Dummy coil question

[David Schwab]

In the absence of ferromagnetic cores (poles and magnets), one must match the area-turns products of the two coils (active plus dummy) to achieve cancellation.

Now how does having, or not having a core effect the situation?

For instance, Alembic doesn't have a ferromagnetic core in their dummy coils, but many pickups do. Does the inductance have to match?

[Joe Gwinn]

Now how does having, or not having, a core affect the situation?

A ferromagnetic core makes the effective area-turns product larger by a ratio more or less equal to the incremental permeability of the core, which is the same as the ratio of inductance with and without the core in place. The actual value of inductance doesn't matter, only the ratio.

For instance, Alembic doesn't have a ferromagnetic core in their dummy coils, but many pickups do. Does the inductance have to match?

Inductance does not have to match. If it did, the big dummy coil would not work.

The whole point is that a big coil of few turns can have the same induced hum voltage as a smaller coil having more turns, and yet their inductances will be greatly different. The coil with few turns will have much lower inductance, and thus little effect on the tone.

[EtLa]

The coil with few turns will have much lower inductance, and thus little effect on the tone.

in that case the dummy need to be connected in series with the pickup because in parallel the dummy with few turns(low resistance) will create a path to ground?

[Joe Gwinn]

in that case the dummy need to be connected in series with the pickup because in parallel the dummy with few turns(low resistance) will create a path to ground?

Yes. Only series will work if the coils are different sizes.

[David Schwab]

Thanks Joe.. that was a very clear explanation.