Following on from the stablization of sine wave oscillators, the same problem applies to more complex state variable type oscillators, such as the primary VCO core of the Oscilloplasm. This is based around the classic state variable circuit, though with some added complexities. Here is a photo using classic analogue computing notation. Just as an aside, when this circuit was first envisioned, opamps were realised with vacuum tubes, and the multipliers were electromechanical devices that relied on physical rotation. This technology was used in early rockets and aerospace machines as well as the laboratory. Don’t knock the humble transistor!
T is effectively a saw wave that frequency modulates one side of the sine core, the potentiometer 2 adjusts the time constant of the other side to vary the frequency content of the output.
Zener diode negative feedback and diode clipped positive around integrators 3 and 4 is what was used in the first iterations of the module, but the sound was not pleasing. Simply allowing the VCO to distort by clipping the rails actually results in an interesting sound and this was genuinely considered, the problem with this however was not the distortion but the amplitude instability, as frequency stability depends on this, and this is essential for a high performance modern oscillator.
The end of the journey was found in the THAT4301 RMS to DC converter chip. Previous RMS to DC converter ICs had no provision to vary the conversion rate. This really rules them out for use in an oscillator core, as there is a sweet spot where settling time is fast enough but the conversion rate is low enough to minimise distortion artefacts, which depends on oscillator frequency.
The integration time constant on the 4301 IC is controlled via a current into pin 2. The datasheet suggests generating this through a fixed voltage into a resistor. A variable voltage could also theoretically work, and indeed it works in practice (I suspect the design team for the IC knew this). We simply use the same voltage that is controlling the VCO frequency to control the conversion rate.
The principle is the same as other AGC (automatic gain control) techniques. We use some component detect the amplitude level, in this instance averaged over several cycles, relative to some reference level, and apply this to a gain control element that allows for proportional negative or positive feedback around a specific node in the VCO loop (in this instance an integrator in the VCO core).