DIY Electronics Question

[Paul Maddox]

Paul,

> Basically: it all works fine from CV 1 to 2 but I need to block the flow
> from 2 to 1.

ok, disconnect what ever you have feed the LPF2 CV, then insert this

 unity gain, non-inverting, summing, buffer circuit (bad ascii art alert)

              ______________ to LPF1 CV input
              |
CV1 ---+-----|\
                      | \_________ to LPF2 CV input
                      | /
CV2 ----------|/


this way, CV1 and CV2 will be summed and control LPF2, but with an
opamp (and the way it works) CV2 will not be passed back to LPF1.

This article talks about non inverting summing opamps,
http://masteringelectronicsdesign.com/the-transfer-function-of-the-summing-amplifier-with-n-input-signals/

They're not common for a number of reasons, as it says.
so you might be better sticking with a two stage approach, an
inverting summing amp, than an inverting amp to get what you need.
This can still be done with one chip (TL072 for example as it has two
opamps on a single 8 pin chip).

Paul