Controfuzz: Crossing over to the frizzy side

[Mark Hammer]

So, waiting for more streamed video of the Obama visit taking place a few blocks from me at work, I'm surfing around, and end up listening to a demo of the Z-Vex Machine on youtube.  I had never really paid much attention to it before, and it seemed really interesting.  More importantly, the idea of crossover distortion started to interest me.

I'm not sure why, but the Gretsch Controfuzz came to mind, and I started to ponder whether crossover distortion could be fit into that.  Here is the schematic, courtesy of Alex Petrini.  The original used LM748 op-amps and compensating caps.  I don't know anyone who feels the functioning is compromised in any way by using some other compensated op-amp.  I used NE5534s,simply because I have a lot of them hanging around.

The Controfuzz "works" by blending a clean signal and a heavily clipped signal.  IC1a provides a fixed gain of just over 2200, which is more than enough for the D1/D2 diode pair to clip just about anything passing their way, even when the note is well into its decay phase.  Since the duration of the apparent sustain in the fuzzed signal is much longer than the clean signal, and since the two are combined as difference signals, there is an interesting transformation over time as the note seems to go from clean to fuzzy, rather than the reverse (the normal course of events in a typical fuzz).  Such a simple circuit and such an elegant idea.  Worthy of Tim Escobedo!

With so much gain applied, though, and with the need to balance the signals properly to achieve the degree of cancellation required to produce its unique effect, you'll see that PT1 is preceded by a fixed 220k resistor.  This turns the R6/PT1 combo into a sort of 230k pot that is always turned down most of the way, keeping the fuzz level fairly low.  Make no mistake, it can be a LOUD effect when PT1 and PT2 are dimed, but the fuzz level does need to be tamed for it to work properly.

Okay, back to musings about crossover distortion....already in progress.....

At first I was wondering if there might be some role for crossover distortion to be used instead of normal diodes-to-ground clipping.  maybe a diode pair could be between C4 and R6.  I ruled that out because crossover distortion places no restrictions on maximum amplitude.  Under those conditions, proper mixing/balancing would require R6 to be some ridiculous value in the megohms, and I didn't feel like futzing around to find out.  Then it occurred to me that if the diode pair used for crossover distortion were germanium, the "clamped" level at the junction of C4, R6, D1/D2 would still be a high enough amplitude to get through the forward voltage barrier of a Ge diode.

So, the first thing I did was whip out my perfed Controfuzz, and simply straddled R6 on the solder side with a back-to-back pair of Ge diodes (don't know what specific kind; they were some generic Radio Shack 1N34a subs) .  Sure enough, there was the characteristic sizzle.  But of course, it was way too loud to get the transformation/fading-in effect.  So, I quickly tacked on a 47k resistor in series, between the Ge diode pair and the R6/PT1 junction.  That got me the characteristic "sizzling bacon" edge of x-over at a level that was manageable by adjustment of PT1. 

What I would suggest for a final arrangement is keeping R6 as is, and replacing the tacked on 47k with a 33k resistor in series with a 500k log variable resistor (pot).  The idea is that the amount of additional x-over distortion can be dialed in, and the relative balance of "normal" diode-to-ground and x-over clipping can be adjusted.  If you make the series resistance after the Ge diodes large (500k+33k), then the 220k resistor presents an easier path for the once-clipped signal to follow, and the twice-clipped is way in the background.  As the 500k pot resistance is decreased, more of the twice-clipped signal is mixed in with the clean at IC1b.  Of course, if you crank PT1, it becomes a serious fuzz machine as the clean signal gets lost, and adding in the x-over distortion just makes it meaner....and louder.

One of the neat things about this is that one can achieve double clipping without having to add another gain stage (à la BMP).  And, although it isn't somethng you'd notice terribly often, because the clipping threshold of the Ge diodes is lower, you'll still get some x-over distortion even after the IC1a signal is insufficient to squeeze clipping out of the 1N4001 pair.  In effect, it extends the period where clipping can occur.  So, if adjusted right, the fuzz-fade-in effect can occur over a longer period and be more noticeable.

Such a simple mod, and so many extra tonal colours to be achieved.  Daddy LIKE!

[jacobyjd]

Wow...that is cool.

I bet it would be even more awesome if I could see the schematic 

[Mark Hammer]

Do you mean the original or what I added.  If it's what I added, just stick a back-to-back Ge pair in series with a pot and fixed resistor, and put THAT in parallel with R6.  If I could post pictures from work, I'd do it, but for now your imagination will have to do.

[jacobyjd]

Do you mean the original or what I added.  If it's what I added, just stick a back-to-back Ge pair in series with a pot and fixed resistor, and put THAT in parallel with R6.  If I could post pictures from work, I'd do it, but for now your imagination will have to do.

The original--I was just poking fun at this:

Here is the schematic, courtesy of Alex Petrini. 

I remembered that you couldn't post pics from work--there are no images showing on your original post--I got all excited to see the schematic, but there was nothing there. I felt totally punk'd. 

I found the link when I quoted your OP, but it's giving me trouble. I went ahead and google-fu'd it and found a lovely schematic on none other than Beavis Audio, complete with lovely breadboard layout, courtesy of the illustrious Dano.

Here's a link to the PDF: http://www.beavisaudio.com/bboard/projects/bbp_Gretsch_Controfuzz_Rev1_1.pdf

[Mark Hammer]

Well now that's weird, because I linked to Alex' schematic GIF in the original post (I can link from work, but I can't see or use image archive services like flickr or photobucket), and it shows up wonderful on my screen right now.

Dano's PDF file is nice and clear.  One would get the impression that all it does is allow clean/fuzz blending, much like a Sparkle-drive does. Nuh-uh, or should I say "No you di-int!"   The blending also occurs in time.  That's where the Contro part of the name comes from.

What the lower clipping threshold of the x-over distortion adds on top of this is that not only can one adjust the flavour of the fuzz that eventually fades in, but if you hang around long enough (And the real effect only emerges if you sustain notes or chords.  Playing fast doesn't let the magic through.), you hear a transition from clean to normal+x-over to x-over alone.

I suppose if one wanted maximum flexibility, you could stick something like a 50k pot between the 1N4001 pair and ground, to reduce the clipping.  Or better yet, make R4 a little bigger, or stick a 1k pot in series with it (to change/reduce IC1a's gain), so that the changeover point where the normal clipping ends and only the x-over remains can be moved around.

I think I have my weekend project identified! The 4-knob Controfrizz.    And what's neat is that I think this can be "bypassed" with an SPST switch the same way as the Small Clone ( http://www.diystompboxes.com/smfforum/index.php?topic=74196.0 ) if I stick the FET between C5 and IC1b.

[Arn C.]

Mark,
    This sounds too cool.  Please keep us posted on your progress, I think I would like to try this too!
Thanks for all this great info you put out here for us!

Peace!
Arn C.

[Mark Hammer]

Just a side note.  Although it will "work" with any op-amp, looking over the original schematic that used a 748 and 25pf compensating cap, and the LM748 datasheet, and considering the voltage gain of the thing (2200 or roughly 67db), I'm wondering if recapturing the original sound, and relative balance of lower, middle, and higher-order harmonics, might not ask for a small value cap in parallel with R3.

[jacobyjd]

I've always been a distortion/overdrive guy--it lends itself to the styles I generally play with my band...however, fuzz pedals keep sneaking their way into my live rig.

I used to look at those guys who build like 30 different fuzz pedals and laugh, saying, "I'm addicted to caffeine, but that's nowhere NEAR as pathetic as being addicted to fuzzes like THOSE guys. That'll never happen to me."

All of a sudden, I'm all "Clean-channel" this and "Fuzz Face" that, and before you know it, I'm up at 2am building some harebrained backward fuzz pedal with twitchy hands.

I feel an intervention coming.

I think I need a smoke...where's my pipe?

I SOLD IT FOR MORE TRANSISTORS!?!?!?



Here I am, hopelessly tossed in the throes of addiction, and what do you people do? Feed me more of that sweet, sweet fuzz...

[dano12]

Yay! Controfuzz post!

This really is an overlooked effect. Thanks Mark for bringing it up and suggesting some very cool mods.

Oh yeah, Happy Friday, and I'm glad we were able to send a much better President up to say hello.

[Mark Hammer]

Yay! Controfuzz post!

This really is an overlooked effect. Thanks Mark for bringing it up and suggesting some very cool mods.

You're welcome!  Although, as noted, this isn't anything that a shredder would find all that interesting (because of the time required between successive notes for its personality to emerge), those who like ambient or psychedelic stuff will like it.  I would imagine that any sort of slow-ish filtering, like a phaser, flanger, or auto-wah set for really slow decay, would get along nicely with it.  Having changing harmonic content to filter, and changing filtering of that harmonic content seems to always have a happy ending in my experience.

Oh yeah, Happy Friday, and I'm glad we were able to send a much better President up to say hello.

So am I.  Thanks!!  Now let's see if the beavertail** he had here starts sweeping the nation.  And have a frizzy weekend yourself!

EDIT:  Oh yeah, one last thing.  I think I might also remove, or toggle-enable, the cross-over distortion diodes in my Boss HM-2 and make it more normal sounding.

(**"Beavertails", while available any time of year are best eaten in the winter, generally after skating on the Rideau Canal, when your toes are frozen and the dough is steaming.  It is a beavertail-sized piece of deep-fried bread dough, similar to a Hungarian langos, but normally served as a dessert.  This one is sprinkled with cinnamon and sugar.  The one Pres. Obama had evolved at one commercial outlet over the preceding week and was named for him.  It apparently had both whipped creme and a large chocolate-syrup O.)

[petemoore]

  Not to 'contra' the thread focus...
  Perhaps a tube phase inverter designed to behave in a crossover distortion creation role would be interesting ?
  There's plenty [I just read tons] about phase inverters in audio amps, different types, how to balance them.
  And also for guitar amps, different types, how much imbalance is right for you etc.
  But none taking the PI distortion to such extremes.
  It would seem if it was of any use at all in a thread such as this, all the mods would apply .

[Lurco]

copy `n` paste in new window:
http://topopiccione.atspace.com/pjimages/GretschCtrfzzSch.gif

[nelson]

I've been giving some thought to the idea of effects slowly fading in on note decay. I'm working on a state variable filter that blends in on sustained notes. It uses two OTA's, one for the state variable filter and one for the two VCA's. Of course, this way of processing effect dynamics could be utilised with just about any effect.

I think an effect becoming more apparent the longer a note is held, although not new, is a very under used dynamic.

[Mark Hammer]

I think an effect becoming more apparent the longer a note is held, although not new, is a very under used dynamic.

Yep.  Those sounds aren't just for synth players.  Guitarists who like fuzz boxes can provide "pads" too.

[Mark Hammer]

Built.  And let me tell you, it rocks.  The mods shown dramatically increase the sonic possibilities well beyond the original.

First, being able to adjust the amount of cross-over distortion introduced varies the tonal palette of the Controfuzz effect (where the fuzz lingers after the clean note has decayed). 

Second, the Gain control on the fuzz gain stage allows one to reduce the intensity (hence duration) of the fuzz component.

Third, reduce the Gain enough, and crank up the Crossover component (while keeping the Distort control down) and you get a different kind of Controfuzz effect with a little more dynamics.

Fourth, adjust the Gain and Distort just right, and it can actually be used exactly the way you'd use a Sparkle Drive (or any of the other clean+distort pedals out there or on your breadboard).

Fifth, turning up the Distort control gets you mostly distorted signal, so you can use it as a fuzz if you want  Lots of output to punish amps and ears.

Sixth, when use as a distortion in this manner, the balance between normal and cross-over distortion components can be altered for different flavours of distortion.  The bit-of-clipping-lots-of-crossover sounds fabulous.

The original appears to have included a 470nf (.047uf) rather than 100nf input cap on the 2nd op-amp.  It doesn't make a huge difference in bottom end, but I included it just for "authenticity".  I used an internally compensated TL072, as shown in the Beavisaudio drawing.  The original used LM748 op-amps and 25pf compensating caps.  Given the gain-bandwidth limitations of the 748, and the very high gain used in the original, I included treble-cutting feedback caps for both op-amp stages to "recapture the magic".  That change makes the unit much more usable in the absence of tone controls.  If you think of it, the stock circuit has nothing other than those compensating caps to keep the ultra-top end under control, and with THAT much gain, you need something.  Especially with a high slewing op-amp like the TL072.   There is still more than enough treble to respond to guitar tone-control changes, though.

I haven't dickered much with crossover distortion, and certainly not with different orders of clipping->crossover vs crossover->clipping.  But I think I like this for bridge pickup VERY much.   Crossover distortion only clips the ascending and descending "slopes" of the waveform, but does not clips the peaks.  The neat thing here is that, while crossover distortion normally doesn't constrain the dynamics enough, preceding the crossover part with an amplitude clipping part (and modest gain so that clipping is not applied across the entire lifespan of the note) keeps those peaks under control, whilst still allowing for some moderate dynamics.  That's why it sounds so good on bridge pickup.

I'll see if I can post a sound sample whole my wife is busy watching the Oscars tonight.  In the meantime, if you have the parts, BUILD IT or breadboard it.  Took me about 2hrs to perf and wire the thing, even with interruptions, so it is a one-afternoon project.

As an aside, I'll bet you that a basic MXR Distortion+ or DOD250 would make one helluva pedal if the diodes to ground were 1N914 and you stuck a series pair of Ge diodes between there and the volume control.

[dano12]

Very nice Mark. I'll give this a whack.

[Mark Hammer]

Thanks for the easily-adapted drawing.   Saved me a heckuva lot of work.

BTW, the 100uf cap is there for the usual reasons.  Original had nothing to smooth power.

[nordine]

something tells me this would sound badass on bass

will try it out, thanks Mark!

[nordine]

breadboarded it and something weird happens

oscillation,

triple checked everything, any ideas?

[puretube]

...  More importantly, the idea of crossover distortion started to interest me...

I'm not sure why, but the Gretsch Controfuzz came to mind, and I started to ponder whether crossover distortion could be fit into that...
and since the two are combined as difference signals...

Okay, back to musings about crossover distortion....already in progress.....

Such a simple mod, and so many extra tonal colours to be achieved.  Daddy LIKE!

wink-click