[sdiy] Basic Synth Patch? VCO-VCF-VCA

[harrybissell]

Hi John (et al) inline

john mahoney wrote:

> Mike,
> I'm no Synth God, but I'll take a stab at it. :-)
>
> I've had the same thought: Without a VCA in front of the VCF, you don't have
> voltage control of the filter's input level, so you can't overdrive the
> filter under VC in that way, which could be a useful technique. Actually,
> you don't need an AM control on each VCO, all you need is one VCA after the
> VCO mixer. (You need multiple VCAs to do VC mixing of the VCOs, but that's
> another story.)

You could also get the AM from the VCA either before or amter the VCF...

>
>
> On the other hand, if you are *almost* overdriving the VCF (usign a good
> amount of resonance), it will overload when the center frequency hits a
> "sweet spot" where the input signal is strong, e.g. near the fundamental
> frequency. This is voltage controlled overdrive of a sort, but not through
> simple amplitude control.

This is MUCH less true with the Moog Filter. The Moog ladder has a fixed
peak level... so the passband (below cutoff) amplitude is reduced when resonance

is increased.  The ladder filter may actually sound quieter when the resonance
gets
higher.  This is of course offset by the pronounced peak that is really EASY to
hear...
so its kind of a wash...   (My ladder has an added VCA to increase the amplitude

when the resonsnce is increased... making for a constant perceived volume
operation).

The distortion of the Moog Ladder comes from the overload of the input
differential
transistor pair... it gives a rolled-over top on the waveform... maybe akin to
the
classic "tube distortion" sound (now I said 'akin' so don't FLAME me, Eric... if
you are
listeneing :^)

Moving into this overdrive region probably won't get you into the nasty filter
overload you are
describing... but that is NOT true of just about every other filter in the
world... State Variable
filters, Sallen-Key filters.. have peak amplitudes above the passband (with
resonance) and they
do not overdrive politely... they will clip hard (can I say transistor
distortion ??? :^) and will probably clip other stages like VCA following... you
amplifier, mixer etc...

Moving the VCA before the filter can be done but usually you still NEED (or want
badly)
a final VCA if only for a noise gate...

H^) harry

>
>
> You can get a great "growl" effect when the filter sweeps through a sweet
> spot and goes into overdrive.
> --
> john
>
> ----- Original Message -----
> From: Michael Boracci
>
> Dear Synth Gods,
> I need to understand the basic synth patch of VCO-VCF-VCA. It seems to me
> that one more VCA would be needed between the VCO and VCF. There has been
> mention, several times on this list about overdriving the Moog filter. How
> would this be done with VC, if a VCA is not used between the oscillator and
> filter?
>
> In addition to the basic amplitude control function I would think that since
> there is an FM input on most VCO's, there should be AM as well right?
> [snip]