Building the Chopped OC2

[Marcvv]

The output of the LM324 is a squarewave, so it sounds like that's what you're hearing. Does the fuzz change or go away if you put a buffered pedal in front of it?

Ok, now we are onto something.
I used a BB preamp clone (No gain just unity signal) in front of the chopped OC2 and now the fuzz on the clean signal is gone. (You can still very, very slightly hear it on the octave but maybe that is part of the design?) If I bypass the BBpramp the fuzz is back.

So what can I do now?

Thanks for chiming in Slacker.

Marc

[slacker]

Sounds like it could be a similar thing to this thread where the guy had problems with an LFO getting into the input http://www.diystompboxes.com/smfforum/index.php?topic=72395.0. Sticking a buffer in front solves it because the output of the buffer effectively lowers the impedance of the input.

What you could try is similar to what R.G. suggests in that thread and lower the input impedance. Trace the circuit from the "In" to the first 1Meg resistor and try putting say a 100k resistor in parallel with that and see if that helps.

I'd also make sure your input wire goes no where near the board, especially the LM324 and 4013 part, and make sure your ground wire from the board is a good solid connection straight back to your DC jack or battery and not via the input or output jacks.

[Marcvv]

Sounds like it could be a similar thing to this thread where the guy had problems with an LFO getting into the input http://www.diystompboxes.com/smfforum/index.php?topic=72395.0. Sticking a buffer in front solves it because the output of the buffer effectively lowers the impedance of the input.

What you could try is similar to what R.G. suggests in that thread and lower the input impedance. Trace the circuit from the "In" to the first 1Meg resistor and try putting say a 100k resistor in parallel with that and see if that helps.

I'd also make sure your input wire goes no where near the board, especially the LM324 and 4013 part, and make sure your ground wire from the board is a good solid connection straight back to your DC jack or battery and not via the input or output jacks.

Hi Ian,

I soldered 100k paralel to what I think is the first 1meg. (There is no pcb layout available so it is hard to be sure which is the right one. Maybe Taylor can tell me?)

No difference. Should I change the value?
Just to be sure: is that the very first 1M  or the one just after that?

[slacker]

I think reducing either or both of the 1Megs on the input might help, Taylor can tell you which ones they are.

You could try lower values, and see if that helps, but at some point you'll start to lose some treble.

[Marcvv]

I think reducing either or both of the 1Megs on the input might help, Taylor can tell you which ones they are.

You could try lower values, and see if that helps, but at some point you'll start to lose some treble.

I will try your suggestions when Taylor chimes in on which 1 meg's I should adjust.

In the meantime I searched some on the forum and ran in to a few posts on the same subject but with no clear solution.
I am also wondering, if this is a matter of impedance, why I have this in my build and others don't? Or did they not notice it??

Thanks, Marc

[Morocotopo]

Guys, I want to take advantage of this topic to ask you something:

I made my own layout for this, just testing it, works OK, but when feeding a distorted amp, I can hear a quiet "rrrrrrr" in the background. Into a clean amp it´s not audible.The octave pot controls it, when lowered from max it almost dissapears. Do your builds do the same? Just to know if it´s par for the course or I should start debugging. Tried a buffer before it but no change. Moving wires around made no difference.

Thanks.

EDIT: I have wires coming out of the PCB instead of the 330K R´s before U1D for testing the variable filter mod, and the thing is not boxed... could be a factor?
EDIT 2: On sustained notes, it jumps from octave to octave semi-randomly (not all the time...sometimes). Feature or malfunction?
EDIT 3: The pots are connected to DC (Vref), so when turned, they make scratching noises... do yours do that too? Do you know if the original did it too? it stikes me as weird...

[Marcvv]

If you read older posts about the chopped OC2 that were not based on this pcb, you read similar problems.
Apparently it is part of the design.

I hope Ian's suggestion on the impedance is correct. I will try that as soon as Taylor chimes in again on which 1 meg resitors to adjust as I do not have a pcb layout...

[Marcvv]

I am about to put mine in a box. I used a Jfet buffer in front to deal with the impedance issues. Works fine now.

I included the envelope synth mod and i want to combine this with a green ringer for an oct up.

The green ringer get its input from the jfet buffer (actually a splitter/ double jfet buffer) That also feeds the chopped oc2, and the output of the GR goes to a 100k pot to a 100k resitor to the base of the 2n3904.

At the base of the 2n3904 are now two extra signals, green ringer and envelope synth.

It all works but the added signals are not loud enough.
I read in the pdf that i might need a mixer. Anyone tried this already and what solution did you find?

Thanks, Marc

[Marcvv]

Solved it.

Green ringer get its signal from pin 1 from IC 1 as suggested in other threads. For the envelope synth out I used a setup as suggested for the square wave signals. That was my mistake  
I can mix clean, clean octave down, green ringer octave up and envelope synth. No need for a mixer as all the signals are in balance.
Pedal level is slightly over unity.

[Taylor]

Cool mod!

[karter2000]

I seem to be getting the faint fuzz signal as others have found.  I tried grounding the top copper pour and the buffer as others tried.  Is there anything I can try?  I've not got in an enclosure yet, just a breadboard.

Thanks!

[Marcvv]

I seem to be getting the faint fuzz signal as others have found.  I tried grounding the top copper pour and the buffer as others tried.  Is there anything I can try?  I've not got in an enclosure yet, just a breadboard.

Thanks!

Input buffer. I used a Jfet input buffer. Solved. It seems to be an impedance issue, read back in this thread.

[karter2000]

That's strange as I already tried an input buffer (Orman FET).  Which one did you use?

[Marcvv]

I used thishttp://www.muzique.com/lab/splitter.htm as I first wanted to use the splitter for combining the Chopped OC2 with the green ringer. I later changed that but kept the buffer.

[UKToecutter]

Taylor
The PCB's are showing as out of stock.
Any idea when you'll be getting a new batch?
Regards

Andy

[Taylor]

No, not sure yet unfortunately.

[jeremypb]

I just built my chopped oc2 from the schematic with my own layout.
I'm getting fuzz, like a lot of other people but mine is right away, at pin 1 of the tl074.
any ideas? Is it grounding or do i need an input buffer?

[jeremypb]

I added a buffer and the distortion is gone but the circuit is still not working right. I get no octaving effect, but I might have figured it out.
Looking at Taylor's schematic on musicpcb.com and comparing it to the schematic I built from:
http://www.aronnelson.com/gallery/main.php/v/Processaurus/OC_2_chopped.gif.html?g2_imageViewsIndex=1
I notice in the one i used, R4, D10, R49 and Q8 are connected to Vref. Is that my problem?
Also, on Q8, the J201, which is the source and which is the drain?

[Taylor]

Yes, there's an error in my schematic. Use Ben's rather than mine if you're not using my PCB.

[jeremypb]

thanks Taylor,
Can you clarify the fet transistor for me? which end in the schematic is the source and which is the drain?