[sdiy] linearize OTA for VCF?

[Ian Fritz]

Hi Terry --

That sounds like a sensible way to set up the limits. I set up my 13600 VCA
to clip at input levels of ~7 V, which seems good as far as S/N goes, but I
have to be careful what I put into it!

  Ian


----- Original Message -----
From: "Terry Michaels" <104065.2340 at compuserve.com>
To: "Ian Fritz" <ijfritz at earthlink.net>; "synth"
<synth-diy at dropmix.xs4all.nl>
Sent: Monday, September 10, 2001 11:21 AM
Subject: Re: [sdiy] linearize OTA for VCF?


I set the input clip level equal to + and - 12.5 volts, this is the maximum
voltage swing available in other circuits (mixers, etc.) where the signal
passes through an opamp, which will deliver nominally + and - 12.5 volts in
a system with + and - 15 volt supply rails.  A quick bench test of my
CA3280 based VCA reveals the following.  If DC is applied to the control
voltage inputs so the VCA is full on (unity gain, Iabc = 0.5 mA), the
output clips when the input reaches + and - 12.5 volts.  The output swings
+ and - 12.5 volts also.  If I reduce the control voltage to, say 20% of
full on (Iabc = 0.1 mA), it still takes + or - 12.5 volts at the input for
clipping to show up at the output.  The output level is now about + and -
2.5 volts though.  If I apply enough control voltage to drive the VCA to a
gain of X2 (Iabc = 1.0 mA, the VCA has an internal limit at that point),
the output clips when the input reaches + and - 6.25 volts, the output
swings + and - 12.5 volts.  Since I don't usually run the VCA's at a gain
greater tha X1, they don't clip before other modules do.

Since my VCO's generate + and - 5 volt waveforms, I basically have X2.5 of
headroom.  This seems to work out well, if I mix the full output of 2
VCO's, l would expect the instantaneous sum of 2 VCO waveforms would not
cause clipping in the VCA's, mixers, etc.

Terry Michaels