I agree with Kevemaher, tube preamplifiers can be as distortion free (as tested by listening) as solid state, and where a solid state amplifier may measure lower, it is not a difference we can hear. So when tube preamplifiers sound different from solid state, it is not due to THD differences. The last 30 years of audio "science" makes this pretty clear. And there are a lot of decent theories out there to explain the observation. First of all, THD is not the culprit. What adds audible harmonic distortion in a tube amplifier, which may be pleasant or euphonic, is the output transformer, phase splitting, etc. The industry was pretty successful for a long while back in the late 20th century in convincing audiophiles of the inherent superiority of SS, but it just ain't so.
Rainy Day Musings
It is a dark and rainy afternoon in N. California. Satiated with tunes, Snoopy's mind wanders........to distortion in preamps.
I have recently transitioned to tube phono and preamps. I had used a ML #38s for several decades and some solid state phono preamps. I was quite pleasantly overcome with how lively, detailed, warm, human the sound coming from my speakers have become with tube gear
The reason? I don't know. I've thought a quite bit about this, but have no conclusions to offer. However, I do have some ideas about what doesn't appear to be as big a problem as some would opine.
I have become quite facile in using REW to measure the sonic behavior of my system. REW can also used to measure purely electrical signals from any component.
Aside: it is my impression that the particular "sound" of an instrument is due to the spectrum of its overtones (not distortion, but modes of the instrument). If each instrument could only create a single frequency, then one could not tell the difference between instruments. I assume that electric guitars also mimic the overtones (and are sometimes deliberately driven into massive distortion).
Electronic components are designed expressly to not exhibit modal behavior. When excited by a single frequency, components are designed to reproduce that frequency and only that frequency (within stated limits).
When an electronic component creates other frequencies (almost always higher multiples of the exciting frequency), those other frequencies are called distortion. This clearly is a simplification as sum and difference frequencies can also be excited).
These distortions occur at the same frequencies as the overtones in the musical instrument (or artificially (digital or analog) created instrument for that matter). These overtones are present in the musical signal throughout the audio system.
My conundrum is that although one wants lots of the former (overtones) and none of the latter (distortion) and they occur at the same frequencies, how can one distinguish one from the other? Do overtones swamp the distortion that exists or are the distortions so high that they dominate? I really don't know since I am not familiar with the detailed characteristics of musical instruments.
My phono circuit is all tubes (phono and preamp). I routinely measure less than 0.01% from those components (Paragon "E" (phono), ARC SP-6B (phono) and BAT VK-33SE preamp). The ARC SP-6B even has an adjustment to make the distortion in the phono section smaller (I have seen 0.0001% distortion during my adjustments). This distortion value is not as low as that achieved with well designed solid state equipment. However, this difference is inaudible to me. Far greater is the noise and distortion from the vinyl pressings themselves and the rising noise from 1KHz down to 20Hz one sees due to the application of the RIAA curve in the circuit. Noise is also not a problem for listening to my system. I only begin to hear noise in a speaker when I am less than 6" away when the gain set at normal listening level.
From this I must conclude that tube preamps designed to have low distortion (less than 0.1% or ???) are indistinguishable from solid state components in this category. Tube preamps have drawbacks. They run hot, are not energy efficient, need care and replacement for tubes that wear out, and need 10s of minutes to achieve stability. These seem to be better criteria for tube/solid state decisions, not distortion.
Power amps are a different. I don't fully understand what's going on, but I can see where there may be significant challenges, especially matching to a speaker. Seems to me that this is one aspect where solid state has an definite advantage. I use an ML No. 27 power amp.
I really would appreciate comments, especially critical ones. Tell me what I'm missing.
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@OP - here's an accessible piece that might be helpful to understanding how transformers can affect sound. https://korneffaudio.com/what-the-heck-is-up-with-transformers/ Well designed output transformers can eliminate a lot of the hysteresis and saturation effects referred to in the link. However, there is also the fact that tube power amplifiers tend to have relatively high output impedance, the interaction of which with the speakers impedance curve causes non linear frequency variations. But tubes themselves also cause distortion - see this link: https://www.analogethos.com/post/understanding-distortion In addition, tubes are are more susceptible to microphony than solid state devices. Lastly there is the matter of signal to noise ratio. John Atkinson's measurements of various SS and Tube power amps covers this topic pretty well.
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