Dr Quack, heavily Hammerised with blending mod

[markusw]

In the last few weeks I have built a Quack with the following mods (mainly, if not all, suggested by Mark Hammer) in order to be able to use it also with my bass: gain pot, attack pot, decay toggle switch, sweep toggle switch (one octave down). Up to now everything works fine and sounds great (at least IMHO). Then after browsing this forum I wanted to include a blending stage to add same dry signal. See: http://diystompboxes.com/sboxforum/viewtopic.php?t=17994&highlight=quack+neutron

I gratefully followed Mark Hammer´s suggestions and built another PCB containing the blending stage (similar to Mark Hammer´s mods of the Bi-Filter Follower: see http://hammer.ampage.org/

1. The output of the original Quack (after the 0.05 µF output cap and the 470k pulldown R) now goes directly into two consecutive inverting unity gain stages (each with a 10k R in the feedback loop) without a cap between/in front of them. The outputs of these two stages feed a toggle switch that allows me to change between inverted and non inverted wet signal. Subsequent to the toggle switch there is a 0.22 µF cap and a 1k resistor in series.

2. The dry signal goes from the point after the 10 µF electrolytic cap following the input buffer of the Quack in another inverting unity gain stage (again with 10 k in the feedback loop). The output of this stage again feeds  a 0.22 µF cap and a 1k R in series.

3. Both signals go to the outside lugs of a 25k pot (for blending). The wiper goes through a 10 k R into the final inverting stage (with a 10 k R and 50 k log pot for volume control) and subsequently to a 4,7 µF output cap (with a 22k pulldown R). From there it goes to the 3PDT switch as previously the original output of the Quack.

(VCC+ and VCC- of the op amps (NE5532AN) go to the red and black wire of the 9V battery; the non inverted inputs of the op amps are connected to the non inverting input of IC1B in the Quack).

So far, so good. The inverted/non inverted toggle switch as well as the blending pot and the volume pot work fine.
Unfortunately, there is some weird effect when I turn the Normal/Bass DPDT toggle in the Quack to Normal (at least I assume; the position in which the signal gets fed directly into the subsequent 0.01 µF cap):  either I do not get any effect or (when the gain and/or sensitivity is increased) a get a popping noise (clipping?).  Interestingly, I did not observe this phenomenon before adding the whole mixing PCB!
However, I have to admit that last night (it was rather late) when I added the two OP amps, by mistake I plugged them in the wrong way and then connected the battery. After correcting this bug, I also tried it with two new op amps but I still get same popping/clipping noise.

Finally, I rewired the original Quack output back to the 3PDT switch and removed the wire from the mixing output (while leaving all other connections between the two PCBs intact) and interestingly I still get this popping/noise. Tonight I am going to restore the original wiring of the Quack by entirely removing the mixing PCB. Lets see if I still hear this phenomenon.


Now to my questions:

1. If my plugging in the op amps in the wrong way has caused one or several components to fry, which of them are most likely defect.

2. As mentioned above, I left the original output cap (and pulldown R) of the Quack in place and used the output of the Quack as a wet signal. Should I remove the R or the cap or both of them?

3. Looking at the schematics of the Hammer-modified Bi-filter, what are the four 1k resistors (feeding the filter and dry level pots) for.

4. Should I leave the 4.7 µF ouput cap and the 22 k pulldown R at the output of the mixing stage or should I replace them with the values used at the Quack output.

Sorry for the long post.

Thank you very much in advance for any suggestions.
:?:  :?:

[markusw]

Hi all,

an update.

I have added another toggle switch that allows me to entirely remove the dry signal from the mxing stage. Furthermore, after playing a while I think I will ommit the option of adding the inverted wet signal to the mixing stage since IMHO it is not really useful. The mixing stage per se is really great (thanks Mark Hammer!!).


Restoring the original Quack showed that everything is fine with it. So I added the mixer again. But now the problem with the normal/bass toggle switch in the Quack is even worse. In the setting where the signal goes from the normal/bass toggle switch directly to the 0.01 µF cap (Bass?) everything is fine(Sorry I mixed it up in my last post). In the setting where the sinal goes to the 47K R and subsequently into the two 0.0047 µF caps I get a severe drop in volume, even if the sensitivity and the gain pot in the Quack (and also the volume pot in the mixing stage) are turned up completely.  Actually I just hear a very faint sound although the LED flickers while I am playing.

Is it possible that the battery is too weak for this setting or is it more likely that I completely fried some of the components.

Any suggestions before I put the whole stuff into a box?


Thanks a lot.

[Mark Hammer]

1) To the extent that it has improved matters, you're welcome.

2) To the extent that it has made your life a living hell, my sincerest apologies!

3) I think the "problem" is that the original component values for the filter-mode switch in the Dr Q simply did not anticipate running anything else in parallel.  As you recall, the stock DQ (sans Orman mods) has the guitar effectively plugged in directly to the mode switch.  I'm not the expert here by any means, but my gut sense is that a second 10uf polarized cap going to the dry path from the source of the FET may provide sufficient decoupling of the two paths, or at least enough that their individual resistor values don't interact poorly.

Let us know if that solves the problem.

[markusw]

Mark,

thank you very much for your reply. I will try it this evening. Since I am rather used to new things not working at first (I am working in research although on an entirely different topic) its far from making life a living hell. Browsing this forum on the other hand I really got the impression that you are on of the experts here.

In addition to the second 10 µF cap (or alternatively) would it also make sense to increase the resistor values of the first inverting unity gain stage in the dry path of the mixer from 10k to some higher values (e.g. to 47k or 470 k)?

[Mark Hammer]

I'm an "expert" the way folks who have been fired from dozens of jobs or gone into bankruptcy repeatedly are experts in the workplace and business, or the way people who have been in dozens of failed relationships are experts on love. :wink:   I just have a longer track record of mistakes than most.

[smoguzbenjamin]

I like to think that making a mistake once teaches you far more than doing it right 10 times. :roll:

[markusw]

I agree with smoguzbenjamin on making mistakes.


Here is an update again.

After some hours of debugging and repairing a bad solder joint I could restore the original issue: when the dry signal (taken from the point after the 10 µF cap subsequent to the JFET input buffer) is not connected to the mixing stage everything is fine in both settings (normal and bass). When it is connected to the mixing stage everything is OK in the bass setting, but in the normal setting I either get a more or less dry signal (despite the fact that the blending pot is turned to wet) or (when the gain and/or sensitivity pot is turned up a little more) something like a severe overload/clipping that even causes the preamp of my bass amp to clip.

Then I added another 10 µF cap in the dry path between the JFET buffer cap and the mixing stage, but the result was similar. Finally, I followed exactly the suggestion made by Mark and took the signal directly from the source of the JFET. Bingo: now everything is perfect. The additional second 10 µF cap now is not even needed (many thanks to Mark Hammer!!!!!!).    


One question to all experts and non-experts: why does it make such a huge difference to take the dry signal directly from the source of the JFET and not from the point after the 10 µF cap subsequent to the JFET?
:?:  :?:  :?:

[Mark Hammer]

Congrats, and thanks for the tip!


Why does it make a difference to do it the way you did?  Because the FET doesn't see pathways, it sees resistors, and the assorted resistors in different parts of the pedal, that are nicely visually organized and segregated for our comprehension, can easily turn into a bunch of parallel resistors that interfere with the normal functioning of the device.