Yeah, I would have to try it in physical reality. So between TL072, LM393/339 and NE555 you would reccomend better the TL072 right?

The TL072 is fine there's plenty of opamp choices, pretty much any opamp will work.

What do you mean by voltage divider equation with R19 and R21? How do you come to that equation?

There's many ways to look at it, however looking it as a resistive mixer covers the Schmitt trigger and the combination of the bias Trimpot + LFO

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/averager-summer-circuits/Consider the case where you only have two voltage sources V1 and V2 and they feed their corresponding resistors R1 and R2. The voltage at the output is a blend of the two input voltages.

Vout = (R2/(R1+R2)) V1 + (R1/(R1+R2)) V2

If you look at only one of the voltages, say V1, with no V2, you get a simple voltage divider equation,

Vout = (R2/(R1+R2)) V1

If you view circuit from V2's perspective, with no V1, you also get a voltage divider equation,

Vout = (R1/(R1+R2)) V2

The two input circuit is just the sum of both those separate results. That works because of superposition theory.

You can also view this as a blender,

Vout = k * V1 + (1-k) V2; where k = R2/(R1 + R2)

k always between 0 and 1 depending on the resistor values.

k=1 select V1, k=0 select V2, k=1/2 is equal amounts of V1 and V2

For the schmitt trigger case,

V1 = the opamp output voltage, which is either 0V or 9V ; actually more like 1V or 8V

V2 = Vref = 4.7V

R1 is R21 in the schematic

R2 is R19 in the schematic

From the two opamp voltage you calculate two voltages on U1b's +input and they are the thresholds.

Alright, that I can deal with. But we're talking about the C10, right?

Yes.

And, I know you said to ignore C7 and R24, but how do they prevent those clicks? I can't see how..

It'a hard to expain. When the opamp output tries to change state the cap C7 shifting the voltage. Kind of like pulling a piece of cheese on a string that a mouse is trying to get. Have a play in spice with different C7 values.

What do you actually mean? Because the 1M+trim

..

It's a little confuse to me, the offset it's

If you look at this circuit as a resistive mixer, similar to the above,

Output = JFET gate voltage

V1 = Bias Trimpot

R1 = 1M + adjustment trimpot

V2 = LFO out

R2 = 3M9 resistor.

The LFO output can be broken down in to and AC component and a DC component,

VLFO = VLFO_AC + VLFO_DC

The gate voltage is then,

VGS = k * VBIAS + (1-k)*VLFO

= k * VBIAS + (1-k)*( VLFO_AC + VLFO_DC)

= {k* VBIAS + (1-k)*VLFO_DC} + { (1-k) VLFO_AC}

From the perspective of the JFET gate,

- the first {} is the true DC level and it depends on the bias trimpot adjustment and the also the DC level coming out of the LFO.

- the second part shows how much AC part of the LFO get through. ie. the how the peak to peak at the LFO output

gets reduced by a factor (1-k) before getting to the JFET gate.

Since the 3M9 resistor is somewhat larger the the 1M you will find k is about 0.8, so 1-k is 0.2

so the LFO swing at the gate is reduced somewhat compared to the swing the LFO output.