Serge-style CV inputs

Christopher List Christopher_List at
Wed Mar 13 14:29:57 CET 1996

Howdy DIYers,

This relates to a current AH thread on Serge CV inputs. Going through their 
catalog it seems that there's a lot of modules that have CV's with CV amount 
knobs that have + and - amount (depth). When the knob is centered, the CV has 
no effect, when it's all the way to the right it has max "+" effect, all the 
way to the left, it has a max inverted effect. 

It seems like there must be a simple way to do this - but I'm not savvy enough 
to figure it out off the top of my head. Possibly using the pot to somehow 
simultaneously control feedback in a follower and an inverter so that one is 
canceled out completely, and the other canceled out partially. (Thinking out 
loud here...) perhaps when the pot is centered, the normal and inverted signals 
are summed in equal amounts and when you go more to one side or the other the 
summing is weighted towards the inverted or normal input...

Anybody have any ideas? Seems like, if there is a simple way to do it, this 
would be a great way to control CV depth and eliminate the need for an inverter 
module. It also seems like this should be a stock circuit for any builder's 
notebook so I'd like to find the most elegant solution. 
How about this:

    |------------------------------+         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? 

- Thanks for any tips,

I know you guys are saying "build it and try it yourself", but my modular 
(including the power supply) are visiting a friend this week.

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