Just finished a Nurse Quacky modded for bass

[Earthscum]

Like the title said.

First I just boarded it as was. The sensitivity seemed to always be high to get it to quack. Figured it's the nature of the circuit so it can be used with hotter pickups.

First mod was to the input of the filter. Since I'm using this for bass, I just popped in a .1uF cap at the input. I tried others, but I decided on the .1 just to ensure my lows could always get through. Not sure if it will really make much of a difference until I get back on a stage that I can DI through their subs. I also changed the output cap to .22u, and both are probably overkill, but since I don't get any flubbiness or anomalous behavior, it works for me.

Anyways, I tried a bunch of feedback caps, mainly referring to the Morley schems for reference. I finally decided on the NQ's .0047u at the output side, and swapped up to a .022u on the inv. input side. Makes my bass say "WOW!"

I was pretty happy with it, so I started populating the PCB. When I got done and tried it again, I wasn't as happy with it. Still worked the same, just wasn't as flattering... the lackluster wore off or something. So I googled, and got a brainy idea.

I increased the feedback resistor in the filter, as per Mark Hammer's advice. I just added in a 150k for preliminary. I'm almost totally happy with it now, but I think I may add in a 200k or 250k pot instead, to adjust the Q, and take up pedal face. 

Last mod is my brainy idea to just allow more bass to enter the envelope section. It seemed like I was having to pound the E and B strings to get the same sweep as D and G, so I changed the .01u input cap up to a .047. Now I actually have to tap the higher strings just a bit harder to get the quack back, and that's a GREAT thing for bass, IMHO. The lower strings are a bit easier to control, and now I don't get the overshoots that make your ears bleed when I start plucking higher on the fret board.

I'll get some pics up when I get it boxed up, and maybe rearrange some recording gear and get a soundclip. This thing's awesome sounding with dead strings, and mind blowing with new strings. My GF's cheap Silvertone really agrees with this thing, so I may be making another one, lol.

[funkxero]

thank you! exellent!.

you can put a scheme, Do not speak Inglés
Greetings from CHILE!

[Mark Hammer]

The thing with envelope-controlled filters is they have to be acting on frequency content that is actually available, in order to have any audible effect.  That had two aspects.  First, the signal source actually has to produce that content, and second, the content still needs to be around as the filter does its sweeping.

In the case of bass, all of that harmonic content is shifted downwards, but because of the increased stiffness of the strings, it also doesn't hang around for as long as it does with a guitar.  So, when adapting an auto-wah for bass, not only do you need to shift the range of the filter downwards, via the filter caps, but you probably also need to reduce the decay time so that the filter is on the downward part of the sweep while there is still harmonic content to filter.

[funkxero]

how I can avoid(delete) the saturation?

[Mark Hammer]

What do you mean by "saturation"?

[funkxero]

mmm distorcion! a little on the lower strings

[funkxero]

That happens when the POT RANGE is nearing the end 

[Mark Hammer]

That is "envelope ripple".  When the note decays, it does not do so evenly.  Remember, as well, that the diode has a forward voltage, and unless the signal is above that forward voltage, nothing reaches the transistor.  So if the signal falls just above and just below that forward voltage, very quickly, by very small amounts, as the note decays, you will hear that as a very very quick up-down sweep of the filter.  So fast, that it sounds like distortion.

There are three "cures".  One is to mute your strings as fast as possible so that there IS no decay.  Another is to use a much larger value than 10uf (22uf) to smooth out the ripple (which the Quacky does), but that results in the filter taking a much longer time to sweep back downward.  A better solution is to use a smaller value cap to produce a faster decay, so the sweep goes back down before he string reaches the point where ripple starts to occur.

[funkxero]

el cap de 22uf reemplazarlo por el en10uf?

[Mark Hammer]

No.  I was thinking about the Dr. Q, which DOES use a 10uf cap.

To use the Nurse Quacky with bass, you should try a capacitor value smaller/lower than 22uf, like 10uf, or even 4u7.

[funkxero]

Excuse me, but to cap you mean in the scheme?? http://www.home-wrecker.com/nurse-quacky.html

[Mark Hammer]

Si.

[Earthscum]

On analysis of the envelope section, I noticed that at max Sens. the 47k and .01u form a HPF at about 340Hz. At halfway down, it passes starting around 164Hz.

With the .047u, mine is passing 72Hz at max, and at half Sens. starts about 35Hz. This explains why the Sens. pot seems to start to lose "adjustability" after about halfway. I didn't really look at the section as close before, and chose the .047 as a crapshoot and it worked great. Now I may bump up to .1u based on what I found.

I'm also going to swap back to the 10u I breadboarded with (don't know why I put the 22u in population, honestly). As well, I'm bumping the 3.3M up to a 4.7M or 5.6M to compensate. With those mods, I may leave the .047 alone. Right now, my Range knob sits at about 4/5 the way up on average. With the faster decay I was able to coax more vowel sounds out of it.

BTW, if you want a  round-about of what it sounds like with the caps I selected, it can reproduce the bass to a 't' in these songs: Ween - I Can't Put My Finger On It and (bad word warning!!!) Ween -  Big Fat F***. I love the sound of the  bass in the second one. (the band-passed breaks are actually the keyboardist). The first one is a Brass Master and a Dr. Q, if I remember right.