Octave from Op Amps?

[GibsonGM]

Hi Guys,

I've been following this forum for a while now, have learned some really great stuff! This is my first post -  1st pedal was a Si Fuzzface...I modded it totally with switched input caps, tone stack & gain recovery, and have something like a really cool Sabbath sound!   I also noticed a tendency towards octave in that circuit, so:

My next problem to solve is: can op amps be used for a decent octave up? I've been messing around in simulation with rectifying a signal, phase inverting 1/2, and recombining them.  The waveform looks right, but something's missing - possibly, should the output be made to swing negative so it's actually an AC signal??  What I have now is doubled, and 1/2 sine wave (doesn't cross 0).   I used 2 741's and a 1458 to test this idea (I know, noise, slow slew, but all I have here), and you can hear the 'chimes' in there, but it's not pronounced enough.  I'll try to get a schematic up soon.  Seems like a good idea, what does everyone think??     Thanks - keep up the awesome posts!!!!!!

[GibsonGM]

Check this out, schematic:

[christian]

Thats pretty much what you see in this schematic

Just ignore the very high gain gain stage and signal clippers in the end. This is meant to be a fuzz effect.
One thing that could improve the fullwave rectifier is using active rectifier, which would eliminate the diode drop.

This is pretty much the standard way to create an octave-up effect.

ch.

[GibsonGM]

Thanks, Christian...maybe I'm trying to reinvent the wheel or something, but I think what I'm really getting at is octave with little fuzz (or at least less of the original sound present)...I think it might be minimized by "bringing the signal back to AC"...in the pic below, the blue wave is my output and brown the input.   Notice the doubled output freq., but that the negative peaks were removed in rectification.   What I'd like to do is somehow "shift" the wave 'down', so it's balanced around zero.   Like, if it goes from 0 to 1 now, can it be made to go from -.5 to 0 to +.5??  I'm assuming the fuzz comes from the assymetrical shape.   If the zero crossing point could be shifted to where the dotted line is, that would give a 'balanced wave'...anyone have any idea of how this could be done?  I'm just playing, lol, and don't know a lot about op amps, etc.; trying to learn!   

[Quackzed]

capacitors block dc i think, maybee putting on an output cap would center the signal around 0v..
try a .1uf ....

[christian]

What I'd like to do is somehow "shift" the wave 'down', so it's balanced around zero.   Like, if it goes from 0 to 1 now, can it be made to go from -.5 to 0 to +.5??  I'm assuming the fuzz comes from the assymetrical shape.

Yeah, just DC blocking the signal with capacitor after the last gain stage works. At least thats what you should do anyways, if youd plan on building this, since there shouldnt be no DC floating around between equipment (unless its really for control-voltage uses..). Another option would be to use voltage shifting on the last gain stage.

ch.

[GibsonGM]

Can you explain voltage shifting, Christian? A DC blocking capacitor does just that...kills the output, since the wave never crosses 0.  I actually built this thing, and it works to a slight degree...just have to get that output to cross 0 and go negative! 

[christian]

Can you explain voltage shifting, Christian? A DC blocking capacitor does just that...kills the output, since the wave never crosses 0.  I actually built this thing, and it works to a slight degree...just have to get that output to cross 0 and go negative! 

Well, if the signal is always positive, then you could put a resistor from last opamps inverting input to +Vcc, equal value to the one to ground.

But the output capacitor followed by a resistor to ground does work. You may need to boost the input signal quite heavily and tweak the resistors after the diodes, their values affect the current flow which affects the effect, you might wanna bring them up to 100k or so. It also wouldnt hurt to add yet another 100k to ground after the 2 mixing resistors.

Also, active rectifier may work better than this passive one.

ch.

[R.G.]

Do a google search on "precision full wave rectifier" and either "opamp" or "operational amplifier".

It's an old, well known circuit.

John Hollis used it in one of his circuits.

[GibsonGM]

Thanks a lot, guys...I'm sure those 'mods' can only improve things!  I did a total noob...one of my 1K to ground resistors after a diode wasn't placed correctly in my breadboard, and the 2.2K gain resistor was 220 ohm (red red RED lol).  Another example of working in simulation but build errors.  Once I fixed those, this circuit has a VERY pronounced octave up - similar to the Danelectro & others!  I'll keep messing with it (esp. the caps, for noise!)...using better op amps of course.  I've seen a few 'real' oct circuits, I just wanted to start from scratch to see if I could just wing it together.  It was quick & easy.  I'm happy with the results, and now know where to look.   
Keep posting   

[GibsonGM]

Just an update for anyone who might want to try this...here's a link to a sound clip.  I think it sounds pretty good, considering the only critical thing in this would be the 2 opamps for rectification (I'm using a dual), the rest are just gain and mixing. Diodes can be played with - these are Ge.  I'm going to keep messing with the input levels & active rectification - I'm just trying to minimize the fuzz since most other oct circuits add that! 
Thanks again for the input...

67 Gibson Melody Maker thru Fender Hot Rod Deluxe clean channel

http://members.aol.com/bandseeker/octave.mp3