Accursed Nurse Quacky

[mlabbee]

She won't Quack!!!  I built off the home-wrecker.com/GGG schematic.  No sweep!  The filter section works fine - I get a wah effect if I fiddle with the range when I play, so I'm guessing my problem is in the envelope generator.  Does anyone have the pin voltages on Q1?  I'm getting .3 V on the base and 0 on everything else - any thoughts?

[Mark Hammer]

Simplest thing is just to check for signal with your meter.  Leave your instrument plugged in, and check for *AC* voltage at the output of the sensitivity control and at the output of the  envelope follower gain stage.

You should see at least 50mv at the wiper of the sensitivity pot when you strum, and a few volts at the output of the op-amp when you strum hard.  If you don't then perhaps something is amiss with the pot or the op-amp.   It also happens sometimes that people get the FET wired up wrong, and still hear enough signal to notice the functioning of the filter, despite not having enough signal making its way through to the envelope follower to force a sweep.

[B Tremblay]

What LED activity are you seeing?  One should be constantly lit and the other will light in resposne to your playing.

What transistor are you using and are you sure of its pinout/orientation?

[mlabbee]

Well, I thought I had it.  Before my last "fix" one LED was always on and the other never lit up.  While followig the suggestions, I notice I had the unlit LED in backwards.  I tried turning it around, but now nothing works. Aaargh.  

I'm using a 3904, I've checket the pinouts on my DMM and the HFE looks good . . .

[Transmogrifox]

I agree this is the accursed circuit.  How did EH even market these things?  It bewilders me what would tempt a guitarist to pay $230 for the original unit...that is, if it's priced like other EH pedals.  Even to pay $50 for it would be insane.

Now that my rant is over, I think your problem is most likely in the op amp stage somewhere.  Either you got a resistor with the right colors, but were off by a factor of 10 on one color, or you just accidentally connected something wrong and overlook the same mistake each time you go over it.  We all know that the circuit from that schematic does work, so it's just a matter of re-checking the entire circuit, including the JFET input as Hammer suggested to be sure pinouts are correct and that everything is consistent with the schematic.  I know debugging can be a pain in the arse, and especially with this Nurse Quacky circuit because it's so finnickey.  It honestly took me quite a little toying around to get mine to work right.   I never did like it.  It's too distorted sounding and its response is hard to deal with.

Therefore I built my own envelope filter with a 5 pole smoothing filter:

www.geocities.com/transmogrifox/ef1

[Mark Hammer]

In 1980, the piano player in our band paid $30 new for a DQ, and maybe $40 for a Small Stone.  How the prices went up is anothermatter, but they were quite cheap when they were young.

[Transmogrifox]

If I were only 20 years older--maybe I would have bought a bunch of them then.  You could sure make a killing on the real legitimate "vintage" EH pedals now if you could buy them for $30 then.  I bet the Dr. Q was pretty cool at the time when the envelope filter was a relatively "new" thing for guitar.

[Ge_Whiz]

I'm trying to suppress a grin while reading this thread.

One of the first pedals I wanted to build was a Dr Q, then I heard about how finicky it was. I don't like the idea of a design that is overly-sensitive to the type of op-amp used in it.

Then I discovered the Nurse Quacky and set about designing a veroboard layout for it. I kept making silly mistakes and got frustrated so I gave up.

Then I asked myself if could really find a use for this type of effect - I ain't a 'funky' player - and threw together a quick 'n' dirty build of Tim's 'Phuncgnosis' to trial the sound.

I liked the Phuncgnosis so much I boxed it up and never looked back!

[Torchy]

crp

[Mark Hammer]

The basic circuit is a pretty easy perfboard build, all things considered.  It is a simple affair to add another op-amp or transistor stage to boost the level, and an equally simple affair to add a volume pot afterwards to adjust effect/bypass level-balance.

If one is willing to use vertically oriented resistors, and can score some modest-sized caps and small LEDs, the whole thing can be stuffed into a 1590B with pots for sweep width, attack time, and output level (in addition to a Q and Mode switch) with little difficulty.  If you want the deluxe version, grab a 1590BB and you can add a toggle for decay range, reroute the trimpot to a chassis-mount for range fine-tuning, and throw in another few bits and pieces for downward sweep (someone described this here long ago using a P-channel FET in place of the NPN bipolar), as well as a jack for external keying (for example by an LFO, an envelope generator, a drum box, or what have you).

Of course I say this NOW, after having built a number of them successfully, and preceded by spending several exasperating years trying to debug one that was built with a buggy switch and wire that was fractured inside but had the insulation intact.

Once you get over the construction hurdles that are common to ALL perfed or PCB projects (e.g., misreading pinout or LED orientation, cold solder joints, bad pots, etc.), this IS a relatively easy project with easily identifiable sources of difficulty, and easy remedies.  Hell, don't you wish ALL projects came with an LED indicator to let you know if there was signal or not?

You may have noticed me mentioning the Q, and I imagine some of you went "What?  I can adjust the Q?  How?  How?  Tell me NOW!".  IN that type of filter topography, the Q is essentially given by the gain of the filter section formed around the op-amp.  In the case of the DQ/NQ, the 470k feedback resistor can be increased to up the gain and selectivity of the filter, or decreased to do the opposite.  Just note that it gets UGGGGGGG-LEEE with noticeably more gain.  I wouldn't go any higher than maybe 820k.  Also note that if the Q is reduced to have a subtler, less resonant sweep (and maybe some of you who have built an MXR Envelope Filter clone can attest to how nice a slower, less resonant sweep can sound in some cases), the volume will be reduced and the effect/bypass level-match buggered up even more.  This is why I suggest maybe tacking on a gain stage of x5 or x10 after the filter section, with a volume pot to trim back.  This will let you compensate for too much and too little level at the filter output.  I will also point out the benefit of having yet another op-amp stage where you can stick in a well-chosen feedback cap and trim off some of that hiss.