"Major Variola (ret)" <mv@cdc.gov> wrote:
Yes, in the linear part of their operation. But its the *distortion* (large signal behavior) which differs ---tubes distort differently when "overdriven". I believe the difference when driven with a square wave is that tubes have a more RC-like output function, vs. a sharper (faster slew) transistor reproduction.
It's true that tubes distort differently, but not for the reason you claim. If you compare the transfer characteristics (Vce vs Ic with Vbe as a parameter) of a tube to those of a transistor, you see that (using BJT terminology) the transistor passes through saturation and reaches linear operation at a relatively low Vce (triode/linear, saturation, and Vds respectively, for you FETholes). On the other hand, the tube does not saturate quickly in this way. If you draw a load line over these transfer characteristics and grind through the math (or have Matlab do the work), you'll see that BJTs and FETs produce mostly odd-order harmonic distortion, whereas tubes produce more even-order harmonic distortion. The audible difference in this distortion is consistent with empirical results from acoustic musical instruments: violins (and other stringed acoustic instruments), which produce mostly even-order harmonics, have a drastically different timbre from, say, a clarinet, which produces mostly odd-order harmonics.
One little known fact is that humans actually prefer a small amount of distortion in their listening. The THD of amps with a lot of decimal-zeroes, is a good technical spec (easily attainable, cheaply, nowadays), but is totally a marketing scam. First you can't hear the difference between .01 and .001 % THD, and second you prefer ~ .1%
While I'm sure you're right that most people can't tell the difference between .01% and .001%, I'm not sure I believe that _everyone_ likes distortion (or, if they do, that everyone likes distortion of the same kind or level.) It seems that when you _re_produce a recording you may as well do so with as much fidelity as possible, leaving it to the artist to introduce whatever distortion they intend when they record it. When I listen to Ormandy and Philadelphia perform Mahler 2 I want the utmost fidelity; at the same time, I don't complain about the distortion in the recording when I listen to the Smashing Pumpkins's latest album. -- Riad Wahby rsw@jfet.org MIT VI-2/A 2002