Hi!
That’s a great project. I believe Gaudi’s circuit used a DAC to apply a DC voltage to the VCA. It can be difficult for people unfamiliar with analog circuits to understand.
For driving the VCA, the TPIC2050 datasheet is a good reference. It uses PWM. For precise control, you may need to connect a resistor in series with the VCA.
You can also do this with a microcontroller and a motor driver. Just be careful of overcurrent. If you ask ChatGPT or something similar, I think you’ll get a better idea of how it works.