Nurse Quacky or Dr. Q mod for bass?

[JimRayden]

Do I have to change any caps and by how much?


----------
Jimbo

[Bernardduur]

I thought there was something on tonepad about this......

I adjusted it by replacing the filter caps from 0.0047 µf to 0.01 µf

here is some explaining of possible mods

[Mark Hammer]

Yup. You can set those caps to anything you want, but dropping them by an octave is a good start.  The range of the filter will be adjusted proportionally to the change in value.  So if the caps are doubled, the range drops to 1/2 (an octave down), and if they are quadrupled, the range drops to 1/4 (2 octaves down).

The other issue with using any autowah with bass is envelope ripple.  You will note that guitar synths were around for a number of years before bass synths entered the market.  The reasons? pitch detection time (when waveform peaks have more time between them, it takes longer to figure out the average inter-peak interval) and envelope ripple.

In general, more smoothing of the envelope signal will reduce ripple and eliminate the annoying trill/burr/buzz/gurgle as the note dies out.  This is done by the medium-value cap to ground after the diode and before the transistor (22uf in the case of the Nurse Quacky).  The problem with that is that,,,,,well, don't get us started on the "ideal" envelope follower again .

My advice for a bass-friendly NQ would be the following:

1) scrap the attack control shown (the 1k pot), and settle on one attack time /resistance you can live with.  Values for that resistor between 47R and 82R ought to be fine (I'm thinking 68R might be close to ideal).

2) stick a 33uf cap in place of the 22uf cap.

3) in parallel with the 33uf cap to ground, stick a 100k fixed resistor in series with a 500k log pot (linear in a pinch).  This will allow you to vary how long that 33uf cap takles to discharge, permitting different lag/decay times.

4) change the .0047 filter caps for .01 - .018uf (.01 will still get you mileage out of a guitar, .018 if you aim for bass-only terrain).

5) install a SPST switch/toggle that inserts a 100k-180k fixed resistor in series with the 470k feedback resistor in the filter stage (the one that straddles input and output) to get higher and lower resonance for more squawk..

[Buzz]

Hi all.

Firstly, sorry for the necro - bump. I have been meaning to make a nurse quacky for a long time, and for a bass guitar. This thread seemed suitable albeit old.

I have a lot of questions, but I will start with one. Have I got the right resistor for the switch mentioned in point 5?

[Buzz]

I am referring to the 470k marked with red asterix.

[Buzz]

Original layout:

http://www.generalguitargadgets.com/wp-content/uploads/ggg_nurse_quacky.pdf

Schematic:

http://www.home-wrecker.com/nurse-quacky.html

I have changed GGG's layout file. I hope no-one minds.

[duck_arse]

that looks to best fit "straddles input and output", so yes. have you looked at orman's "dr quack"? it has a bass/norm switch standard.

[Buzz]

Thanks Duck, I'll have a look.

[Buzz]

I applied the mods mentioned here to mirosol's layout. http://tagboardeffects.blogspot.com.au/2013/01/nurse-quacky_12.html

Ended up with this....



I threw it together today.

I don't know what I did wrong, but the results were kind of comedic.

If I hit the string softly, I get a clean note with some volume drop.

If I hit the string hard... whoop! Super loud quack. Doesn't matter what fret I play, I get pretty much the same pitch quack. Really LOUD!

I may have made the best prank pedal ever. Nearly gave me a heart attack.

I'm actually losing interest in making a bass autowah. The part of the one song I wanted to use it in is played in sixteenths at a lively pace. Too fast to work well with autowah methinks. I think I'll get closer to what I want with a phaser.

I would still ( for learnin' purposes ) like to know what went wrong.

A couple of differences from the above layout that might be at play:

I used an LM358AN. Does it differ from the LM358?

Out of 33uf electros atm. So I chucked in a 22uf + 10uf in parallel.

I'll get a recording of it and post it. It really is funny.

[duck_arse]

buzz, I ain't got the nurse schem, but looking at the doctor/dr schem, they have a 10uF after the rectifier, followed by a signal diode. what's your rect cap, 100uF?! by increasing the 100R in the rect you vary the attack time, and by stringing a pot (is that the 500k?) across the 10uF rect cap, you can vary the decay time.

[Buzz]

Here's the nurse schemo:



Here's one with the mods marked in red....



I might have buggered up the decay pot on the vero layout... vero does my head in.

Should have etched a board I guess. They have better visual logic.

[Buzz]

buzz, I ain't got the nurse schem, but looking at the doctor/dr schem, they have a 10uF after the rectifier, followed by a signal diode. what's your rect cap, 100uF?! by increasing the 100R in the rect you vary the attack time, and by stringing a pot (is that the 500k?) across the 10uF rect cap, you can vary the decay time.

The Nurse changed the 10uf cap to 22uf.  Mark suggests changing this to 33uf for bass. Yeah, that's where the 500k pot should be.

Nurse also puts the attack mod in, by swapping the 100R with 51R + 1k pot.

By undoing that mod I've actually made a redundant resistor... I've got 51R + 68R giving me 119R in place of the original 100.

Using the two resistors let me hack mirosols layout with more ease.

Apart from the whole failure thingo. lol.

[sevrien]

buzz?

should there still be a link parallel (under) with the 150K resistor and the switch?
seems weird to me, but what do I know about electronics, I'm allready happy when the LED turns on.
Perhaps the lower connection of the resistor and switch should be one row lower?

[Eb7+9]

I don't know what I did wrong, but the results were kind of comedic.

If I hit the string softly, I get a clean note with some volume drop.

If I hit the string hard... whoop! Super loud quack. Doesn't matter what fret I play, I get pretty much the same pitch quack. Really LOUD!

I may have made the best prank pedal ever. Nearly gave me a heart attack.

I don't think you did anything wrong,
it's one of those educational circuits where one stands to learn something by asking "why?"

two things going on here:

(a) active filter circuit tend to have a logarithmic or inverse SQRT response to variable (tuning) resistance

which means, the filter will traverse way more quickly thru the low-end part of the spectrum
read: hit the sub-audio range like butter, or very quickly stuck at a minimum value (yielding almost instantaneous loud peaking)

go look around on YT and you'll see this in pretty much every State variable filter demo ...
the same applied to Transmogriphox's SK-wah as well

[the std. two-transistor (Kushner-Plunkett) wah, otoh, sweeps the response via a linear scaling of the loop gain]

to remedy this issue you'd need some kind of expo-conversion to reverse the filter's non-linear response to "tuning" resistance
and that brings us into an overly complex design arena for most stompboxers
(let's leave that stuff to the synth guys ... eh)

(b) the side chain circuit ends up with a zero-bias BJT acting as a variable resistor

again, this is highly non-linear since we are looking at the diode response of the Vbe junction
as the "turn-on" function ... which is almost a sudden as a brick wall (hardly any play room)

again, one of those cases where "it works" // but, not well enough for good performance ...

you might want to try an n-channel jFET (2n5457) in lieu of the poor BJT
or convert the circuit to an opto-coupler approach

either way, you're looking at the 1/x response of the jFET VCR
or the slightly more stretched-out exponential turn-on specs of an opto-diode and resistor combo

both qualitatively similar to the BJT response, with the opto case probably being the slightly better one
(ie., Mutron-III)

in either case, there is no amount of capacitor tweaking that will fix these two obvious (shall we call 'em) flaws
the only reason why this might work better for guitar is that detection range is a little more "stretched out"
in the gtr frequency range ... but not by that much, since we're only one decade away in general

the "quack" appellation is fittingly well deserved ...
(notez: I don't have anything against ducks or a fast auto-filter responses, they're just not that easy to work with ...)

[R.G.]

Hi, J.C.   

[sevrien]

first of all, HAPPY NEWYEAR!

second,

I made my first PCB design based on previous posts.
Is there someone out here who could check this visualy and give me a second opinion?
I've checked it a few times but I'd like to be sure.

Greetings

[Mark Hammer]

I would suggest dropping the value of the 100k in series with the decay pot to 22k or so.  With a 33uf averaging cap you'll need to discharge that sucker faster to get a decay more suitable for bass, and that means a lower resistance.

I'm not clear why the pic shows TWO "Decay" controls.  Care to explain?

[duck_arse]

sevrien - please, edit the whitespace outta yer images. it will make it easier for normal people to view w/out scrolling.

there is a resistor from pin 5 to ground, next to the 47uF electro, is that meant to be 470k? and yer power supply - for battery powered doesn't really matter, but for external DC, +9V should connect to the 100R resistor, then the  100uF cap connects to the other end of the 100R, and that is your internal V+. the top of the two resistor voltage divider would connect to that point.

[if you remove the +9V green link, and shift the 100R to those two pads, then link across where the 100R was, it would be as I'm suggesting.]

there is little reason to have a 51R in series w/ a 68R when you could replace the pair with a single 110R or 120R resistor.

I haven't gone right through, but it looks ok otherwise.

[Mark Hammer]

There are a couple of redundancies in what you've drawn, such as the dueling decay pots, and the dual attack-time resistors (51 + 68).  I say this while simultaneously applauding the initiative you took to draw up a layout.

May be time to do a redraw, though.

[sevrien]

update:

This version is based on the homewrecker PCB.
That board had 2 resistors in series that doubled as bridges, this is reduced to 1 single resistor.
The power supply is kept as it was and is the same as the homewrecker board, not really sure why I should move the 100R resistor.
The double pot was a copy-paste error
Since I'm planning to etch the board using vinyl as an insulator, I tried to keep the traces big enough.