Serge-style CV inputs

gstopp at gstopp at
Wed Mar 13 21:51:38 CET 1996

     You drew what is called a "Reversible Attanuator". These are found on 
     Moog modulars, the Micromoog, the Multimoog, and probably a bunch of 
     other machines as well. Some comments about the ASCIImatic:
     1. You can make the output op-amp a buffer, rather than an inverter.
     2. You need to use a linear pot.
     3. You need to hand-select the pot to find one that is exactly 50% of 
     resistance at the center of rotation (they're all different).
     4. You need to drive the thing with a low-impedance output from the 
     source, or else you won't get exact cancellation at the center of pot 
     5. You can buffer the input to solve the problem in (4) above, and 
     just use the wiper of the pot as the output, but that should be 
     buffered too if you want to maintain your output impedance standards. 
     Yes that makes 3 op-amps, but hey buffers are easy to build.
     - Gene
     gstopp at

______________________________ Reply Separator _________________________________
Subject: Serge-style CV inputs
Author:  Christopher List <Christopher_List at> at ccrelayout
Date:    3/13/96 2:29 PM

    |------------------------------+         1K
    |             100K             |    +--/\/\/\-+ 
    |          |-/\/\/\-|     100K \    |  |\     | 
    |          |        |     Pot  /<---+--|-\    | 
    |  100K    | |\     |          \       |  \---+---- CV Out
CV -+-/\/\/\---+-|-\    |          /       |  /
                 |  \   |    1K    |    ---|+/ 
                 |  /---+--/\/\/\--+    |  |/
             ----|+/                   ---
             |   |/                     -
(I hate ASCII-matics) If your CV generators have a 1K output impedance, this 
should work out OK, right? I suppose it'd make more sense to throw another 
op-amp in there to buffer the non-inverted input and insure a 1K impedance for 
both voltages into the pot, but this means an odd number of op-amps, and who 
likes that? 

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