studio synth circuit

I originally wanted to call it Chimera , but then I remembered there`s already a synth company using that name, so I went with Cerberus to avoid confusion, and because Tricephalic Filter  just doesn`t have the same ring to it. I didn`t bother renaming the mp3 ²s below. But I digress Like its namesake, this filter has three heads , or in
studio synth circuit - schematic

this case, filter modes. The main influences here were RenG© Schmitz` Late MS-20 Filter, Osamu Hoshuyama`s VCF 1984B and fake SSM2040 VCF, and Ray Wilson`s VCSVF (this was another reason I wanted to use Chimera, to mark it as a composite beast- oh well). Like all of these filters (as well as many others), the Cerberus is based on OTA`s (Operational Transconductance Amplifiers)- in this case, the LM13700. The LM13700 datasheet contains a schematic for a single-ended voltage controlled resistor, which is how it`s being used here- in fact, the circuit used in the filter(s) is not much different from the application circuit shown in the datasheet. Each filter stage (of which there are four here) consists of one such VC resistor as part of an R/C filter network, with each stage buffered before going on to the next. This is a handy thing about the LM13700- it includes a buffer for each OTA on the chip. These four stages arranged in a serial configuration give us the core  of a 4-pole lowpass filter. That`s cool, but what if we want other filter types This is where Ray Wilson`s filter comes in. Nice guy that he is, he explains how his VCSVF works quite well at his site. In a nutshell, the highpass filter is achieved by subtracting the lowpass signal from the input, and the bandpass is the highpass signal through the first filter stage. This actually creates a BPF which is steeper on the highpass side than the lowpass side. Since...

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