Switchable additional winds

[Antigua]

Interesting thread, but here's something I never usually see discussed.

We tend to think of coil-tapping in terms of "additional winds". That is, wind enough to make a viable coil, then add some more turns to make a hotter coil, but retain the selectable connection to the point where the added turns begin. Because the additional turns are on the outside, the added circumference makes for greater resistance per turn.

But how does that compare to making the additional turns on the inside? For example, put 1500 turns snuggled up against the polepieces, run a lead out to a solder terminal, then add another, say, 6500 turns. I won't pretend to understand Eddy currents or magnetism in general, nearly as well as my colleagues here. But at the very least, the scenario I describe moves the "start" of the default coil outwards, and obviously changes the circumference of the turns.

So what I'm wondering is: how does the relocation of the additional turns to the "inside" change things with respect to inductance, resonant peaks, etc., and which would be the preferred arrangement - added coils on the outside, or the inside?

I can't answer this question directly, but I can tell you that when the inner and outer coils are the same turn count, the plots overlap almost identically:



Based on this, I'd think it wouldn't make much difference if you tapped inward versus tapping outward.

[Helmholtz]

I can't answer this question directly, but I can tell you that when the inner and outer coils are the same turn count, the plots overlap almost identically:



Based on this, I'd think it wouldn't make much difference if you tapped inward versus tapping outward.

Inductance of the windings is essentially determined by the square of the turns (with a small additional influence of dimensions, forget resistance). Inductive coupling forces the resonant frequencies of both coils to be identical (see link on coupled windings) even if their inductances differ.

@Antigua, you can improve S/N ratio of your plots by choosing the Automatic Voltage Scale option in the bode plotter of the PCSU 200.

[Mark Hammer]

Not to be too much of a pest, but would you expect those plots to be similar if we were talking about a flatter coil, like a Jazzmaster, as opposed to a coil where inner and outwer turns have very similar circumference?

I ask because the overall shape of the coil can have an influence on tone, so naturally I'm curious about whether coil shape/dimensions also have an impact on the inner/outer turns aspect. Like I say, I don't know enough about this stuff to have any opinion. I'm just going on superficial aspects of pickups.

[Steve A.]

3 wire vs. 4 wire tapped pickups

Perhaps this has been mentioned already but using the lower resistance tap with a pickup using 3 wires instead of 4 would leave the outer winds connected to the hot signal output but not terminated by a path to ground, a situation which I suspect would make the pickup more susceptible to noise as the unterminated winding would act as an antenna. Which would be another reason to use 4 eyelets and leads instead of 3.

Correct me if I am mistaken about that.

Thanks!

Steve A.

[Joe Gwinn]

>Not true. There is always some effect, and adding the interwinding layer reduces that effect. Interwinding layers are standard practice in transformer building. As are interwinding shields, for which one would can use metallized mylar. The shield metal must be grounded.<

Not convincing. In high frequency transformer applications open (non-terminated) floating windings are typically avoided.

We are talking audio transformers here, not high-frequency (like RF) here. And in RF floating windings are used, such as for feedback in oscillators.

A grounded interwinding shield effectively reduces interwinding capacitance (capacitive coupling between windings) but inevitably increases ground capacitance for both windings, which seems counterproductive in PUs.

True. People add some insulation between to reduce capacitance to ground (versus the other coil) if it's a problem.

Can you show measurements and/or an equivalent circuit?
BTW: I generally assume the inner start of the active pickup winding to be grounded.

No, I haven't built this recently. But what I'm saying is straight out of books on designing audio transformers. It's actually a complicated bunch of balanced compromises.

>It's true that the loss of the connected coil will increase. The intent is to make it flatter, to reduce coloration. This must be assessed by ear - instruments are not much help here.<

In my experience most guitar players prefer some coloration i.e. resonance peak. Otherwise they could as well back the the tone control a bit to get a similar result.

Then use a larger resistor, to allow some coloration from the floating coil through. As I said, the correct instrument here is a golden ear.

[David King]

Mark Hammer,

I can tell you that some pedal steel pickups designs leave an airgap of .015-.020" between the magnets and the inside (start) of the coil to kill off some highs. I'd imagine that switching out the inner coil tap would have the same effect. Never tried it but it might have a use on an otherwise too bright guitar.

[Helmholtz]

We are talking audio transformers here, not high-frequency (like RF) here. And in RF floating windings are used, such as for feedback in oscillators.

No new arguments here. I am familiar with scientific literature on transformers. And I know and understand the detailed equivalent circuit, which is the same for RF and LF applications. My assessments are based on this knowledge as well as basic physics.

If a winding is used for feedback purposes, it has to be connected/terminated. This creates a situation that is not comparable to a tapped PU with an open winding.

I repeat: I do not see how the interwinding capacitance (which is rather low for a cleanly wound PU anyway) could influence the actice coil's resonance. It is not in the circuit when the secondary is floating. And it is essentially shorted when the two windings are connected at the tap (3 wire type).

My general resume regarding tapped singlecoils, based on theory and the measurements of Antigua, is that tapping inevitably introduces unwanted additional capacitance in the active coil and thus tends to prevent getting really different sounds (i.e. resonant frequencies) from different configurations (full vs tapped).
If you just want an additional sound with a lower resonant frequency it is much more efficient (and easier) to hardwire a suitable capacitor across a non-tapped PU. And I would start out with a lower wind non-tapped PU to allow for a wider range of useful resonances.

[Helmholtz]

Not to be too much of a pest, but would you expect those plots to be similar if we were talking about a flatter coil, like a Jazzmaster, as opposed to a coil where inner and outwer turns have very similar circumference?

I ask because the overall shape of the coil can have an influence on tone, so naturally I'm curious about whether coil shape/dimensions also have an impact on the inner/outer turns aspect. Like I say, I don't know enough about this stuff to have any opinion. I'm just going on superficial aspects of pickups.

You state that the shape of the coil can have an influence on sound. The question is why. Is it really the shape of the coil or just caused by different resonances?
The sound difference between a Jazzmaster and Strat PU can be explained by different resonances (frequency and Q). I you want to study the single influence of coil shape, all other influencing factors - especially resonance - need to be equal for the two PUs. This can be achieved by varying the number of turns and - if necessary - additional capacitors and resistors.

[Joe Gwinn]

No new arguments here. I am familiar with scientific literature on transformers. And I know and understand the detailed equivalent circuit, which is the same for RF and LF applications. My assessments are based on this knowledge as well as basic physics.

If a winding is used for feedback purposes, it has to be connected/terminated. This creates a situation that is not comparable to a tapped PU with an open winding.

I repeat: I do not see how the interwinding capacitance (which is rather low for a cleanly wound PU anyway) could influence the actice coil's resonance. It is not in the circuit when the secondary is floating. And it is essentially shorted when the two windings are connected at the tap (3 wire type).

My general resume regarding tapped singlecoils, based on theory and the measurements of Antigua, is that tapping inevitably introduces unwanted additional capacitance in the active coil and thus tends to prevent getting really different sounds (i.e. resonant frequencies) from different configurations (full vs tapped).
If you just want an additional sound with a lower resonant frequency it is much more efficient (and easier) to hardwire a suitable capacitor across a non-tapped PU. And I would start out with a lower wind non-tapped PU to allow for a wider range of useful resonances.

Well, as I said upthread, all these effects are present to one degree or another, simultaneously. Various ways to increase or decrease this or that effect was discussed. So, one listens, and uses these tricks to steer the pickup towards a desired sound. The role of the theory is to give us clues as to what alterations are likely to move things toward or away from a desired direction.

[Helmholtz]

Well, as I said upthread, all these effects are present to one degree or another, simultaneously. Various ways to increase or decrease this or that effect was discussed. So, one listens, and uses these tricks to steer the pickup towards a desired sound. The role of the theory is to give us clues as to what alterations are likely to move things toward or away from a desired direction.

Yes, Sir. Right you are.

I love and support experimenting and listening tests. Experience and theoretical analysis of the dependencies may help to concentrate on the more promising experiments.
The easiest way to study the influence of the interwinding capacitance and other parameters of the tapped SC would be to simulate the appropriate equivalent circuit in LTSpice.

[Antigua]

The easiest way to study the influence of the interwinding capacitance and other parameters of the tapped SC would be to simulate the appropriate equivalent circuit in LTSpice.

I use LTSpice quite a bit. If you could propose an equivalent circuit for the apparent high capacitance of a tapped coil I would certainly give it a go. I did read the PDF you linked to discussing transformer capacitance, but I'm not 100% clear on how to translate that to the tapped coil model.

You said further up that you tested foil tape around a single coil, and did not get as high of a capacitance as a was apparent with the tapped coil, but isn't a secondary winding of a tapped pickup extremely close to the primary coil? Even foil tape will contain a little more distance between the windings than will the continued winding of the secondary coil. Being able to model this in LTSpice and see the "apparent" high capacitance without actually assigning a high capacitance would be very informative.

[Antigua]

Not to be too much of a pest, but would you expect those plots to be similar if we were talking about a flatter coil, like a Jazzmaster, as opposed to a coil where inner and outwer turns have very similar circumference?

I ask because the overall shape of the coil can have an influence on tone, so naturally I'm curious about whether coil shape/dimensions also have an impact on the inner/outer turns aspect. Like I say, I don't know enough about this stuff to have any opinion. I'm just going on superficial aspects of pickups.

The belief that the shape of the coil effects the tone isn't well founded. The coil merely translates flux change into a voltage. If anything about the geometry of a single coil were to effect the tone, it would have to be the pole piece, because that along with the string determine the flux change, and while guitar strings are geometrically simple and uniform, pole pieces can be had in a variety of shapes and sizes. For example, if the pole piece is a long "blade", even harmonics caused by side to side movement are suppressed. If the pole piece is very narrow, they're increased. If the pole piece were very wide enough,like half an inch or more, comb filtering would cancel out higher harmonics, when the field is about twice the width of those harmonic divisions along the string. Humbuckers do this, insofar as having two rows of small pole pieces is similar to one very wide pole piece.

[Antigua]

If you just want an additional sound with a lower resonant frequency it is much more efficient (and easier) to hardwire a suitable capacitor across a non-tapped PU. And I would start out with a lower wind non-tapped PU to allow for a wider range of useful resonances.

The thing about that is that if you use a cap to get the "full" sound, you wouldn't get an associated voltage boost for having more turns of wire in series. Maybe an inductor across the pickup can be used to "unwind" it, but the issue with that is that the load decreases the resonance. Maybe if you used a 1meg volume and no-load tone pot, you could retain enough resonance to allow a parallel inductor to work effectively, but then you'd need to replace it with parallel resistance when you switched the inductor out of the circuit, which isn't too much to ask. The harder task is finding a suitable inductor. The Bill Lawrence "Q Filter" is an inductor that supposedly works well, but I don't know too much about it.

OTOH, even with just a cap, I think you can get really close to P-90 sound with a Strat if you pick a cap value that brings the resonant peak down to about 2kHz. I suspect the reason that's not often done is that people associate tone with the guitar as a whole, they don't expect a Strat with three little pickups to have a big, warm P-90 tone. Similarly, you rarely see Les Pauls fitted with Fender single coils.

[Helmholtz]

I am well aware of all this and if the loudness step is what you are after, my proposal won't do.

My point is, that the unavoidable inductive coupling between the winding parts prevents a tappable overwound SC from convincingly reproducing the brighter sounds a a lower or "standard" wound PU, no matter how you split the coil or separate the windings, as the unused part of the coil always increases the effectice capacitance across the active coil by a considerable amount. As a result the two sounds available won't differ much - except when using very long guitar cables. But find out yourself.

The parallel capacitors method, instead, allows selective tuning of the resonance and thus for really different sounds without loudness steps, which I consider favorable.

In my experience the sound and dynamics of a good P-90 cannot be emulated by a Strat type design with alnico cores. The passive filter characteristics of a PU cannot explain the total response of a PU, even if the aperture is equal. But this would be a completely different topic.

[Mark Hammer]

I'm the last one to impart magical properties to shape. However, coil shape is a "marker" for other things that can be predicted to affect the electrical properties, as in Helmholtz' example of resonance. Certainly the properties of the magnet or polepieces (or slogs) play a significant role, however their role is not in spite of the coil properties, but in concert with them.

I am assuming that if I used normal Strat polepieces, and a conventional number of turns with very conventional wire and tension, but inserted a spacer between the polepieces and coil, such that the coil really only "started" 1/8" or 3/16" out from the polepieces (which, of course, would not fit inside the usual plastic pickup cover), it would likely end up sounding different, if only because of the distance of so many turns from the polepieces themselves. If a pickup has 800 additional turns that are switch-selectable, I would not expect that pickup to sound much different when those turns are added from the location closest to the polepieces vs the outside of the overall coil, simply because so few turns creates a negligible difference in distance from the polepieces.

But when one has a flatter pickup, like a Jazzmaster or even P90, it doesn't take as many turns to create distance from the polepieces (or center bar and slugs). And I would think that where perhaps it might not make that much difference if additional turns were situated on the inside or the outside of a Strat-type pickup, they just might for a flatter wider profile. Again, taking into consideration that there are more, and less, consequential numbers of additional turns.