VCA discussion

[gstopp at fibermux.com]

>
>Does this mean that an opamp/transistor lin/exp converter will work 
>very nicely to control the circuit gain?  I hope so.
>

It appears that it's already exponential as it is. I hope that one of the 
transistor theory gurus can verify this in theory at least (Joachim? Juergen? 
Magnus?)

>
>BTW, did you _actually_ get a flat trace out?  I was still getting a 
>very small signal on my output.  I'd like to see the output go completely flat 
>on mine.
>

Not actually flat, but down to around 10-20mV (about the same amplitude as the 
noise) which is really "down in the mud". I suspect that this is okay for a 
normal mix. I wonder - what do commercial machines go down to? Anybody know 
this?

>
>What kind of input and output signal levels do you think are 
>adequate?   BTW, that strange function output was what I was getting when I 
>first fired up the circuit.  I guess you know why I was a little distressed!
>

I like +/- 5 volt signal levels from VCO's, cut down to around 2V p-p for audio 
inputs to VCF's when I want the resonance to be clean. I want them full strength
for moduation sources, and I like to use VCA's to provide voltage control of 
modulation source amplitudes, so the VCA's need to be able to tolerate full 
strength VCO outputs as well as 10V envelopes and other hot signals. Keep in 
mind these desires are for modular systems, and not preset machines which have 
much more restricted signals in the audio path generally.

>
>I think this one lends itself nicely to audio only.
>What about CV-modulating VCA's.  What would you use them for?
>

Yes, audio only, if you want to reserve your 3080 stock for other modules. An 
example of using a VCA to modulate CV's would be delayed vibrato, where an 
LFO triangle or sine goes into a VCA, and an AR envelope with a long attack 
setting opens up the VCA, which drives the frequency input of the pitch 
VCO(s). Another example would be velocity control of VCF envelope depth, with 
a VCA controlling the amplitude of an envelope into the filter, and the 
velocity voltage as the CV to the VCA.

- Gene
gstopp at fibermux.com



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