Wednesday, March 14, 2012

tubes vs. transistors: is there an audible difference?

milbert |ABSTRACT - Engineers and musicians have long debated the question of tube sound versus transistor sound. Previous attempts to measure this difference have always assumed linear operation of the test amplifier. This conventional method of frequency response, distortion and noise measurement has shown that no significant difference exists. This paper, however, points out that amplifiers are often severely overloaded by signal transients (THD 30%). Under this condition there is a major difference in the harmonic distortion components of the amplified signal, with tubes, transistors, and operational amplifiers separating into distinct groups.

As recording engineers we became directly involved with the tube sound versus transistor sound controversy as it related to pop recording. The difference became markedly noticeable as more solid-state consoles made their appearance. Of course there are so many sound problems related to studio acoustics that electronic problems are generally considered the least of one's worries. After acoustically rebuilding several studios, however, we began to question just how much of a role acoustics played.

During one session in a studio notorious for bad sound we plugged the microphones into Ampex portable mixers instead of the regular console. The change in sound quality was nothing short of incredible. All the acoustic changes we had made in that studio never had brought about the vast improvement in the sound that a single change in electronics had. Over a period of several years we continued this rather informal investigation of the electronic sound problem. In the past, we have heard many widely varied theories that explain the problem, but no one, however, could actually measure it in meaningful terms.

Anyone who listens to phonograph records closely can tell that tubes sound different from transistors. Defining what this difference is, however, is a complex psychoacoustical problem. Any investigation of this admittedly subtle phenomenon must really begin with a few human observations. Some people try to point out and describe valid differences. Others just object to the entire thesis and resort to spouting opinions. It is the listener's job to sort out the facts from the fiction.

Electrical engineers, especially the ones who design recording equipment, can prove that there is no difference in tube or transistor sound. They do this by showing the latest specification sheets and quoting electronic figures which are visually quite impressive. It is true, according to the parameters being measured, that there is only a marginal difference in the signal quality. But are there some important parameters which are not being measured? One engineer who admits that there might be some marginal difference in the sound, says, "You just have to get used to the nice clean sound of transistors. What you've been listening to on tubes is a lot of distortion." Of course the question which comes to mind is: What is this distortion and how is it measured?

Psychoacoustically, musicians make more objective subjects than engineers. While their terms may not be expressed in standard units, the musician's "by ear" measuring technique seems quite valid. Consider the possibility that the ear's response may be quite different than an oscilloscope's.

"Tube records have more bass....The bass actually sounds an octave lower," says one rock guitarist. A couple of professional studio players have pointed out on numerous occasions that the middle range of tube recordings is very clear, each instrument has presence, even at very low playback levels. Transistor recordings tend to emphasize the sibilants and cymbals, especially at low levels. "Transistor recordings are very clean but they lack the 'air' of a good tube recording." "With tubes there is a space between the instruments even when they play loud...transistors make a lot of buzzing." Two people commented that transistors added a lot of musically unrelated harmonics or white noise, especially on attack transients. This same phenomenon was expressed by another person as a "shattered glass" sound that restricted the dynamics. It was generally agreed that tubes did not have this problem because they overload gently. Finally, according to one record producer, "Transistor records sound restricted like they're under a blanket. Tube records jump out of the speaker at you....Transistors have highs and lows but there is no punch to the sound."

When we heard an unusually loud and clear popular-music studio recording, we tried to trace its origin. In almost every case we found that the recording console had vacuum-tube preamplifiers. We are specific in mentioning preamplifiers because in many cases we found hybrid systems. Typically this is a three- or four-track console that is modified with solid-state line amplifiers to feed a solid-state eight- or sixteen-track tape machine. Our extensive checking has indicated only two areas where vacuum-tube circuitry makes a definite audible difference in the sound quality: microphone preamplifiers and power amplifiers driving speakers or disc cutters. Both are applications where there is a mechanical-electrical interface.

As the preliminary basis for our further investigation we decided to look into microphone and preamplifier signal levels under actual studio operating conditions. Hoping to find some clues here we would then try to carry this work further and relate electrical operating conditions to acoustically subjective sound colorations. Our search through published literature showed that little work bas been undertaken in this area. Most microphone manufacturers publish extensive data on output levels under standard test conditions [1], but this is rather hard to convert to terms of microphone distances and playing volumes. Preamplifier circuit design is well covered for noise considerations [2], but not from the standpoint of actual microphone operating levels. Distortion has been treated in numerous ways [3-5], but with very few references to musical sound quality [10].


umbrarchist said...

WOW!  That brings back memories of old hi-fi days.

John Kurman said...

I've described the word "timber" as the shape of your voice, which makes sense to me, since the lungs, chest, neck, sinuses and head influence sound as much as the larynx does. As such, the timber of tubes is to transistors the way firelight is to fluorescents (IMO). Of course, tubes are still used in the big radio stations, above 10,000 watts. Which just goes to show you, vacuum tubes, buggy whips, what have you, nothing ever goes away. Why, you can even still buy flint knapping kits nowadays.

Felix Vaughn Cheyne said...

Fascinating article, BTW. TYVM. There is a pioneering (but quite informal and subjective) blind study done by some record producers in, oh, perhaps 1973 (with which you seem to be familiar) which is quite illuminating in terms of elucidating the differences AT THE TIME of tube, transistors as discrete components, and IC designs as far as -IIRC- cutting-head lathes for records, and they determined that tubes (for that specific purpose) had a greater dynamic range and were "louder"; that discrete component machines were clear present and "3-dimensional"; and that IC designs made the sound flat and tinny. Altogether they preferred the discrete component transistor sound.

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