Different ways of clipping. Noob question?

[ercsguitar]

Pardon this noob question, but I'm trying to figure some things out. Recently I've been doing some research on clipping and the various ways to achieve it. I was hoping to find some sort of scale that listed all the various ways to cause clipping within a circuit and a corresponding description of the harshness of the clipping (from very light to over the top square wave). I've read the Distortion 101 article on GeoFX, but honestly it was mostly over my head and didn't seem to give me much on Ge clipping is softer than Si. I'm not even sure if all this is possible since so much of the intensity of the clipping has to do with other features of a given circuit.

[demonstar]

This is a good read and may be what you are after...

"http://www.muzique.com/lab/tclip.htm"

[demonstar]

Also...

"http://www.muzique.com/lab/zenmos.htm"

[earthtonesaudio]

Don't forget, diodes are not the only way of clipping your signal.  You can also bias transistors into cutoff or saturation to clip them... and pretty much anything (transistors, opamps, tubes) can be made to clip if the signal tries to swing past the supply voltage rails.

[ercsguitar]

Right. I'm kinda looking for a comprehensive scale of clipping methods that would include diodes, transistors, mosfets and whatever else can cause clipping from subtle to outrageous.

[R.G.]

The problem is, you're asking the question incorrectly, although that's the way most people understand and pose it.

The actual device doing the clipping doesn't have much to do with whether it's soft clipping or razor-edge ear-bleed clipping. For instance, there are ways to make a 12AX7 triode clip off the top of a signal as flat and level as any semiconductor.

Every clipping device, whether a diode, zener, or active device faking a clipper somehow, has a "clipping knee". This is the region where it goes from not clipping the signal at all to being as clipped as it's going to get. To a huge degree, the relative size of the signal and the clipping knee determine how drastic the clipping is.

For instance:
Silicon diodes got a rap for harsh clipping at one time. But the Thomas organ Vox amps use silicon diodes as a LINEAR MODULATOR for their tremolo and there is no hint of clipping there. They have quite small signal levels going through the diodes and the conduction knee is much smaller than the signal. So no perceptible clipping happens.

But if we take a sine wave and amplify it up to, maybe 10V and run that into a pair of silicon diodes back to back, we get so close to a ruler flat clipped waveform that it's hard to tell it started as a sine. The signal shoots through the clipping knee so fast that the knee cannot add any rounded over clipping, and that's what we perceive as slowly increasing distortion. Note that with this same test, Ge diodes and LEDs will give similar results, as long as the signal is much bigger than the clipping threshold.

If you want soft clipping, set up a clipper where the signal voltage is comparable to the clipping knee voltage.

This concept is getting an entire chapter in the book I've been writing on musical distortion, but I figured I could save you some grief while I finish it up.

[ercsguitar]

Thank you so much R.G. Somewhere in the grey matter I was beginning to tell that was what actually was going on, but I don't think I had quite made all those connections yet.

So if I want some really soft clipping I need to regulate the voltage going into whatever I'm using to clip (be it diodes or transistors or whatever). For example, if I were to drastically reduce the voltage coming into the clipping section of a fuzz face I could clean it up nicely, right?

Then that poses the question do the different ways of inducing clipping have different characteristics? And if so (because I presume they do or no one would make switchable clipping sections) what are those characteristics? OR is it just the fact that they have different voltage cut offs that cause them to clip differently and that's why people insert switchable clipping sections? In other words could I, by regulating voltage, make a zener diode sound like a Ge diode or a diode sound like a mosfet or a transistor or whatever? This bit of information has really opened my eyes and piqued my interest.

[earthtonesaudio]

I think you're on the right track with your thinking. 

But keep in mind that not just voltage but current through the diode affects the clipping characteristics.  Ge diodes only have low voltages for low currents, at higher currents the threshold voltage can go way up (orders of magnitude type of increase).  Look at some diode datasheets, and pay attention to the curve that has voltage vs. current, and note the "knee" that R.G. was talking about.  Then build your multi-clipper and note how the differently shaped knees sound different at one setting.

Then you could be really slick and not just have diodes to ground, but diodes to ground through something like an adjustable current sink (or an adjustable resistor for a simpler version; see the AMZ articles on clipping and "warp" controls).  That might be an area ripe for experimenting.