I added an buffering opamp on the output. Now, the output load doesn’t affect and modify anymore the signal frequency.
An updated and improved version is available!
The core oscillator is unchanged:
It’s the classic 2 op amp configuration. The first part (U1A, R1..R4, C1, C2) is an integrator circuit. The second part (U1B, R5..R8) is a schmitt trigger comparator.
U1A output produces the alternating “ramp” signal, giving us the “triangle” signal while U1B output is a square wave signal, 90° phase shifted.
The frequency range is selected by switch (or jumper) S1.
Here’s the new output stage:
U1C is a non inverting amplifier. The optional potentiometer R9 allows you to adjust (reduce) the output signal. The amplification gain is A = 1 + (R10 / R11).
The LED driver is different. Now it’s an op amp instead of a transistor:
The PCB is 5×5 cm wide.
Here are some pictures showing a double eurorack LFO:
About the BOM
Many component values are not critical in the Simple LFO. It’s a robust circuit which can tolerate many experiments and modifications of the values.
The 2 input capacitors C5 and C6 can have any value between 1µF and 100µF. A common value is 10µF. One important thing is the minimal operating voltage of C5 and C6: it mustn’t go below 25V.
TL084 or TL074?
The TL08x and TL07x are interchangeable. They are pin-to-pin compatible, the voltage levels are identical.
The TL07x family is a lower noise version of the TL08x family.
However, the signal levels involved in synth modules is probably somehow immune to noise problems.
So feel free to use whichever version you want.
I made two panels with FPE: