Today I have had a second look at my NGF DC VCA http://www.haraldswerk.de/NGF/NGF_VCA_DC/NGF_VCA_DC.html with active feedthrough compensation. I want to see if there is another improvement with respect to input offset voltage and input bias current. I take out my proto board and tried to compensate for those faults with different methods to no real success.

Proto Stripboard

But using an OpAmp optimized for DC performance like OP27 drops the CV feedthrough nearly down in the noise floor. With active compensation it is down to 1mV over a CV sweep from 0..5V. Without active compensation it is about 2mV. This is a 5000 to 2 ratio.

Screenshot CV vs feedthrough

I think it is not worth the afford to use active compensation any longer. And you can have two VCA out of the LM13700. The key for low CV feedthrough is to lower the I_abc through the OTA and use DC optimized OpAmps. I use OP27 and 250uA now. The THD is still at 0,2 %. My knew DC VCA: NGF VCA DC due. http://www.haraldswerk.de/NGF/NGF_VCA_DC_due/NGF_VCA_DC_due.html