Clipping diode flavors

[Rodgre]

Can anyone give me a definitive run-down of their favorite configurations of clipping diodes and what their characteristics are?

I have some assumptions/observations, but I wanted to get some other input.

Also what rules of thumb do you follow with configurations of diodes, i.e. three diodes in series does this, two this way, and one that way does that, a germanium and an LED together do this...

Roger

[R.G.]

Can anyone give me a definitive run-down of their favorite configurations of clipping diodes

Good, valid question. Everyone has their own set of favorites, and they may make suggestions you like. Be very aware that whether someone else likes a particular **anything** has little to do with whether you like it.

and what their characteristics are?

If you mean sonic character, you're going to have to listen yourself. For instance, what does a "liquid" clipping sound like? or harsh? or "chewy"?

If you mean measurable diode characteristics, that's easy, you get that for free below.

Also what rules of thumb do you follow with configurations of diodes, i.e. three diodes in series does this, two this way, and one that way does that, a germanium and an LED together do this...

Once again, if you want sonic character, that's hard to get. Our language does not have precise terms. However:
1) diodes start to conduct at different voltages depending on what their materials are; germanium is about 0.15 to 0.3V, silicon is 0.45 to 0.7, Schottky diodes are usually half a silicon, LEDs vary by color and other factors from 1.2V (old, low efficiency reds) up to about 2.5 (high output green) and then the wild ones - silicon carbide at about 3.5-4V and other composites and montages of LEDs. Count on testing every LED if you're trying to get to some specific voltage.
2) diodes don't strictly turn on at a specific voltage. They start conducting quite low levels of current from 0V on up, but the amount of current varies exponentially with the voltage, so there is some voltage where the current suddenly shoots for the moon, and it takes literally huge, smoke making, device-exploding amounts of current to make the voltage across them go much higher.
3)The characteristic of exactly **how** the diode transitions from dopey, little current conductor to current-eating monster is called the knee of the diode's conduction curve. Some diodes are quite abrupt in going from little to a lot of current, some have a smoother, rounder translation from almost off to pretty much on.  The sharpness of that knee compared to the size of the signal pressed on it determines the sharpness of the clipping, and hence the degree of harsher, higher order harmonics generated. Example: two 1N4148 diodes used as a clipper. If you feed these things a 2V peak to peak signal, the will clip off the upper and lower 0.3V of signal, and you get distortion, and not a whole lot in this case. If you feed them a 50V p-p signal, they produce nice, solid rectangular waves with vertical sides, flat tops, and sharp cornered edges, and sound quite buzzy. The same diodes fed a different signal level, and from different source impedances sound different. If you stick a resistor in series with the diode clipper, it changes the sound because it changes how much current the diodes can get.

Let me re-emphasize: it is the relative size of the signal being clipped, the amount of voltage to get to the diode knee, and the sharpness of the knee that will determine much of the sound of the diode clipper if all other things are equal.
4) all other things are never equal: pre and post clipping EQ affects sound as much or more than the clipping characteristic. Diode imbalance can add even harmonics to the otherwise odd harmonic mix from a balanced clipper. Unless you do something special, diodes are never matched, but the degree of mismatching is unknown.
5) stacking diodes does a couple of things.
5a) stacking more diodes clips at a higher level, and so the output signal is bigger (that is, less is clipped off), less trebly (fewer high frequency harmonics), and may sound better just because it's louder. Humans are like that.
5b) stacking diodes can introduce much larger imbalances than just using like diodes. For instance, a silicon in series with a germanium clips at a significantly higher voltage than a silicon by itself. That makes the clipping be asymmetrical (i.e. different on positive signal peaks than negative ones) and Fourier's Theorm says that causes even as well as odd harmonics to be generated. How much and what kind depends on how unbalanced you get. One diode and no diode is pretty unbalanced, and makes an audible "octave" sound.
5c) stacking diodes can give subtle cork-sniffing changes to knee sharpness. Whether this is good or not depends on whether you like the cork.

[brett]

In both the clipping feedback circuits (TS, BluesBreaker) and the clip-to-ground circuits, I like asymetric clipping very much.  2 x 1N4148 and 1 x 1N4148.  

1N4148s have a slightly rounder "knee" than 1N400Xs, and sound slightly softer.
cheers

[vseriesamps]

I asked essentially the same question a few weeks ago, and a long thread emerged. If you search under my username for the thread "which diodes sound like what?" you'll find it. When I did the search it was on page 2. I'd post a link but I don't know how.

Check it out - happy experimenting
K

[WGTP]

In my experiments with the Blues Breaker and Muff Fuzz, I tend to like SI/LED combinations, or a single SI or LED.  That way the Op amp clips on the other side of the signal.  This is with a solid state amp at low volumes and mayb be too wooley for tubes at high volumes.   Besides changing the harmonic content, it seems to change the "feel" as well.