First time poster. Please be kind!
I'm working on an analog synthesizer project and recently started looking at Voltage Controlled Filter designs. I've scoured the internet for a low pass configuration that would allow me to control the filter's cutoff frequency with a control voltage (from a low frequency oscillator) without resorting to the complicated mathematics of, say, the Moog ladder filter. So far, the top contenders are the filter for the Gakken SX-150 and the filter for the Korg Monotron series. Unfortunately, the physics of even these relatively simple topologies are a bit above me. I've tried doggedly to try and resolve the components in both circuits to the classic Sallen Key filter but without much luck. To date all I concretely understand is that the BJT transistors must function as resistors whose base junction current determines resistance. This resistance then works in conjunction with the capacitor impedance to selectively allow current flow as in a basic first order topology. That's not so bad. Exactly which components achieve this effect however, is still a mystery to me. I like to believe I have done the requisite amount of googling but even the most accessible webpage (Tim Stinchcombe) wasn't much help. The addition of more than two capacitors in both schematics gives me much pause and I'm still not convinced that the feedback diodes are even strictly necessary. Would someone mind explaining to me which components I should focus on and how they work together?
EDIT (Included Monotron Schematic)