I just had a little tussle with with the local blatherskites over at one of the less hospitable internet audio forums. The actual subject of the exchange is unimportant. However it got me to thinking about distortion and how today the concept of distortion has become seriously misunderstood.
One of the most unfortunate consequences of the “Stereo Wars” of the 1970s was the amount of misinformation and propaganda spread concerning audio quality. And on no subject was this more pronounced than that concerning distortion. We went from the 1950s when we had truly great sounding amps with nominally less then 5% harmonic distortion to the mid 1970s when we had crappy sounding amps with far less than 1% (or even 0.1%) harmonic distortion. But the marketers and salesmen of the time convinced buyers that the overall Total Harmonic Distortion or THD number was some kind of metric by which all amplifiers should be judged..
Some make the argument that it was very high gain amplifiers with massive feedback that drove the reduction in distortion numbers; which is technically true. But Harold Black invented the feedback amplifier in 1927 and published his seminal paper “Stabilized Feedback Amplifiers” in 1934. By the early 1940s we had lots of high gain signal chains with lots of feedback, they just weren’t used for audio amplifiers. But why?
The answer to this question lies in the understanding of distortion. Those designers of the 1940s and 1950s knew that not all distortion was created equal. They knew that odd order harmonic distortion (3rd, 6th 7th, etc.) was the enemy of a musical amplifier and that the even order harmonic distortion (2nd, 4th, 6th, etc.) was the key to enjoyable listening. They understood the action of the human ear and how we perceive sound. They knew that a few percent of even order harmonic distortion (of decreasing magnitude with order) was not only acceptable, but desirable. And this is exactly the type of harmonic structure produced by triodes.
But this is not the type of distortion produced by transistor amps. These amplifiers are rich in odd order harmonics that sound harsh and tiring. So to get transistor amplifiers with even acceptable sound quality, they had to drive the distortion way down. Down, in fact, to far less than 1% THD. And those unscrupulous marketers, salesmen, and designers of the 1970s made THD their whole marketing pitch.
But we know better.
However, due to the decades of misunderstanding and misinformation concerning amplifier performance, the word distortion carries with it a very negative connotation. As such, I have a proposition to make. When addressing the performance of single ended vacuum tube audio circuits, I think we should abandon the entire notion of Total Harmonic Distortion. Instead I think we should talk about two different metrics for our amplifiers.
The first we shall identify as “Color”. The amplifier “Color” should be calculated as the total even order harmonic distortion and reported in percentage (%). The second we shall identify as Total Odd Order Distortion or TOOD. This should be calculated as the total odd order harmonic distortion and reported in percentage (%) as well. What this does is allow us to separate the two different auditory phenomenon associated with harmonic distortion. The TOOD is what we would call “bad” distortion; that which should be minimized. The “Color” is what we would call beneficial distortion; that which we should target just enough to give the amplifier a pleasing richness and fullness. If we base our numbers on the great amplifiers of the 1950s and 1960s maybe we should strive for less than 1% TOOD and maybe around 4% Color.
As an example, let’s look at the 6V6 Marblewood amp. Subjectively, this amplifier sounds much better than one would think based on the THD numbers. It’s been described as “rich”, “full”, and “musical” by those I’ve invited to listen. On the Marblewood amplifier page I posted the distortion characteristics for the amp using the 6V6 power tube. Here is that same table but with the added calculations of “Color” and “TOOD”.
This shows us something very interesting. When the distortion is broken down into “Color” and “TOOD” we see that the numbers say that the discordant distortion is very low and the color is moderate. Does this provide us some more clues as to why single ended tube amplifiers sound so good? Perhaps the “Color” is providing the fullness and richness; keeping the amplifier from sounding sterile or strident. While at the same time, the discordant “TOOD” numbers are far less than 1%.
So what do people think? Am I on to something here?
As always, questions and comments are welcome.