Harmonics

[MV]

I've been reading about valves and that even harmonics makey their hot tone... But I'm interested how to amplify even or/and odd harmonics, what's their relation to THD and how does clipping affect to it. Why does valve amplify even harmonics?

[teemuk]

Distort the wave asymmetrically => prominently even order harmonics
Distort the wave symmetrically => prominently odd order harmonics

Has nothing to do with tubes, except that a generic common cathode gain stage tends to distort asymmetrically.

[MV]

Thanks, I appriciate it

[MV]

But why?

[midwayfair]

Wave cancellations.

http://en.wikipedia.org/wiki/Tube_sound#Harmonic_content_and_distortion

[PRR]

> Why does valve amplify even harmonics?

It doesn't *amplify* the evens any more than the odds or the fundamentals of the original signal.

_All_ simple amplifiers amplify (everything), but also _distort_.

_All_ simple amplifiers distort by adding additional tones exactly related to the original tones. (This is un-like string harmonics which are not exact multiples of their fundamental.)

_All_ simple amplifiers add primarily even-order at medium level. _All_ simple amplifiers add odd-order at high level.

This transition from clean to even to odd is part of the "flavor" of electric guitar. E-guitar needs such transition to emulate the changes in voice woodwinds and horns from medium to loud. Often the raw power does not increase, but stronger harmonics add emphasis to musical phrasing.

Simple BJT transistors can add a LOT more even-order distortion than tubes. It may be too much of a good thing.

So while the question is simple, the answer is murky.

Most distortion effects are developed with a hint of inspiration/theory and a TON of playing-around. Build stuff. Abuse it different ways.

[ashcat_lt]

I've always felt it to be a little dangerous to talk about distortion devices as though they were "adding" harmonics.  We're not talking about additive synthesis here, but about waveshaping.

While it is possible mathematically to represent a square wave as a combination of a series of harmonics, that ain't what an overdriven amplifier or a Rat pedal does.  These things just make square waves - or approximations thereof.  Our brains hear this as an increase in harmonic content, to a certain extent pulling this naturally generated square wave apart into the component sines, and we can even see them in analysis.  That, however, is a bit of an artificial creation - a way for us to make sense out of the phenomenon.

I have trouble expressing my concern in a way that makes sense, but if it comes down to a chicken and egg kind of thing, I want to say that the distortion comes first, and the percieved harmonics are a byproduct.

[Kesh]

We're actually pretty bad at perceiving the phase of harmonics, which also give a wave its shape, but very good at perceiving their level. So two utterly different looking wave forms would sound almost identical if they had the same harmonic content. (We are quite good at perceiving phase of transients, as these help locate sound sources)