Tube Guitar Amplifier Overdrive Webpage

[printer2]

Haven't read your distortion page Rob, just killing a few minutes with this thread. Had a couple people going on about soft and hard clipping between tube and SS in one forum. Did not match what I have seen on my scope. Thought I would back my opinion with a picture of a Champ type amp just at the onset of clipping and a little way into it rather than crank it up all the way where you would get a square-ish wave.



Not rounded even though the level is not much greater. Sorry for the poor capture, the camera I was using was not cooperating. Maybe you can use the Champ as an example of clipping waveforms as well as a push-pull amp, maybe a 5E3. Maybe a shot just starting to clip, mild clipping and hard clipping. It can lead into bias shift, cross over distortion, why biasing the output stage hot helps smooth it out when overdriven. The Champ may be driven hard like the P-P amp but it does not produce crossover distortion but on an 18 Watt it can make the amp sound buzzy, even transistor like (tongue in cheek).

A lot of good points from all here, but some of it may just glaze over some eyes. I think the amount of gain and NFB an amp is an important component of the 'transistor sound' as some complain about. Funny that there are so many SS fuzz pedals and so few tube ones. Maybe it is partly a design thing rather than a device thing. Glad to see everyone willing to pick up the ball and help you run with it here. Probably the best forum of guys that know their stuff.

[uneumann]

Haven't read your distortion page Rob, just killing a few minutes with this thread. Had a couple people going on about soft and hard clipping between tube and SS in one forum. Did not match what I have seen on my scope. Thought I would back my opinion with a picture of a Champ type amp just at the onset of clipping and a little way into it rather than crank it up all the way where you would get a square-ish wave.

Rob - I think there's growing awareness that the whole SS/tube debate is a red herring. Since both technologies are moving targets, it may be best to avoid that argument or issue completely. If you want to do something useful to introduce non-EEs to overdrive, maybe just show some examples as printer2 and Juan produced. (Pictures are worth 1000 words, and whatever.) If you captured waveforms under a few standard conditions for a few common amps - Champ, 5e3, Bassman, 18W, (even SS amps ) etc, you'd have some pages that introduce people to the variety of conditions that occur. You don't have to draw any grand conclusions - just show the data with enough explanation that helps people understand what they're looking at. Interested people will start wondering why and how those things occur and a curious mind will seek out answers. They are out there.

I know that's a lot of work - and you may not have access to that many amps - however, there are lots of people out there and maybe if you start the site, you may find that people will contribute "pages" about their favorite amps. I'll offer my early pages on the univalve as a start and an example. Let me know if you want that data - I'm happy to send it - or just grab it off this site... https://sites.google.com/site/string...alve-mod/mod-d

[Malcolm Irving]

... picture of a Champ type amp just at the onset of clipping and a little way into it rather than crank it up all the way where you would get a square-ish wave.
....

Interesting to note the wave-shape on the left hand scope picture (at the onset of clipping). Notice how the top of the wave is a bit broader than the bottom. That's a result of a certain amount of 2nd harmonic. I think good 'clean' amp sounds (whether tube or solid-state) introduce some useful harmonic distortion well before clipping.

[Usable Thought]

Interesting to note the wave-shape on the left hand scope picture (at the onset of clipping). Notice how the top of the wave is a bit broader than the bottom. That's a result of a certain amount of 2nd harmonic. I think good 'clean' amp sounds (whether tube or solid-state) introduce some useful harmonic distortion well before clipping.

OK I must turn off my Romulan cloaking device & stop lurking for a moment to ask a question -

This thread is starting to get really interesting along the lines of what harmonics are/aren't musically desirable & how these relate to particular amp designs. The territory involved would seem to include waveforms and circuits, but also psychoacoustics & whatever other terms would be appropriate for "why I like that particular tone" - and at a level that goes beyond "tubes are great man, they have even-order harmonics, and hey who can tell me about Billy Gibbon's rig* so I can get his sweet sound?" etc.

Recently I was Googling for academic research on musical tone & perception but what I seemed to find was that most university-level research is still centered on acoustic instruments, often orchestral; or on singers. Now and then electric guitar/amp sound does get studied, obviously, since we can all cite a study or two. But there don't seem to be that many studies & most seem to be pretty moldy; or else sometimes Google will retrieve a grad student's thesis which covers only the most basic territory & thus is not of interest.

So I guess I am asking - what books/articles exist on electric guitar + amp tone that address harmonics of clean tone, overdriven tone, etc., in a way that covers both the production of these sounds in various circuits & the psychoacoustic/musical aspects as well - and are not a mere repetition of the same few myths or generalizations? I can't recall a single article or book that does this. Usually what gets written for guitar-oriented magazines doesn't get into such deep waters but sticks pretty close to repeating the same old cliches. Or in a technical book such as Merlin's there is are mentions of harmonics etc. but only in passing, never as the main topic of interest.

- - -

* Exhibit A.

[teemuk]

I'm pretty sure you will NEVER find a book that can describe "psychoacoustic" or "musical" effects of distortion, since these are largely subjective issues (each individual perceives them differently and has more or less unique taste and preferences) and largely dependent on context and application: Distortion considered "musical" in death metal music probably isn't that in context of classical music, or even hard rock music. Tones that work with music styles orginated in 1960's are likely irrelevant to 1990's music, and so on. Some people may hate certain sound only because they know the sound is produced with technology they have prejudices again et cetera. It's too subjective issue to draw universal, objective conclusions. One person may prefer certain distortion sound while another person hates it. We change musical style and context and their opinions about "musicality" of certain type of distortion may flip around or probably at least change entirely. Which one of them will define an objective and universal terms to explain musicality? In what context? That's right, neither can.

Do we have books that define which is "better" more "artsy" art: cubism, photorealism, installations, modernism, dadaism...? Is it more "artsy" if its painted or drawn with charcoals? No, all such things fall to the realm of subjectivity, just like evaluating "musicality" of something.

BUT, I recall reading some very decent primers about distortion from literature aimed at sound engineers and alike. You know, guys recording, mixing, mastering (etc.) sound. They will encounter distortions and they need decent primers to explain what different types of distortions (symmetric vs. asymmetric, "soft" vs. "hard" clipping, and so on) actually do to the signal in -objective- terms.

THAT can be defined.

One can conclude objectively things such as...
- Increase in harmonic distortion results to increase in intermodulation distortion
- Symmetric clipping dominantly generates odd order harmonic distortion, and in lesser amounts even order
- Asymmetric clipping generates BOTH even and odd order harmonic distortion, by good chance in greater percentage than symmetric clipping
- Soft clipping generates more harmonic distortion than hard clipping because of increased non-linear range of operation at "soft clipping zone"
- Below clipping threshold hard clipping generates less harmonic distortion, but when distortion appears it does so more abruptly and generates more high order harmonics
- Human hearing is insensitive to transients so we may not perceive transient distortion. Hard clipping at transients may go undetected but similar soft clipping setup may easily reveal distortion because greater area of non-linear operation expands distortion to appear outside transients.

We can define objective conclusions how all these interact with each other since they are phenomens that follow strict, definable laws. What such and such amount of harmonics does for overall IMD and so on.

Answering how each individual subjectively perceives that stuff in terms of "musicality" is, however, practically impossible. And we would have to rewrite those definitions for each and every musical context out there. Then they would likely change just because of human nature to seek new thrills whenever of getting bored and fatigued to things he has already gotten accustomed to. Definitions of what was "musical" yesterday would end being being definitions of what is "boring" today. We can't evaluate subjective things in objective terms. That's simply impossible.

Personally, I don't even need literature that tells me what to like and what not to like. I'm quite well aware of that myself already, thanks. But I do appreciate literature such as these, which explains what different phenomenons may do in terms of sound, but do not try to enforce ideas how we should subjectively regard them or react to them.

http://www.simulanalog.org/clip.pdf
http://sound.westhost.com/articles/intermodulation.htm

[Usable Thought]

I'm pretty sure you will NEVER find a book that can describe "psychoacoustic" or "musical" effects of distortion, since these are largely subjective issues (each individual perceives them differently and has more or less unique taste and preferences) and largely dependent on context and application

I disagree completely with just about your entire post - the only part I can't disagree with is the articles by/for studio recording techs which I find interesting.

All arts, including music, depend on the combination of native human physiology (what we tend to like as animals of a particular species w/ears and eyes and our other senses & emotions & bodies & minds) and personal and cultural history (whether our parents listened to opera or took us to the opera as kids; whether rock music was widely available to us growing up in this era, versus the classical music that would have been available in the 1800s in Vienna); and to some extent the ineluctable predilections we all have as individuals. And all artists, musical and other, depend on their audiences knowing the same genres they do - genres we did not know as infants but learned on our way to and into adulthood. We don't just learn but but share and develop these genres together.

To say this richness of human experience, both what is innate and what is acquired, can't be talked or written about because taste is "subjective" is to dismiss what has been written about the arts since at least the time of Greek classical civilization and on to today.

And specifically, if your view is correct, and it's all "subjective," then how do you account for the confluence of interest in particular tones - indeed for your own activity in furthering the development of SS circuits that produce such tones? So we DO talk about all of this in a musical context . . . and like it or not, you are part of this conversation even if you think you are not - even if you think you are only writing about technical or objective matters.

No one is telling you "what to like." But people obviously do like to gather together and talk about the things they like and what tastes they have in common - as well as how their tastes differ and why. Hence this forum, hence the Gear Page, hence Rolling Stone. Etc. etc.

FYI I mentioned academic musicological and physiological research not because I do such research nor think it is the only way to talk about music - but because some of this research does in fact focus on (a) what kinds of sounds particular instruments produce, and (b) why we as humans on the whole like these sounds and not others. The topic is very broad and I at any rate find it potentially quite interesting. You are nibbling around just one edge of it; I'm interested in the other edges too.

I thank you though for the reference to sound engineers & how they might view the topic - I can bookmark that as a possible lead.

[Malcolm Irving]

IMD - Something New

Thanks very much for posting that link. Very interesting information about IM and symmetrical distortion (for a symmetrical input waveform).

[Mike Sulzer]

@Malcom Irving ****Edit: I am sorry, this should read @Usable Thought. Thanks to g1 for pointing it out.

"This thread is starting to get really interesting along the lines of what harmonics are/aren't musically desirable & how these relate to particular amp designs. The territory involved would seem to include waveforms and circuits, but also psychoacoustics & whatever other terms would be appropriate for "why I like that particular tone" - and at a level that goes beyond "tubes are great man, they have even-order harmonics, and hey who can tell me about Billy Gibbon's rig* so I can get his sweet sound?" etc."

I think the first thing to consider is that the tone you like now is a function of what you heard and liked in your youth, and that is a strong function of all kinds of factors not directly related to the actual sound. Furthermore, people of the same generation tend to like the same things, but there are strong changes on the time scale of a generation.

But moving from the "high level" down to the "low level", the ear and immediate associated brain processing is most sensitive to two factors, at least for the purposes here:
1. The relative power levels of different frequencies;
2. The time variation of the signal on a time scale considerably longer than the periods of the audio signals themselves.

An important result of 1. and 2. is that changes in the phases of the various signal components have no effect unless drastic enough to affect the variation described in the second. (That is, stretching out a transient, etc.)

As for the electric guitar sound, the nonlinearities involved were something completely new, causing audible effects of both types, even creating the sensitivity to aspects of the signal from a stringed instrument that otherwise would not be audible. The new types of harmonic structure required mental adaptation, most successful in the young, of course.

The significance of differences in SS and tube is that they can help us understand how all this works. The complexities of nonlinear processing suggest that you should look first at the last stage of the processing (after the nonlinearities), and that highlights the output impedance of the amplifier and the input impedance of the speaker, an interaction often referred to as "damping factor", although the concept of impedance is more general and useful.

[Malcolm Irving]

Intermodulation is a 'bad thing' in general, as new frequencies are created which are not harmonically related to the original signal frequencies (and therefore sound 'unmusical').
Certainly for hi-fi, PA systems, etc. IM is avoided as far as possible.
In guitar amps, players using a lot of distortion often tend to use 'power chords', which include octaves and 5ths, but avoid major or minor 3rds. One reason for that may be to minimise IM products.
However, I'm wondering if IM can sometimes be 'good' for distorted guitar and it occurs to me that a common technique is to play two notes, where the lower note is a tone below the higher note, and then ‘bend’ the lower note up to unison with the higher note. In this case the difference frequency will start low and go sub-sonic, while the sum frequency will bend up towards the octave above the two unison notes. Could be a nice effect?

[Usable Thought]

I think the first thing to consider is that the tone you like now is a function of what you heard and liked in your youth, and that is a strong function of all kinds of factors not directly related to the actual sound . . . As for the electric guitar sound, the nonlinearities involved were something completely new . . .

I think you've got it half right. As I mentioned in my reply to teemuk, all art - including music - has not only a cultural basis but a physiological one. So the "actual sound" matters much more than you might think. Someone with a really good musicological/psychoacoustic background might be able to show that there are many similarities between the distorted electric guitar & acoustic instruments such as the cello etc. - not to mention the human voice. If there were not a strong physiological basis for our interest in electric guitar sounds, my bet is the instrument would not still be popular.

The question is similar to asking why video games became popular. There is actually an enormous amount of research, both academic and commercial, around this question; many theorists have tried to answer it without success. Video games combine storytelling plus games, but sometimes seem hard to classify as either. There have been attempts to say games demonstrate something radically "new" in our response to them - but these attempts have never persuaded enough people to become the accepted wisdom. Why? Because human physiology took millions of years to develop & does not change in a generation nor in 40 generations. So if we like something "new" such as video games or electric guitar, you can bet your last dollar our appreciation for this "new" thing is based only superficially on technology and trends - the deeper reason is our evolutionary makeup & physiological capacity.

[Mike Sulzer]

Intermodulation is a 'bad thing' in general, as new frequencies are created which are not harmonically related to the original signal frequencies (and therefore sound 'unmusical').
Certainly for hi-fi, PA systems, etc. IM is avoided as far as possible.
In guitar amps, players using a lot of distortion often tend to use 'power chords', which include octaves and 5ths, but avoid major or minor 3rds. One reason for that may be to minimise IM products.
However, I'm wondering if IM can sometimes be 'good' for distorted guitar and it occurs to me that a common technique is to play two notes, where the lower note is a tone below the higher note, and then ‘bend’ the lower note up to unison with the higher note. In this case the difference frequency will start low and go sub-sonic, while the sum frequency will bend up towards the octave above the two unison notes. Could be a nice effect?

The fifth and octave power cord is the basis for much of electric guitar playing for just the reason that you say. But it is the controlled addition of more, disharmonious by its nature, that gives depth and results in music that remains appreciated over time.

[Mike Sulzer]

I think you've got it half right. As I mentioned in my reply to teemuk, all art - including music - has not only a cultural basis but a physiological one. So the "actual sound" matters much more than you might think. Someone with a really good musicological/psychoacoustic background might be able to show that there are many similarities between the distorted electric guitar & acoustic instruments such as the cello etc. - not to mention the human voice. If there were not a strong physiological basis for our interest in electric guitar sounds, my bet is the instrument would not still be popular.

The question is similar to asking why video games became popular. There is actually an enormous amount of research, both academic and commercial, around this question; many theorists have tried to answer it without success. Video games combine storytelling plus games, but sometimes seem hard to classify as either. There have been attempts to say games demonstrate something radically "new" in our response to them - but these attempts have never persuaded enough people to become the accepted wisdom. Why? Because human physiology took millions of years to develop & does not change in a generation nor in 40 generations. So if we like something "new" such as video games or electric guitar, you can bet your last dollar our appreciation for this "new" thing is based only superficially on technology and trends - the deeper reason is our evolutionary makeup & physiological capacity.

If you understand me to be understating the importance of the physiological, then I am not communicating well. As physicist and engineer, I put that first by my nature, and include the higher level phycological effects only out of necessity!

However, I think you are way underestimating how different the IM effects are from anything that has come before. Where in the acoustic world does one have to reduce down and control the production of sum and difference frequencies?

There is a somewhat difficult way to show just how important these effects are. That is, construct a system in which each string is distorted separately. The result is just much less interesting than one might expect.

This is not to say that there are not interesting and surprising effects with single string playing. For example, there are the two kinds of harmonics: those generated by nonlinearities from the string fundamental and harmonics are exact, while those from the natural string vibration are shifted off by an amount that increases with the harmonic number. The two must coexist while clashing. This is most significant in the picking transient before the higher harmonics die out, and this is something new also. Acoustic stringed instruments produce huge overshoots in power in the initial transient. Even in relatively clean electric guitar playing, the transients are clipped off, and it is the effects of the distortion that contribute heavily to the audible effect.

[Usable Thought]

However, I think you are way underestimating how different the IM effects are from anything that has come before. Where in the acoustic world does one have to reduce down and control the production of sum and difference frequencies?

Hmm, I think we are talking at cross purposes now. I can't understand a word you say because I just don't have your technical background. If I look at what you are saying in a slightly different way, however, I think you are clearly onto something - I just don't know what it is! As I say I used to be an editor & writing coach for professionals; and I developed a sense early on for the relative degree of BS (and I don't mean bachelor of science!) vs. credibility in someone's writing even if I did not share their technical background. I get a sense of credibility from you so that's good.

Anyway I don't think right now I have any more to contribute to the discussion so I'll go back to lurking. For me to really grab onto these topics I'd have to immerse myself for a long time in them on my own . . . and my problem is, there are so MANY topics I would like to do that with, that I have to pick and choose. Learning math; learning about basic circuits; learning jazz progressions on the piano; learning better classical piano technique; and much more. And due to health reasons I have only a couple of good hours per day - though I shouldn't kvetch about it. I just wish there were time enough for me to really do it all.

However this is definitely a topic I hope to hang onto and come back to - whether sooner or later.

[robrob]

what books/articles exist on electric guitar + amp tone that address harmonics of clean tone, overdriven tone, etc., in a way that covers both the production of these sounds in various circuits & the psychoacoustic/musical aspects as well - and are not a mere repetition of the same few myths or generalizations?

You won't find a better book on this subject than Neumann and Irving's Guitar Amplifier Overdrive A Visual Tour. The book includes a lot of graphical content to help make it understandable and math is kept to a minimum.

[robrob]

Thanks for the suggestion Dr Neumann, I'll read your linked webpage and peruse your website, it looks very interesting.