Varactor Diode in Capacitance Multiplier Configuration to make Env. Filter Thing

[doitle]

I recently came across a demonstration of a capacitance multiplier and it renewed my interest in a part I previously saw but wrote off as unusable. I was reading about varactor diodes and how their capacitance is determined by the voltage across them. I was thinking wouldn't it be interesting to use this as part of a filter circuit to move the F0 point based on how hard you are playing? Also as the note decays the filter would move as well. It could sound terrible it could sound completely awesome! Unfortunately though the biggest varactors I could find were in the  neighborhood of 100pf. Very small. This multiplier however could convert that 100pf into a .1 uF fairly easily! Then we could be in business for a filter! It would swing much farther than the 100pf would and with the added range might give you a really cool sweep or something. I'm gonna start trying to research it more and figure something out and maybe borrow a Varactor diode from the lab to bread board it. Anyone have any thoughts? Think it will work? Think it is a stupid idea?

[WangoFett]

Hi guys, long time reader first time poster here.

Would these work in audio-range applications?
I had a quick look and couldn't find any varactors with usefully high capacitance (highest I found was 100pF @ around 1V reverse bias).   Perhaps by putting a few in parallel to ground in place of the C in a passive low-pass filter configuration, one could build an audio-range low-pass filter with a corner frequency changeable by moving the bias point around by a few volts.
But as a guitar's signal bias point normally doesn't move as it decays I don't think the filter would behave like an envelope filter at all.   
How about robbing part of a swell circuit that works by varying bias point to control gain, and follow that with a varactor based passive filter section?

[doitle]

Howdy there, welcome to the forum.

As per the post,
I didn't mean that it would behave like an envelope filter, but that it would be envelope controlled. As the amplitude decays the center point of the filter would move. At least I think... I've got to get something together in a simulator to see what it really does because this is all just speculation.

Also yeah they are usually pretty small but that's why I mentioned the Capacitance Multiplier. It should allow the usage of a 100pF Varactor to get equivalent capacitances in the 1uF range.

Strangely enough I couldn't find any online above about 110pf but I go and look in our lab at the school and there are plenty that are 450pf or higher. Must not be made in large sizes anymore or something.

[snap]

at A=440Hz it would move up and down 440 times per second?

[doitle]

Maybe but I dont know how "fast-acting" the capacitance change is and it may have a low pass effect instead taking the envelope of the guitar signal peaks instead of the raw guitar signal. As I said I really need to try to get something together in a simulator and find out what LTSpice thinks would happen. I've been too swamped with school work though to test it out yet.

[WangoFett]

Sorry doitle I think I only read half your post    and thanks for the welcome.
The capacitance multiplier thing looks interesting.
I think this would sound weird, but it could be a great kind of weird.