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Here are two easy to build filter circuits based on the cd4069 chip. I´ve used them in my recent projects and I´m quite pleased how they work. These schematics are examples meant to be a starting point and the component values are examples so feel free to experiment with different values. First a lowpass filter It uses three inverters. So you can build two with a single 4069. R1 and R2 are the cutoff frequency use a 10k-20k stereo pot or two ldrs and leds driven with a cv source (lfo,sequencer etc.). Cap values are drawn 0,1uF but I´ve built one with 0,047uF and it worked fine. The 100k pot is resonance, and this filter will oscillate when resonance is turned up. There are two output options drawn in the schematic "out" (wich is more bassy) and "out2". You might want to use a output cap (1uF). Second one is a bandpass filter. This thing uses only one inverter. Try different cap values instead of 0,01uF ones(maybe from 0,001 to 0,068) and they dont all need to be the same. This is also voltage controlled with the npn-transistor you can use a pot to ground, ldr etc. insead if you like.