[sdiy] CA3080 has gone where noone has ever been

Benjamin Tremblay btremblay at me.com
Thu Apr 8 15:07:16 CEST 2021


Awesome. I blew out a 3080 on the Radio Shack Moog in 1984. I had never heard of an OTA until I read the description in the RCA replacement parts catalog at Mountain States Electronics in Colorado. At first I thought, this is why it’s a Radio Shack product, Moog would never use this little chip in its own products. Of course I was wrong, and the schematic for an OTA sure looks like the schematic for a VCA with current mirrors on the differential inputs.  My tiny teenager brain could not process this. If it’s a current amplifier, why not just use the cheaper LM3900?  My mental functions could not get me to the logical next step: In the state variable filter circuit you built using a stereo potentiometer to control the cutoff frequency, two OTAs could replace the pots as “virtual resistors”. It never occurred to me that I could breadboard the OTA and control its gain with current without needing a voltage processing circuit. 
This is a perfect example of the walls that kept me from going deeper into synth-building. The first principal of the OTA is “current-controlled differential current amplifier”, but every article or schematic I could find jumped over that into paragraphs about how clever they were to normalize the input to modular CV and provide an exponential-to-linear converter. 
And of course my own experiments building VCAs were disappointing because I could not trim out all the thump. I thought it was because I was a dummy who could not grasp how it really worked. Nobody told me a good VCA thumps if the input is near-DC and the envelope is too spiky. 

Benjamin Tremblay

> On Apr 8, 2021, at 8:43 AM, Neil Johnson via Synth-diy <synth-diy at synth-diy.org> wrote:
> 



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