A general purpose OTA?

Discussion in 'General Electronics Chat' started by spyrusthevirus, Nov 20, 2012.

  1. spyrusthevirus

    Thread Starter New Member

    Nov 20, 2012
    Hello fellows,
    I have been looking into some analog synth modules, and no matter where I look the OTA seems to be the heart of every one of them. Or something equally important anyway.
    As you are surely aware, the rediculously widespread CA3080 is now rediculously priced and out of production (since 2006 I believe?), and the only "alternative" that I know of is the 13700 and 13600 chips. Which is not an alternative at all, since those are dual OTAs, and 2 euros a piece is a considerable amount.
    Which brings me to the big question, are there any general use, low cost and still in production OTAs? nothing fancy, just something to get the job done nice and easy. I'm looking for a price like 0.8 euros, not in huge quantities (something like 5 )

    *I am shopping from the Bay, because the shipping fees from major electronics stores on the internet are pretty high and require big orders (I'm located in Greece). Even if there are a couple of stores remaining that won't drink your blood to give you a CA3080, the question of what to use in its place in the future remains.

    Also, I'd welcome a few ideas of doing the same thing with normal opamps and transistors-just a rough guideline to tailor it to each circuit's needs.

    Thanks a lot
  2. Audioguru

    New Member

    Dec 20, 2007
    OTA ICs are difficult to find and are expensive today because they are old and nobody uses them anymore. Soon the LM13700 will also be obsolete.

    About 40 years ago (when everybody except me made horrible distortion from their geetar circuits), normal opamps and transistors were used to make OTA circuits. The opamps worked poorly and the OTA circuits also worked poorly.
    I was designing and selling extremely low distortion products that worked very well and sounded wonderful.

    I worked with a Norwegian large intercom system that sounded very good. The Canadian government wanted bids for a new PA system in Canadian airports. Their consultant listed FM radio station hifi spec's (I know, the consultant was crazy because the annoucements were speech, not hifi music).
    I slightly modified the circuits in the Norwegian system (PAMEX), it met the spec's and won without any competitors.
    It used one of the the original OTA ICs, the NE572 "compandor" IC for auto audio level control (mainly to compress the audio if it was too loud so it did not have clipping distortion).
  3. spyrusthevirus

    Thread Starter New Member

    Nov 20, 2012
    sounds fun to me. I'm sure the deadlines and absurd demands of clients were not but hey, can't have everything.
    Anyway, any chance you can elaborate on those "semi-discrete" current/voltage controlled amplifiers? or direct me to a book/website/similar resource. I value the theory as much as the practical application, I like knowing what the heck is going on in my circuit. So the question remains, how do you modernise all those old designs? I haven't seen any current ones, using today's parts, most rely on either OTAs or various old obsolete stuff to make things work. Propably has to do with the fact that people don't want to make a huge modular synth anymore, there are VSTs for that.:(