Doctor Q trouble

[KaptenSpark]

I set up the Doctor Q envelope filter on my breadboard yesterday, but i had no .05 caps so I used .047 (and .0047 for .005) and the thing didn't work at all. I could sense that it was going somewhere near what it's supposed to do but I only got a loud crack when i played with lots of attack.
Then I tried the "Doctor Quack" and it didn't work at all.

Also, I used LM358 and 2n3904.

Do I have to get some .05 caps for it to work or is there anything else I can do?

Doctor Q schematic: http://filters.muziq.be/files/schematics/eh_doctorq.gif
Doctor Quack schematic: http://www.muzique.com/schem/quack.gif

[twabelljr]

I've used .047uf in in place of .05uf before with no problem at all. I think the fault is somewhere else in the circuit. Recheck your layout again, and again....

[Mark Hammer]

Most of the time, .047 IS .05 (the same way 500k is 470k, etc.).  Keep in mind that caps are rarely as close a tolerance as resistors are, so if you figure +/-10% (or even a little more), it becomes clear that the values shown in schems canusually withstand a bit of variation; especially if stating a value doesn't assure you will GET that value, even if it says so on the component.

Tnere are many ways for any project to go wrong.  IN the case of the DQ, there are essentially 2 things to concernyourself with primarily.  One is to assure that the filter section works properly as a filter, and the other is to assure that the envelope follower (AKA rectifier) is providing the sort of output that can drive the filter adequately to hear a sweep.  Sitting in between these is the transistor, which should (hopefully) be oriented properly.

So, keeping these three things in mind, take your meter and verify that:

a) There is some signal reaching the rectifier gain stage through the sensitivity pot.  Plug in, set the pot for max sensitivity, and strum. You should get something like 50mv AC at the wiper of the sensitivity pot (yes, I know hot pickups can deliver more, but you're not power chording here...you still need to hold the probe with one hand, right?).  Now touch your probe to the output of the op-amp.  You should have at least a couple of volts (AC).  You should have a little under half of that at the junction between the second diode and the trimpot input.

b) When in the Bass mode, you get the same 40-50mv at the input of the  filter (- pin) when you strum.

Finally, verify that the pinouts on the transistor correspond to what you *thought* they would be.  There are so many variations in pinout, and recommendations floating around about what is an acceptable substitute.  The sub recommendations are valid, but they don't always have the same pinout.