Well, I still think that it's a VCA, but I found that the gain goes ∗up∗ with increasing resonance. The gain increases until you reach a value of about 4 on the resonance knob. Above that, the gain stays the same.
More analysis still forth-coming...
Chip
--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
>
> Hi Johannes,
>
> A few posts ago, you said:
>
> > I always wondered why the Resonance control is part of this.
> > Similar to the output LPF there is a controlled (by the
> > Resonance knob) LPF at the input.
>
> Looking at the schematic, I believe that this LM13600 (the first half of IC20) is actually wired as a Voltage Controlled Amplifier, not as a VCF. Yes, there is a cap to ground after the OTA, but there is no negative feedback. Without the feedback, I don't think that it can be a VCF. So, I think that it is just a VCA.
>
> Looking below this chunk of circuitry, you'll see that the other half of this LM13600 is wired in a similar way (though with different component values). It, too, has no negative feedback. This second half of the LM13600 is implementing the VCA MOD and the Program Volume, so one can probably assume that they definitely intended this LM13600 stage to be acting as a VCA.
>
> Due to the similarity, it's my conclusion that both halves of IC20 are acting as VCAs.
>
> Going back to the first half of the VCA (the part that seems to be controlled by the resonance setting), it seems to me that this circuit is simply adjusting the amplitude of the audio based on the resonance setting. To my eye, it appears to reduce the amplitude of the audio as the resonance setting is increased. I guess that the idea is to keep the amplitude of the self-oscillation under control. Just my conjecture...
>
> Chip
>