Nurse Quacky for Bass: Is there an official schematic? (PIC)

[barret77]

Hello all

I'd like to build a nurse quacky for bass; I checked the schematic at home-wrecker ( http://www.home-wrecker.com/nurse-quacky.html ) and the post "Modding a Dr Quack for bass"   ( http://www.elixant.com/~stompbox/smfforum/index.php?topic=17064.0 ).

I'm having trouble to follow all the Mark Hammer advices - some of them are already at the home-wrecker schematic, some are not - and I'm getting a bit lost. (I'm still a newbie)

So, is there an "official" schematic or layout for the bass quacky? Are there further recommendations from Hammer's post?
Has anyone built it?

Thanks a lot!

[Mark Hammer]

There is no "official" bass schematic, to the best of my knowledge, but the recommendations for bass use are typically as follows:

1) The range of the bass is obviously lower, so the filter has to be tuned lower to provide the same "character" to lower notes on the bass that it provides to higher ones on the guitar.  You can start by increasing the value of the two .0047uf caps to at least .01uf, and probably higher than that.  Making them .022uf will drop the filter down by a bit over two octaves.  Something around .015 to .018 may be better for folks who like to play on the higher strings of the bass.

2) The envelope follower that sweeps the filter has to respond to the bass signal appropriately.  Stock, the Nurse Quacky tends to ignore much of the low range where many of the bass notes live, such that it would tend to respond only to higher notes.  You can fix this by increasng the value of the .01uf cap after the sensitivity control to .022 or .027uf.

3) Variable attack time is extremely useful for rhythm players, but much less so for bass players, who generally want a fast attack time to emphasize the percussive aspect of the bass.  So, leave the 51R resistor in place (it can actually be anything between 47R and 68R, depending on what you have on hand) but iognore the suggested 1k pot.  Instead, you may find more use in a decay-time pot or preset.  This will be a resistance to ground in parallel with the 22uf cap to make that cap drain faster.  That can simply be a toggle and a 100k resistor in series.  Open is slower decay, and closed is faster.


In the absence of the input buffer, like the Dr Quack has, I am hesitant to recommend any sort of blending circuit for the moment.  Let's just try these changes for the time being and see if it gets you where you want to go.  If not, then we can discuss blending clean and filter a bit more.

[barret77]

nice. I'll work on it!

Thanks

[Ashurbanipal]

I just built a nurse quacky for my bass.

I have a switch to go between 2 .0047's and 2 .01's. I like the 0047s for single coil bridge, and 01 for humbucking p-bass. I preferred a 10uF cap instead of the 22uF because then the decay was faster which gives more opportunity for the filter to close all the way before the next note. I got rid of the sensitivity pot because it always worked best with the sensitivity all the way up. I kept the attack pot. It really works well and goes from really fast to too slow. I use a buffer and compressor before it. I preferred an ne5532 like someone else did who posted a while back. A tl072 was suprisingly bad in the circuit.

I had to experiment a lot with this one. I would advise using a switch for the 0047s/01s/etc. and try several values until you find what you like, especially since you may like different values depending on whether you use the neck or bridge pickups. And then the 22uF decay cap was a smoother decay but just wasn't funky enough, so I would plan on changing it or doing what Mr. Hammer suggests.

[Mark Hammer]

The switchable filter range is an excellent idea, especially if it lets you easy convert from guitar to bass and back.  Obviously if you decide to let the pedal go, that also doubles the potential market for selling it, right?  (Well, realistically, let's say increase it by 20% since bass players aren't as pedal-crazy as guitar players  )

The best way to range switch, would probably be to replace each of the existing .0047uf caps with two .01uf caps in series.  In series, they provide an effective capacitance of .005uf.  Shunt one of them in each pair, and the capacitance becomes .01uf.  You can use a DPST or DPDT switch to do this shunting.  The reason I recommend doing it this way is because it will provide reasonably pop-free switching of range, compared to switching between .0047 and .01 caps.

If .01uf does not shift the filter range downwards far enough, you can simply use a .0068uf and .015uf cap in series instead of two .01uf values.  IN series, they produce a value .of .0047uf, and if you shunt the .0068uf caps, it drops to .015uf.

[Mcgiver69]

3) Variable attack time is extremely useful for rhythm players, but much less so for bass players, who generally want a fast attack time to emphasize the percussive aspect of the bass.  So, leave the 51R resistor in place (it can actually be anything between 47R and 68R, depending on what you have on hand) but ignore the suggested 1k pot.  Instead, you may find more use in a decay-time pot or preset.  This will be a resistance to ground in parallel with the 22uf cap to make that cap drain faster.  That can simply be a toggle and a 100k resistor in series.  Open is slower decay, and closed is faster.

Mark does this toggle or pot have to be inserted before or after the 22uf ? I mean if by ignoring the 1k attack pot we can place there the toggle with the 100k resistor in series and change the 51R resistor for a 68R, is that what you mean?

[Mark Hammer]

The rate at which the 22uf cap charges up is determined by the current limiting resistor between the diode before it, and the cap.  So, with a 68R resistor, standing between the rectifier and the cap, more current comes through and the cap charges up faster, compared to having a 100R or greater resistor.  This is why the 1k "attack pot" has the unfortunate side-effect of interacting with sensitivity.  Yes, an extra 1k in series extends the time taken for the cap to fully charge, but it also mimics having a less sensitive rectifier or weaker input with which to supply that current - a bit like the way you can feel poor if your super-rich dad gives you an extremely meagre allowance.

The larger value resistor goes in parallel with the cap such that the +end of the cap has a path to ground.  It already *does* have such a path through the trimpot connected to the transistor, so it's not like there is an open circuit between one end and the other of that cap, but the extra parallel resistor will speed up the discharge of that cap even faster.

Generally speaking, a smaller-value cap will also produce a faster decay, but unless it is VERY small (like 2.2uf or less in this instance) you will hear a very pronounced and ugly envelope ripple in this circuit as the note decays.  The decay has to be VERY fast for it to come and go without being able to hear the ripple.  My sense is that it is better to have a larger value cap (22uf vs 10uf or less) and parallel resistance to achieve a smoother quick decay.

The circuit itself does not really "care" if that parallel resistance is drawn on the left or the right of the cap.

The replacement resistor does not HAVE to be 68R.  That is a suggested value intended to provide something fast but not ridiculous or unmanageable.  A value of 47R, 51R, 82R or even 100R may be more to your tastes.  Note that the time to charge up the cap to ground will be a joint function of the cap value and this resistor, such that the same chargeup time (attack) can be achieved by having a lower resistance when the cap is increased.  A 47uf cap and 47R resistor will yield roughly the same chargeup time as a 22uf cap and 100R resistor.  Similarly, bigger value caps require smaller drain resistors than small caps do to acheive drain in the same time.

[barret77]

Hey

Can anyone check if I understood it right? the capacitor changes are not shown here, I'm just pointing the alternate values... I'm wondering about the 100k resistor...

[Mark Hammer]

Looks fine, generally.  The only thing I question is the inclusion of .068uf caps in the filter, though I suspect this is simply a typo.  They should be .0068uf (= 6n8 = 6800pf).  And again, to do what was suggested with a range-switching toggle, these should be in series.

[barret77]

oops, really a typo. I'll fix that...

Thanks for all your assistance, Mark!

[Mark Hammer]

Certainly let us know if these changes accomplish what you wanted.  If not, then we'll figure something else out.

[Mcgiver69]

Mark I've been trying to find the BC549B but seems that there is a shortage of them here in England can I use the BC549 or the BC549C instead? do you think is going to work well just like the BC549B ?

[Mark Hammer]

The B and C designations are for different hfe ranges.  In this application you are not interested in the gain potential of the device.  Rather, it is merely being used as a kind of voltage-controlled resistor.  Consequently either will do.  Pity you had to waste all this time trying to chase down something you didn't really need.

[Mcgiver69]

Yep the sad story is that I'll have to wait until they deliver the BC549C because now my nearest shop don't have it in stock.
BTW Mark I'm using Stripboard Magic, well I've been using it since I began building effects, do you know something more updated and as simple as this software ? It is very good but the designs are not very small, I've had to tweak a lot to get a fairly sized one. Apart from the size of the projects all of them have worked very well no complains and I've never had a project that didn't work, well I guess I've been lucky.

Thanks for your help you are an ACE !!!

[Mark Hammer]

Yep the sad story is that I'll have to wait until they deliver the BC549C because now my nearest shop don't have it in stock.

Almost any "utility NPN" will do for now.  Keep in mind that while you're waiting for something that gives what we *think* may be optimum sweep, there is still the matter of nailing down the optimum caps for setting the range.

BTW Mark I'm using Stripboard Magic, well I've been using it since I began building effects, do you know something more updated and as simple as this software ? It is very good but the designs are not very small, I've had to tweak a lot to get a fairly sized one. Apart from the size of the projects all of them have worked very well no complains and I've never had a project that didn't work, well I guess I've been lucky.

Stripboard is actually very hard to find where I live, even though other things seem to be plentiful.  So, personally I just use perf or PCB.

Thanks for your help you are an ACE !!!

  Not really, just persistent, and a tireless rehasher of what others have contributed.  But thanks.

[Grocha]

Hi. Any news in this topic??

Grats
Grocha

[Mark Hammer]

What specifically did you want to know?

[Grocha]

Excuse... There is a vero layout for bass mod of nurse? And the only changes are the capacitor values? what the "officialls" values?

Thanks,

[Mark Hammer]

No "official" values.  The filter range for bass needs to be set lower than that for guitar.  How much lower will depend on how you play bass and what sort of bass you use.  If it is a traditional 4-string bass, then one to one and a half octaves lower (multiply filter cap values by x2 or x3) is about right.  If it is a 5 or 6 string bass, then filter range might depend on whether the extra strings are higher or lower and where you like to play (higher strings, or lower notes).

On the other hand a fast decay is probably desirable regardless of the filter range.  Personally, I think if you follow the posted schematic earlier in this thread, using .015uf caps for the filter, a 47R (not 68R as shown) attack resistor, and a 10k fixed resistor in series with a 250K variable resistor (instead of the 100k switchable resistor as shown), you'll be happy with what you get.