Moog VCF Controls

Controls

• One-pole pole location: controls cut-off frequency
• Feedback gain: controls resonance

Resonance

• The phase of each pole at $s=-a$ is $45^{\circ}$ when $s=ja$
• At this frequency ($\omega=a$), the phase through all four filters is $180^{\circ}$
• The gain of each one-pole at $\omega=a$ is $1/\sqrt{2}$
  $\Rightarrow$ total gain is $1/4$
• Therefore, with a feedback gain of $k=4$, the loop has in-phase, positive feedback at frequency $\omega=a$