Basic theory of an octave fuzz? Is there a Geo article?

[strassercaster]

hello i just made a PE COB . for a friend. I started building pedals 7 months ago and it was my 45th build to date. I started out years ago in pinball machines and restorations. i had a tech that was a cool guy and he taught me a lot about electronics.He taught me well it got to the point he hated coming because i had serious tough issues ha ha. i really got into it for several years buying and selling about 80  machines spanning 6 decades. I prefered the 90's machines because of the challenge and profitability ha ha. I no longer have any pinballs and decided to get into pedals.So i know some basic theory and do really well at trouble shooting but most of what i know has nothing to do with tone audio or sine wave/triangle waves or how these circuits come together.Pinball is more about switches ,connectors,timers, clock chips and cpu issues.

i have read most of the Geo articles . i have built univibes,chorus ,compressors,delAys, fuzz ,rangemasters,flangers ,all on stripboard except the flanger and univibe which i am in the process of doing.

I havent seen a good article on the technology of an the octave fuzz. I love the way RG goes into each circuit. My addiction started with modding my wah back in december. the wah article is a masterpiece
.could someone walk me through this circuit .I know its splits at some point but how or where is the octave coming from? I know its q3-4 but what going on there? while i have studied all the RG Geo fx articles .I have to read them several times and sometimes i still cant wrap my brain around some of the more complex stuff.i admit i am still painting by numbers in a lot of ways and realise I dont know much.I saw a comment RG made on the prescription electronics build and it got me thinking.

The build i just did i started with  the reccomended 2n3904 and it was like circus lemonade. it was mild And nothing to get excited about. they were all at 330-355 hfe. so i upped them to 2n5088 550-590 hfe and i was pleasently suprised. So i saw where he said that q3-4 were the octave section so i upped those to 2n5089 and it was really octaving but also noisier and some unwAnted distortion. so i backed q3 down to a 2n5088 and it was right where i wanted it. just on the verge of insane with the octave knob at 9 and insane  totally dimed.

i read that a few people match  these transistor pairs as close as possible. i was wondering because i already mailed it to him . Did I mess up . it sounds good bit could it have been better had i matched the sets as close as possibel? i did like it better with q4 being hotter but its a quirky effect doesnt do much below the 9th fret.

Thanks in advance and I am 90 percent done with mine I just need to hook up the 3pdt switch which ill do in a couple of days. i am pedaled out  I built a fuzz face and a small clone chorus today on stripboard plus the second C O B of the week.Thanks again

[strassercaster]

Is it just creating 2nd harmonics?

[hymenoptera]

I'm curious too.

I just figured the waveform was getting folded asymetrically as one side smashed the rail, giving a taste of 2nd harmonics on the loudest notes.

If it's something else then I'd love to hear more about it.

[R.G.]

The basic theory of an octave fuzz divides into three branches. One of those branches can only be done by mathematical operations inside a digital signal processor (DSP), so that's not how most pedals work.

The way most pedals work is that they full-wave rectify the audio signal. Some few of them with a more subtle octave effect just clip asymmetrically. These all work on the principle that any time you do an asymmetrical process to a signal, you generate second harmonic distortion. The second harmonic is heard as an octave. The details of exactly how and how much the distortion process bashes the waveform accounts for the peculiar sound of the octave effect.

The Fender Blender, Green Ringer, Fox Tone Machine, and Univox Super Fuzz (and many others, variants, and knockoffs) do electronic full wave rectification. The Tychobrahe Octavia uses an actual transformer and diodes, power supply style, to do full wave rectification. Rectifying small signals with semiconductor diodes is a problem because the diode forward drop eats up some of the signal you're trying to rectify, causing an even uglier distortion than the full wave rectification. All of these except the Octavia use electronic schemes to bias their rectification so it doesn't have as big a problem with diode forward drop. The Octavia uses the transformer to boost the signal a lot, and germanium diodes with smaller forward drops to make the problem smaller.

A full wave rectified sine wave can be run through Fourier frequency analysis to find that the output has its biggest component at twice the input frequency, and progressively smaller components at 4x, 6x, 8x, 10x and so on, pretty much out to infinity. 2X, 4x, and all the other powers-of-two harmonics are octaves. The others are progressively more discordant. The human ear picks out the 2x as an octave, but also hears the others as distortion.

That's on a sine wave. On complex waves, which can be thought of as an additive mixture of frequencies, you not only get 2x F1 and 2x F2, you get F1 +/- F2, F1+/- 2xF2, 2xF1 +/- F2, and an infinite string of sum-and-difference frequencies of the original frequencies and their harmonics. This is intermodulation distortion, and is responsible for more of the harshness in FWR distortions. Guitars do not produce sine waves, so intermodulation is always there.

The details of filtering before and after the distortion process affect the intermod nature of what comes out, as do the details of the intermod process itself.

You get similar results, but less second order content, by doing anything to one polarity of signal that you don't do to the other polarity of signal. Banging the signal against one or the other but not both power supplies will do it. So will half wave rectification, wave folding, etc.

The third method exploits the harmonic structure of distortion. The MOS Doubler uses the second harmonic distortion of MOSFETs (and JFETs in the earlier version) and extracts this by adding two out-of-phase versions of the same signal as amplified by FETs. Adding out of phase signals cancels the original signal and any odd-order distortion, but reinforces the even order, especially the second harmonic.

There's about a week's worth of typing to get the details of each one, but that's the basics. Bottom line is you do something to a signal wave, and then take the Fourier transform of the result. A prominent 2x the original frequencies means you got an octave.

[Ben Lyman]

Good question and thanks for the great answer RG!
Is the Jordan Bosstone supposed to do an octave thing too?
I'm getting a weird Octavia-ish thing happening with a Bosstone on my breadboard, you know the thing when you play on your strat neck PU between the 10-15th fret area w/vol and tone slightly rolled back.

[Mark Hammer]

Some fuzzes with produce either octave-up or octave-down artifacts; the Boss-tone being one that will yield apparent octave-down if you keep enough bass in the input signal and pick just right.  I welcome an explanation of how such artifacts occur, but suffice to say that these circuits are not deliberately designed to produce such things.

I'll try and convey what RG said in a different way (since one explanation, no matter how good, does not necessarily click with every learner).
Imagine a "pure" waveform on a scope.  Wave sweeps up, wave sweeps down, up, down, etc.  The "downs" fall halfway between the "ups".  Let's say we split the signal so that the ups and the downs each temporarily followed a separate parallel path, with the "ups" having just a flat "nothing" between each upward blip, and each "down" having a flat nothing between each downward blip.  Now, we electronically invert one of those.  Doesn't really matter if it's the ups or the downs.  Having the "in-between" blips pointing in the same direction as the other blips gives us twice as many blips per second as we started out with.  Ergo, one octave up.

For a variety of reasons whose math is way over my head, bending notes, when using one of these circuits, yields sideband products.  That is, sum and difference, like a ring modulator.  Bend your string up, and you'd swear you heard a note descending at the same time...because it does.  This is, of course, why the Green Ringer is called the Green Ringer.  It is a simplified octave doubler, but because the octave is not drastically more robust than the sideband products, it tends to have a more pronounced ring-modulator flavour than the Foxx and many others.

One of the consequences of using diodes in series with the signal to effectively lop off half the wave form is that we don't get to hear the entire half cycle.  The signal has to reach the forward voltage of the diode in order to pass.  If the forward voltage is much higher than the signal level, then the first and last little bit of each half cycle is lopped off, because it doesn't "get through".  One of the more interesting artifacts of that in some circuits is that the signal can appear to have a slight delay when you pick.  Not delay in the sense of echo, but rather a bit like the several milliseconds offset that happens with older MIDI devices and guitar synths.  Personally, as much as I'm a big booster for the Foxx circuit, the old transformer-based Tycho Brahe Octavia has a certain charm stemming from the audible delay/stagger of this sort that it introduces.  If I was going to attempt to play the Hendrix tune "One Rainy Wish", there is no other octave I'd rather use, just because the delay produces a softening of the sound.

[anotherjim]

I suppose you might think you hear an octave down if the IM produced harmonics happen to be odd harmonics of the octave down fundamental together with the original fundamental heard as it's second harmonic. Put another way, our brains know that if there's 200Hz and 300Hz together then there ought to be 100Hz below. It's called a "missing" or "phantom" fundamental.
https://en.wikipedia.org/wiki/Missing_fundamental

As of the original post, I've no idea why Hfe should matter so much to the rectifier mixer transistors -  unless resistor values are leaving lower Hfe examples starved of base current.

[PRR]

> just creating 2nd harmonics?

Evens on top of the fundamental just makes a sweeter/richer fundamental. The perceived pitch does not change; you don't say "octave up".

> match these transistor pairs as close as possible

R.G. gave an excellent overview of the ideal doubler process.

If *everything* (not just transistors) is not well matched, in addition to the even-order effects you get input bleed-through plus odd-order effects. Input bleed tends to spoil the "octave" effect (your ear goes to the lowest pitch it can make out). A little 3rd harmonic will mask quite a lot of 2nd harmonic and give a very different sound. This 3rd is *not* a simple overtone of the 2nd, and spoils the "octave up" deception.

On the simple "octave up" schemes, balancing is needed to reduce the fundamental and to reduce the odd harmonics.

The bit about diode (or transistor) small-signal error deserves consideration. A "perfect rectifier" leaves spikes at the zero-crossing. As these *are* 2nd harmonic, they are good; as these are infinitely sharp-pointed they throw even harmonics to infinite frequency, which is shrill. The popular "octave up" schemes do im-perfect rectification leading to rounded transitions at the zero-crossings. But this also rounds-away soft sounds. The balance between rounding and loss of small signals must be critical.

As R.G. says, pre- and post-EQ is usually wanted. Broad chords will throw IM hash all over the place. A rising pre-EQ response may help for musical sounds (which tend to be bass heavy). A falling post-EQ helps deemphasize the inevitable hash. (As an extreme you break the input into multiple sub-octave bands, handle each one separate, filter, and recombine.... but that's a lot of stuff.)

[anotherjim]

I can't find a schematic of the COB -  does anybody have a link?
I can find a strip-board layout, but there's hidden track cuts below components so it's hellish hard to trace.
Anyway, it's looks like a pretty low input impedance - which takes care of some of the pre filtering -  provided you don't have a buffer before it. It doesn't look like it has a differential pair to mix a  polarity split to make a full wave rectified octave-up. Maybe it's an asymmetric clipper type since transistor gain is found to matter a lot.

[Cozybuilder]

Do these help?

http://www.sabrotone.com/?attachment_id=3813


posting #6
http://www.diystompboxes.com/smfforum/index.php?topic=77090.0

The cuts are easily found on both layouts

[amptramp]

If you want visualization of waveforms, go to this site:

http://www.falstad.com/fourier/e-index.html

and go sine then fullwave rectify and move your cursor over the peaks and you will get coefficients of each harmonic.  It may get a little confusing but DC is on the left and you gat amplitude and phase of all successive harmonics.

[Cozybuilder]

I traced Harold Sabro's layout, hopefully got it right:



Edit: Inserted missing R18 & R19 (thank you Duck), values of components added

[anotherjim]

Russ's trace is good enough for me. I don't think I totally understand it!
I'll try to explain what I can figure out and see if anyone agrees or disagrees?

I can see Q4 is a phase splitter and the diodes rectify (blocking alternate cycles) and passively mix to give a full-wave rectified output. Apart from the 100k resistors limiting the diode currents and making sure C7 & 8 function when the diodes are off and R16 & 17 both being equal value, I can't see anything  to match for the purest octave up.
The input seems to have some kind of envelope dependant bias from Q2 back to Q1 base - see the 120k/100nF. I think that gives DC bias from the average level on Q2 emitter. And it does it again from Q4 back to Q3. I don't know if this is just an auto biasing technique or does something for the audio!???
Additionally, C5 is high frequency negative feedback on the octave signal and the only thing I can see cutting the top end anywhere.

[PRR]

> some kind of envelope dependant bias from Q2 back to Q1 base

Where is the envelope developed? Unless things clip, the cap just holds the average DC, which will not change with audio.

I too am not sure about some bias details, but I ass-ume this is largely two linear stages with a phase-splitter and diode-net on the end.

The key *is* the diodes and (three) 100Ks. It is "octave up" because a full rectified wave has some similarity to twice the original frequency (it repeats twice as often), and because the original is fairly well suppressed.

For a 1KHz sine, the output is a lot of 2KHz and declining amounts of 4KHz, 6KHz, 8KHz and 10KHz. The 2KHz spacing confirms (to the ear) that the fundamental "is" 2KHz.

The caps matter too, because this technique (most doublers generally) throws a big DC component (almost equal to half the peak of the original wave). We don't want that to swamp subsequent audio stages, yet don't want it to bleed-off too fast. In this plan the caps are large and a DC offset is passed along the audio path. With some BIG amps and speakers this could even result in a cone-jump, giving an attack you can feel in your chest; other amps may fart a bit until they bleed-off the DC in their own hi-pass networks.

Link to sim run. The linear stages are simplified and totally ideal. 1KHz 200mV input.


http://oi60.tinypic.com/24enx4m.jpg

With ideal diodes and huge input levels, the output should be pointy at the zero crossings. But these real and biased diodes come to zero more gently. There may be a smaller input level where the point-rounding is similar to the sine-side and the result is nearly sine. At larger levels the pointiness and upper harmonics come out, in proportion to level, which is a very musical thing (voice, reed, other instruments, but not-so-much naked steel strings).

[LightSoundGeometry]

hello i just made a PE COB . for a friend. I started building pedals 7 months ago and it was my 45th build to date. I started out years ago in pinball machines and restorations. i had a tech that was a cool guy and he taught me a lot about electronics.He taught me well it got to the point he hated coming because i had serious tough issues ha ha. i really got into it for several years buying and selling about 80  machines spanning 6 decades. I prefered the 90's machines because of the challenge and profitability ha ha. I no longer have any pinballs and decided to get into pedals.So i know some basic theory and do really well at trouble shooting but most of what i know has nothing to do with tone audio or sine wave/triangle waves or how these circuits come together.Pinball is more about switches ,connectors,timers, clock chips and cpu issues.

i have read most of the Geo articles . i have built univibes,chorus ,compressors,delAys, fuzz ,rangemasters,flangers ,all on stripboard except the flanger and univibe which i am in the process of doing.

I havent seen a good article on the technology of an the octave fuzz. I love the way RG goes into each circuit. My addiction started with modding my wah back in december. the wah article is a masterpiece
.could someone walk me through this circuit .I know its splits at some point but how or where is the octave coming from? I know its q3-4 but what going on there? while i have studied all the RG Geo fx articles .I have to read them several times and sometimes i still cant wrap my brain around some of the more complex stuff.i admit i am still painting by numbers in a lot of ways and realise I dont know much.I saw a comment RG made on the prescription electronics build and it got me thinking.

The build i just did i started with  the reccomended 2n3904 and it was like circus lemonade. it was mild And nothing to get excited about. they were all at 330-355 hfe. so i upped them to 2n5088 550-590 hfe and i was pleasently suprised. So i saw where he said that q3-4 were the octave section so i upped those to 2n5089 and it was really octaving but also noisier and some unwAnted distortion. so i backed q3 down to a 2n5088 and it was right where i wanted it. just on the verge of insane with the octave knob at 9 and insane  totally dimed.

i read that a few people match  these transistor pairs as close as possible. i was wondering because i already mailed it to him . Did I mess up . it sounds good bit could it have been better had i matched the sets as close as possibel? i did like it better with q4 being hotter but its a quirky effect doesnt do much below the 9th fret.

Thanks in advance and I am 90 percent done with mine I just need to hook up the 3pdt switch which ill do in a couple of days. i am pedaled out  I built a fuzz face and a small clone chorus today on stripboard plus the second C O B of the week.Thanks again

hey Billy, Its me john . the COB sounds great. I am going to build me one soon. I might even have parts on hand already, and if so, i can put one together by tuesday ..i like this one much more than the green ringer I am using..seems to have a more pronounced octave effect and the ringer is really bad outside of the sweet spot for much of anything .

Billy , you are way ahead of the curve. your stuff looks like you been doing this for 5 years already ..those pinball machines are like computers/atches and old radios ..full of physics and math lol

[strassercaster]

Thank you all for the  knowledge and wisdom. I told my friend that RG replying to my post was like gettiing a one on one guitar lesson from Eddie van Halen or Ritchie Blackmore LOL. I am still wrapping my head around these explanations but i have a basic understanding now. I lNow have work and studying to do THANk YOU ALL  i really appreciate it.I will have more questions after i get a handle on all this information. I finished the second one yesterday . i used mismatched 2n5088 forq3 and 2n5089 for q4 again but  I am going to try matching as close as possible and report back.I placed an order for a transformer to do the tycho brahe today as well.

[strassercaster]

hello i just made a PE COB . for a friend. I started building pedals 7 months ago and it was my 45th build to date. I started out years ago in pinball machines and restorations. i had a tech that was a cool guy and he taught me a lot about electronics.He taught me well it got to the point he hated coming because i had serious tough issues ha ha. i really got into it for several years buying and selling about 80  machines spanning 6 decades. I prefered the 90's machines because of the challenge and profitability ha ha. I no longer have any pinballs and decided to get into pedals.So i know some basic theory and do really well at trouble shooting but most of what i know has nothing to do with tone audio or sine wave/triangle waves or how these circuits come together.Pinball is more about switches ,connectors,timers, clock chips and cpu issues.

i have read most of the Geo articles . i have built univibes,chorus ,compressors,delAys, fuzz ,rangemasters,flangers ,all on stripboard except the flanger and univibe which i am in the process of doing.

I havent seen a good article on the technology of an the octave fuzz. I love the way RG goes into each circuit. My addiction started with modding my wah back in december. the wah article is a masterpiece
.could someone walk me through this circuit .I know its splits at some point but how or where is the octave coming from? I know its q3-4 but what going on there? while i have studied all the RG Geo fx articles .I have to read them several times and sometimes i still cant wrap my brain around some of the more complex stuff.i admit i am still painting by numbers in a lot of ways and realise I dont know much.I saw a comment RG made on the prescription electronics build and it got me thinking.

The build i just did i started with  the reccomended 2n3904 and it was like circus lemonade. it was mild And nothing to get excited about. they were all at 330-355 hfe. so i upped them to 2n5088 550-590 hfe and i was pleasently suprised. So i saw where he said that q3-4 were the octave section so i upped those to 2n5089 and it was really octaving but also noisier and some unwAnted distortion. so i backed q3 down to a 2n5088 and it was right where i wanted it. just on the verge of insane with the octave knob at 9 and insane  totally dimed.

i read that a few people match  these transistor pairs as close as possible. i was wondering because i already mailed it to him . Did I mess up . it sounds good bit could it have been better had i matched the sets as close as possibel? i did like it better with q4 being hotter but its a quirky effect doesnt do much below the 9th fret.

Thanks in advance and I am 90 percent done with mine I just need to hook up the 3pdt switch which ill do in a couple of days. i am pedaled out  I built a fuzz face and a small clone chorus today on stripboard plus the second C O B of the week.Thanks again

hey Billy, Its me john . the COB sounds great. I am going to build me one soon. I might even have parts on hand already, and if so, i can put one together by tuesday ..i like this one much more than the green ringer I am using..seems to have a more pronounced octave effect and the ringer is really bad outside of the sweet spot for much of anything .

Billy , you are way ahead of the curve. your stuff looks like you been doing this for 5 years already ..those pinball machines are like computers/atches and old radios ..full of physics and math lol

Thank you John .mitsna cool corcuit. I was going to start on a Foxx or tycho brahe one afternoon and my friend requested the clean octave blend which i never heard of. i was pleasently suprised after i did some tweaking. i would have never known . I have owned boss pitch shifters and whammy pedals. they always sounded sterile or cold to me kind of fake. these fuzz octavers have a warm fuzzy feel ha ha. It a definite keeper and Ill be building the octavia soon.

[strassercaster]

here is a link too a short demo and expkanation for my firend before mailing it. i wanted to make sure he liked. it was 2am after a 12 hour day at work and 4 hours building this one. please forgive my playing and poor demo skills ha ha.

http://youtu.be/gYx7XmnOPng

[hymenoptera]

I enjoyed the demo vid, thanks!

[LightSoundGeometry]

hello i just made a PE COB . for a friend. I started building pedals 7 months ago and it was my 45th build to date. I started out years ago in pinball machines and restorations. i had a tech that was a cool guy and he taught me a lot about electronics.He taught me well it got to the point he hated coming because i had serious tough issues ha ha. i really got into it for several years buying and selling about 80  machines spanning 6 decades. I prefered the 90's machines because of the challenge and profitability ha ha. I no longer have any pinballs and decided to get into pedals.So i know some basic theory and do really well at trouble shooting but most of what i know has nothing to do with tone audio or sine wave/triangle waves or how these circuits come together.Pinball is more about switches ,connectors,timers, clock chips and cpu issues.

i have read most of the Geo articles . i have built univibes,chorus ,compressors,delAys, fuzz ,rangemasters,flangers ,all on stripboard except the flanger and univibe which i am in the process of doing.

I havent seen a good article on the technology of an the octave fuzz. I love the way RG goes into each circuit. My addiction started with modding my wah back in december. the wah article is a masterpiece
.could someone walk me through this circuit .I know its splits at some point but how or where is the octave coming from? I know its q3-4 but what going on there? while i have studied all the RG Geo fx articles .I have to read them several times and sometimes i still cant wrap my brain around some of the more complex stuff.i admit i am still painting by numbers in a lot of ways and realise I dont know much.I saw a comment RG made on the prescription electronics build and it got me thinking.

The build i just did i started with  the reccomended 2n3904 and it was like circus lemonade. it was mild And nothing to get excited about. they were all at 330-355 hfe. so i upped them to 2n5088 550-590 hfe and i was pleasently suprised. So i saw where he said that q3-4 were the octave section so i upped those to 2n5089 and it was really octaving but also noisier and some unwAnted distortion. so i backed q3 down to a 2n5088 and it was right where i wanted it. just on the verge of insane with the octave knob at 9 and insane  totally dimed.

i read that a few people match  these transistor pairs as close as possible. i was wondering because i already mailed it to him . Did I mess up . it sounds good bit could it have been better had i matched the sets as close as possibel? i did like it better with q4 being hotter but its a quirky effect doesnt do much below the 9th fret.

Thanks in advance and I am 90 percent done with mine I just need to hook up the 3pdt switch which ill do in a couple of days. i am pedaled out  I built a fuzz face and a small clone chorus today on stripboard plus the second C O B of the week.Thanks again

hey Billy, Its me john . the COB sounds great. I am going to build me one soon. I might even have parts on hand already, and if so, i can put one together by tuesday ..i like this one much more than the green ringer I am using..seems to have a more pronounced octave effect and the ringer is really bad outside of the sweet spot for much of anything .

Billy , you are way ahead of the curve. your stuff looks like you been doing this for 5 years already ..those pinball machines are like computers/atches and old radios ..full of physics and math lol

Thank you John .mitsna cool corcuit. I was going to start on a Foxx or tycho brahe one afternoon and my friend requested the clean octave blend which i never heard of. i was pleasently suprised after i did some tweaking. i would have never known . I have owned boss pitch shifters and whammy pedals. they always sounded sterile or cold to me kind of fake. these fuzz octavers have a warm fuzzy feel ha ha. It a definite keeper and Ill be building the octavia soon.

its good to know its verified - cozy traced it and you built it ..Im good to go !

I got the tycobrehe working and didnt like it ..I may have made an error in the build but it was some nasty tone destroying fuzz and not much octave lol

tag board has two new ones up ..a liquid phaser and a tap tempo trem. I saw the huge phaser board and thought of you haha