This VCF is a straight forward 24db/octave filter based on a late 70's design. The actual sound specs are vague and the noise floor isn't great, but the sound is phenomenal. I built 2 of these units for studio and live use and ran them into each other. It has been one of the trademark sounds for the "The modifiers" project, although the filter in "take me higher" was from an akai s612. Like all our custom gear, the actual circuit is simple to build and really cheap. In fact the most expensive part is the box it sits in. This filter works of 24 volts dc so you need a power supply or you can build your own. There is a Modifiers track coming out this year on VU called "tropical rain" The lead synth line uses one of these filters and it was recorded during a live show. If you want you can substitute the 741 for a higher quality part. This is the beauty of analog electronics. You can always experiment and hot wire stuff.

This is is a resonant filter which means that a frequency is chosen and is then amplified to the point of resonance. The center pot chooses the frequency while the pot on the left amplifies it and the one on the right is used to regulate of much a voltage source from the oustide can affect the circuit. The transistor applies the voltage that regulates the filter the right pot is lets you change that for an outside control voltage. Audio goes in one end, you noodle with the knobs and a filterred version comes out the other end. How swell.

The parts list

One op amp, one transistor, 6 resistors, 4 caps and 3 rotary pots and thats the whole thing. About five bucks worth of parts and you are done.

RESISTORS= R1,R2=15Khz/R4,R5=100Khz/R6=10Khz/R8=150Khz

POTS= R3=250Khzaudio/R5=100Khz linear/R7=10Khz linear

CAPS= C1=1uF/C2,C3=.01uF/C4=5uF

MISC= 1x741 op amp/1xgeneral transistor.