[DIY-PCB] Negative / Positive CV input circuit

[Harry Bissell]

Harry Bissell writes:

John: Thanks for the kick in the seat of the pants (gets me off my dead ass).
You're right. But with a slightly different circuit. If you ues the wiper as
the input you are always screwed. The circuit attached will do the trick. Make
all resistors the same value as the pot, except the two feedback and the
resistor from stage 1 output to stage 2 input. I can't claim the circuit, PAiA
has used this as a "mixer". My spin is to be a positive/negative CV input
(with multiple stages).

I didn't like the 100K values (too high for me). The actual input impedance
here is 28.5K with the wiper centered. and 30K at either extreme. Close enough
(for rock and roll).

If any DIY-PCB folk have comments or questions, please contact me... or I'll
put it in our schematic.... Harry


JWBarlow at aol.com wrote:
Hi Harry:

Thanks for this info! I've been using this as a CV input for VCFs and VCAs 
for a while but I'd not even noticed the gain change due to the fact that I 
have several different module types that are all mixed in my system, so I 
often have to add gain just to make things compatible. I'm wondering if the 
gain problem might be correctable by choosing different feedback resistors in 
order to compensate for this -- any thoughts here?

John Barlow

In a message dated 5/8/99 6:32:31 PM, harrybissell at netscape.net writes:

>Harry Bissell writes: That might work. I think the gain might be a little
>funny because when you put the wiper of the pot all the way to one side
>you
>would have effectivly, 200K in series to one op-amp, and 100k to the other.
>With 100K feedback resistors, that would give unity gain in the first stage,
>and -1/2 gain in the other. So the circuit would divide CV by two, which
>could
>be scaled back later (in the scanner itself). Or if the input CV was high
>enough, you wouldn't care.... Nice Idea I will remember it... If noone
>has
>objections for other reasons we can use it, so I'll wait for more comments.
>
>The "varying input impedance" of the circuit as I drew it is usually only
>a
>problem when it is driven from a high impedance source, or when the source
>changes frequency response with different loads (like a guitar pickup,
>microphone, piezo sensor etc...) Usually CV is buffered output with about
>a
>series 1K resistor, this makes the error usually about 1-2%
>:-) Harry
>


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