Slightly modified Shocktave

[Heemis]

So I recently breadboarded the shocktave, and I loved the octave down, thanks Joe Davisson!! But I wasn't crazy about the unison signal, so I decided to rework it a bit.

Here is a very rough drawing of the altered schem:  http://farm4.static.flickr.com/3424/3390154798_7d0bc61203_b.jpg  (The part values for the original part of the schem didn't scan very well, so here's a link to the original schem:  http://www.diystompboxes.com/analogalchemy/sch/shocktave.html)

I basically used a slightly modified distortion plus circuit for the input boost, and then broke the output of that off into the octave generator through a .47mf cap, and a low pass filter set around 280hz.  I wanted to achieve good octave tracking without having to totally dull the unison sound by using the neck pickup, and rolling off the treble on my guitar.  This works well as far as tracking goes, but it looses some volume and therefore the octave doesn't sustain very long, but long enough for me.

Since I was using a TL072 for the D+ style input stage, I decided that I wanted to use the other op amp for something.  My first thought was as an active low pass filter with a boost going into the octave gen, but that resulted in a constant low frequency drone.  So I dropped that idea and went with the passive filter and decided to add a switch to send the Octave down signal to a separate buffered output to drive another amp.

Basically I just wanted to post up the schematic and have someone let me know if anything looks technically wrong with it.  I currently have it on the breadboard and it sounds great and works exactly as I want it to, just wasn't sure if I messed something up from a design perspective because I'm fairly new to tinkering with circuits like this.

Thanks for any comments/constructive criticisms in advance!

[Heemis]

Also... any idea why this circuit would oscillate like crazy when I turn it on without a guitar plugged in?

[Heemis]

Ok, I know the schematic drawing is ugly, but I really just want confirmation that I set it up correctly before I commit it to perf.  Anyone?

[DDD]

Hi Heemis,
the circuit may oscillate because of the penetrating of the output signal to the input - it's usual for the hi-gain devices with the unplugged high impedance input. So, when you shunt the input with guitar pickup, the effect goes away.
Also, I've breadboarded the JD Shocktave and have to say it works great. But I feed the oscillator (some people mistakenly treat it as a trigger) not through frequency-to-voltage converter, but through the half-way rectifier, composed of a serial capacitor and couple diodes.

[liquids]

Seems like a good idea.  I tweaked the circuit around a bit and personally found that the more gain I could feed the octave circuit the better.  I went with no 'clean' signal at all as that was the kind of tones I prefered.  Your idea is an interesting variation.

I'm curious as to why you would feed the f-to-v section before the diodes.  I think the clipping, however slight, might help the section as I found it to track more consistently with a dirty signal, to a point, though you'll loose volume, maybe.   I would hence continue to mess with using the second op amp as a boost into octave side.  Op amps are going to generate quite a bit of gain potentially...so play with a pot (which you can then turn into a set resistor if you prefer) in the feedback loop of the second op amp stage for example. To much gain and yes, you may get oscillations or so much gain that the octave tries to read it...so control gain carefully. The first op amp stage slamming a second--even if the second is not set too 'hot'--may in theory also produce some dirt = harmonics and compression that may help the octave track better.

I see the .1uF cap to ground, and that kind of drastic!  At least from my experience.  It does make the low strings track nicely and with minimal parts, and if it works for you in this design, go for it, but I found too heavy on the low pass filtering to mean I 'lose' the high E and or B strings in some way, particularly higher frets.  I ended up going for multiple stages of slight filtering to compensate.  Don't be afraid to use two or more smaller RC filter stages (10k/.01 is a good start, .022 works but to my ears results in lost gain and tracking on the high strings, and you can tweak the resistor, etc) instead of the one 'big one' for the octave side.  Following this, if you loose gain, you can use the second op-amp as a gain recovery stage for whatever you loose from filtering, if nothing else.

I'll be gone a few days but am going to follow this one and mess with the idea myself when I get back.  I have built veroed and boxed tweaked octave, but I continue to breadboard ideas that I get as I go along, and learn more...so I may breadboard this version as well and compare.   I love the shocktave's core tone! Happy tweaking.   

[Heemis]

Thanks for the replies guys.  Since my post I've added a simple transistor boost stage after the low pass filter on the way into the frequency to voltage converter.  I've been having great luck keeping the unison signal bright, and still getting perfect tracking... and now I have normal sustain as well! 

I plan on tweaking the low pass a bit more as per your suggestions Matthew, and also switching the location of the clipping diodes to see what works best.  In the meantime I've been having a ball putting two practice amps on either side of me and playing with the octave down out of one, and the unison guitar signal from the other, true stereo operation!

[Quackzed]

i have a little switch on mine that shorts one of the .1 uf caps in the flip flop section, (either one will do). it switches from down octave sound to a same octave sound that has a weird soft attack, may not be a really usefull sound, but its easy to try out...  also that .47 off the opamp to the flip flop could be bigger for more gain into the flip flop... big like 10uf even... and i agree that feeding the flip flop a n already distorted signal works well. might try a dual low pass as suggested,
(2x) in<--10k---,-------10k---,--->out
                      .01             .01
                        l                 l
                        L__________L__ ground
that wont steal as much signal and will have a steeper roll off of highs. for better tracking AND less fizz ...
---i remember mucking around with the one i built and i felt that the flip flop liked a nice sustained sound with limited dynamic range
--feed it too much signal and the added high frequencies confuse it and make it jump between same oct and down oct..
--dont feed it enough and it loses sustain and fizzles out too fast...

                                                                               

[Heemis]

Since my original post I had since put this project aside, and built a Ropez phaser, and a modified DOD 250 overdrive, but I've come back to this one with a passion!

I've completely reworked the idea, and at this point, my main goal is to just maintain and buffer my dry signal, and either mix that with the octave down, or send the octave to it's own buffered output.  I've been working on it for the last couple of days and I'm pretty close to something very usable.  I'm definitely going to experiment with a dual low pass, and also your flip flop cap mod Quackzed.

I hope to post a full schematic once I've got it working exactly how I want.