Nurse Quacky trials and tribulations

[Samuel]

My nurse quacky build took a turn for the interesting last night. I was having the "undersignalitis" that Mark Hammer described, where I was getting no envelope follower action except for when I really leaned into the guitar, and on the bridge pickup.

Was chatting with The Tone God, who suggested some resistor replacements to get some more oomph out of the opamp. So I started replacing things, and it was helping, but I hit the level of diminishing returns and wasn't getting quite enough out level out of it to drive the envelope filter still. On a whim, I pulled the TL072 that was in there and replaced it with a JRC4558. Big Difference!!!! Just need to set some of the resistors back to stock, cause I was sending the 4558 far into overload territory, but the thing was quacking like a charm!

Thanks Tone God! Now its time to read up on the specs of various opamps...

[Samuel]

Nurse Quacky success! Ended up going with a 10K attached to the sensitivity pot, I thought that gave me the best range...Here's some samples:

sample one

sample two

[smoguzbenjamin]

Hey Sam!
Although I decided to build the EA tremolo this holiday, this is sure as hell gonna be my january build... Could you give a schemmo for the parts you replaced? Cos your samples sound cool and funky! :twisted:

[Ge_Whiz]

Hmmm... curious. Can anyone explain why such an op-amp substitution should lead to such a big difference in sensitivity? I'm baffled.

[gez]

Hmmm... curious. Can anyone explain why such an op-amp substitution should lead to such a big difference in sensitivity? I'm baffled.

I haven't checked the data sheet, but it probably has more swing at the output so you end up with more control voltage (many op-amps can only swing within a volt or two of the rails - some are better that others)

[Mark Hammer]

I've quite forgotten what op-amps gave me what, but I tried substituting a variety of op-amps for the 1458 on the orginal Dr Q, and some gave diddley-squat in the way of sweep while others gave a sweep that was several times in excess of what the 1458 yielded.  With the stock circuit, a 1458 gave the ideal amount of sweep.

Given that, ultimately, all you want from the envelope follower is something that will not only track the sound but deliver a suitable voltage to the transistor serving as control element, I suppose tinkering with the trimpot and parts just ahead of it could resolve any problems created by use of a non-1458 op-amp.

Jack Orman's fix that became the Dr. Quack, and eventually Nurse Quacky, was intended to mimic the limits on voltage swing inherent to the 1458 but with any op-amp.  Samuel's solution indicates there is more than one way to skin that particular cat.

I might point out that it is really only the envelope follower section of the DQ/NQ where the voltage swing is pertinent.  The filter section can use whatever the heck you want.

[Samuel]

smoguz: two permanent changes -

1) Used a JRC4558 for the opamp. There is no opamp specified on the schem at runoffgroove I don't think, so this isn't a *change* per se, but TL072 was no dice and the 4558 made everything happy...

2) Swapped the 47K (the one tied to lug 2 on the sensitivity pot and the .01uF tied to the neg. input of the first opamp) for a 10K. I tried a few different values, and this one seemed to give me the most usable range for the Sensitivity control.

3) When I was having problems with the OA, I raised the 3M3 resistor in the feedback loop of that first opamp, to get a little more sugar to the envelope follower...When I swapped in the 4558 with that resistor at 4M7 or 10M, I was in distortion-land, so I went back to the 3M3 value.

That's it for changes! Great circuit, so thanks to Tremblay for the schem and perf layout, and huge thanks to The Tone God and Mark Hammer for all the debugging assistance.

[Samuel]

Also keep in mind that all these changes were made to make up for what I assume are shortcomings in my guitar pickups, your mileage will amost certainly vary, and the "stock" parts  may work just fine for you.

[smoguzbenjamin]

Yup, sounds good. By the time I get the EA trem working it'll be the 15th of January (pocket money), so the Dr. Quacky will be next :twisted:

Anyway I was thinking (trying to be versatile), would it be possible to use an external expression pedal to control the sweep (asin turn it into a regular wah pedal)? 'Cos that'd be cool aswell

[Samuel]

Um, from my (extremely limited) experience I would say no, an expression pedal wouldn't be very useful. The range of sweeps that sound decent (as controlled by the "range" pot) is pretty limited. That plus...um...that would...uh...as you said...turn into a...erm...regular wah pedal.

[smoguzbenjamin]

Ahm, ya but I always think of useless things. But anyway, just for kicks, could you easily convert the Dr. Quacky so you could use an optional expression pedal?

[Samuel]

I'm definitely not the guy to ask, I've never done anything with expr. pedals...

[Mark Hammer]

Tim Escobedo's assorted filter designs on his "Circuit Snippets" page often use the transistor in the basic transistor-to-ground bandpass filter like you seein the Quacky, as a sort of crude control-voltage mixer.  The default voltage applied to the transistor is the one coming from the enevelope follower, but Tim adds on a pot that adds a sort of variable bias voltage.  Given that it is steady, I suppose one says the envelope is added on top of that rather than saying it is added on top of the envelope.

It is simply a pot with one end going to V+, the other to ground, and the divided-down voltage from the wiper feeding the transistor base through a resistor.  He uses it for tuning the filter, but it is essentially no different than an expression-pedal arrangement, and there is no reason on earth why it could not be built into a foot-controlled device.  Indeed, any of you that have built a DQ/Quack/Quacky can vouch for the fact that you can actually work the trimpot as if it were a wah control.  Hell, I mounted mine on the chassis fore filter tuning.  Tim's added pot steps that up a bit so that you use the trimpot to set the overall sweep range, and use the added pot for finer adjustments.