Dual Wire Wind? (Alternative to tapping/splitting)

[John_H]

Means that a higher C lowers the Q-factor.

But more important than Q is that a large C means a low resonant frequency and thus poor HF response.

Thanks Roland.

[Chuck H]

Already discussed and pointed out repeatedly is the additional capacitace caused by the unused portion of a coil. My question now is whether that still applies if BOTH ends of the usuaed portion of the coil are switched open. With no landing place for any semiconductivity I wouldn't think so. It seems to me that this common problem might be solved with a little more switching circuitry effort?

[Helmholtz]

Already discussed and pointed out repeatedly is the additional capacitace caused by the unused portion of a coil.

The windings on the same bobbin are magnetically coupled as with a transformer.
Each winding is loaded with its own self-capacitance (which means an impedance load).
By transformer action each self-capacitance reflects to all other windings.
So the effective capacitance of all windings is increased no matter if the windings are connected or not.

[Chuck H]

The windings on the same bobbin are magnetically coupled as with a transformer.
Each winding is loaded with its own self-capacitance (which means an impedance load).
By transformer action each self-capacitance reflects to all other windings.
So the effective capacitance of all windings is increased no matter if the windings are connected or not.

Thank you. I think I finally have it.

[J M Fahey]

Shortest answer is: bifilar winding is BAD, except on PT primaries where we deal only with 50/60Hz so it does not matter.

[Chuck H]

Shortest answer is: bifilar winding is BAD, except on PT primaries where we deal only with 50/60Hz so it does not matter.

Ok. So is this problem somewhat mitigated if you tap a series wound coil? Also considering that you could disconnect both ends of the "additional" winding.?.

I think this should just act like a copper wrap around the pickup with this arrangement. Which I've done for some extreme shielding efforts. I remember it changing the tone of the pickup but it wasn't that bad.

Apologies. I'm sure this has already been covered but I don't remember.

[Helmholtz]

When you tap a series wound PU winding, the capacitance of the full coil will stay the same.
But the effective capacitance between the start of the winding and the tap will be increased by the reflected capacitance from the unused part of the coil.
In fact the capacitance between tap and grounded coil start will be larger than the self-capacitance of the full coil.
Reflected capacitance is caused by transformer or magnetic coupling and inversely depends on the turns' ratio squared.


Bifilar winding causes additional capacitance (and sometimes additional secondary resonances) due to direct capacitive coupling between the windings.

[Helmholtz]

With a series resonant circuit like a PU the Q (and thus the height of the resonant peak) depends on the square root of L/C:
https://en.wikipedia.org/wiki/Q_factor (scroll down to RCL circuits).
Means that a higher C lowers the Q-factor.

This statement of mine from post #15 is wrong, sorry for confusion.

The transfer response of a magnetic PU corresponds to a second order low pass filter, which cannot be described by the formulas for a simple LCR resonant circuit.
In his book Zollner shows a formula for the Q-value of a loaded PU, based on a simplified equivalent circuit.
This Q-formula is somewhat complicated and not easy to interpret in terms of capacitance.

But, getting back to the question how capacitance (PU self-capacitance or cable capacitance) affects the resonance peak.
There are real PU measurements showing that the resonant peak tends to increase in height with larger capacitance.
So it seems that pickup Q increases with capacitance while the resonant frequency drops.