I didn't know what else to call it. It was a little fuzz that I found at http://www.geocities.com/j4_student/cookbookfuzz.gif  It's a fuzz circuit made up to show electronics students distorted wave forms I guess. Anyway, it's not bad for what it is. It uses one op amp out of an LM324 quad op amp. Why this chip was chosen instead of a single op amp, I have no idea. It makes a whole lot better fuzz for rhythm than a lead fuzz. It didn't show an output cap, so I used a 1uf there. I tried the 1N914s they showed, 1N270 germaniums, and lastly red LEDs. The LEDs turned it into a crunch machine! If you just want something to power chord the night away on, this will do just fine. For leads it's crap. It has no sustain to speak of, but I guess it wouldn't with a single op amp. It's really more of an overdrive than a fuzz. You have to play the strings pretty aggressive with your guitar volume all the way up to make it sound right. Turn the "Depth" and "Intensity" pots wide open too. I just wanted to hear what it would sound like, so I threw it together on perf board in about an hour. It's not on par with Joe's Easy Drive for tone, but it's not too shabby.

Hey, Jeff!

Try changing the depth pot to 200K or more.  This should increase the gain and might put it into lead land.  The single op-amp thing isn't the issue.  Witness the Rat, the Tube Screamer, the Distortion+, the DS-1 and the ROG Tube Reamer.

I used 1M for the "Fuzz Depth" pot. Cranks the gain right up.

Keep in mind this uses an inverting op-amp configuration, which means that the input and output are phase-reversed.  Not a problem in itself, but if there is any splitting of the signal BEFORE this circuit, and subsequent recombining, there WILL be cancellations.  Using a dual op-amp, with the second stage functioning as an inverting gain recovery amp (generally needed to overcome the output ceiling set by the diodes in the stage you've shown), you'll have more output, and it will be in phase with the input.

To do this, simply duplicate what you've shown, minus the diodes, and minus the volume pot between stages.  You will note the schematic shows a polarized cap after the 100k volume pot.  Let that be the 1uf cap shown as an input cap on the 2nd stage.

For pop-free switching, tack a 2.2meg pull-down resistor to ground on the FIRST 1uf cap, and stick the polarized cap on the final output BEFORE the 100k volume pot rather than after it.  That way the volume pot serves as the pull-down resistor for the cap.

If you want to get nuts, stick a pair of LEDs across the gain pot in the 2nd stage for double clipping, and use a 500k pot in the first stage instead of 100k.  To keep fizz out, stick a 100pf cap in parallel with the diodes in the first stage.  This will provide very little treble rolloff at low gains, and at max gain (with a 500k pot) will roll off treble around 3.1khz for a nicer sounding fuzz.

Thanks all! This ought to be a learning experience for me. Mostly, I'm a "paint by numbers" kind of guy, as R.G. calls it. I build the circuit the way it's presented on the schematic. I have been doing a little experimenting lately with input and output caps to do tone changing. Most of the time I don't know the math behind what I'm doing, just the mechanics of it. I go to the "Simple Mods and Tips page", along with R.G.s rules, and do the modifications that way. This circuit looks like distortion 101 to me, so I ought to learn something from the mods that you've suggested. Unless I've misunderstood, as resistance goes up, the gain also goes up, so that is why you're having me up the pot value. True? That seems backwards to me.

As "fuzz depth" resistance increases, negative feedback (which reduces gain) goes down, hence gain of op-amp increases.

Quote from: jmusserUnless I've misunderstood, as resistance goes up, the gain also goes up, so that is why you're having me up the pot value. True? That seems backwards to me.

True.  All op-amps have what's called an open-loop gain.  That is some maximum amount of gain produced if there is absolutely nothing in the feedback loop, whether that feedback loop is between the output and the '-' input pin or the '+' input pin.  Think of this like how fast a vehicle *could* go in neutral if it was allowed to roll down a steep hill.

When you provide something in the feedback loop, you are essentially stepping on the breaks and attaining something less than an all-out roll downhill.  The amount of such negative feedback provided determines how much gain is produced.

Gain in an IC op-amp is always determined by the relationship/ratio of two resistances.  One is the feedback resistance from output to '-' pin, and the other is the resistance from the '-' pin to ground (for non-inverting op-amps) or the resistance leading up to the '-' pin (for inverting op-amps, such as what was shown in the initial illustration/schem).

Why is this the case?  Think of these two resistors as forming a kind of volume pot.  With any volume pot functioning in the usual way, it is the ratio of the resistance on one side of the wiper to the resistance on the other side of the wiper that sets how much of the original signal is going to be available at the wiper.  When the input-to-wiper resistance is much larger than the wiper-to-ground resistance, there is very little signal available at the wiper.  This is true no matter what the actual pot value or taper is.

In the case of op-amps, the two critical resistances are analogous to the two halves of a pot.  When the feedback resistance is large, relative to the other resistance, it is as if you've "turned down the volume" on the feedback signal.  With less feedback, it's just like lifting your foot off the brakes and letting the car roll downhill a little more freely.

Because it is the ratio of those resistances that does this, that means you will see the gain of op-amps adjusted by the feedback resistance, the input resistance (if inverting), OR the resistance to ground (if non-inverting), OR both at once.  For instance, some inverting gain stages will sometimes use a single pot with the wiper tied to the '-' pin, one leg of the pot forming the input resistance, and the other forming the feedback resistance.  As you move the pot, both resistances change, altering the gain.

So, the reason I suggested upping the value of the pot is because 510k (a 500k pot + 10k fixed resistance in series) is quite big, compared to the 10k input resistor.  You COULD also leave the pot at the same value, but reduce the 10k input resistor down to a much smaller value, like 1k or even less, to change the ratio and achieve much more gain, however that may produce problems by being so low, so we go with increasing the pot value.

Of course, it doesn't have to be the pot specifically that changes the maximum feedback resistance value.  If you change the value of the 10k resistor in series with the pot to 100k, then the maximum feedback resistance (with pot at max value) would be 200k instead of 110k, producing a maximum gain of 20 instead of 11.  Many folks, though, would  tend to prefer use of a bigger pot since that provides the greatest range of control.  For instance, a 390k fixed resistor and 100k pot would get you anywhere between a gain of 39 and 49, where a 10k resistor and 500k pot would get you a gain range from 1 to 51.  Clearly one of those arrangements will feel like a "control" where the other will feel like a fine adjustment.

make sense?

You were wondering about the quad op amp, obviously the intent was to make 4 for these and hook them together.  

Check out the Muff Fuzz circuits around to see how to use 2 of the op amps in a really nice sounding combination.  

Didn't someone have a 4 op amp Dual Muff posted around here the other day?

The Mockman at ROG is also of the same basic design.  

Well Mark, I hope you're very pleased with yourself. You just couldn't leave well enough alone, until you up and made me learn something! As always, thanks for sharing your wisdom.

Hi....
  ...im NEW to opamps...im very comfortable in the realm of transistors and fets...no problems there...BUT...opamps are a new territory for me.
  so...with that in mind...referencing the schematic, i see that there are 2 pins marked Vs...im assuming that these "tie together" to the voltage source, since the LM324 only has a single voltage pin that supplies all 4 opamps with power?
  also, wouldnt it be possible to "chain" up to 4 of these circuits together on this single opamp, cascading them ? perhaps using different clipping sections in each, or having a single section with no boost at all?
 sorry for the NOOB questions, but as i mentioned...im new to opamps....

  thanks....now where are those dang 2N2222's ??  
  Andy

In this circuit, Vs should more properly be labelled V+ or +9vdc (though it doesn't HAVE to be exactly 9v).

The little 2-resistor/one-cap thingie in the bottom left divides the supply voltage in half (that's why the resistors are equal value) to provide a reference voltage halfway between zero volts and the full supply voltage.  The audio signal will use that as its' "centre", and fluctuate above and below that.  You will see variations of that little circuit in just about anything involving op-amps and single 9v battery supplies.  In the case of transistors, of course, the individual transistor can be biased to sit at the mid-point when nothing else is going on.  With op-amps, you have to "feed" them a reference point.

Quote from: Mark HammerIn this circuit, Vs should more properly be labelled V+ or +9vdc (though it doesn't HAVE to be exactly 9v).

The little 2-resistor/one-cap thingie in the bottom left divides the supply voltage in half (that's why the resistors are equal value) to provide a reference voltage halfway between zero volts and the full supply voltage.  The audio signal will use that as its' "centre", and fluctuate above and below that.  You will see variations of that little circuit in just about anything involving op-amps and single 9v battery supplies.  In the case of transistors, of course, the individual transistor can be biased to sit at the mid-point when nothing else is going on.  With op-amps, you have to "feed" them a reference point.

ok ...im understanding a bit better now....basically its "biasing" the opamp...at least the bottom left hand resistor/capacitor network.

so 9v+ feeds the positive input of the opamp as well as the voltage input (which just has the single cap to ground ...to filter the voltage, im guessing) ?

thanks Mark...

oh...and Happy New Year !
Andy

Yep, you've nailed it.

Happy New Year back at you.

Quote from: Mark HammerYep, you've nailed it.

Happy New Year back at you.

stay tuned, if you would...i have another question for you, but i need to draw it up first...

thank you SO much for your time and generosity of knowledge...You, Gus, Aron, and a few others have basically been the cause of my DIY stompbox hobby/addiction ....  

Quote from: Mark HammerYep, you've nailed it.

Happy New Year back at you.

so Mark....theoretically, you could set up the opamp circuit like this on the LM324, correct? (even adding up to four clipping stages on it, since its a quad opamp ?) ....

i made a couple small changes to the original schematic, including the 100pF cap parallel to the clipping diodes and 500k pot change you suggested, and a second clipping stage that uses germanium diodes...

so...am i correct in assuming that this circuit should work, and that you could use the fuzz intensity pots to control the silicon/germanium diode "balance"

thanks again for your help ?

In principal that ought to work.  Just note that since the possibility for combined gain is VERY high (gain of 51 times a gain of 51 is.....BIG), keeping the leads for the two gain pots shielded and away from each other is a good idea if you aren't keen on squeals being part of yoiur "signature sound".

You can probably use just the one Vref source and tie pins 2 and 6 together.

A DC blocking cap between pin 7 and the output pot is a good idea.  Go with 2.2uf - 10uf.  

A Rat-style variable lowpass tone control between that cap and the output pot is also a good idea.  Try a 1k resistor in series with a 10k pot and a .01ufcap to ground at the input to the VOLUME pot.  This will give you a rolloff from about 1.45khz to just under 16khz, enough to tame any and all fizz resulting from the different combinations of gain in each stage.

Wow, this thread has evolved! I'm glad I checked back. One thing that would be wonderful, is to have individual components highlighted as to what they're doing, and what is happening to the wave form at certain points in the circuits. That way, when someone says "pull down resistor", "coupling cap", "low/high pass filter", "clipping diodes", etc., it would be cataloged. Then you'd just have to go to the FAQs to know what the other person was talking about. This circuit right here would be great, because it's like 101. Certain amounts of this are covered in tib bits everywhere, but are not combined in one comprehensive area. It's like you just said, that the 4.5, (or 1/2 Vcc) was the reference line for the sine wave to go positive or negative of that point. I had no idea that this was anything more than just power supplied to the chip. It makes perfect sense. Then when you rectify the signal above or below the line, you have up or down octave. Like I said, you're going to full around here a little too long on this stuff, and I'm eventually going to catch on! You give great thread! Ha!

Quote from: Mark HammerIn principal that ought to work.  Just note that since the possibility for combined gain is VERY high (gain of 51 times a gain of 51 is.....BIG), keeping the leads for the two gain pots shielded and away from each other is a good idea if you aren't keen on squeals being part of yoiur "signature sound".

You can probably use just the one Vref source and tie pins 2 and 6 together.

A DC blocking cap between pin 7 and the output pot is a good idea.  Go with 2.2uf - 10uf.  

A Rat-style variable lowpass tone control between that cap and the output pot is also a good idea.  Try a 1k resistor in series with a 10k pot and a .01ufcap to ground at the input to the VOLUME pot.  This will give you a rolloff from about 1.45khz to just under 16khz, enough to tame any and all fizz resulting from the different combinations of gain in each stage.

more like this ?

In my experience with Muff Fuzz type circuits, the 10K resistor at the input does not provide enough impedance and the circuit sounds too woofy.  The stock MF uses a 100K there and Gus' mod's uses a 220K.  This will make it sound more like a distortion and less like a fuzz.  Of course a 250K pot there greatly increases flexability.  Should sound really good.  

Just a suggestion, but you might try the GE's and the SI's reversed, because the first stage isn't going to have as much juice going thru it as the second.  I think you will need the lower clipping threshold in the first stage.  

Forgot, Check out the Mosferatu Schematic in the the Schematics 2 for a very similiar design.