The Feague is a 4 pole VCF and quadrature oscillator with 0, 90, 180 & 270 degree outputs. It is based on the design in electronotes 41.
It features a lo/hi switch to run it at audio rates or LFO rates, plus a nonlinear Q pot which makes for interesting behaviour when using as a VCF.
When using as a QUO, the nonlinQ pot is used to increase the amplitude of the output signal, which is known to shrink somewhat at low frequencies in these kind of oscillators.
SIGNAL: audio input
CV: for controlling cutoff in VCF or frequence in LFO/VCO
SYNC: sync in/freeze/hold control
1V/OCT: for controlling cutoff in VCF or frequence in LFO/VCO
Each ouputs are 90 degrees in out of phase : 0, 90, 190 and 270.
In VCF, the two outputs closest to the side edge of the panel are low-pass outputs, and the other two are band-pass.
The trimpot is used to tune for 1V/oct, turn up the Q pot, get the circuit oscillating and tune it as per any regular analogue VCO.
Use as a VCF:
Keep the nonlin Q pot at zero. Patch in an audio signal and adjust the Coarse freq pot and Q pot to use as a filter. Keep the switch on Hi, add CV as you like. Once sounding good, try tweaking the nonlin Q pot and see what you get.
Use as a Quadrature Oscillator:
Keep the nonlin Q pot at zero. Turn up the Q pot until the module starts to self-oscillate (around 8-9). Adjust the frequency with the freq pots. When the switch is Hi the range is at audio rates, set the switch to Low for LFO rates. When using the module as an oscillator, the nonlin Q pot can be used to adjust the amplitude of the output signals. Don’t forget to play with the sync input; it is more like a freeze function.