Switchable additional winds

[Antigua]

Not it's it's open. A capacitor has each conductor connected in the circuit, and the dielectric between them. If you lift one leg on a cap, say in a tone control circuit, what do you get? Nothing.

The other end of the cap is the primary coil. You know a metal cover capacitively couples with the coil, no dispute there. Now consider that the secondary relates to the primary exactly as a metal cover does, with the double jeopardy of being much closer to the primary than any cover, (save for copper tape wrapped coils, a common practice in low priced Chinese pickups).

[Antigua]

Seems like this could be settled with a quick experiment. Then again that might short circuit a lot of discussion based on not totally well understood science. Carry on!

I do have experimental evidence, with a tappable SSL-3, SSL-4, SSL-5 and a Stra-Bro 90. You just take the resonant peak and the inductance of the tap, from that you can determine the capacitance.

[David Schwab]

The other end of the cap is the primary coil. You know a metal cover capacitively couples with the coil, no dispute there. Now consider that the secondary relates to the primary exactly as a metal cover does, with the double jeopardy of being much closer to the primary than any cover, (save for copper tape wrapped coils, a common practice in low priced Chinese pickups).

Right, but primary coil is a complete circuit. Current flows though it. Just like if you wrap copper tape around a coil, you don't want to close the loop, or current flows and you get eddy currents. Disconnect the ends and no current flows. A metal cover is also a closed loop because it surrounds the coil. The "H" cutout on Filter'Trons is intended to break that up a bit.

Bill Lawrence had a patent showing a closed secondary coil around the primary to change the tone. But the coil has to have its ends connected. There's also a patent with two coaxial coils with opposite ends disconnected. But one end of each coil is in the circuit, i.e., one is ground, and the other is hot.

So I'm saying if the secondary coil has both ends not connected to anything, how would it effect the primary coil?

Tests need to be done!

[Antigua]

Right, but primary coil is a complete circuit. Current flows though it. Just like if you wrap copper tape around a coil, you don't want to close the loop, or current flows and you get eddy currents. Disconnect the ends and no current flows. A metal cover is also a closed loop because it surrounds the coil. The "H" cutout on Filter'Trons is intended to break that up a bit.

Eddy currents are separate and distinct from capacitive coupling. You can have one and not the other, and vice versa.

So I'm saying if the secondary coil has both ends not connected to anything, how would it effect the primary coil?

Tests need to be done!

Think about a "capacitive plate", you have a large area that is only connected in one spot, usually the center of the "plate". The whole secondary coil is a plate, it need only be connected at one spot.

I've done tests, here's a SSL-3T example:


DC Resistance:
full: 15.53K
tapped: 7.80K

Inductance:
full: 7.842H
tapped: 2.173H

Resonant Peak:
full: dV: 10.7dB f: 4.84kHz (black)
tapped: dV: 9.4dB f: 5.00kHz (red)

Loaded Resonant Peak (200K ohms & 470pF):
full: dV: 2.8dB f: 2.03kHz (green)
tapped: dV: 4.8dB f: 3.51kHz (gray)

Calculated C:
full: 128pF (138-10)
tapped: 456pF (466-10)

[David Schwab]

The whole secondary coil is a plate, it need only be connected at one spot.

I've said several times...

"The secondary coil has both ends not connected to anything..."

Now do the tests.

[Antigua]

I've said several times...

"The secondary coil has both ends not connected to anything..."

Now do the tests.

I missed that, but when you have a three conductor coil tap, it is not the case that the secondary is disconnected at both ends, and so this is what I am suggesting to the OP, use four conductor cable and wire it as shown in that diagram I posted above in order to fully disconnect the tapped coil.

With the secondary coil open at both ends, the coil will still be slightly reactive, because the secondary will capacitively couple along its length, but that effects is a lot smaller than when the coil is connected at one end, based on similar testing with humbucker coils.

[Chuck H]

Yep. One end connected makes it a capacitor. Same effect you get with shielded cable. The shield presents a capacitance to ground because of it's proximity to the conductor. I don't think anyone would argue that or the similarity in the circuits. Both ends disconnected from the secondary coil eliminates the problem BUT FOR ONE THING!!! If you shield an inductor with copper (grounded or not!) it changes the performance. I can't say why because I don't have the tech chops, but I can assure you that it does because of my own experience shielding pickup coils with copper tape. Copper has a unique ability to bend EMF and I have to assume it's this property that is responsible.

[Antigua]

Yep. One end connected makes it a capacitor. Same effect you get with shielded cable. The shield presents a capacitance to ground because of it's proximity to the conductor. I don't think anyone would argue that or the similarity in the circuits. Both ends disconnected from the secondary coil eliminates the problem BUT FOR ONE THING!!! If you shield an inductor with copper (grounded or not!) it changes the performance. I can't say why because I don't have the tech chops, but I can assure you that it does because of my own experience shielding pickup coils with copper tape. Copper has a unique ability to bend EMF and I have to assume it's this property that is responsible.

Probably eddy currents http://kenwillmott.com/blog/wp-conte...r_Geometry.pdf conductive areas that intersect the lines of flux (covers, pole pieces, copper tape) form eddy currents which manifest as increased series resistance.

If you were to cut the copper tape so that it doesn't have continuity around the coil, it will prevent the eddy current loop, but then if it's not grounded, it won't block electrostatic noise, but if it is grounded it will put capacitance across the coil, so copper tape shielding coils is a lose-lose proposition, in my estimation.

[Helmholtz]

DC Resistance:
full: 15.53K
tapped: 7.80K

Inductance:
full: 7.842H
tapped: 2.173H

Resonant Peak:
full: dV: 10.7dB f: 4.84kHz (black)
tapped: dV: 9.4dB f: 5.00kHz (red)

Loaded Resonant Peak (200K ohms & 470pF):
full: dV: 2.8dB f: 2.03kHz (green)
tapped: dV: 4.8dB f: 3.51kHz (gray)

Calculated C:
full: 128pF (138-10)
tapped: 456pF (466-10)


This is an interesting discussion. Thanks for the measurements. (How do you measure the frequency response? By field injection via transmitter coil and integration of the output signal?)

Such a big increase of capacitance (over 300pF) cannot be explained by capacitive coupling.

My explanation is the following:

When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

This effect does not depend on electrical connection between the coil parts.

[Antigua]

DC Resistance:
full: 15.53K
tapped: 7.80K

Inductance:
full: 7.842H
tapped: 2.173H

Resonant Peak:
full: dV: 10.7dB f: 4.84kHz (black)
tapped: dV: 9.4dB f: 5.00kHz (red)

Loaded Resonant Peak (200K ohms & 470pF):
full: dV: 2.8dB f: 2.03kHz (green)
tapped: dV: 4.8dB f: 3.51kHz (gray)

Calculated C:
full: 128pF (138-10)
tapped: 456pF (466-10)


This is an interesting discussion. Thanks for the measurements. (How do you measure the frequency response? By field injection via transmitter coil and integration of the output signal?)

I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.

Such a big increase of capacitance (over 300pF) cannot be explained by capacitive coupling.

My explanation is the following:

When using the tap as output, the disengaged part of the winding is still there and remains to be inductively coupled with the rest of the coil. It becomes the secondary of a transformer. This secondary appears to be open but is actually terminated by its own self-capacitance. The latter gets transformed/reflected to the primary side (the active part of the coil). And thus the reflected capacitance of the secondary adds to the capacitance of the primary - as long as you don't physically remove the disengaged part of the winding from the pickup.

This effect does not depend on electrical connection between the coil parts.

You might be right. It's a certainty that the unused coil will still be in circuit when disconnected at both ends, since both capacitive and inductive coupling are still in play. That can be observed with humbuckers too, and the effect is very small, though of course they're not going to couple as well as two co-axial coils. I'll see if I can model this with LTSpice, but if that doesn't give a clear picture, I'll probably have to make a four conductor tapped single coil to test out.

Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation.

[Helmholtz]

>I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.<

This is what I thought and described above, the method of my german physicist colleague Lemme. I have been using this for many years. But there are different possibilities to take care of the necessary 1/f (-6dB/octave) compensation: A constant current source for the excitation coil with 1/f sweep amplitude or integration of the output signal. I use a power amp driven by the PCSU 200 signal generator and a big air core choke in series with the low impedance exciter coil to provide a current source with 1/f characteristic. If you don't use a constant current source with 1/f characteristic or integrate the output signal afterwards, you will not get the true PU's frequency response as the measured frequency response will be distorted by the frequency dependance of the impedance of the exciter coil and the non-ideal coupling. An exciter coil driven by a low impedance (50 Ohms) source will also load the PU and reduce the resonance peak.

>Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation<

A closed copper loop around a pickup coil is not a good idea, as this is represents a shorted secondary coil in a transformer, loading down the PU. Consequently output signal, inductance and Q will be lowered. Connecting the loop to ground will strongly increase the PU's capacitance to ground in addition.

Do you have numbers for the capacitance increase with a non-closed and non-grounded copper foil? This would be a better capacitive equivalent to the disengaged windings in a tapped coil.

[Antigua]

>I use a Velleman pcsu200 USB oscilloscope + function generator. With a small exciter coil hooked up to it's function generator, the impedance can be plotted with the bode plotter (or seen by viewing the phase by frequency). The inductance can be determined this way by measuring the resonance with a larger known capacitance, but that's tedious to do, so I use an Extech, which seems to work fine.<

This is what I thought and described above, the method of my german physicist colleague Lemme. I have been using this for many years. But there are different possibilities to take care of the necessary 1/f (-6dB/octave) compensation: A constant current source for the excitation coil with 1/f sweep amplitude or integration of the output signal. I use a power amp driven by the PCSU 200 signal generator and a big air core choke in series with the low impedance exciter coil to provide a current source with 1/f characteristic. If you don't use a constant current source with 1/f characteristic or integrate the output signal afterwards, you will not get the true PU's frequency response as the measured frequency response will be distorted by the frequency dependance of the impedance of the exciter coil and the non-ideal coupling. An exciter coil driven by a low impedance (50 Ohms) source will also load the PU and reduce the resonance peak.

For the integration I use an integrator amp circuit designed by Ken Willmott Electric Guitar Pickup Measurements | kenwillmott.com , which is why you don't see a +6dB/oct curve in the pickup plot on the previous page. He offers the schematic for free, but if you contact him he might send you a printed circuit board, as he had a handful of them produced, if you're interested.

As for the coupling between the exciter and the pickup, I've found that the coupling between them is so low that it's almost impossible to get the exciter to interfere with the impedance of the tested pickup, at least with the Velleman bode plotters. Maybe in other test setups it's more of an issue, though.

>Make note that copper tape around a coil does add a lot of capacitance as well. This pickup in the image below shows to have about 275pF capacitance. A typical Stratocaster pickup of this sort is usually around 120pF, +/- 20pF, so the copper shield is adding something like 150pF capacitance, which is somewhat less than what is calculated for a tapped single coil, despite being a geometrically similar situation<

A closed copper loop around a pickup coil is not a good idea, as this is represents a shorted secondary coil in a transformer, loading down the PU. Consequently output signal, inductance and Q will be lowered. Connecting the loop to ground will strongly increase the PU's capacitance to ground in addition.

Do you have numbers for the capacitance increase with a non-closed and non-grounded copper foil? This would be a better capacitive equivalent to the disengaged windings in a tapped coil.

I agree, I'd advice people with copper taped coils to carefully remove it, if at all possible, or at the very least snip the ground connection. I don't have other values with/without, but I can gather this info in the next week or two and send you a private message when I get it. If you don't visit this forum to often, private message me your email address and I'll send you additional test results that way.

[Helmholtz]

Thanks for your replies.

I think I will stick to my trusted measuring setup as I have verfied its linearity to over 20kHz. It doesn't need the integrator because the 6dB/octave behaviour is already implemented in the drive current for the exciter coil.
If you use e.g. a 3mH exciter coil in series with a 100 Ohm resistor driven by a voltage source, the drive current will decrease above 5kHz by 6dB/octave caused by the increasing reactance of the coil, respectively the RL time constant. Consequently the PU's output will show a reduced high frequency response. Of course this can be improved by a lower inductance and higher value series resistor.

It is easy to verify my "capacitance transformer theory" indirectly by the following experiments, if you have a tapped single coil PU (which I do not):

1) Connect an external capacitor of e.g. 100pF to the tap and the outer end of the coil (i.e. in parallel with the disengaged part of the coil).
Measure the respective increase of capacitance in the active coil.

2)Short-circuit the disengaged part of the coil and note the corresponding decrease of capacitance in the active coil. In this case other parameters (L,Q) of the tapped PU may change as well because of the "eddy current effect".


I plan to keep track of interesting threads regarding pickups and tube amplifiers.

[Helmholtz]

I just did the experiment with copper foil wrapped around a strat pickup. I used 2 wraps of back insulated foil, ends not shorted to avoid eddy currents.
The results are:

Without foil Cp=87pF, with foil (not connected) Cp=88pF, foil connected to output Cp=89pF and with foil connected to ground Cp=123pF.
The tapped coil situation would correspond to "foil connected to output".
I think the results show clearly that it does not matter, if the disengaged windings remain connected to the tap or not. It also shows that a dramatic increase of capacitance in the active coil of over 300pF cannot be the result of capacitive coupling.

[Antigua]

Thanks for your replies.

I think I will stick to my trusted measuring setup as I have verfied its linearity to over 20kHz. It doesn't need the integrator because the 6dB/octave behaviour is already implemented in the drive current for the exciter coil.
If you use e.g. a 3mH exciter coil in series with a 100 Ohm resistor driven by a voltage source, the drive current will decrease above 5kHz by 6dB/octave caused by the increasing reactance of the coil, respectively the RL time constant. Consequently the PU's output will show a reduced high frequency response. Of course this can be improved by a lower inductance and higher value series resistor.

The Velleman PCSU has an output impedence of 50 ohms. I'll get specs on the exciter coil, it's about 100 turns of 44AWG, here's a pic http://i.imgur.com/iJ4ejoR.jpg

I haven't come across this problem myself. For example, here is a P-Bass pickup that measured well up to 14kHz

It is easy to verify my "capacitance transformer theory" indirectly by the following experiments, if you have a tapped single coil PU (which I do not):

1) Connect an external capacitor of e.g. 100pF to the tap and the outer end of the coil (i.e. in parallel with the disengaged part of the coil).
Measure the respective increase of capacitance in the active coil.

2)Short-circuit the disengaged part of the coil and note the corresponding decrease of capacitance in the active coil. In this case other parameters (L,Q) of the tapped PU may change as well because of the "eddy current effect".


I plan to keep track of interesting threads regarding pickups and tube amplifiers.

I can do that, though it might be a week or two before I can get to it. I think this is a high value area of exploration since it means endowing a single coil pickup with multiple usable electrical values, without the drawbacks of what is currently available on the market.