Clipping

Clipping is a simple concept, but the nuances of managing it can be a bit slippery to hold on to. First we'll talk about what it is. Then discuss why it can be difficult to manage. Then we'll move on to the main topic of what you can do to configure good clipping options for your module.

Catching a wave

You should know that the electricity that carries our music moves in complex combinations of sine waves, caused by the guitar strings vibrating above your pickups. The taller the sine waves, the louder the sound can be. The width of the sine waves in a period of time determines their frequency. If the waves are narrow, you have treble sounds and if they are wider, you have bass.

The sound of those sine waves can be considered "clean" if the shapes of the sine waves are not changed by their passage through an effect circuit. A "clean boost" would leave the shapes and widths intact, but make them taller. An effect might leave the shape and height mostly the same, but change the width of the waves. That results in a frequency change in the sound. Or an effect might block or reduce waves of specific widths, resulting in other effects such as a "treble boost". Or add copies of the waves, like a delay. There are all sort of ways to change the waves. That's what effect circuits do.

Clipping Distortion

There are some specific types of changes to waves that are of interest for this discussion. These types of changes result in distortion of the sound in different ways and to different degrees. Clipping is one of the more common ways to create distortion. Clipping just means that the tops (and bottoms) of the sine waves are being chopped off, left more or less flat. Overdrives, distortions, and fuzz pedals often use clipping to achieve their instantly recognizable sounds. For ease of discussion we'll just refer to all of these as "distortion" effects.

There are multiple ways to cause clipping. One way is to overload a component with a voltage outside its operating range. Many effects run with 9V power supplies, usually limiting the operation of key components such as transistors and op amps to a range of 0V to +9V. If you send a signal through them that dips below 0V or gets boosted above +9V, then those signals will get clipped. Many distortion effects do something along those lines to mangle the waves in the signal. Distortion of that type sounds different for different frequencies. If you clip bass signals, it can sound muddy. If you clip high signals, it can get aggressive. If you just do a "light clip" of the waves, you get less distortion and keep most of your volume (height of the wave). If you clip heavily, you get more distortion, lose more volume, and often end up with a tone that seems darker. Any of those effects of clipping may be desirable or unwanted, depending on the effect. You've no doubt heard "muffled" fuzzes and "screaming" distortions. Most distortions will manipulate certain frequency ranges and volumes either before or after the distortion to achieve their specific sounds.

Clipping distortion can also be caused in another way. Diodes are simple devices that allow electricity to flow through them in one direction, while blocking flow in the other direction. They allow different voltages (wave heights) to pass cleanly, or they clip them to a maximum allowed voltage. Diodes have a property called their "forward voltage" or Vf. That is the voltage to which they will clip a signal of a higher voltage. If the Vf of a diode is 0.5V, then waves of less than 0.5V will pass through cleanly. Waves of more than 0.5V will be clipped to 0.5V. That results in distortion, very similar to the method of overloading a component, described previously. It provides a different way of clipping, which has some different properties. For instance, depending on the strength of the signal and the Vf of the diode, the signal may not need to be boosted to be clipped. When you boost a signal, you also boost the noise in the signal. If you don't have to boost, then your resulting signal doesn't have to include boosted noise. Or your effect doesn't have to try to chop out noise (and usually some of your good treble content) to prevent a build up of noise when the signal is boosted.

To summarize this, there is a property of diodes, their Vf, that can be used to control the amount of distortion and volume when they are used to clip the signal. And using clipping diodes to generate distortion may well be accompanied by other design decisions in the circuit to produce a signal with different volumes and frequencies for clipping. Once the clipping is done, the volumes and frequencies may again be adjusted by other parts of the circuit to produce the overall sound of the effect.

Hard and Soft Clipping

So you see, we barely got past describing diode clipping before we noted that other parts of the circuit are probably manipulating the signal specifically to influence the resulting sound of the clipping. Control of clipping is already slipping out of our grip. But we aren't finished just yet. Let's add another dimension to the picture.

You may have heard the terms "hard clipping" and "soft clipping". Technically, they are similar. Diodes are still clipping the signal waves the same way. You might think of the difference between hard and soft clipping as having to do with what happens to the parts of the signal that are clipped. This is a little inaccurate, but close enough for the purposes of this general discussion. In hard clipping, the tops/bottoms of the sine waves of your signal are lopped off, resulting in flattened peaks. In soft clipping, the waves are not flattened, but are "squished" down a bit, still rounded. The hard clipping results in a harsher tone, often with loss of dynamics. The soft clipping is smoother and retains more of the dynamics.

How soft clipping and hard clipping are implemented in a circuit varies. Gain, perhaps frequency-dependent gain, is often carefully controlled, along with the limiting voltages. Those help determine the amount of clipping that can occur, as do the specs of the devices (eg diodes). That's difficult to see when you look at a schematic. You would need to know details about the components, the signal, and intended effect to do the math to figure it out. However, a large portion of the time you can tell just by looking at the circuit layout whether you have hard or soft clipping.

Hard clipping often has the clipping diodes sending the clipped portion of the signal to ground. The clipped portion of the signal is completely lost and cannot be further manipulated. That configuration tends to give that flat top/bottom to the sine waves. As more hard clipping occurs, the wave gets closer to a square wave which will usually sound more like a fuzz.

Typically, soft clipping occurs in the feedback loop of a gain stage, such as an op amp or transistor. The signal can go through multiple paths that handle clipping differently instead of dumping signal to ground as typically happens with hard clipping. The clipping diodes are chopping the portion of the signal that goes through the diodes. But some of the signal doesn't go through the clipping diodes. The clipped and less-clipped or unclipped signals are then combined to yield that smoother top/bottom to the sine waves, but "squashed down" a bit from the original signal. The flatter portion of the signal from one path is softened by the more rounded portion of the signal from another path. In addition, other components may be limiting both the amount of clipping, which frequencies are clipped, or perhaps adjusting the gain of different frequencies in order to clip different frequencies by different amounts. There are more options than are usually available with hard clipping.

In either style of clipping, the gain may be manipulated in the circuit to cause other devices, such as transistors or op amps to clip to voltages the devices can handle. That can provide nuanced clipping that behaves differently as your signal strength varies. Or the clipping may only be applied to certain frequency ranges. For example, it is common to control the bass frequencies because they tend to become muddy when clipped. Or there may be multiple signal paths though the circuit, with different levels of clipping. The point is that diode hard or soft clipping may not be the only clipping taking place and the clipping may change depending on what and how you play, so you might not hear what you expect all the time.

Hard and soft clipping will behave differently, depending on what you are playing. With hard clipping, too many simultaneous notes will tend to produce a muddy mess of harsh tones and harmonics. It is easy to lose the tone and feel if there are more than a couple of simultaneous notes. That may be fine for leads and power chords, but probably won't be quite what you want for full chords with lots of simultaneous notes or leads with lots of bends. Soft clipping produces different harmonics and doesn't get quite as harsh. While any type of distortion can get muddy, especially with more bass content, soft clipping will usually retain more clarity and dynamics as you play more simultaneous notes, such as full chords.

Most distortion circuits use either hard or soft clipping, but usually not both. The MXR Distortion + uses a famous hard-clipping circuit to generate lots of distortion, but with a reduced volume caused by chopping the signal peaks off. The Marshall Bluesbreaker is a famous soft-clipping circuit. When you stack overdrive, distortion, and even fuzzes, you may want to consider their clipping styles and experiment with the order to find what you like best. You might think of a clean boost as having the least amount of clipping - none - that's why it is a "clean" boost, of course. But there may be enough boost to cause your amp or something else to begin distorting. You might also think of a fuzz as the most extreme sort of distortion. You might want to start with a clean boost and a fuzz as you begin experimenting with stacking drives. That gives you two extremes, clean and fuzz, which may help you more clearly hear the differences caused by switching the order. Be sure to try different music and playing styles as you go, as your results will probably vary. Be patient, as it may take some time to figure out which drives in which order produce the best stacked results. Sometimes you may find a pair of effects that work great for some of your needs stacked one way, and better for other needs when the order is reversed. That may be a lucky find, as there are "swappers" that do nothing more than switch the order of two effects in your chain. Check it out: Swapper.

Clipping Diode Configurations

After considering what else is happening in a circuit, you can see that the overall sound of the diode clipping distortion isn't really something you can control unless you start making changes throughout the effect circuit. At that point you aren't really customizing the clipping, you are designing a new effect and you will encounter lots of other issues to manage. So what can we control by customizing the clipping diodes? You can control the selection of the actual diodes and you can control their configuration in the circuit. That doesn't "control" the resulting sound, but it certainly can influence it. In Diodes we discuss specific diodes, so we won't go through that here. Let's concentrate here on the configurations of those diodes.

The audio signal is an AC electric current. AC, or alternating current, just means that it travels in the form of sine waves and that the waves go to some distance above 0V and some distance below 0V. So when we sloppily say we have a 1V signal, we really mean it goes from +1V to 0V to -1V and back to 0V in one sine wave. We've already described clipping as lopping off the tops (and bottoms) of those waves. Doing that requires two diodes, one to chop off the tops and one to chop off the bottoms (the most positive and most negative voltages). Using two diodes is the simplest and most common clipping configuration.

Symmetric and Asymmetric Clipping

If you chop off the tops and bottoms of the waves by an equal amount, that is called symmetric clipping. If you chop them off in unequal amounts, that is called asymmetric clipping. How does that impact the sound? The sound of a tube amp when the preamp tubes are overloaded and clipping is by many considered to be The Tone. To some extent, transistor and op amp based circuits are often trying to imitate that clipped tone. Because of arcane technical details, those with sensitive ears find that asymmetric diode clipping sounds more like the clipping from an overloaded tube than does symmetric diode clipping. Whether you can hear a difference depends on your ears and some of the details about the configuration.

Let's suppose we keep the simple 2-diode clipping config. Suppose we use two identical diodes, which specifically have the same value for Vf. We'll also assume our signal varies as much below 0V as it does above 0V. If this case we would have perfectly symmetric clipping because the diodes clip at the same voltage, and the tops and bottoms will be reduced not only to the same amount, but also by the same amount. But suppose our 2 diodes aren't an exact match for Vf. In that case, they will clip to different voltage levels and remove different amounts of signal. That would yield slightly asymmetric clipping.

It's difficult to say how different the Vf values for the two diodes would have to be before you could discern some difference in sound. There's all that stuff going on in the effect circuit. You have unique ears. Your music may have different content and volume than others', etc. It will depend on a variety of factors and there isn't really a general answer. If the Vf values are both around 0.65V, a normal Vf for silicon diodes, you would probably have trouble hearing something as small as a 0.05V difference unless you were carefully controlling your experiment. In a band situation, it's very unlikely anyone would notice something that small. But if you get it up to maybe a 0.1V or more difference, good ears may well pick it out. Usually the difference would be a bit higher than that, but even then you may have to pay attention to notice it.

A common way to get asymmetric clipping is to use three identical diodes, two on one side and one on the other. Putting two diodes in series like that causes the effective Vf for that side to be the sum of the Vf values for the two diodes. So with those 0.65V silicon diodes, that would give you 0.65V clipping on one side and 1.3V clipping on the other. You can probably hear that if you listen closely.

Tailoring Clipping Diode Configurations

We've already discussed three different configurations, which we will include in our suggested configurations below. And we've also already covered the "trick" you can use to build these configurations. You can use more than one diode on each side to change the effective Vf for that side of the clipping configuration. And whenever you have multiple diodes involved, you can have multiple different types of diodes. Different types can have different Vf values you can work with to affect (but not completely control) the symmetry of the clipping, the amount of distortion, and the volume level of the resulting signal. Different types of diodes also add their own color to the clipping, giving you another way to affect the result.

All the configurations below are just examples of using different numbers and types of diodes to change the sound of the clipping. Modules where clipping customization is an option describe the normal diode configurations that are used. That doesn't mean a different effect will have that sound if you select that clipping configuration due to all the other signal manipulation in each circuit. But it might, or at least it might be similar. Feel free to try them or change them up.

Here's another miscellaneous consideration. JFET and MOSFET transistors function as diodes if you use only two of their three leads. Those diodes have a different flavor to their sound, in the same way that silicon and germanium diodes and LEDs all sound a little different. To some ears, JFET and MOSFET transistors and diodes yield tones more like tubes than other transistors and diodes. That increases the flavors available from three to five. The only catch is that allowing them as options takes up some more space on our boards, space that isn't available, or at least not yet available, on some boards. Not all of our modules support JFET and MOSFET transistors used as clipping diodes - check the descriptions of each module or ask us. Even if it is not an available option, we may still be able to fit them in. The Vf values for those diodes is usually in the 0.6V to 0.85V range. Check Transistors for options. We can give you Vf for them if needed.

Some Guidelines for Choosing Clipping Diodes for GT Rack Effects Modules

Here are a few things to keep in mind, especially if you are choosing multiple clipping configurations for some switched clipping option in one of our modules.

The type (germanium, silicon, LED) and Vf have more impact than brand or model. Vintage-correct diodes may be available, but low-cost substitutes may be indistinguishable from them. Conversely don't expect some magic tone difference due to use of vintage-correct diodes.
If you want to select from a variety of clipping tones that have significant differences, then select diodes of different types and/or select diode configs with different Vfs.
If you want to select from a variety of clipping tones that are all still similar, then use diodes of different types and the same Vf as the original, or diodes of the same type with slightly different Vf values.
Be aware that switching between different types of diodes and significantly different Vf configurations will often result in considerable differences in volume.
It is OK to use both hard and soft clipping at the same time in circuits that allow it. It might get muddy to distort a distorted signal. And you might have a significant volume drop. Or it might not really work at all like you expect. Here's why. Suppose the soft clipping comes before the hard clipping in the circuit. If you put a low Vf diode in the soft clipping section, then there may be no signal to clip when it gets to the hard clipping diodes with the higher Vf. It will appear that your hard clipping isn't working.
If the circuit is designed to normally use say, single silicon diodes for each side, clipping around 0.65V, and you switch in a pair of germanium diodes clipping at half that, you can lose a lot of volume, gain a lot of distortion, and perhaps lower the signal enough that whatever comes after the clipping in the circuit behaves in a different way. Good or bad - you decide. But you may instead want to consider doubling the germanium diodes on each side. You'll still get the overall germanium tone coloring, but your Vf will be nearly the same as the normal config, letting downstream parts of the circuit behave more like they normally would, while also keeping similar levels of distortion and volume.
Since the Vf of germanium diodes can vary considerably, you should specify a range of Vf for them in your diode configs. We'll look for ones in that range and let you know if we don't have them, discuss options, etc.
We recommend that if you are going to have clipping configurations on a switch, make one of the configs the original clipping config for the circuit. Of course, if you are specifically customizing the module because you don't like the original circuit's sound, then feel free to choose anything you like!
The most common silicon clipping diodes are 1N914 and 1N4148. They sound nearly identical and are used in the majority of modern production effects that use silicon diode clipping. These two can also be used in place of most uncommon vintage silicon clipping diodes with little, if any, notable difference in most cases.
The most common germanium clipping diodes used to be 1N34A, 1N60, and 1N270, back when germanium diodes were the norm. They are still used in vintage circuits today. Many current reproductions of vintage circuits are now using other germanium diodes that are more available (often from Russia), or the "orange fakes" with the same part numbers, described in Diodes.
The most common LED clipping diodes are red, by far. They are a great choice for many overdrives.

Clipping Diode Configuration Possibilities

There isn't really a "top" and "bottom" in your signal. We just use those terms to refer to the two sides of the configuration and the two "sides" of the signal above and below 0V.

In our notation below, "Si" refers to any silicon diode, "Ge" refers to any germanium diode. "LED" refers to any LED, but almost always a red one. MOSFET refers to any MOSFET transistor, often 2N7000. "JFET" refers to any JFET transistor such as 2N5457

1. Top: Si Bottom: Si

The most common clipping configuration. Often used with 1N914 or 1N4148 diodes.

2. Top: Si Si Bottom: Si

The most common asymmetric configuration. Often used with 1N914 or 1N4148. Sometimes the extra diode is a 1N400x (eg 1N4001).

3. Top: Ge Bottom: Ge

Common in older circuits, usually with 1N34, 1N60, or 1N270 diodes. Because of variations in quality and manufacturing, the Vf values could be similar or different over a significant range of values, yielding variation in sound from one pedal to the next.

4. Top: Ge Ge Bottom: Ge

Most of the effects old enough to be designed with Ge clipping diodes were available before asymmetric clipping became fashionable. This config is more likely to be a boutique design nowadays, and isn't seen often.

5. Top: LED Bottom: LED

This is a very popular config that is often described as "crunchy", like a driven Marshall tube amp. Probably the most popular clipping alternative in many effects that provide alternatives. Provides a good alternative that is easy to distinguish from silicon and germanium.

6. Top: LED LED Bottom: LED

Less commonly used, but still a good option.

7. Top: MOSFET Bottom: MOSFET

Perhaps not as different from silicon as LED, but provides some subtle "something something" that just may be the little difference you're looking for

8. Top: JFET Bottom: JFET

Similar to MOSFET config, above

9. Top: Si Si Bottom: Si Si

Keep the silicon sound, keep more volume, tame the distortion

10. Top: MOSFET Ge Bottom: MOSFET

Maybe a good choice for those seeking a smooth, tube-y tone

11. Top: MOSFET Ge Bottom: Ge MOSFET

Maybe a good choice for those seeking a smooth, tube-y tone

OK, OK - Now what's the takeaway?

If you're considering some options for clipping in a GT module, here's what it comes down to. It's messy.

Generally, the biggest differences you can hear between clipping diodes is due to their Vf, or clipping voltage. Adding a second clipping configuration with diodes of a similar clipping voltage may result in your two diode configs sounding pretty much identical. Maybe that's what you want, maybe not. Extra switches, extra cost, maybe no audible difference. If you want options that sound more different, choose options with diodes that clip at different voltages.

The second biggest difference you can hear between clipping diodes is due to their configuration - symmetric, asymmetric, one pair, two pairs, mixtures of different types of diodes, etc. If you choose a second clipping configuration that uses the same diodes but in a different configuration, the difference can range from subtle to somewhat pronounced. But these differences are likely to be smaller than the differences between different types of diodes.

Almost all silicon diodes clip in a very small range of voltages, at around 0.6V. Choosing different silicon diodes will not likely have much, if any, audible effect. It is rare that a builder will test the clipping voltage of any silicon diodes because they are so consistent, and the differences are so small that it isn't usually worth the time and effort. Here at GT, we rarely test the voltages on silicon diodes individually. We generally test different brands and models just in case they vary significantly from the 0.6V norm.

Germanium diodes can have wide variance in the clipping voltage, even ones of the same brand and model. We do individually test germanium diodes because of the variation. Circuit designs often expect germanium clipping to be at around 0.3V, so when picking germanium diodes we often test until we find some in the 0.25 to 0.45 range. That is enough lower than a silicon diode that you get the expected germanium difference. Of course, certain circuits have some sweet spots for germanium diodes, so we will look for the right values for those circuits. If a circuit is meant to have symmetric germanium clipping, then the diodes need to be matched. Or you can get asymmetric with only one diode on each side by choosing diodes with different clipping voltages.

LEDs are a bit odd. The older LEDs have colored lenses and are sometimes referred to as "diffuse" LEDs. The newer high-efficiency LEDs have clear lenses. Those two technologies affect the clipping voltages. The color of the LED also affects the clipping voltages. Almost all clipping LEDs are diffuse, red. Very rarely, you'll find a diffuse, green. The other colors and the newer high-efficiency LEDs generally clip at voltages that are just not that useful. It's about 99.99% true the "LED clipping" means "red diffuse LED clipping". Unless you have some specific knowledge about a circuit or are selecting one of the very few circuits that use diffuse, green LEDs for clipping, just stick with red, diffuse LEDs. If you Google "forward voltage of different color LEDs" or something similar, you'll get charts showing conflicting information. That may be due to the way they are tested and it may also be due to different LED technologies. What you'll generally see is that regardless of the actual ranges of values, you'll see that red has the lowest clipping voltage. The others are usually all higher, too high to be that useful in most circuits.

Know that when you are selecting different clipping configurations, you are really just selecting different clipping voltages. For example, if you choose clipping configuration #9 above, the voltages of the two silicon diodes on each side add together to give you the overall clipping voltage for each side (about 1.2V), which would be double the normal silicon clipping voltage in this case. Choosing different types of diodes adds in different amounts. The more different the voltages are on the two sides, the more asymmetrical the configuration and the more you'll likely hear the effect of the unbalanced clipping voltages.

FET transistors used as clipping diodes clip at about the same range as silicon diodes, perhaps a little less, around 0.5 - 0.6V. While the voltage isn't that different, some think they have a slightly different character in their sound. But they tend to be used in asymmetric configurations, so perhaps that is really the difference being heard.

Listen to effects that use different types of clipping diodes - silicon, germanium, and LED. They will sound different. You can pick the adjectives you like best, depending on what your ears tell you. I think of germanium as smooth, dark, compressed, and lower volume. Silicon seems brighter, louder, more aggressive. LED seems crunchy and tubelike, maybe with a bit of midrange tweak. But that's my ears. Once you know what your ears hear, then you can make good choices for alternate clipping configurations in a particular effect circuit.

If a circuit doesn't come with LED diodes, that is often one of my favorite alternative configuration choices, especially in overdrives where a nice tubey crunch can be very appealing. LEDs can also restore a bit of volume lost by germanium diodes in some circuits.

If the default clipping for a circuit is symmetrical, consider an asymmetrical configuration as an option. Changing diode type and configuration will generally give you options that sound more different.

It's generally a good idea to make one of your clipping choices be the original clipping configuration of the circuit. That config was picked by the circuit designer for a reason and may well be a key component of the tone of the circuit. Sometimes it is good to have the "standard" as a choice. Not all options are "better", another reason to consider keeping the original configuration as one of your options.

While they are different, silicon and red LED are more similar than silicon and germanium. Sometimes big clipping changes don't really sound all that great in a circuit. It may be better for the choices to be a bit more similar to retain some of the nature of that circuit. Generally, only pretty old effects used germanium clipping diodes. Most newer ones use silicon or perhaps red LEDs. Switching from silicon to germanium would likely be very noticeable, including a significant drop in volume. While technically OK, going from silicon to germanium may not work out as well as you might want. However, switching from silicon to red LED can be quite nice.

Clipping options can be a great way to refresh an old circuit's sound for your ears. It can also be a great way to give those one-trick circuits a second trick, again maybe giving you something familiar but different to work with when you get stuck in a tone rut.

It's difficult to predict what you'll get tone-wise from a clipping change. There are just too many other factors that can impact the result. Chances are someone may have tried what you're considering, so ask around and let Google be your buddy to see if you can find the opinion of someone that tried the same thing.

Why is an asymmetric configuration more tubey sounding? Most amp tubes have more than once section in them. The sections are used independently, sort of like multiple transistors. But the manufacturing of tubes leads to performance differences between the sections. They will operate and clip at different voltages. If the voltage differences are small, the tube may be described as "balanced". But if the differences are significant, the tube is "unbalanced" and the sections will clip at different voltages, similar to the different voltages on the two sides of an asymmetrical clipping configuration. Asymmetrical is trying to emulate the manufacturing variance in tubes, a sound that many find very pleasing.

If between you and us we are unsure about choosing some clipping option, it may be the case that we can audition the configuration as we build your module and figure it out that way. We can discuss that and all your other options as we configure your order.