Phuncgnosis, How does it work?

[lapsteelman]

I'm breadboarding this circuit right now. The demo's I have heard sound great (Thanks pinkjimi!) and I will probably end up incorporating deadastronaut's mods to it. but first........

I really want to understand how this circuit works. I understand the filter section, and I understand the envelope detector, but how does the diode interact with the filter?  I have built similar circuits with LED/Photocell arrangement , and I have seen it done using a Jfet as a variable resistor.

Any help with this would be much appreciated, Thanks.

[Seljer]

Diodes have a parasitic capacitance that varies with the reverse voltage applied. This is specifically used in varicap diodes (also know as varactors) but all diodes exhibit this behaviour to some extent http://en.wikipedia.org/wiki/Varactor#Operation

[Earthscum]

When you vary the voltage (what the follower is doing), you vary the "apparent" resistance of the diodes.

It all has to do with Ohm's Law. If you have 10k resistor from 9V+ to diode to ground, your diode is .6V, making the voltage across the resistor 8.4V. V/R=A, so 8.4/10000=0.84mA, and across .6V this makes the diode appear as a 666.6R resistor(). I think my math is off. If you increase the voltage, the current increases across the diode, making it appear as a smaller resistor. The current into the caps fills them up faster. If you lower the voltage at the junction of the diode/resistor, it sinks off some current, making the diode appear as a larger resistor. Less current into the caps, they fill up slower. The speed at which your caps charge and discharge determine the frequencies that can pass.

Sorry for the horrible explanation (trying to do multiple things at one here), but hopefully it gives you some idea of what's happening.