One of the things to be aware of when you want to control an analog circuit from a digital output is that the logic level can vary slightly depending on what the other gates in the package are doing as they load the on-chip supply rails. When you show us the schematic, we'll be able to advise if that might be modulating your control voltage and maybe producing the unwanted echo effect.
If you don't have a favourite schematic package yet, simply grab a sharp BB pencil, white paper and a ruler and give us a hand drawn schematic. Scanned is better than photographed, but if you haven't got a scanner, and you are careful with focus, lighting and to have the lens centered over the paper, you can get good enough results. HB and harder pencils often don't leave a clear line when photographed, and its a PITA having to go back over inking lines, or working in ink with all the problems that brings correcting mistakes, just to get a good enough image to share.