[sdiy] Re: Seeking examples for Wed lecture

[Grant Richter]

You can do that "inject to different stages trick" with an OTA  
Butterworth low pass. Just use the Sallen Key feedback scheme with  
the Butterworth "cells".

I built one where each stage cap goes to a selector switch which goes  
through a cap to the feedback amp.

So you can add feedback from the output of stage 4 to stage 1,2,3 or  
4. The "cells" have to be non-inverting.

The difference shows up when the filter is overdriven. Because the  
feedback signal is filtered by 24, 18, 12 or 6 dB.
The less filtering on the feedback signal, the more "snarly" sounding.

Another variation on a theme.