fun vcf is a double state variable filter with the common controls and separate outputs. its core is based on the soviet polivoks synthesizer's filter featuring new stock ??140??1208 and ??140??18
operational amplifiers. original schematic was redesigned in the following way:
high pass output was added
full frequency sweep in the hearing range is now possible 20 hz - 20 khz
no artifacts for the filter to pass unwanted high frequencies at low pass output
inherent distortion when the frequency is set to its low range is eliminated (compensation can be omitted with the jumper on the pcb).
two identical state variable filters are available in one module, each of them has separate input, three outputs - high pass (12 db/oct), band pass (6 db/oct), low pass (12 db/oct), plus a common for both
filters frequency modulation (fm) input. both filters are factory fine tuned to be as identical as possible for good stereo operation.
but the module is not limited for stereo filtering. the user can take any output of one filter and patch it to the second filter's input. this way not only the usual combinations of 12+12=24 db/oct low pass
and high pass, or 6+6=12 db/oct band pass filter modes are possible, but also any in-between modes like passing 12 db/oct high pass through 6 db/oct band pass. the user is welcome to check the possible \
it should be mentioned that subject filter's resonance is self oscillating and has a huge impact on the sound. which becomes merely monstrous in case a serial connection of the both onboard filters is