Lesson 6 of 6

Building the VCA

Put everything together and build a Vactrol-based VCA.

Okay. Now we have all the tools to build the VCA. Check out the following diagram and inspect it. You will find familiar sections.

Vactrol-based VCA schematic

Let me walk you through it:

  • Both the U4A and U5A opamps are buffers for the input voltages.
  • The Vactrol's LED side is wired the same way as in the previous lesson.
  • U2A is wired as an inverting amplifier.
  • Without the Vactrol, this opamp has unity gain (R4/R5).
  • But with the Vactrol, the gain looks like this: R4 / (R5 + Vactrol)
    • The Vactrol's resistance is about 10M ohms when there's no current through the LED side.
    • So, if CV_IN is 0V, the gain is almost 0.
    • As CV_IN increases, the Vactrol's resistance goes down, so the gain goes up to unity.
    • That's literally a VCA.
  • The R6 resistor sets the output impedance to 1k.

🎉 Yeah! Now you understand the complete VCA circuit, as we promised in the beginning.

Once you build it, you should see something like this on the oscilloscope:

VCA output on the oscilloscope

Try the following experiments:

  • Feed the 16bit's A2 output, which gives a super-saw waveform.
  • Feed some external audio via the line in.
  • Use the envelope generator as the CV.