[sdiy] CA3080 has gone where noone has ever been

Spiros Makris spirosmakris92 at gmail.com
Thu Apr 8 15:36:12 CEST 2021


Part of why I found this little fact amazing was exactly that shortly after
the chip went into obsolescence (and has since reappeared in new fresh
prints, I believe?). They only use one OTA and LM13700 is a dual package.
Perhaps the answer is way simpler, and the designer behind this module just
happened to know of CA3080 and used it without overthinking it.
CA3080A was used, which has a slightly tighter gm spec and extended
temperature range (-55C to 125C). There is a note that says "Ordered and
received 300pcs - not one D?C -returned to vendor. REORDERED by RJ"
Berkeley wanted to be absolutely sure they won't run out, so according to
this spreadsheet they kept 150 units in stock, just in case.

You can have a look here, apollo.ssl.berkeley.edu enter this in an FTP
client (ie, Filezilla) and log in with a blank username and password. There
are a few other mission folders too, but none are as detailed as MAVEN is
(you will even find code, schematics, gerbers, tons of goodies).
AFAIK this is public access and, I guess, legal? I found the link by
googling.

On Thu, 8 Apr 2021 at 16:10, Benjamin Tremblay via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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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