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Many of you reading this will have heard of clipping diodes before, but how much do you really know about them? In this guest blog, Alex Millar of Zander Circuitry explains what clipping diodes are, where they are found and why they are used in guitar pedals...
Diodes can serve quite a few purposes in a guitar pedal circuit but in the vast majority of cases they’re used to create either hard clipping or soft clipping in overdrives and distortions, so that’s what we’re going to take a look at.
Clipping is a type of waveform distortion that occurs when a signal is pushed into a circuit with more output voltage/current than the circuit can handle. Probably the most recognisable example of this is when you turn up a tube amp to the point when it starts to distort on its own. Obviously, that requires a lot of volume and not everyone is able to play at those levels, and that's where pedals come in.
In many cases, overdrive and distortion pedals allow us to create distorted tones at any volume - without needing to crank the amp. A common way of creating clipping with a guitar pedal is with diodes.
One of the biggest factors affecting the performance of a clipping diode in a guitar pedal is it’s forward voltage (FV). All diodes will have differing FV values, but generally they can be grouped by type: Germanium (~0.3), Silicon (~0.7), Red LED (~1.7-2.0).
The 3 listed above are the most commonly used diode types for this application, although you can use other components such as transistors and MOSFETs as clipping diodes when configured correctly. So what does the forward voltage do? Well, in short it sets the clipping threshold for your signal. A typical guitar signal can range from 1-4 volts depending on the type of pickup and the rest of the circuit in any pedals you have on.
When a signal hits the diode, anything that peaks that voltage will be clipped. So if you have a pedal with both germanium and red LED clipping options, you’ll find the germanium mode will be quieter and more distorted. Which brings us quite nicely onto...
Hard clipping refers to diode configurations that are tied to ground on one side. This creates a "hard" cut off point where your signal (a nice smooth sine wave) is chopped off and given flat peaks (closer to a square wave). This creates a very aggressive and ‘obvious’ effect and is typically used in effects that are advertised as distortion pedals.
Famous hard-clipping diode pedals include: Boss DS-1, Proco RAT and the MXR Distortion+ / DOD 250.
Soft clipping refers to clipping diodes that are added inside a feedback path, typically in op-amp based circuits. Placing diodes here softens the edges of the clipped signal and offers what people usually refer to as a smoother, tube-like tone. These pedals are often marketed as overdrives, although the lines are a little blurry on that. Without doubt the most famous soft-clipping diode pedal has to be the Ibanez Tubescreamer and its many clones/variants.
Diodes are polarised parts, meaning they have a positive and negative side, like a battery. They are often found placed in back to back pairs in a circuit, facing opposite directions. This is known as symmetrical clipping, because you have the same number of diodes on each side. This means that the signal is clipped evenly on both the positive and negative sides of its waveform.
Asymmetrical clipping is where you have more diodes (in series) on one side than the other, or if you are using more than one diode type (i.e one germanium and one silicon), which throws the balance of the clipping off.
When using the same type of diode for both configurations, it is generally considered that asymmetrical clipping tends to sound louder, clearer and crispier, whereas symmetrical clipping is quieter, smoother and more distorted. One of the reasons asymmetrical clipping tends to sound louder and clearer is because placing diodes in series increases the effective forward voltage (i.e if you have two 0.3FV germanium diodes in series, the total FV would be 0.6; three diodes would be 0.9FV and so on…)
Personally, I love hard clipping setups because of the dramatic difference they make to the way a circuit sounds and reacts. At Zander Circuitry we place a clipping diode rotary switch on almost all of our dirt pedals to give the player a quick and easy way to experiment with them.
The Big Muff, which our American Geek (transistor) and Siva(op-amp) pedals are based on, is technically a couple of soft clipping stages chained in series, even though it is commonly advertised as a fuzz. Most traditional fuzz circuits do not use diodes for clipping, which explains why the Big Muff is often referred to as sounding very smooth with plenty of sustain compared to something like a Fuzz Face.
We could quite literally talk all day to the Nth degree about clipping diodes and their almost infinite configurations, but I think this is a good signing off point that gives you a good overview of the basics.
