I'm quite new to electronics and trying to understand how this voltage-controlled filter (VCF) schematic works exactly.
It's a 12db/oct lowpass filter without the use of capacitors but uses the inherent Miller capacitance of the opamp (between the output and inverting input i guess?)
From what I understand, the circuit involving the first op-amp will set the cutoff frequency by applying a DC voltage on the "Effect control in" input pin. It is a voltage controlled filter after all.
Someone online also told me this but I didn't quite understand it yet...:
Those opamps, the К140УД12 ones, are special (by modern standards) because of pin 8. Pin 8 sets the quiescent current of the opamps, which in turn sets the frequency response.
R5 and R8 make sure that the two opamps share the bias current. It's worth noting that the КР140УД8Б opamp is capable of directly outputting a current with the transistor. As long as R5 and R8 match, their value doesn't matter too much; although, they do set a hard limit to the maximum current pulled from the pin8's. It appears that all the signals other than the output are current-mode.
I found out the КР140УД8Б can be replaced by a TL072
And the К140УД12 can be replaced by: uA776, NTE888, ...
- How does this pin 8 work?
- How is it exactly filtering the signal?
- Is this a 12db/oct filter because there's two op-amp stages in the filter section?
Thanks in advance