Building the Chopped OC2

[Taylor]

Hi, can anyone tell me what the 1uf cap from pin 1 of U1A is for? All it seems to be doing is putting a loud clean signal into pin 5 of U1B, which overpowers the octave down, I only got results after I removed this. Another thing, is using a j201 neccesary? I'm using a 2n5485.

To understand that, you'll need to get a better sense of how the whole circuit works.

The circuit splits your signal into two paths, one audio and one tracking. The tracking circuit (I'm leaving out a lot here of course) creates a square wave that's an octave below the input frequency. The audio path goes through an amplifier (U1B) whose phase will flip when the gate of the JFET goes high. The JFET is fed from the octave down square wave, so that the phase of the clean signal flips every other cycle, giving a note that is an octave below the input but with much of the timbre of the original signal intact.

So, that cap is a coupling cap between the two amp stages. If you remove it, then none of your original signal will get to that amplifier, so you'll probably just be flipping the phase of some DC level (due to the diode) with the octave down square wave. This will just give you a buzzy square wave sound with no timbre of the original, which is cool, but it's not what the pedal is supposed to do, and you can also have the octave down square in addition to the regular octave sound if you check the mods section of the documentation.

If you get no octave with that cap in, there's something wrong with your build - maybe it's getting grounded when it's in?

The JFET is not really critical, since it's being used as a switch. Just be sure you get the gate in the right way if it has a different pinout.

[megalodon]

Yeah, it makes sense now, thanks a lot

[megalodon]

OK, I did a vero layout of my own and it seems to be shorting out somewhere. I've checked for solder bridges etc. and can't find anything, can anyone see anything that I'm missing?

[Taylor]

You'd probably get better results starting a new thread, or checking out one of the other threads for this circuit (I believe there's already a vero as well). This thread is for issues dealing with building the PCB I make.

BTW, if you are using the schematic from my site to do your vero, don't! It has errors that will mess you up. The PCB is fine, but since you're not using my board, you need to go find the original schematics.

[p_wats]

Oh man! After setting this build aside for months and months because I couldn't get rid of the background fuzz I pulled it out again last night (we just moved, so I was organizing the new place and found it). I disconnected the mixed in mods (square wave, etc.) and, low and behold, the errant fuzz dropped considerably. I swear I had already done this to no effect, but I also changed 4013 chips and a capacitor that seemed troublesome, so who knows.

All I know now is that I've got a SPST toggle that takes the 3 extra signals out of the chain when I want the clean octave down and it seems much, much better than I remember.

Closure is imminent!

[telebiker]

Waiting for the PCB...

[Beo]

Taylor,
  I'm getting a really loud pop when I engage. I'm going to disable the led and see if that helps. I might choose to replace the input and output decoupling caps. Can you identify which cap is the input 1uF? The output 10uf cap is obvious, its the only one on the board.
  Have you had other reports of popping issues with this circuit?

[Taylor]

There should be no more popping with this circuit than with any other true bypass effect. I believe the input cap is the one right below the J201.

[Valoosj]

If I were to use the schematic from this topic (http://www.diystompboxes.com/smfforum/index.php?topic=84023.0) with the added mid boost section and the push me pull you, could I still incorporate your mods? (tracking knob, synth mods and variable filter)

[Taylor]

Sure, why not? But things might get hairy with all the mixing of extra voices.

[Valoosj]

I'll just use the schematic from this topic then to etch my own layout with your mods, as you mentioned your schematic contains errors. (Haven't immediately seen any apart from a different labeling of pins with one of the ICs)

[Valoosj]

I can't figure out the pins of the J201. Is D going to pin 5 of the TL074 or to Vref? S is, I assume, going to the two 1M resistors. Right?

[Valoosj]

Anyone?

[slacker]

Drain goes to pin 5 of the TL074, Source to vref, Gate to the two 1Meg resistors.

[Valoosj]

Thank you!
Is it standard that Source goes to Vref? On the schematic it is not really that clear to the novice in electronics which of the pins is Source and which is drain.

[Taylor]

In this kind of configuration I think most common JFETs are interchangeable between S and D, which I believe is why they're drawn that way.

[slacker]

I think Taylor's right. As it didn't say otherwise I just assumed the fet was drawn the right way up with the drain at the top.

[telebiker]

Is there a chance to see this pcb somewhere?

[Taylor]

Hmm, it's been a really long time but I believe there were some minor errors on the board that needed to be bodged, and I just haven't had time to rework this stuff. But, knowing that somebody's still interested, I will have a look at the board again and perhaps I'll be able to make a run of the redesigned board at some point.

So, maybe, but it could be a while.