Saturday, 12 January 2013

Dr. Quack

Jack Orman's fixed version of the original EHX Doctor Q with buffer at the input to increase input impedance plus many more fixes..

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!


  1. Alright, i just built it, but i get a poor effect in terms of volume, also, only one of the LEDs works, the one next to the ic doesn´t, are both supossed to light up? Tomorrow i´ll check everything. I also used 47n intead of 50n, could that be the problem?

  2. I was checking the layout and noticed that, according to the schematic, the 10k resistor that is next to D1, should be 1 row up, making contact with D1 positive and pin 3 of the ic. I made that change and now it works great! it has a very nice sweep and gets a volume boost in the "bass" setting and also gets bassier (obviously). Also, D2 lights up according to your playing and D1 just stays lighted. So, i guess you can tag it as verified after fixing the 10k in the layout. :)
    Greetings from Ecuador!

    1. Great! Thanks for noticing and verifying! I fixed that 10K and tagged it!

  3. Just built this as above, and it doesn't work very well at all. Had a good check over the layout against the schematic and it looks like 2 of the 50nF caps coming off the collector of Q2 need to be 0.005nF, like in the EHX Dr. Q layout. So I swapped them for 2 x 0.0047nF caps and it now works great.

    1. I genuinely thought i had fixed it. Sorry for that.. Layout updated.
      And thanks!

  4. Can i use a 20k pot instead of 25k pot?

    1. I think I used a 20k (possibly 22k) on the Nurse Quacky so seeing as they're similar circuits I'm sure it'll be fine.

  5. This is verified? The sensitivity seems to be a rate, since it speeds up the oscillation, and the range seems to be a sensitivity, since it changes the effects sensitivity to attack. Also, both pots do next to nothing before fifty percent. Maybe use reverse logarhythmic or such? The LED between the resistors does nothing, just stays on, so I think I will use a 3pdt to switch it off at the cathode. Also, rc4558 has kind of a dirty wah, and tl072 makes it cleaner. Btw, my only deviation is that I used a 1n4148.

  6. Whoooooooo!!! Try ic Opa2604 for mi it's the best choice, great tone and filter more open, the pots turns very interactive, i think.
    Great board!!!! Thanks!!!

  7. Hi, can I change the size of D1 and D2 from 3mm to 5mm without adjusting other parameters? Do the changes affect the sound? I'd like to change the LED size, because all my other pedals are equipped with 5mm LEDs, too.

    1. The easiest way is to socket the LEDs and try them. It shouldn't make a difference

    2. I will do, thanks. But I still have to order the parts.

    3. Another build is verified! I just wanted to let you know that it works great with a 5mm LED (I used a green one instead of the 3mm red D2). I also attached a standard red 5mm LED to the foot switch (True Bypass). And I added a SPDT to be able to switch between the 2.2M resistor after D2 and another resistor. But I'm still trying to figure out the best value of the second resistor. Regarding the IC I'm still thinking wether to go with the 4558 or the 5532. Both sound really nice. And I subbed the 5nF with 4n7 and the 50nF with 47nF.

  8. hi,
    i am having similar problems, as the ones stated above.
    i can;t get the D2 to respond to my picking unless i pick real hard.
    basic components are: Q1 2N5457, Q2 2N3904, IC TL072.
    i have placed a 1M resistor in series to the 2.2M, and replaced the 3mm D2 Led with a 5mm yellow one.
    it's a bit more responsive, but the output is almost identical as my clean tone.
    i have noticed that if i touch the 2.2M resistor with a screwdriver, the D2 Led is working as it should, but with a lot of noise.
    can someone please explain what is going on? or
    can someone provide working voltages?

    1. hi George!
      do you mean 2x1M resistors in series cause you didn't have a 2.2M?cause i can't see the reason why you made it 3.2M.
      anyway. i think that there is no problem with this layout.

      thought i've only build the e-h doctorQ, i can't guess why AMZ replaced the trimmer with a pot, cause i remember that there was only one spot of it's sweep that provides the effect.did you found any spot on that range pot that gives you a better effect? if not i guess that there is a problem with your build man. you could post some voltages and photos.

    2. woo hoo!
      Savvas, i got it working!
      i indeed measured the voltages, and although i am a new to diy, they didn't seem strange to my eyes,so, i tried different Leds for the D2 position.
      my build likes best 5mm blue led for D1 and 3mm red or 5mm yellow.
      blue/yellow led combo sounds better and it's more visually appealing.
      i have also switched the IC from TL072 to JR5458, and added an extra 10K (in parallel) to the 10K between the 9v and ic1A, following this suggestion (last comment)

      my voltages (pots at max. position) are :
      Q1 (2n5458)
      D: 8,76 (same as my battery)
      S: 5,86
      G: 2,92

      Q2 (2n3904)
      E: 0,00
      B: 0,22
      C: 0,00

      IC (TL072)
      1: 3,20
      2: 1,84
      3: 3,20
      4: 0,00
      5: 8,59
      6: 4,38
      7: 4,37
      8: 4,27

      D1 (5mm blue Led)
      (+): 3.20
      (-): 0.00

      D2 (5mm problematic-blue Led)
      (+): 3.20
      (-): 0.61

      D2 (5mm working-Yellow Led)
      (+): 2.80
      (-): 1.33

      ps: the resistors in series are 2.2M +1M. i followed this mod: "The 3.3M gain resistor, raised from 2.2M in the original, results in a stronger envelope signal. (Mark Hammer)"

    3. well done george! i'm glad you made it!
      i was about to open mine and take voltages.
      to the next one!

    4. thank you Savvas!
      now i know what it means to be build a sensitive circuit.
      it has a fairly small sweet spot and the leds must be matched.

  9. It does seem weird that Q2 has 0v on collector and emitter. Does that change with the envelope?

    1. emitter is fine cause it's connected to ground. but for collector you could be right Travis..even if sw4 and sw5 are connected, there is still a 47k so 0 voltage is weird.
      but maybe i'm totally off on that one..i can't say for sure...

  10. Oops sorry I didn't reference the layout very carefully obviously. Just found it odd that there's 0v on the collector really

  11. This is a great build, sounds fantastic. Used 2N5457, 2N3904, 3mm red LEDs and TL072. Put it before my Mutron III clone and with both going you get some wacky sounds. Going to have alot of fun with this one.

  12. Yes. Anything up to 22 wide will fit a 1590B

  13. hai..saya telah membuatnya dan berhasil dengan sangat baik dengan J201 sebagai jfet nya.tl072.dan led 5 mm biru terang..tetapi saya mendapatkan sinyal output yang kecil....apakah dapat membantu..?
    terima kasih untuk layout yang hebat ini..