What Are Clipping Diodes?

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...

All about clipping diodes

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.

Forward voltage

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 vs Soft Clipping

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.

Symmetrical vs Asymmetrical Clipping

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…)

Clipping diodes: Summary

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.

Try These Pedals With Clipping Options

[product=zander-circuitry-siva-v2] The Zander Circuitry Siva takes the late '70s IC/op-amp Big Muff - as used so effectively by The Smashing Pumpkins - and drags it kicking and screaming into the 21st century. The Siva features dual footswitches, an EQ section with Shift controls, and 8 clipping options, covering a host of germanium, silicon and LED configurations. [/product]

[product=drunk-beaver-bat-v2] The Drunk Beaver Bat V2 is a mutant Rat circuit, using NOS LM308 and OP07 op-amps as well as 6 different clipping modes to give 36 different iterations of the Rat circuit, enabling you to dial in your perfect sound! Stefan Karlsson of the Guitar Pedal X blog calls Drunk Beaver founder Vitalii Bobrov 'the master of Rat Distortion', and it's easy to see why - Vitalii has crammed so much into a small chassis and it sounds phenomenal. And he hasn't stopped at just one Rat-based pedal: he has created an entire family of Bats! [/product]

[product=walrus-audio-eras] The Walrus Audio Eras is a no apologies, high-gain distortion that’s ready to chug, shred, and palm mute with tight response and multiple clipping options. The Blend control gives you total accuracy to dial in your clean or pre-Eras dirt sound with the Eras distortion, making it a super-versatile - and super-heavy- distortion. The Eras features a five-position rotary control to give you 5 different clipping options. [/product]

[product=kink-guitar-pedals-defender-of-the-hate] The Defender Of The Hate+ is a Muff-style pedal with - among other things - 2 switchable clipping stages: Gain Stage 1 (Top) - silicon diodes or LEDs; Gain Stage 2 (Bottom) - asymmetrical silicon or germanium diodes. As well as Tone and Mid-Shift controls, the right toggle allows you to bypass the tone stack for a direct, aggressive character. There's also an internal Gate control which can be adjusted if your rig is too noisy. Awesome! [/product]

[product=red-noise-stone-blender-fuzz] The Red Noise Stone Blender is a modern take on the Sola Sound Tone Bender, one of the most famous names in the world of fuzz pedals. But far from being a one-trick pony, the Stone Blender features a neat 4-way Diode Switch which lets you select one of four different clipping diodes for diverse voicing options. The innovative De-Clip control adjusts the tonal balance and presence of the mids, enabling you to get your sound sitting perfectly in a busy recording or live scenario, alleviating those mid-scoop issues once and for all! It can help you increase pick attack, and works wonders with bass guitar (Founder Ariel is a bassist and this pedal is more than capable of some 4-string thunder!). [/product]

[product=drunk-beaver-heavy-bat] The Drunk Beaver Heavy Bat is the "ultimate Rat-style pedal" (at least according to Stefan Karlsson of the Guitar Pedal X blog), who says that the Heavy Bat is "probably my favourite ever Rat to date - even against something as excellent as JHS Pedal’s PackRat." The Heavy Bat takes the guts of the Bat and Bat Cold War and adds tons of additional switching and modifications for what must be one of the most complete pedals in this genre - ever! [/product]