Chopped Boss OC-2 getting faint static crackle and thinking the schematic is bad

[bushidov]

Hi Guys,

So, I build a Chopped Boss OC-2 based around this schematic which I made in a post earlier:


While drawing it, others noted mistakes which were corrected and eventually led to this one. I did this because my previous Boss OC-2/3 clone, which in turn ended up being a clone of a different octave down pedal, led to a similar problem.

However, the problem is a tad different now. Now, I only get a faint fizzle/distortion/crackle on the clean level only. The fizzle decays with the note played on the guitar, so it there isn't any note, there isn't any fizz. If I turn the octave effect to 0, and turn the main level up, the louder the main level knob is, the louder this faint distortion is. If I turn the main level to 0 and then turn the octave pot up, it works great and is as clean as a whistle.

With everything plugged it, I probed with an audio probe. Going into pin 3 of U1A, I get normal sound. On pin 1 of U1A, I get the fizzle, and it's actually louder than it would be at the end of the circuit (I guess all that filtering on U1B, U1C, cleaned some of it out)

So I went to go isolate it.

First thing I did was take out R18. With the octave knob at 0 and the level at 10, I still get the fizz. Pin 1 of U1A still made that louder fizz.

Keeping R18 out, I then took out C5. Fizz disappeared.

I put R18 back in, but kept out C5. Fizz returned, but it was much less noticeable. I had to crank my amp to hear it, but it was still there.

I then returned it all back to normal. Then I removed D2, thinking maybe that half wave rectifier was clipping somehow. Nope.

I returned D2 and then removed Q1 thinking maybe the JFET switching was introducing the noise (which would explain why when R18 was in, but C5 was out, why some fizz was still making it out.) But, nope.

Then I returned it all back to normal. Removed U2 and U3. Still, the fizz happens.

Is there something incorrect on the schematic that still needs added or changed? Any ideas? Am I destined to never understand octave down pedals?

[Rob Strand]

You have done a great job with your debugging technique here.  I read the problem and look at the schematic and the rest of your post pretty much answered all the questions!

So one obvious problem is the 1uF input cap C3 is around the wrong way. 

Having the fizz in the tail means it's not the opamp clipping.  However with such a simple circuit, to have fizz in the tail of the signal might point to an opamp problem.   The fact pulling C5 helps might also indicate an opamp problem.  Do you have any other dual opamps? preferably one from a different batch, even a different type.
 

[bushidov]

So one obvious problem is the 1uF input cap C3 is around the wrong way.

To rule out flub, I removed it and replaced it with a 100nF film cap, and I still get the fizz, but it did seem to make it "less". Not sure, maybe that was just wishful thinking in my head playing tricks on me.

Do you have any other dual opamps? preferably one from a different batch, even a different type.

Unfortunately, this PCB I made was set for quad op-amps. But I did pull out my TL074 and replaced it with another LM324. Still getting the fizz.

I am wondering if this design might need to be broken up into dual op-amps because that might be causing it?

Here is the layout

[Rob Strand]

Unfortunately, this PCB I made was set for quad op-amps. But I did pull out my TL074 and replaced it with another LM324. Still getting the fizz.

I am wondering if this design might need to be broken up into dual op-amps because that might be causing it?

I've got my doubts.

What's very strange is this build and the last build have similar problems.  There's got to be a common factor.

With everything plugged it, I probed with an audio probe. Going into pin 3 of U1A, I get normal sound. On pin 1 of U1A, I get the fizzle, and it's actually louder than it would be at the end of the circuit (I guess all that filtering on U1B, U1C, cleaned some of it out)

So I went to go isolate it.

First thing I did was take out R18. With the octave knob at 0 and the level at 10, I still get the fizz. Pin 1 of U1A still made that louder fizz.

Keeping R18 out, I then took out C5. Fizz disappeared.

I put R18 back in, but kept out C5. Fizz returned, but it was much less noticeable. I had to crank my amp to hear it, but it was still there.

All that is very strange.    I understand what's happening but I cannot see a cause.

It seems the power supply and/or VBIAS is somehow related to the fizz.    It's like your 220uF cap (C2) is doing nothing.   Are you sure these are OK?   Maybe they are fakes with nothing inside?  Do you have any big caps you bought from elsewhere?  Anything from 47uF up would be OK.

[bushidov]

Well, now I am really confused.

I broke out the TL074 into individual TL071 op-amps on a breadboard.

I added them one at a time

One was adding back U1A. No fizzle, and it was able to get to the output jack via the level knob maxed.
Next one was adding back U1B. No fizzle. Output made it out.
Next one was adding back U1C. No fizzle. Output made it out.
Last one was adding back U1D. Fizzle returned, but octave effect worked.

So, then I said to myself "I remember pulling U2 and U3 and it didn't stop the fizz, so it must be in the circuit of U1D.

So then I disconnected R18 and R21. Fizzle went away.
Then I jumped a wire from where R18 was, where it connected to C10/R19 over to where R21 was, where it connected to C13/R22 and the fizzle returned. This means U1D couldn't be the problem.

So now I am confused.

[Rob Strand]

So then I disconnected R18 and R21. Fizzle went away.

So the consistent thing between that and the last build is disabling that rectifier part of the circuit seems to stop the fizz.

Then I jumped a wire from where R18 was, where it connected to C10/R19 over to where R21 was, where it connected to C13/R22 and the fizzle returned. This means U1D couldn't be the problem.

I can't quite work out what you did here.

I was thinking to bypass IC1D:  So you would lift the IC1D side of R21 then wire it to the output of U1A.
Then after that, see if the fizz is present with R18 lifted and R18 loaded.