups.1093066/ , and the split is about 75% and 100%

This gives me an idea though, a Rail style humbucker could be designed more around the idea of being a two tone single coil, where you have one blade/coil with an inductance of 2 henries, and another blade/coil with 1 henry, which could be split to 2h or series to 3h, and then you'd get a choice of vintage or "hot" single coil tones. In series it would humbuck partially also. The Fender Fireball pickup in the Meteora model works on this basis, but in a full sized PAF form factor. I think in a single coil sized rail humbucker it could be a killer single coil stand-in product. 

Regarding the coil size and sensing area, my understanding is that the coil has almost nothing to do with the sensing window. It's acting as an antenna, which if placed far away, pickup up little sound, and if placed close, it picks up more sound. Having the coil be wrapped around the pole pieces is just the most idea place that it could be, but in fact there are pickups that place the coil outside, such as the TV Jones / Gretch HiLoTron.

Suppose that you magnetize just the bridge pickup, but you activate the neck pickup, and also suppose that the neck pickup has no magnetization at all. Very faintly, you would hear the sound of a bridge sound from the neck pickup, because that's where the string that is being magnetized. The magnetized portion of string is broadcasting a signal.

Although, if a coil is arranged in such a way that it receive both positive and negative flux at the same time, which must happen to some degree, since the coil and the guitar string are not in the same exact place, then you get some proportionate cancelation. That cancellation shouldn't be especially frequency dependent, unless the part of the signal that cancels and the parts that does not are of difference frequency profiles. To some small extent that would happen, because the guitar string's harmonic output varies along its length. If more content from further down the string cancels as opposed to content at the bridge, then more high end treble will have been preserved and lower harmonics will have cancelled out. 

Yea, I really think the possibilities of rail pickups for selectable inductance is really under-exploited... in the space of a Strat single you can easily have four outputs on offer: "dark boost pedal" full series, hot coil, cool coil, and jangly parallel. With four of them in the space of a humbucker the possibilities quickly get dizzying (or freeing, depending on your perspective).

You description of the strings as transmitters and coils as receivers was a breakthrough in my understanding of electric guitars! Thank you for that. I feel like it also helps me understand some other designs or aspects that I was struggling with. For example I now understand the DiMarzio Bluesbucker design with magnets under one coil but none under the "dummy coil". It also gives me some thoughts and ideas as to why it's so hard to replicate the classic Strat sound with noiseless designs like rails, since a rail would inherently be magnetizing a larger area of the string which should result in comb filtering even when splitting coils (the proportion of signal would still follow inverse square law, and the rail coils just aren't that far apart from each other). Accordingly I would think that designs that allow the pickups to be placed closer to the strings would maximize the inverse square difference for a more "narrow" single-coil tone, right?

[stratotarts]

I've never seen a Strat sized quad. The ones I saw were humbucker sized.

[sumitagarwal]

Mar 7, 2024 17:50:05 GMT -5 stratotarts said:

I've never seen a Strat sized quad. The ones I saw were humbucker sized.

Sorry, should have clarified that for that I was referring to the humbucker-sized ones

[stratotarts]

Like this?

[sumitagarwal]

Mar 8, 2024 8:49:52 GMT -5 stratotarts said:

Like this?

Yup, that's right. Although some quads like the DiMarzio MultiBucker expose leads for each coil.

Both the MultiBucker and the Godin Tetrad (first version, with rails) are really just two Strat-sized double rail pickups together. In fact, now that I have them in hand I see that the Godin Tetrad are literally two Strat-sized double rail pickups separately screwed to the guitar body.

Now that with antigua 's help I understand the relationship between the magnets and coils much better, I have some additional thoughts on rails and other designs.

First is that it's inherently difficult to get a proper single-coil sound out of a split PAF-sized pickup with two sets of poles, because while the split can get the inductance and resonant peak right you still have two magnetized sections of string in close proximity that will introduce comb filtering.

The DiMarzio Bluesbucker is a very interesting take on this issue, magnetizing only one bobbin and allowing the other to be just a dummy coil:




Unfortunately for me, it's wound and designed for a darker hotter P-90 voice rather than glassy Strat voice.

For aesthetics most "single-coil sounding" PAF-sized hum-cancelling pickups seem to stick the pole pieces along the center of the pickup, in either a stack design or with smaller bobbins around a larger central one (DiMarzio Phantom P90, Duncan Silencer P90). But the Bluesbucker approach seems marginally better because in most guitars it would allow the magnetized portion of the string to be closer to the 24th fret harmonic as is desired by most people and moves the hum-cancelling coil slightly further away from the magnetized portion of the string.

I wonder if this same approach could be applied to Strat-sized rail pickups by simply eliminating the second rail from the second coil. Or could it even be possible to place a single rail in between the two coils? Or would that simply cause the coils to cancel each other out?

[ms]

"First is that it's inherently difficult to get a proper single-coil sound out of a split PAF-sized pickup with two sets of poles, because while the split can get the inductance and resonant peak right you still have two magnetized sections of string in close proximity that will introduce comb filtering."

The comb filtering happens when the outputs from the two coils are added.  If you are using just one of the two coils you do not have that.

The problem is getting a high enough Q with steel rather than Alnico poles.

[sumitagarwal]

Mar 8, 2024 10:20:01 GMT -5 ms said:

"First is that it's inherently difficult to get a proper single-coil sound out of a split PAF-sized pickup with two sets of poles, because while the split can get the inductance and resonant peak right you still have two magnetized sections of string in close proximity that will introduce comb filtering."

The comb filtering happens when the outputs from the two coils are added.  If you are using just one of the two coils you do not have that.

The problem is getting a high enough Q with steel rather than Alnico poles.

Thanks! That makes sense, that the magnetized area of the string would be continuous but the distance between the string and the coils would differ slightly and introduce comb filtering. Am I understanding that correctly?

[stratotarts]

No, it's that when the signals from the two coils is added, signals of opposite polarity cancel out. In reality those are harmonic nodes on the string. Suppose the fundamental is A, the signal is A+A, the same in each coil because the string is moving in the same direction in relation to each coil. So, 2A. But harmonic B is a much higher vibration of the string, such that the crest of a wave is opposite one coil, and the valley is opposite the other - so one is B and the other is (-B). When added, B+(-B) = 0. They cancel each other. This action depends on short wavelengths, so the effect is that generally, higher harmonics cancel out more than the fundamental and the lower harmonics.

Consider the article by Tillman, linked in the reference thread:

till.com/articles/PickupResponse/index.html

[antigua]

Mar 8, 2024 10:27:37 GMT -5 sumitagarwal said:

Thanks! That makes sense, that the magnetized area of the string would be continuous but the distance between the string and the coils would differ slightly and introduce comb filtering. Am I understanding that correctly?

The string moves up and down in a broad movement, that's the fundamental, it represents the lowest bass frequency. But then all the treble you hear is harmonic movement. In the vibrating string the harmonics are all moving up and down at the same time, at different points along the string, in intervals based on the harmonic number. You can get an idea of this from this graphic:



As an aside, when you do a harmonic at the 12th fret, the fundamental, the largest one at the top, gets snuffed out, but all the higher harmonics keep on going. 

For all the harmonics, if the string is moving up over here, they it has to be moving down over there. The comb filtering happens when  you have two coils, and a given harmonic is moving up over one coil, but down over the other, that particular harmonics cancels itself out. The distance between the coils determines what harmonic intervals / frequencies will cancel, because this is a geometric situation between the placement of coils and the physical string width of the moving harmonics. When you combine two pickups, like neck and bridge, the pickups are so far apart that there is significant audible comb filtering, and that's a defining aspect of that sound, but with a PAF style humbucker, the coils are so close together than comb filtering is only at high frequencies and is non existent for all intents and purposes 

But there's also just the fact that you get more treble harmonics nearer to the bridge, regardless of whether you have one coil or two or four, so if you have a a pickup like a Lace Dually, of course the coil that's closer to the bridge gets more treble than the coil that's further away. With a PAF style humbucker, you get a combination of the bridge sound that is both near and far from the bridge, which gives it a richer sound, having more base and more treble and a single coil, as well as more overall output. 


 

[aquin43]

The comb filtering due to adding two pickups is usually shown as continuing right up to the highest harmonics. The impulse response of such a filter would be two time spaced identical impulses for one input pulse. That may nearly be true for a humbucker pole spacing but we know that the travel of the wave on the string is dispersive, i.e. the impulse changes shape as it travels. So the two output pulses in the impulse response will have different shapes. The pure cancellation at short wavelengths seems unlikely.

[ms]

Mar 9, 2024 5:46:16 GMT -5 aquin43 said:

The comb filtering due to adding two pickups is usually shown as continuing right up to the highest harmonics. The impulse response of such a filter would be two time spaced identical impulses for one input pulse. That may nearly be true for a humbucker pole spacing but we know that the travel of the wave on the string is dispersive, i.e. the impulse changes shape as it travels. So the two output pulses in the impulse response will have different shapes. The pure cancellation at short wavelengths seems unlikely.

Some years ago I measured several db of comb depth on the E6 string with a humbucker.  It is a difficult measurement.

[aquin43]

I was thinking more of widely spaced pickups where dispersion has time to do its work.

Treated as a filter, it is an unusual kind. In the steady state it can remove harmonics but without phase shift, as far as I can see. On the other hand, the response to an impulse is two impulses which is unlike any of the usual filters that we come across in the analogue domain.

[stratotarts]

Mar 9, 2024 5:46:16 GMT -5 aquin43 said:

The pure cancellation at short wavelengths seems unlikely.

Yes, the instrument would be frustrating to play if that were so. Because of the fretting in different places, notes would have effectively arbitrary volume. I think it's reasonable to believe that the effect exists, but is diffuse due to the phase spreading that you mentioned. In other words, affects all notes almost equally (with a gradation from very low to very high). Tillman's diagrams show the behaviour of an extremely simplified mathematical model.

[sumitagarwal]

Thanks for all the education on this!

So from the sounds of it if a traditional humbucker has everything too close together to create comb filtering within the audible range, then it would have to be that for a Strat-sized dual blade that's even more of the case.

Which would leave the tonal difference, as stated, largely due to the difference in Q from Alnico pole pieces versus having a bar magnet on the bottom with two metal rails that aren't magnets, right?

I'm not aware of any dual-rail pickups that use alnico magnets as the rails themselves. I assume this is for some reason not viable?

Zexcoil Z-Series use metal pole pieces but then do use individual magnets at the bottom of each pole. Would this give a a result more similar to traditional alnico pole pieces instead of rails?

[ms]

Mar 9, 2024 13:38:15 GMT -5 stratotarts said:

Mar 9, 2024 5:46:16 GMT -5 aquin43 said:

The pure cancellation at short wavelengths seems unlikely.

Yes, the instrument would be frustrating to play if that were so. Because of the fretting in different places, notes would have effectively arbitrary volume. I think it's reasonable to believe that the effect exists, but is diffuse due to the phase spreading that you mentioned. In other words, affects all notes almost equally (with a gradation from very low to very high). Tillman's diagrams show the behaviour of an extremely simplified mathematical model.

The short wavelengths are a very small fraction of the energy of the string vibration.  I do not think that they affect the perceived volume very much.  

I think that more rapid damping of the higher harmonics is a more important effect than dispersion.  Yes, the higher harmonics are not exact multiples of the fundamental but they are not that much off.

[stratotarts]

Mar 9, 2024 15:14:39 GMT -5 sumitagarwal said:

I'm not aware of any dual-rail pickups that use alnico magnets as the rails themselves. I assume this is for some reason not viable?

Well, the Firebird pickups do. They are not rail pickups per se, but internally they have alnico bars inside the bobbins. There are multiple reasons why not rails. First, manufacturers hate to use multiple magnets because they are a big part of the pickup cost. But also, the magnets are brittle so would break easily if left exposed. You may say, what about the round magnets? I think those are harder to break because they are thicker and don't have corners, only edges. Aesthetics probably comes into it as well. Exposed bars wouldn't really look very good IMHO.

Forgot to mention, manufacturability. That's really what limits the Firebird design. It is difficult to position all that stuff without a frame, and hold it in place. The Firebird PU uses the cover as a kind of fixture to hold everything together, assisted by the subsequent potting.

[stratotarts]

Mar 9, 2024 15:14:39 GMT -5 sumitagarwal said:

Zexcoil Z-Series use metal pole pieces but then do use individual magnets at the bottom of each pole. Would this give a a result more similar to traditional alnico pole pieces instead of rails?

No. The Zexcoil designer has explained that the poles material is the main determinant of the sound of those pickups. In fact, he offers different pole materials as options. But they are fundamentally like steel poles on other pickups, because they are all different formulations of steel (last time I looked at the Zexcoil site IIRC). The Neo's don't play much of a role, other than to effectively magnetize the steel.

[sumitagarwal]

Mar 11, 2024 15:20:31 GMT -5 stratotarts said:

Mar 9, 2024 15:14:39 GMT -5 sumitagarwal said:

I'm not aware of any dual-rail pickups that use alnico magnets as the rails themselves. I assume this is for some reason not viable?

Well, the Firebird pickups do. They are not rail pickups per se, but internally they have alnico bars inside the bobbins. There are multiple reasons why not rails. First, manufacturers hate to use multiple magnets because they are a big part of the pickup cost. But also, the magnets are brittle so would break easily if left exposed. You may say, what about the round magnets? I think those are harder to break because they are thicker and don't have corners, only edges. Aesthetics probably comes into it as well. Exposed bars wouldn't really look very good IMHO.

Forgot to mention, manufacturability. That's really what limits the Firebird design. It is difficult to position all that stuff without a frame, and hold it in place. The Firebird PU uses the cover as a kind of fixture to hold everything together, assisted by the subsequent potting.

I know that PRS aren't always looked at with kindness outside of PRS Land (where they are looked at with too much kindness), but they seem to be attempting more interesting pickups than most of the big names.

The new Narrowfield DD pickups in the NF-53 are reported to sound quite good, and they use 12 rectangular magnetic pole pieces with 8 metal inserts in between them. Not exactly the same as using magnet rails, but possibly similar in practice?

[ms]

8 ==> 10

If the "in between" pieces are not magnetized, you need take into account the possibility of decrease in volume with string bends.  So there might be more to the design than meets the eye.  These are taller, thinner bobbins, and so I guess that you need lots of steel to make sure that the windings furthest from the strings are effective (unlike the Alnico in Fender SC, which allows field to escape).

[antigua]

Mar 9, 2024 5:46:16 GMT -5 aquin43 said:

The comb filtering due to adding two pickups is usually shown as continuing right up to the highest harmonics. The impulse response of such a filter would be two time spaced identical impulses for one input pulse. That may nearly be true for a humbucker pole spacing but we know that the travel of the wave on the string is dispersive, i.e. the impulse changes shape as it travels. So the two output pulses in the impulse response will have different shapes. The pure cancellation at short wavelengths seems unlikely.

Another factor I forgot about, if we're talking about higher frequency harmonics, they don't last long after the transient. So if you say the comb filtering only effects higher harmonics, then it will be limited to a "pick attack" modification of the sound.

I think low harmonics are definitely filtered as the math describes, because you can fake the sound of "notch positions" with a graphic EQ by copying the comb filter, but not only that, if not for the comb filtering, the notch positions would sound more like a true blend of constituent pickup signals.

[antigua]

Mar 9, 2024 15:14:39 GMT -5 sumitagarwal said:

Thanks for all the education on this!

So from the sounds of it if a traditional humbucker has everything too close together to create comb filtering within the audible range, then it would have to be that for a Strat-sized dual blade that's even more of the case.

Which would leave the tonal difference, as stated, largely due to the difference in Q from Alnico pole pieces versus having a bar magnet on the bottom with two metal rails that aren't magnets, right?

I'm not aware of any dual-rail pickups that use alnico magnets as the rails themselves. I assume this is for some reason not viable?

Zexcoil Z-Series use metal pole pieces but then do use individual magnets at the bottom of each pole. Would this give a a result more similar to traditional alnico pole pieces instead of rails?

The Mojotone Quiet Coil is a rail with AlNiCo bars, like a miniature Firebird pickup. What makes it less than viable is the low permeability and lower strength of the bar magnet form factor of AlNiCo as compared to steel. The sound is weaker than what you get with the larger round AlNiCo pole pieces. I made a post about the pickups a few years ago. 

For Zexcoils, the individual magnets don't mean much other than to control the polarity pole by pole. The Q factor would be higher because the potential for eddy currents is lower with those thin steel blades. I'm not familiar with the idea that they use different steel grades to get different sounds, but I think all an all it would just mean a weaker pickup instead of a stronger pickup when you opt to use a steel with lower permeability. The steel has just one role to fulfil, which it either does well or it does poorly. 

 

[antigua]

Mar 11, 2024 15:20:31 GMT -5 stratotarts said:

Mar 9, 2024 15:14:39 GMT -5 sumitagarwal said:

I'm not aware of any dual-rail pickups that use alnico magnets as the rails themselves. I assume this is for some reason not viable?

Well, the Firebird pickups do. They are not rail pickups per se, but internally they have alnico bars inside the bobbins. There are multiple reasons why not rails. First, manufacturers hate to use multiple magnets because they are a big part of the pickup cost. But also, the magnets are brittle so would break easily if left exposed. You may say, what about the round magnets? 

My understanding is that because AlNiCo is a self reinforcing magnet, where one side of the magnet helps keep the other side magnetized, They tend to work better when they're long, with the poles at the far ends. The cylinder shape is probably close to ideal with a 1:4 or 1:5 length to width ratio. I think this also explains why the bar magnets have such non uniform flux density along the length. 

[antigua]

Mar 11, 2024 20:59:29 GMT -5 sumitagarwal said:

I know that PRS aren't always looked at with kindness outside of PRS Land (where they are looked at with too much kindness), but they seem to be attempting more interesting pickups than most of the big names.

The new Narrowfield DD pickups in the NF-53 are reported to sound quite good, and they use 12 rectangular magnetic pole pieces with 8 metal inserts in between them. Not exactly the same as using magnet rails, but possibly similar in practice?

I read about them here prsguitars.com/blog/post/the_story_of_narrowfield_pickups , PRS's market position is to be for lawyers who can't decide if they like Fenders or Gibsons more because they love Hendrix and Clapton with equal amounts of zeal. So their mini humbucker design continues in the tradition of uncreatively averaging the a Fender and a Gibson design. 

About the metal in between pole pieces, we see that with the DiMarzio inter-pole piece slugs that they actually have a patent for. I would assume it's just steel pieces all the way across with a magnet underneath, because anything else would cost a lot more to produce and just make the pickup not work as well. 

If you took a blade single coil humbucking pickup, and cut little divisions in the blades, the magnetic field wouldn't change much at all, you're just adding some trivial air gap in inconsequential places. But the more important thing would be in terms of conductivity, this could reduce eddy currents to a large degree and increase the Q factor. Knowing PRS, I'm not sure that was even intentional on their part. I googled "Narrowfield DD pickups q factor eddy currents" and there's no evidence of these words all appearing on the same web page at once, at least until I press Post in a moment.

That NF-53 looks pretty nice, but I'd wager dollars to doughnuts that PRS is probably getting help from Dall-E 2 for it's latest amalgamations. 

[ms]

Mar 12, 2024 10:12:37 GMT -5 antigua said:

Mar 9, 2024 5:46:16 GMT -5 aquin43 said:

The comb filtering due to adding two pickups is usually shown as continuing right up to the highest harmonics. The impulse response of such a filter would be two time spaced identical impulses for one input pulse. That may nearly be true for a humbucker pole spacing but we know that the travel of the wave on the string is dispersive, i.e. the impulse changes shape as it travels. So the two output pulses in the impulse response will have different shapes. The pure cancellation at short wavelengths seems unlikely.

Another factor I forgot about, if we're talking about higher frequency harmonics, they don't last long after the transient. So if you say the comb filtering only effects higher harmonics, then it will be limited to a "pick attack" modification of the sound.

I think low harmonics are definitely filtered as the math describes, because you can fake the sound of "notch positions" with a graphic EQ by copying the comb filter, but not only that, if not for the comb filtering, the notch positions would sound more like a true blend of constituent pickup signals.

I think he is saying that the comb filtering resulting from two pickups (say neck and bridge) works for low harmonics, but not for higher ones because of a kind of "blurring" resulting from dispersion.  ("Blurring" might not be a great description, but it is all I can think of now.)    But your decay time argument is right. At a high enough harmonic, or frequency, the pattern is never set up because the energy is gone before enough round trips can occur.  But as long as the energy is there, I think it does.  A given string harmonic corresponds to a very narrow frequency range, and so there is a well defined phase velocity.