[148] Dr. Quack Envelope Filter mods

Date: February 23rd, 2009 | Comments : [5] | Categories: DIY.

Dr. Quack 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!


5 Responses to “Dr. Quack Envelope Filter mods”

[637] Brenton Says: 6:11 pm, March 23rd, 2009

could the filter be full wave rectified if i used a rectifier bridge instead of the red led?

[670] piod Says: 12:25 pm, April 16th, 2009

Sound yes. but not Quack why? When turn pot Range the sound is envelope.

[766] Pete Says: 6:36 am, August 7th, 2009

I built this some time ago, and it didn’t work (sound, but no quacking).
Today I changed the 470k resistor to 2,2M and now it works.
But just with my Tele and very hard picking.
choosing higher values or even leaving the resistor out makes it more quacky, but also adds distortion (bad trans. distortion ;-))

Are there any other mods I can try?

PS: I used RG Keens layout (that’s why I’m a little bit disappointed, because his layouts always worked very well…)

Thanks, Pete

[6424] Veeence Says: 3:16 pm, March 30th, 2014

In regards to the sensitivity, I futzed around with the 470k/2.2 meg switch that Pete mentioned without bringing it into a playable range (using a bass guitar). Anything above 2 megs doesn’t seem to have much affect. After poking around, I ended up halving the 10k between the 9v and ic1a. This pushed the sensitivity way up and didn’t let any magic smoke out . I’m not much of an electronics guy, so any feedback whether this is a good idea or not would be appreciated.

[451348] Hackman88 Says: 9:32 am, December 13th, 2014

In own, i just made some changes some of the parts in the schematic you can still use the indicated potentiometer 100k and 25k try changing this parts.. now.. in the schematic at the INPUT you can see the capacitor is 0.05uf(503) change that into 0.1uf(104) then next is the resistor they said before that the 470k is the original then they change it into 2.2M as you can ee in the schematic its indicated as 2.2M now change that into 1.5M then there is a diode which is the 1n914 you can straight it through or you can put res. from 100 to 1000 ohms but in my own way i did put 1.2k so thats it so good luck……


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