Ken Rockwell recently said as much in this article about audio tubes:
http://kenrockwell.com/audio/why-tubes-sound-better.htm
Ken Rockwell is an electrical engineer and an AES member, but sadly he suffers from
Kenrockwellitis: Posting hastily, presenting a simplified, easy-to-read, entertaining view which may not be totally true. Don't get me wrong, i like his page, and he perhaps has some of the best lens reviews on the 'net.
From his article i can comment:
"Tube amplifiers measure poorly in the lab specifically because of these added distortions, but these distortions are often a part of what make them sound better."
They actually don't measure "poorly", this only depends on the kind of measurement. When testing for intermodulation distortion and crossover artifacts, which are really serious problems, under important conditions (i.e. actual power used at listening levels), good quality tube equipment tests just fine. Also, they present a limited series of harmonics being added to the music, in stark difference to most solid state amps. In this respect they can measure better.
"Ditto for guitar amplifiers used in creating music. The ways that tubes distort when pushed to the edge are much more musical than the artificial sounds that come from transistor amplifiers when overdriven. "
This is a different use of tube amplifiers --
intentional distortion and coloration.
"Tube amplifiers have much more distortion than solid-state amplifiers, but most of it is second-order, which is quite musical. That's why it's called "harmonic" distortion."
Actually, second-order distortion is not so musical once added to certain kinds of music like complex choral music, etc. Which is, coincidently, the music used for giving a serious listening test to an audio system.
The difference between a good tube amp and not-so-good transistor amp, is that the tube amp would perhaps only add 2nd and 3rd harmonics to the signal, while a cheap transistor amp would add also the 4th, 5th, 6th and so on and so on. Which impacts the sound. Therein lies one important difference.
"Even-order harmonic distortion can be so pleasant that back in the 1970s the Aphex Aural Exciter was very popular in recording and broadcast specifically because it was designed to generate and add these harmonic distortions!"
... and other people passionately hate the Aphex Aural Exciter being added to recordings.
"Tube amplifiers overload gradually. Add more input and they distort more, but there is no precise level above or below which they suddenly start to clip.Contrast this to solid state, where there is a very definite clipping point."
Yes, but one buys a transistor amplifier of sufficient power to never, ever, ever, get close to clipping. Also, power is cheaper with transistor amps.
"In contrast, because of the much higher voltages and lower capacitance values needed, tube amplifiers almost always use much better film (or 50 years ago, paper) capacitors for audio coupling."
Good quality transistor amps also use higher quality capacitors. I'm surprised that an Electrical Engineer would use such a cheap argument. Again, it's just
Kenrockwellitis interfering with Ken Rockwell's vast knowledge of electronics.
"Solid-state equipment usually uses polarized electrolytic capacitors to decouple DC, and that's bad."
Solid state amps can be (and ARE routinely) designed so they don't require coupling capacitors at all!! Using electrolytic coupling capacitors in the signal path is so 1970s... Or they can use them, and use higher quality caps. Which should happen on an audiophile solid state amp.
"Tube power amplifiers have higher output source impedances (...) It will tend to lower the low-frequency damping, which can have the effect of blooming the bass, which some may prefer."
Or some others may not prefer at all, because this, by definition, is getting non-accurate bass response.
At the end KR mentions a blind listening test where the audience couldn't reliably identify a tube vs solid state amp. I think this is the really important part of KR's post. Basically, if one wants to design a really high quality amplifier, he/she can do it with tubes or with transistors, no problem with either one!
