Circuit Analysis Help on a Chopped Boss OC-2

[bushidov]

Hi There,

In regards to my failed attempt to get my Behringer UO300 reverse engineered to through-hole and true bypass, I felt it necessary to put it aside and understand the circuit better.

As I've never really looked into clean octave down pedals before in any real detail, I wanted to know if anyone here knows how it works? I got the schematic from this:
https://www.aronnelson.com/gallery/main.php/v/Processaurus/OC_2_chopped.gif

and then made my own


I love ElectroSmash's site and learned a lot from it (as well as from here, too), but ElectroSmash only has so many circuit analysis's going on and nothing in regards to clean Octave Downs (or ups for that matter)

So, what is going on with this schematic?

[EBK]

I think there was a Stompboxology paper on the subject that described it well.  Give me a second to look....
Here it is. I recommend reading the whole thing (there is a part that specifically discusses the OC-2 circuit arrangement):
http://moosapotamus.net/files/stompboxology-freq-div-sub-synth.pdf

[11-90-an]

I too barely understand the schem too but wikipedia has a general statement about it... https://en.m.wikipedia.org/wiki/Octave_effect 

[antonis]

I think there is (was) something like "Going discrete" in stompboxology moosapotamus series posted by Eric, dealing with octave effect (Distort-O-Matic VII something..) but I can't recall it right now..

[Rob Strand]

You have a few bugs on your schematic:

- No resistor between U3A and the JFET gate.
- No resistors between the pot wipers and Q2's base to form a passive/resistive mixer.
- Having the pots DC coupled to Q2's base can cause scratchiness when the pot are turned.

Probably a good idea to bump R5's value to say 4.7k since the original Boss pedal had a three input passive mixer at the output and yours only has a two input passive mixer mixer.   (The Boss pedal can clip with high output basses with 2.7k.)

[bushidov]

- No resistor between U3A and the JFET gate.

Added. Yeah, I missed that on the sample schematic.

- No resistors between the pot wipers and Q2's base to form a passive/resistive mixer.

Added 100K's. I think I am seeing that Boss schematics place that resistor "inside" their potentiometer symbol, which threw me off.

- Having the pots DC coupled to Q2's base can cause scratchiness when the pot are turned.

Added a 0.22uF capacitor to decouple the DC

Probably a good idea to bump R5's value to say 4.7k since the original Boss pedal had a three input passive mixer at the output and yours only has a two input passive mixer mixer.   (The Boss pedal can clip with high output basses with 2.7k.)

I do use a Stingray bass, which has an active pickup, so that's probably a good idea for me.

Below is my corrected schematic.


So I guess I have a lot of questions of what is going on here. I know general things, like input buffers, output buffers, passive filters, and some op-amp topologies, but this thing is a lot of uncharted territory for me to learn.

So, I think I understand that U1A is just an input buffer that adds a little gain in a non-inverting op-amp configuration, and that the Q2 is just a non-inverting emitter-follower buffer. The line that follows the output of U1A to the level/volume pot, through mixing resistor R31 (mixing with R32) going through the DC decoupler C17, and leaving the Q2 emitter-follower is the dry/clean signal. This is about the extent of my "certainties"

As to my "best educated guesses"
Leaving the U1A, and going through C5, we go into U1B and U1C op-amps. I see stuff that looks like partial Sallen-Key filters or just collections of RC filters, but it could be that I "drew them funny" in my schematic, and that's why "I am not seeing it". I also see at the non-inverting pin of U1B, the injection of what I can only gather as the raw octave low signal, which I am guessing at this point is probably a pure square wave, as that is directly coming out of the CD4013BE chip. I am also guessing it just is using that JFET as a means to switch between 100K to bias and directly to bias.

So my first couple of questions there would be:
1. What filtering/filtering topology is going on around U1B and could I have drawn it better?
2. What filtering/filtering topology is going on around U1C and could I have drawn it better?
3. What is the purpose of that Germanium Diode D2?
4. Why as the pots going to bias and not ground?

Now for my "I am kind lost here"
So, I see the clean signal leaving U1A and going to R18 and all through R21 (op-amp U1D) is some sort of big filter.
5. What filtering/filtering topology is going on around U1D and could I have drawn it better?
6. Is R21 - R23 and C13 - C14 and extension of the filter around U1D, or a second filter, prepping the LM324 part of the circuit?

I know LM324's don't typically "sound good" in regards to guitar amplification when compared to better options, like the TL074, which I do use to filter the audio signal. So I am guessing this is used for the frequency division.

7. So, if I am reading the stompboxology article correctly, the whole quad op-amp U2 (LM324) is just being used to take the clean sine signal and convert it to a square wave? And then the CD4013BE is dividing the frequency by 2?
8. And a frequency divided by 2 is an octave lower?

9. I guess if my generalized assumptions are true on 7 and 8, that probably makes those filter questions 1 and 2 as filters to take the output of the CD4013BE and make the square wave back into a sine wave as best as possible?

[EBK]

You're most of the way there on 7&8.  Keep reading though.  The OC-2 uses an additional clever trick....

[ElectricDruid]

1. What filtering/filtering topology is going on around U1B and could I have drawn it better?

It's not a filter. It's an inverting/non-inverting op-amp stage. By turning the FET on and off, you can get either an inverted or a non-inverted output from it. It's probably more closely related to the typical FET phaser stage than anything else.

I don't like Vbias at the top, mostly because if this was a bipolar supply circuit, those would be ground connections, and ground connections go at the bottom (at least in my head they do). But this is personal taste and why it's interesting to read other people's schematics!

2. What filtering/filtering topology is going on around U1C and could I have drawn it better?

I think it's one of these, 3rd order Sallen-Key lowpass:
http://sim.okawa-denshi.jp/en/Sallenkey3Lowkeisan.htm

Could you have drawn it better? Well, when I've got a op-amp stage wired as a voltage follower, I like to always have the feedback loop at the top and the input at the bottom. It gets it out of the way of bias resistors hanging off that +ve input. But note that the filter calculator I linked to doesn't draw them like that either. Beyond just making it clear, it's personal taste.

3. What is the purpose of that Germanium Diode D2?

Not sure. Is it acting as a basic half-wave rectifier?

4. Why as the pots going to bias and not ground?

Because they're not separated by DC-blocking caps, so if they went to ground, they'd have a substantial voltage across them (Vbias at the top, and ground at the bottom) so they'd sound scratchy when rotated. Connecting them to Vbias avoids that problem without requiring more parts.

Now for my "I am kind lost here"
So, I see the clean signal leaving U1A and going to R18 and all through R21 (op-amp U1D) is some sort of big filter.
5. What filtering/filtering topology is going on around U1D and could I have drawn it better?

It's a 3rd order filter like the other one U1C. In fact, I think the way you drew this one is neater, but (as I said already) I'd have put the feedback path along the top out of the way of those ground connections.
I would say that however you choose to draw them, U1C and U1D should be drawn *the same* since they're the same thing.

6. Is R21 - R23 and C13 - C14 and extension of the filter around U1D, or a second filter, prepping the LM324 part of the circuit?

No, it's a separate network. It comes after the filter output at U1D pin 14 so it's a different thing.

7. So, if I am reading the stompboxology article correctly, the whole quad op-amp U2 (LM324) is just being used to take the clean sine signal and convert it to a square wave?

Yes.

And then the CD4013BE is dividing the frequency by 2?

Yes. That's a convenient side-effect of the flip-flop action. Pulses coming in produces pulses going out, but at half the rate.

8. And a frequency divided by 2 is an octave lower?

Absolutely! That's the definition of an octave! x2 or /2!

9. I guess if my generalized assumptions are true on 7 and 8, that probably makes those filter questions 1 and 2 as filters to take the output of the CD4013BE and make the square wave back into a sine wave as best as possible?

In an "ordinary" octave pedal, that would be true, but it isn't true here. A typical octave pedal turns the input into a square wave, divides the square by two to get an octave lower, and then filters the octave signal so it's not a completely harsh buzzsaw.

But that's not what happens here. Instead, all that other stuff along the bottom is just to create a *control* signal for that FET. It flips every other input waveform to create a new waveform that is twice as long (and therefore an octave lower). Since the waveform is not produced by a flip-flop but is instead just a alternately-flipped version of the input, it does all that amazing stuff that octave pedals don't normally do like *respond to changing dynamics* and *show some effect from changing harmonic input* and so on. In short, it's trying to be much more responsive.

Hope some of that gets you a bit further!
Tom

[Slowpoke101]

You may want to bias the base of Q2. 1M to Vref should be OK

[11-90-an]

sorry if this is OT, but bmcan the cd4013be can be replaced with something like this?


Since both are flip flops of some sort?

[Rob Strand]

Since both are flip flops of some sort?

Yes, they are flip-flops.  The versions you gave are the bare minimum circuit showing the principle.   These require switches which go open, so you can't drive them with an opamp.

Real circuits, of which there are many variations are like the ones on this page, you can see capacitors and resistors which create the trigger pulses,
http://www.tpub.com/neets/book9/36d.htm

They are not 100% the same as a 4013 but there are plenty of Octave circuits out there which uses those transistor circuits.     I know duck_arse was playing around with these a while back.

[duck_arse]

wot now?

[bushidov]

You may want to bias the base of Q2. 1M to Vref should be OK

If I don't, what is going to happen?

I kind of figured the VBIAS from the combo of octave and level pots's resistance would have provided enough biasing to Vref.

[Rob Strand]

If I don't, what is going to happen?

I kind of figured the VBIAS from the combo of octave and level pots's resistance would have provided enough biasing to Vref.

The transistor needs base current to bias correctly.  Check out the OC2 schematic.
The coupling cap prevents the bias passing from the opamps to the transistor base.

(In general, BJT input and output buffers need this bias resistor.)

[bushidov]

The transistor needs base current to bias correctly.  Check out the OC2 schematic.
The coupling cap prevents the bias passing from the opamps to the transistor base.

Ah, that's right. Originally, I didn't have that cap there, and that's why it would have worked to a degree. Yeah, I should have caught that.

[duck_arse]

They are not 100% the same as a 4013 but there are plenty of Octave circuits out there which uses those transistor circuits.     I know duck_arse was playing around with these a while back.

if you want a super simple octave down, the transistor flipping flopping is a good way to do it - but don't expect tracking accuracy of any description, or an output that sounds anything like the input, cause it's not going to happen. it's just all octave hopping fuzzy madness comes out the end.

[antonis]

it's just all octave hopping fuzzy madness comes out the end.

Cheers for godfather of my bike flasher impressive naming..

[11-90-an]

if you want a super simple octave down, the transistor flipping flopping is a good way to do it - but don't expect tracking accuracy of any description, or an output that sounds anything like the input, cause it's not going to happen. it's just all octave hopping fuzzy madness comes out the end.

petition for a sample scheme please...

wouldnt that be slightly tamed with a square to sine wave converter? 

[duck_arse]

petition for a sample scheme please...

wouldnt that be slightly tamed with a square to sine wave converter? 

I say simple, but my circuit is way overblown. it started from the shocktave, via Ben Lyman, then went silly. see the whole mess here:
https://imgur.com/a/uC0SI

[ElectricDruid]

wouldnt that be slightly tamed with a square to sine wave converter? 

It would, if such a thing reliably existed. Mostly they require prior knowledge about the frequency and/or amplitude, neither of which are things we know ahead of time for a guitar signal. As ever, you can fake it and get somewhat close.

It's funny, because going in the other direction (Sine->Square) is very simple.