3080 VCA question

[Martin Czech]

:::Sorry if this has been mentioned before but I was wondering about how
:::many dB of dynamic range is reasonable to expect from a 3080 based VCA
:::for audio use?

>From my experience I'd say -90dB or more. (headphones pretty loud,
then turning Iabc down, leakage allmost inaudible). The problem is not
leakthrough (think of s&h applications) but control current rejection. The
more you modulate , the more DC shift will appear at the output, fast
envelope transients will "pop" , even with no audio signal. But this may
be part of the sound of some synths. Think of the single npn common base
vca in the Korg MS20.

Maybe someone comes up with a "pop" compensation, using two otas in a
differential scheme. But then LF13700 of course.

But if you want to control modulation signales etc. as well, especially
for pitch control, you should better stay away from simple otas. Too
much shift. Anyway, a dc blocking cap at the audio input as allways a
bad idea. You might use for dc.

btw. don't follow the linear/expo dual vca path like formant, just give it
an expo converter with linear input, so you have both options, even at
the same time, but only one vca element. In the first moment the knobs seem
to behave strange, but a proper front panel will help.

Iabc=Ilin*expo{U/Ut} in this case, ie. both control inputs work in a 
multiplikative fashion, which is a bit unusual.


m.c.