VCF with inductors

[Rene Schmitz]

At 10:19 02.10.1997 +1000, you wrote:
>I noted Jon's question and started thinking
>
>A crossover in a speaker box uses inductors to define the cutoff frequency.
>Is it feasable to use a transformer type arangement, that is an inductor with
>some extra turns around it, to form a variable inductance ? The extra turns
>would have a DC voltage applied accross them to give a electro-magnetic effect,
>and this DC voltage would be the control voltage?
>
>
>Rowena

It could be done, in a bit different way.
I ve seen this in a Radio Frequency VCO, it used a diff pair of trannies,
so the currents in both windings were out of phase.
By modulating the ratio of both (ac) currents, you can 
modulate the magnetic flow thru the coil.
Thereby variing the inductance. (I guess from the inductance, downto
the stray inductance is the range that could be archieved.)
This is *not* makeing use of the core characteristic, and
could be used over the audio range. And works with no core 
at all.
(Better I stop here before suggesting to move a core in and
out of the coil by a little motor....:-)

Bye 
 René