CV Mixer?

[Harry Bissell]

Hi Paul

For a mixer you are going about this the wrong way....
You should use the opamp as an inverting summer (with the + input grounded,
100K feedback to negative in, and both 100K resistors to negative in.)

This will invert the CV, so you follow this stage with another inverting op
amp...

The reason this works better... the - input is actively held at the potential
of the positive
input (by the feedback). So the voltage at the -in is zero.  The signals
comming in CAN'T
interact with each other... they are completely independant. Moving one pot
will not
affect the other pot.

If it is important to you that the pots be "linear".. then the 100K rsisitors
should be bigger
than the 100K pots so loading does not occur (parallel resistor combination...)
With 100K pots this might not be practical (you would need resistors 10x that
value... rule of thumb... or 1Meg) So use a non-inverting buffer on each pot
wiper (like in your schematic), then into the summer.

Yes its possible to have inverting/non inverting gain. Lets see what everyone
else posts
and I'll submit thatever way they don't show you.....

H^) harry   (available on or off list)

Paul Wilkinson wrote:

> I need to make a CV mixer.  I want to be able to scale a couple of input
> voltages and add an offset.  I was wondering if someone could comment on
> this circuit (no laughing).
>
> Do I need to 100K resistors between the pots and the follower?
>
> Also, how hard is it to have the voltages invert when you turn a pot to the
> left of 12 o'clock?
>
> - Paul
>
> v in
> |
> |
> |
> 100K <----- 100K ----- follower (see below)
> pot
> |
> |
> |
> gnd
>
> v+
> |
> |
> |
> 100K <----- 100K ----- to same follower
> pot
> |
> |
> |
> v-
>
> follower looks like this:
>
> |\
> ---|+\
> |  \________________o  out
> |  /     |
> |-|-/      |
> | |/       |
> |__________|
>
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