Jack Orman's Dr. Quack


  • Range
    controls how much the filter can sweep
  • Sensitivity
    controls the sensitivity of the envelope follower to the input signal
  • Normal/Bass
    switches between 2 frequency ranges, one for guitar, one for bass


PCB available from General Guitar Gadgets

mods by Mark Hammer

  • Find the 100 (or on some schematics 50) ohm resistor after the RED LED (or silicon diode for the Dr. Q) following Pin 1 of the opamp. Also find the 10uF cap at one end of the resistor that goes to ground. The resistor sets the attack time, which Mark explains is the time taken to charge up the capacitor. The capacitor in turn sets the decay time.
  • Make sure the resistor is 50ohms (I used 47 with fine results). Change if necessary. Insert a 1K pot in series with this resistor to act as a 1K variable resistor. To do this, connect the side of the 50ohm resistor that is not connected to the opamp to an outer and center lug of the pot. Then connect the other outer lug to the silicon diode and the 10uF cap going to ground.
  • Add a 10 to 20 uF cap in parallel with the 10uF cap to ground that is switchable. This lets you increase the total capacitance (adding parallel caps adds their capacitance together) to 20 to 30. I added a 22uF cap in parallel with a SPST switch. When the switch is closed the decay time is increased, which you here as a smoothing out of the sound. This makes the unit less squirrelly by letting you strum chords and playing rhythmic stuff without hearing "ripple."
  • Use a low noise opamp, such as the NE5532 or LM833N. This dramatically reduced distortion!
  • increasing the value of the cap to ground in the envelope detector circuit of the DQ will help alleviate much of the perceived distortion it produces, by smoothing out gaps in the envelope signal, which in turn result from use of a half- wave rectified envelope. The down-side is that this extra filtering will extend the decay time of the envelope detector since the cap will take a bit longer to discharge. I used this to my advantage, by switch-selecting between two different decay-setting caps to get two different flavours of response time. The existing schematic for the DQ shows 10uf. Add another 2-5uf on top of that, and the ripple will be reduced a bit without a substantial change in response time. Add 22uf in parallel with the 10uf and you get a longer decay time with very little ripple. MUCH better for rhythm work.

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envelope-controlled envelope follower amplitude controlled fx pedal stompbox stomp box guitar effects pedal filter eq/excite/filter bandpass bp bpf band-pass filter diy
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