[sdiy] CA3080 VCO

[Scott Bernardi]

Try http://members.home.net/sbernardi/elec/lfo.html. This is actually an
LFO, but if you decrease the value of C2 to 1000pF or so it will make an
acceptable VCO. Get rid of the dual color LED (since that is meaningless at
audio frequencies.
Some modifications can be made to improve the exponential generator:
R4 can be a tempco resistor for temperature compensation, if desired
Your CV inputs should be 100K going in to the summing node of U1:A.
I'd increase R5 to decrease the reference current.
R2 isn't needed.
The output of U1:A should be tied to Q2's base, and Q1's base should be
grounded. This will give you a positive increase in frequency for positive
control voltages.
A uA301 can be substituted for the 748 (U3).
Q1 and Q2 could be THAT-120's for better performance. Otherwise, you need
to match them (see http://members.home.net/sbernardi/elec/og2/xxxx for a
transistor matching circuit).
The ends of the FREQ pot should go through resistors to +V and -V. Measure
what voltage range at the wiper of R1 is at to get your desired frequency
range, then choose the resistors to get these voltages.
A FINE FREQ pot can be run between the rails with a 1M resistor to the U1:A
summing node.

Casey Crane wrote:

> Hi all,
>
>         Years ago, back in the 70's when I was getting the electronotes
> newsletter I remember seeing a schematic for a nice triangle/square VCO
> built around a CA3080 OTA. Does anyone know what I'm talking about ? Is
> there a schematic of this somewhere on the web ?
> Casey
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--
Scott Bernardi
sbernardi at home.net