Another project I'm already having fun with, despite being in such an early stage. Aiming to get a great analog sound out of very few parts!

Though I call it a "drone synth" it's also good for percussive tones, reminds me a lot of those very old analog drum machine blips and bloops.

A video on the voice prototype can be found on here.

The Core Circuits

This synth is build around two primary circuits: the 40106 schmitt inverter and a single opamp resonant filter design I use a lot in my designs.

First some details on the 40106:
It's a great little IC for building noise boxes (or fart boxes) though it is possible to make a pretty decent linear or exponential VCO with very few additional components.

The most basic 40106 oscillator you can build only needs a single capacitor and (variable) resistor in the feedback path.

The fun thing about this stupidly simple oscillator is that you can do so many things with it:
- You can get triangle wave from the input side and square on the output side.
- Adding a diode in the feedback path turns it into a sawtooth and pulse oscillator.
- Using two diodes with a potentiometer in the feedback loop allows for PWM.
- Putting a transistor parallel to the capacitor allows for VCO capabilities, you can even get a good 4 octaves out of this!
- And probably many other cool things people have done with this chip!

In RezzoDrone I'm using a pulse oscillator to excite the resonant filter and a square/triangle oscillator to modulate it's resonant frequency.

So let's take a look at this resonant filter circuit, again it's just very few components!

This is derived from the Twin-T circuit, which is normally a bandpass filter but I'm primarily using the low pass section.

I've used various versions of this core circuit as a VCF and kick drum generator already and it can probably used for many more uses.
The feedback resistor sets the gain and is usually an high value around 1 megaohm.
The resistor to ground, between the capacitors, sets the frequency and partly the sustain too.

In RezzoDrone I'm using a transistor in parallel to that resistor to ground to modulate the frequency.

The Resonant Modulated Filter

Now that we have looked over the basics, here's the schematic of a single voice:

PLEASE NOTE THAT THIS IS THE ROUGH CONCEPT CIRCUIT. It's very much subject to changes.

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