As We See It

In the 1952 edition of the Radiotron Designer's Handbook, long recognized as the "bible" of the industry, the permissible level of IM distortion for a high-fidelity amplifier was given as 3%, with the alternate figure of 2% being cited as a "rather extreme" specification. We wonder what the author of that statement would think of today's solid-state amplifiers with their measured IM of 0.01% and less. And we wonder what he would think about the fact that these super-amplifiers still have audible distortion.

The Radiotron Handbook was written during the heyday of the tube, but even in the latter days of that era there were people who maintained that triode output tubes sounded "better" than pentodes (or, rather, than tetrodes). Distortion measurements showed that neither was categorically better than the other, yet the triode-vs-tetrode controversy persisted until solid-state laid the whole issue to rest.

It was soon observed, though, that all solid-state components had their own characteristic sound. They were sharp, crisp, and quite brittle-sounding—fine for reproducing "hard" transients like triangles and xylophones, but not so fine for massed violins, which became annoyingly shrill. By comparison, most tube equipment was rather "sweet" and slightly veiled, which made strings sound fine but took the edge off harder sounds. After the initial infatuation with "solid-state" wore off, it became obvious that the "crispness" of transistors was just as much a form of distortion as the softness of tubes, only different. So, subsequent designs aimed at lower and lower distortion figures until these were in some cases significantly lower than in the best of the tube-type amplifiers. Yet the characteristic difference was still there: Transistors sounded harder, tubes sounded "sweeter." And the astounding thing about it was that the amount of hardness from one solid-state component to another was audibly different when measured distortion figures differed by as little as 0.002%!

The usual IM and harmonic distortion tests do not actually measure an amplifier's distortion, they measure the effects of it—the modulation of treble by bass, the production of sum-and-difference tones, and the introduction of spurious harmonics. But the fact that such minuscule measured amounts of these effects are nonetheless audible through loudspeakers producing hundreds of times as much distortion as the amplifiers raises the question of whether there might not be a different effect occurring simultaneously, one to which the ear is more sensitive than those that we measure. We don't believe, though, that the answer is quite that abstruse.

When an amplifier is adding harmonics, they are not likely to be equally strong. For example, the strongest harmonic from a single-tube amplifying stage will be the second. The third will be weaker, the fourth weaker still, and higher-order harmonics will be weaker still until they die out. (Push-pull operation cancels most of the second harmonic, making the third one the strongest.) In transistors, however, the distribution of harmonic energy is entirely different. Here, the harmonics are likely to be of fairly equal strength all the way out to and even beyond the 10th. There is little tapering off in the higher-order harmonics, as in a tube.

A total-harmonic-distortion test (THD) measures mainly the strongest harmonic. Thus, for a tube-type power amplifier (with push-pull outputs), the reading would be a measure of its third-harmonic distortion. The weaker higher-order harmonics would not register at all. On a solid-state power amp, though, the THD reading (which might be numerically identical to that of the tubed amp) may be that of the second, third, fourth, or fifth harmonic, or that of all the harmonics to out beyond the 10th. Obviously, identical THD measurements do not assure identical distortion characteristics. And since it is a proven fact that the ear is much more offended by the higher-order (fifth, seventh, etc.) harmonics than by the lower ones, it would seem equally obvious that the ear is responding to these little details that the THD measurement overlooks.

This is borne out by subjective observation. In tube equipment, a measured THD of 0.05% through most of the audio range (that is, using test frequencies of 40, 1000, and 5000Hz as the fundamentals) was almost an ironclad guarantee of rich, sweet sound, although there may have been a barely perceptible "veil" over the sound due to the audibility of the lower-order second and third harmonics. Things didn't start to sound hard until THD rose to above about 0.2%. In solid-state equipment, the sound is annoyingly hard at 0.2% THD, and some hardness is still perceptible at 0.05% THD (footnote 1). The "veil" rarely becomes audible, because it is covered up by the screech. Hence, the transistor's reputation for "clearer" sound.

Dynaco's Dave Hafler (who supplied much of the info for this piece) suggests that the industry devise a way of "weighting" the distortion readings to favor the higher-order harmonics, as we now weight signal/noise ratio readings to favor that part of the spectrum where the ear is most sensitive. This seems like a fine idea, but the industry seems in no hurry to consider it. Heaven forbid that the consumer should be given a better way of distinguishing the good from the bad!—J. Gordon Holt

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Footnote 1: Stereophile's Test CD 2 has tracks where you can compare a pure tone with various amounts of second, third, and seventh harmonic distortion.—John Atkinson