<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; "><BR><DIV><DIV>On Apr 5, 2006, at 2:41 PM, Logan Mitchell Sr wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; ">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</SPAN></BLOCKQUOTE></DIV><BR><DIV>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.</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>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)</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Here's the Oberheim SEM schematic: <A href="http://www.synthfool.com/schematics/sem.gif">http://www.synthfool.com/schematics/sem.gif</A></DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>And here's the 3080 data sheet: <A href="http://www.elby-designs.com/datasheets/ca3080.pdf">http://www.elby-designs.com/datasheets/ca3080.pdf</A></DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV>Good luck!</DIV><DIV><BR class="khtml-block-placeholder"></DIV><DIV><BR class="khtml-block-placeholder"></DIV></BODY></HTML>