In the previous articles we’ve looked at the main ingredients of a lead guitar tone, which were; distortion, frequency curves, and dynamics. Today we are going to look more closely at distortion and how we can manipulate it to gives us control and flexibility of tone.
The main aspects of distortion as it relates to tone are:
- Type of Distortion – Hard or Soft Clipping
- Symmetrical or asymmetrical distortion
- Where the distortion occurs – at the pickups, the amp, the speaker or a combination of all three
Equally important, is the use of EQ to control the distortion ‘voicing’, but we’ll focus on that in the next post.
Types of Distortion
Its easiest to classify harmonic distortion as being ‘hard’ or ‘soft’ (or anywhere in between) and as being ‘symmetrical’ or ‘asymmetrical’. I’m not examining the solid-state versus tube debate, or the odd-order versus even-order harmonic distortion debate. From an engineering standpoint, solid-state devices can be designed to clip in ways very similar to old tube designs, while a poor design – whether tube or solid-state – will always sound pretty bad. There is no definitive difference between the sounds of a solid state or tube amp so listen, and decide for yourself.
‘Hard’ and ‘Soft’ Clipping
‘Clipping’ is what causes the effect of distortion, and gets its name from the peaks of the waveform being ‘clipped’ off.
This is a pure sine wave with no clipping
This is the same wave, but after clipping. The dotted line represents the original sine wave.
‘Hard’ and ‘soft’ refers to how ‘squarely’ the waveform is clipped. A very flat, square clipping characteristic is known as ‘hard’ clipping, and a rounder clipping shape is known as ‘soft’ clipping. Hard clipping is typically more complex, sounds more aggressive, and generates more harmonic content than soft clipping, which sounds more ‘overdriven’ and retains focus. Silicon fuzz pedals are a good example of hard clipping, whilst very early vintage tube/valve guitar amplifiers will give you an idea of softer clipping.
In the truer sense of the word, soft clipping refers more to the kinds of distortion found in analogue tape devices and the like. Compared to these soft clipping devices even the gentlest overdrive is really a hard-clipping device. But within the realms of guitar devices the softer clipping devices are usually clean amps, clean boosts, tube/FET based distortion units. The harder clipping units usually usually get their distortion from overdriven transitors.
Its important to understand that how hard or soft the clipping is, has little to do with where the ‘gain’ knob is set. The ‘gain’ controls how much clipping you have in the signal, but the actual clipping characteristic is determined in the design of the pedal or amp. Unfortunately most pedals/amps don’t offer a ‘clipping’ control where you can vary the ‘hardness’ and ‘softness’ of the clipping, so this is something you need to be aware of when purchasing you gear.
Occasionally you may come across ’boutique’ pedals or ‘mods’ for commercial pedals which will allow you to have some control over the clipping characteristic. These pedals usually achieve this by having a switch which will add/rearrange clipping diodes in the circuit. These are a great idea if done well, however be aware that these are only available with a switch – you can’t have a ‘knob’ which gradually changes from hard to soft clipping, its usually just one or the other.
Symmetric versus Asymmetric Clipping
So far all of the examples have been symmetric clipping, which is where both sides of the wave form are clipped the same. Asymmetric clipping, on the other hand, is where each side of the wave form can be clipped differently.
Symmetric clipping is more focused and clear, because it is only generating one set of harmonic overtones. Since asymmetric clipping can be hard-clipped on one side, and soft-clipped on the other, it has the potential to create very thick complex sounds. This means that if you want plenty of overtones, but do not want a lot of gain, asymmetric clipping can be useful. For full-blown distortion symmetric clipping is usually more suitable, since high-gain tones are already very harmonically complex.
Asymmetric clipping also seems to create more apparent intermodulation distortion (the bad kind of distortion). Also, since chords have more harmonic content than solo lines, it is a good idea to avoid chord playing whilst using asymmetric clipping. This helps to avoid objectionable levels of intermodulation distortion. Sticking with symmetrical clipping for rhythm parts helps ensure that your tone doesn’t become muddy and unclear.
Generally speaking, asymmetric clipping is believed to sound more like a real tube amp than symmetric clipping, since most common ‘hailed’ tube amp designs clip asymmetrically. However in my experience they both create very good useful sounds for different applications. Also its my understanding that the more expensive tube amps are actually designed to minimise the amount of asymmetry in the waveform.
Here is a clip with various riffs played at different gain levels. Each riff is first played through an Tubescreamer style pedal and then played again through a Super Distortion style pedal. These pedals are almost identical except that the Tubescreamer clips symmetrically and the Super Distortion clips asymmetrically.
MP3 DEMO OF ASYMMETRIC AND SYMMETRIC DISTORTION COMING SOON
Try these pedals out in your local music store if you want to investigate more. Neither of these pedals are fantastic sounding units (IMHO), but they do highlight the differences between asymmetric distortion and symmetric distortion quite well.
The circuits for the TS-808 and the SD-1 are very similar, so if you decide that you like both of these units for different purposes, you could consider buying one of them and then adding a switch to change the diode clipping arrangement. There are plenty or manufactures who produce kits for this kind of mod, and I wouldn’t be surprised if versions of these pedals exist that have a symmetric/asymmetric switch as standard.
One last note about symmetric versus asymmetric distortion, is that typically asymmetric clipping will have a predominant first harmonic, which the symmetric clipping will not. Many guitarists feel that this harmonic is essential to their tone.
My understanding is that a symmetric clipping circuit doesn’t have to have a diminished first harmonic, but just it is common that they do. Similarly not all asymmetric clipping circuits will have a dominant first harmonic, but it is common. This is because push-pull amplifier designs (which normally clip symmetrically), cancel out the even-order harmonics. Single-ended designs (which clip asymmetrically) do not result in the even-order harmonics being canceled out.
Its probably unclear whether those who favour asymmetrically clipping actually prefer the sound of the asymmetrical waveform, or just prefer the sound of the even-order harmonics. Most likely its a combination of each with more than a heavy hand of hoodoo/voodoo placebo effect thrown in there as well. Realistically they are both valid sounds and it just comes down to choice and personal preference.
Where Should the Distortion Occur
Pickups, OD/Distortion pedals, pre-amps, power-amps, and speakers all distort to some extent. The trick to a good tone is to know where the distortion should occur, and how you can combine distortion from different parts of the chain.
Distortion for High Gain Tones
Generally speaking higher gain tones should only have a few parts of the chain distorting since distorting distortion usually only serves to increase the noise floor, and does not create a very musical tone. This means that for very high gain tones you should probably rely mostly on pre-amp distortion or maybe even a separate pedal. Although there are people who will disagree with me on this point, I feel that high-gain tones need a minimum of pickup distortion and minimum power-amp distortion. Any extra distortion in those areas can only lead to increased levels of intermodulation distortion (I can hear the front three rows hissing and screaming “Blasphemer! Blasphemer! How dare he criticise the power-amp distortion?”) .
In some dedicated high gain amps the master section is relatively linear (i.e. introduces very little distortion), so turning up the master volume will still maintain a good clear, heavily distorted tone. But using a distortion pedal with a vintage style amp with the master volume turned up high, will often take definition and clarity away from the tone, and, in extreme cases, will result in a mushy fuzzy sound, with plenty of inter-modulation distortion (that’s distortion that we don’t want, remember?).
Since high-gain tones sound best with most of the distortion coming from the pre-amp, you should try to use a fairly ‘neutral’ pickup – or even a slightly bright pickup (see ‘voicing’ in the next post) – with a clear ‘hi-fi’ sound. Active pickups can work well, but there are plenty of passive pickups which suit perfectly as well – Dimarzio pickups are very common for this, though I have not used them personally.
As for the speaker choice, it is really personal preference. In my opinion a touch of speaker distortion actually works very well even for high gain sounds, but this is kind of counter-logical and you may disagree.
Overdrive for ‘Clean(er)’ Tones
For clean to overdriven sounds the distortion can be spread out through the chain a bit more. Often players will choose a more characterful, and complex sounding pickup (one with plenty of harmonics, thick sounding, or specific EQ etc), combined with a little pre-amp distortion and a significant amount of power-amp and speaker distortion.
For very subtle clean-overdrive, it is often the power section and the speaker distortion which creates richness of tone. However, Dumble amplifiers – a very revered amp among blues players for ‘mild crunch’ – are known for their use of ‘precision’ power-amps. These power amps were designed to create minimal distortion, so Dumble amps got most of their overdrive from the pre-amp.
For a straight-ahead rock tone – very dirty but not ‘high gain’ – I think that the power-stage is perhaps the most important part of achieving a decent tone. I feel that the power section is so important because it is effectively ‘distorting distortion’. In high gain sounds, distorting distortion is a bad idea since the tone is already very complex and adding more distortion simply makes the tone incomprehensible. But in a rock context in works quite well because it makes the sound very dirty and more complex, but because the tone isn’t soaked in huge amounts of pre-amp gain the overall tone still retains some focus. Hence, the tone sounds ‘very dirty’ but doesn’t sound like your using bucket loads of distortion either. (Afterthought: Maybe asymmetrical distortion might help create a ‘dirty but not high-gain’ sort of tone?)
Beware of intermodualtion distortion though when you try this idea. Van
Halen’s dimed ‘brown’ sound is probably as dirty as you can get before you will need to consider changing over to ‘modern’ high gain amps with more pre-amp distortion and less power-amp distortion. In fact, the intermodulation distortion that occurs in the brown sound is actually already very high, so Van Halen used ‘tempered’ tuning to allow him to play chords but still retain some clarity and focus – otherwise his tone may have just become mushy.
Sonny Landreth / Eric Johnson Lead Tone
Another great tone trick where distorting distortion works remarkably well, is running a fuzz pedal in front of an overdrive, or a dirty pre-amp. Usually we use fuzz pedals to create most of the distortion and then set the rest of the chain relatively ‘clean’, however Eric Johnson and Sonny Landreth, among others, use the fuzz pedal combined with an overdrive of some sort, to great effect.
This usually only works well with lead work, since any chordal playing will highlight the intermodulation distortion. Also it tends to work best with clearer sounding single coils as the more complex humbuckers tend to become too muddy with so much layered distortion. Unfortunately this means that you need to deal with single coil noise, combined with lots of distortion. It can be hard to get just right, but when you do it sounds complex, thick and lush, but with surprising clarity.