Info from AMZ: The AMZ Dr. Quack is an updated and improved version of the E-H Dr. Envelope Filter pedal. Here are a few notes to help anyone building this circuit. Please refer to the complete schematic on this site for reference.
The opamp of choice would be the 4558, which I used in building the prototype. The 5532 has low input impedance but good output drive, and should work as well. The 4559 would also be good. The circuit should be fairly insensitive to the opamp choice.
(1) You can omit the 1N914 as it is not really needed; short it out or substitute a resistor (100 to 1000 ohm),
(2) The transistor choice makes some difference in response of the filter sweep. Don’t use a 2N5089 or other high gain transistor. Use the lowest gain silicon that you have.
(3) The trimpot adjustment is critical and has a fairly small sweet spot. It may take some fine adjustment to make the circuit work well.
(4) The circuit has a half-wave rectifier… this means there will be more ripple in the response and decay than other envelope filters. Not much you can do about that except increase the value of the capacitor after the red LED-100 ohm envelope drive. Make it 22uF and see how that sounds. You can even go to 47uF.
The color of the LED isn’t going to make much difference. You could use a red for the bias and a green for the opamp output but it will make very little difference. The forward voltage of the LED doesn’t change much as the signal varies since we are dealing with small currents in this application. I’d just stick with 2 identical red LEDs.
These tips should be enough to ensure that your quacking filter will be a success!