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For the last few months I have been playing electric guitar on a small 5W tube amp with a nice saturated gain channel. This past weekend I picked up a Big Muff Pi pedal, which I believe is classified as distortion.

In having played the amp alone, on the gain channel, I get a nice swirling distortion. Now, playing the pedal through the clean channel, I notice a couple of things. The distortion is nice. It's fuzzier, a quality I couldn't get with the amp. But I also notice that the distortion itself does not have a variable quality. It seems that the sound has a singular quality, whereas the amp distortion was swirling and would come and go slightly.

What are the physics for these two distortion approaches? Since I named the pedal I will share that the amp is a Marshall 5W tube combo (without reverb).

EDIT: I want to be clear. I am not concerned about whether this pedal is classified among those in the know as a fuzz pedal. At the end of the day, both the amp and the pedal are distorting the signal.

My question is about the physics of why it appears to be different in each case, specifically swirling (in the amp) and a very straight-ahead sound (the pedal)

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    There’s not a set standard for terminology but I think I’m not alone in classifying the Big Muff Pi as “fuzz”. That’s due to the type of sound it has and the way the amount of fuzz doesn’t change very much when the gain is changed. May 3 at 11:35
  • The selected answer of this question, even if the question was mostly about distortion and overdrive might have some useful insights!
    – Tom
    May 3 at 14:41
  • I don't want to appear ungrateful for your clarification. Regardless of whether this pedal is a fuzz pedal, this is still distortion. And this doesn't help me understand the physics of the on-board distortion vs. the pedal distortion. Moreover, the edit to the title of my question is misleading. May 3 at 14:44
  • @JasonPSallinger you say "I am not concerned about whether this pedal is classified among those in the know as a fuzz pedal", but it's actually important to the answer - 'amp like' distortion is generally thought of as the kind of 'soft clipping' associated with valve amplifiers (whether actually produced by a valve amp, or by a circuit emulating one) whereas fuzz pedals generally producer a much squarer, harder-clipped wave. Some distortion pedals try very hard to achieve an 'amp-like' clipping response - fuzz pedals don't.
    – topo morto
    May 3 at 16:19
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    Key to note is that the distortion you often hear in recordings is not just a function of the guitar, or the effects pedal, OR even the amp, but also the speaker cabinet that the amp is playing through as well as the type of microphone that is being used to mic the speaker cabinet (yes, that's how it's traditionally done) and the placement of said microphone. ALL of these things contribute to the tone. For home recording where you can't afford a Marshall 4x12 turned up to 11, there are cabinet/mic simulators (see Pod Studio, etc) that can really let you dial in the tone you're after.
    – J...
    May 3 at 21:15
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As Todd Wilcox commented, there isn't a completely clear-cut classification, however the three categories of saturation-type effects are tube overdrive, fuzz, and transistor overdrive / distortion.

  • Fuzz, which is what most people would also classify the Big Muff as, is the simplest. It involves increasing the signal level (that's all “gain” means, BTW!) by a lot, and hard-clipping everything that's too strong to be handled by the voltage range of the circuit.
    Simple fuzz of a signal for different ampliture
    Note that for any sufficiently strong signal, the result comes out as basically a pure square wave. Only at low input levels are some details of the signal preserved.
    It is the hard edges of the square-signal that gives fuzzes their harsh, raspy buzz. To compensate, there's usually a tone control available, which is a simple low-pass filter. It smoothes out the hard edges somewhat, but the sound will still be pretty much the same no matter the input.toned down fuzz

  • Tube overdrive, which is what most amps produce when you crank the gain (not the Ibanez “tubescreamer” overdrive, though!) is similar in principle, but instead of the hard clipping of the signal peaks they merely round them off.
    Soft-clipping as by tubes
    This also avoids very harsh transients, but responds dynamically and preserves much more of the original signal character than a fuzz. Also, most tube circuits don't clip both halves of the signal in a symmetric manner, which can create even harmonics from odd ones. (This isn't as notable for clean guitar signals because these already come with even harmonics, but in particular fuzz square waves contain only odd harmonics).

  • Transistor overdrive, as well as distortion pedals, use another trick to be more playing-responsive than fuzz: they boost specifically the higher-frequency components of the guitar signal before clipping (typically midrange, somewhere between 800 Hz and 5000 Hz). This way, the fundamental won't dominate and you won't get just always the same square wave. Then, after the clipping, you again filter out some of the high-frequency croak.
    distortion

Source code

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    @user1079505 I actually included that effect in my source code, but didn't explain it yet! May 3 at 16:22
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    @JasonPSallinger distortion is unlikely to cause swirling, but it could accentuate some periodic feature in the sound that's already there - such as a beat frequency or some subtle fluctuations in amplitude or pitch.
    – topo morto
    May 3 at 18:39
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    @topoReinstateMonica: My guess would be that "swirling" is a result of the distortion being sensitive to amplitude and to the relative phases of the harmonics within the signal.
    – supercat
    May 3 at 19:04
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    Tube amplifiers have a feature often called "sag", where playing hard drops voltages inside the amp so that it distorts more without increase in volume. It's one of the reasons why it's difficult to simulate tube amp distortion with solid state components, and it could be part of this "swirling"
    – ojs
    May 4 at 7:12
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    @kebs it's Haskell, with the dynamic-plot library. May 4 at 7:32

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