[sdiy] Technical questions about how to setup a working CA3080 circuit from Logan

[Michael Bacich]

On Apr 5, 2006, at 2:41 PM, Logan Mitchell Sr wrote:

> I recently constructed a Low Pass VCF using a pi-type configuration  
> with the last resistor connected to pin 2 of a CA3080/NTE996 with a  
> 10k resistor also connected to pin 2 & pin 6. I tried to turn the  
> filter on & off by connecting my ADSR's output to the CA3080's pin  
> 5. I put a 2.2k resistor from pin 3 to ground but got no response

Did you connect the ADSR output directly to Pin 5 with no current- 
limiting resistor in between?  If you did, you may have already  
irreparably damaged your 3080.  You have to limit the control current  
(the Amplifier Bias Current, or Iabc) at Pin 5 to an absolute maximum  
of 2 ma.  For a maximum control voltage of 10 volts, that means you  
must have a resistor of at least 5K to limit the input current,  
otherwise, you will destroy the 3080.  However, the 2 ma figure is  
just a maximum rating for the device.  In real-world practice, the  
necessary control current will likely be much lower, and the resistor  
value will likely be much higher -- look at established working  
circuit diagrams for examples.

Many of us prefer to use a transistor rather than a resistor to  
convert the control voltage into a control current, and to couple  
that current into Pin 5 -- usually a PNP transistor such as a 2N3906  
or 2N3906.  This method is also seen in many 3080 circuit examples,  
for instance, it's done that way in the Oberheim SEM.  The SEM VCA  
uses a 2N3906 PNP transistor for this purpose, with its base  
connected directly to ground, its emitter connected directly to Pin 5  
of the 3080, and its collector connected to the envelope generator  
control voltage via 47K resistor (you may need to adjust that value  
somewhat, depending on your actual maximum control voltage).  This  
configuration (and that circuit) works pretty well as a general- 
purpose VCA, and can probably also work (with some tweaking) as  
variable stages in a filter, as you are attempting.  (note that the  
SEM filter also uses 3080's, possibly similar to what you're doing)

Here's the Oberheim SEM schematic:   http://www.synthfool.com/ 
schematics/sem.gif

And here's the 3080 data sheet:   http://www.elby-designs.com/ 
datasheets/ca3080.pdf

Good luck!


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