The Mini-3 VCF A or B
I have included a 4 pole lowpass Moog type and a 2 pole lowpass OBX type
filter. The OBX type is easier to to build but I prefer the sound of the
Moog type, if you have space in your case build both and install a select
switch.
4 Pole Moog Type
The classic ladder type resonance filter can be made to oscillate on
it's own producing a smooth sine. Sounding much like a theremin.
2 Pole OBX Type
The OBX type is a state-variable filter in a lowpass configuration. It
can be expanded to a 4 pole by adding two more stages of OTAs and buffer
amps but if your going to that much trouble build the Moog type insted.
Testing The VCFs
Apply +- 12VDC to the curcit, then hook the output to
a monitor amp and the input to a audio source.
You should hear the filter cutoff change when the CV
is altered by the INIT FREQ trimmer. Changing the resonance pot should add a
hollow quality.
