Chopped Boss OC-2 - true bypass, 1/2 the parts

[Processaurus]

This got lost in the move, I guess.  Here's a re-post:


This was a notion I've had for a while, you can eliminate a lot of parts from a boss octave divider clone by ditching the FET switching and the 2nd octave down.  Now it only takes 3 chips: 2 quad opamps and a CMOS 4013 flip flop.  The boss design sounds "cleaner" than more synthy sounding analog octave dividers, by mangling the original signal into a sub octave, rather than mixing in a filtered square wave.

Using this schematic as a starting point, sometime I wanted to try inserting the tone control from Howard Davis's much more complex Deluxe Octave Multiplexer design, and also a compressor IC in the side chain, a la the Anderton Rocktave divider to try to get the tracking better (though the boss already seems to do better than most analog octaves I've tried like the octabass, octron, blue box, FM4).

[Processaurus]

I had asked about the active lowpass filter used to filter harmonics out of the fundamental extractor part (and also in the synthesized octave down to minimize artifacts), and how to change the cutoff frequency, STM was good enough to post this link:

That topology corresponds to a third order lowpass filter.  Here is an online calculator that would allow you to choose cutoff frequency to your needs:

http://astro.u-strasbg.fr/~koppen/radio/Activee.html

Just make sure you select a third order response like 3rd order butterworth and you're done.

Also, Zero the Hero corrected a mislabeled pin, thanks, and had some good questions:

Sorry for the newbee questions, but i'm trying to divide by blocks this schematic and I do not understand how the lower octave is generated.
The guitar signal enters in the effect and after the first non-inverting op-amp it get splitted in three parts:
- the direct level goes straight to the output buffer Q2 via pot R59

Yes.

- IC1 (pin 12, 13, 14) forms a lowpass filter, then R10 / R32 / C20 another first order lowpass filter

Yes.

then the filtered signal goes splitted and squared (o rectified?)

Rectified.

into both positive and negative half-wave by IC2. What this part of the circuit actually does?

You have it right, so far.

I suppose that IC2 (pin 5/6/7) and IC2 (pin 8/9/10 correct this!) are rectifiers that with C22 and C23 become envelope followers which drive alternatively the filtered signal applied on non-inverting pins of IC2 (pin 1/2/3) and IC2 (pin 12/13/14)

I don't understand this section very well either, but I think this is the clever part that makes the OC-2 track notes a little better than other analog octavers.  The filtered half wave rectified signals drive opamps set up as comparators, notice one half wave goes to the non-inverting input of IC2 pin 12, and the other half wave signal goes to the inverting input, IC2 pin 2.  The half waves are compared to the signal from R10/R32.  This part is mysterious to me, I would be very interested to hear a better analysis.

CORRECT ME PLEASE! all this mess drives the CD4013.

I don't quite get the how and why, but the two comparators put out short square wave pulses.  One side pulses when the signal goes positive, and the other when it goes negative. These pulses alternately pulse the set, and reset pins on the first 4013.  The output of that section of the 4013 is an even duty cycle square wave, at the same frequency as the instrument.  I'm guessing it is an attempt at hysteresis, to prevent glitches when the fundamental extractor would otherwise trigger on little overtones rather than the fundamental.    This square wave from the first section of the 4013 gets divided again to be an octave down by the next, second section of 4013, just like other octave dividers that use flip flops.

I do not understand how IC1 (pin 5/6/7) works and how the divided signal is mixed. The output of the CD4013 is applied to Q8 which is configured as a variable resistor (right?) more than an amplifier. Then I get lost...

Q8 is used here as an electronic spst switch.

Q8 varies the amount of straight signal applied to the non-inverting pin of IC1 (5/6/7), which normally is applied through resistors of the same value on both inverting and non-inverting pins. Since CD4013 returns a square wave whose frequency is half the original frequency, is it correct to say that IC1 (4/5/6) has gain >1 only for an amount of time equal to half the input frequency (actual the inverse of the half of the input frequency)? Thus, the divided signal actually modulates the straight signal rather than being mixed with it.

Q8 is used as a switch, to change the phase at the output (pin 7), at a frequency an octave down from the fundamental.  The Ge diode adds a DC offset when it switches phase.  This Stompboxology article explains it  better than I can, skip to page 14 for the concept at work here.  This Geo article does to, except for what the Ge diode does.

This can't be correct... and explaining everything in English is a torture...
icon_cry

Your English is excellent, thanks for getting me thinking about this, some time I will hook up the scope the fundamental extractor part we are interested in, to see how it works.  Might make a good extractor to graft onto other synthesizing effects, like Escobedo's PWM, or simple square wave shaper, or Blue box

[littlefunky]

does anybody has layout for this schematic?

[Zero the hero]

Processaurus, thanks for replying!
I've read your post just now, that's way this delay in replying.

PS: Those links are gorgeous!

[Processaurus]

On the schematic I added power and ground hookups to the 4013 flip flop chip, and realized I could get rid of the coupling cap and 1M biasing resistor on the output transistor buffer, because everything coming in was biased at V1/2 already.

does anybody has layout for this schematic?

I don't do layouts, but this would be straightforward to do on perfboard or strip board, if you've made a few simpler projects already.  Visually it looks like a lot more parts than it is, but it'll only be 3 chips, a transistor, a FET, and the supporting passive components.  I will gladly help you de-bug it if you decide to make it by hand and run into trouble.

[littlefunky]

thanks for reply Processaurus.I will try to make layout for this schematic,because stripboard and perfboard techniques hard for me :)sorry for bad english

[marcoharder]

Speaking of the OC-2, is there a mod that will make the octave sound more consistent? I often find it dropping out and leaving the fundamental note ringing on its own, and then it comes back randomly. Is there a way to correct this?

[Jaicen_solo]

Not without a more complex circtuit to either compress the driving signal, or a PLL.

[Barcode80]

you can run a compressor pedal before it. that should give it a chance to track a bit better.

[Processaurus]

Speaking of the OC-2, is there a mod that will make the octave sound more consistent? I often find it dropping out and leaving the fundamental note ringing on its own, and then it comes back randomly. Is there a way to correct this?

All the analog octave downs are marginal at tracking pitch, but with what you're saying, I wonder if the signal you are feeding it may be less than it was designed for (perhaps weak pickups?).  Have you tried boosting it before the octaver  a bit to see if it works better?  If it does it would be easy to internally increase the gain to optimize your OC-2 for your instrument (if the trouble is with the signal strength, and not the general fickleness of octavers).  PS Do you use the neck pickup?

Not without a more complex circuit to either compress the driving signal, or a PLL.

Sometime I'm going to try inserting an Analog Devices compressor chip I have kicking around in the sidechain, to see if that improves things. 

[Andre]

Ben,

I think there's a small error in your chopped OC-2 Schema.
It looks like you swapped pins 8 and 10 of IC2. The LM324 datasheet says pin 8 is output.

I also noticed that you left out the FET input stage. Does this influence the input sensitivity ?

BTW. I'm doing a PCB with only the extractor part of the OC-2 for use with guitar tuner, octave up/down circuits and maybe guitar synths.

André

[Processaurus]

Good eye, I fixed the mislabeled pins, that woulda definitely made it not work.  The JFET buffer is in place to buffer the FET switching, it won't help anything in this current version.

BTW. I'm doing a PCB with only the extractor part of the OC-2 for use with guitar tuner, octave up/down circuits and maybe guitar synths.

André

Sounds fun!  I noticed the new Music from Outer Space guitar synthesizer uses a very similar fundamental extractor.

[snoof]

Sounds fun!  I noticed the new Music from Outer Space guitar synthesizer uses a very similar fundamental extractor.

that thing looks awesome, talk about a busy build, geez...

[audioguy]

Is there a layout for this??? I thought I saw one just the other day, but now I cant seem to find it anywhere???

[Processaurus]

Someone here may have made a layout just fo the fundamental extractor, but I haven't seen one for the whole shebang.  I think I'll try a version of this with the boss fundamental extractor and the EHX deluxe octave multiplexer octave synthesizing and tone sections, that one can get inspirationally sub-ey!
Looks like the schematic's link got bungled in a forum move or whatnot, here it is again:

[ConanB]

Good timing; the day I'm looking for a good octave down and you repost your schem

Now to see if I can find a good octave up to blend in with the down signal.

[Processaurus]

Now to see if I can find a good octave up to blend in with the down signal.

The green ringer at GGG with the LP filter mod I hacked together might be the ticket, it's pretty piping.  The old mu-tron octave divider uses the green ringer for the octave up sound (marked ring on the switch).  Or any octavia with a LP filter will get extra octave-y.  The board of the GR I put together was actually in a shape to fit in the battery compartment of an OC-2, the intent to do what you're thinking.  I'd still like to try it, and I'd love to hear yours if you get one lashed together.

[ConanB]

Well the aim is for a clean octave down and a clean octave up in a box, and have it setup like the EH Micro Pog, with blend amounts for the up/down/clean.

Not sure when I'd get it done by, but I was looking at the Green Ringer as the up. Guess I'll have to look into this LP filter. Might have to wait for now as I have to get a smash drive boxed up first

Loving this hobby

[Sweetalk]

I have the layout for the complete OC-2, all octaves + FET switching. I did it by editing an original schematic that has the layout on it. You can choose not to populate the FET switch components and make it true bypass like I did, I also didn't put the BA634 (I couldn't find it anywhere) and works really well.

The tracking it's WAY much better than the MXR Blue Box (the only octave pedal that I build), I get the best sound with my bridge humbucker. I have a single coil in the neck and simply it doesn't work with it so I modded the input buffer and put a "Gain" control to adjust for humbucker-style pickups or single coil-style pickups operation. Also with the Gain all the way up has a very weird distortion effect.

I'll post the schematic / layout if it's allowed to, let me know!!!

[audioguy]

I would love to see that layout!