Full detailed sound at 30 - 40 - 50 dB

But if the total is low, the components must be even lower (or theoretically at most equal).

@willemj The problem is that the ear does not treat all harmonics on equal ground. It is far more sensitive to the higher orders! Think log scale...

We've known this for the last 80 years (see Radiotron Designer's Handbook, 3rd edition) and I suppose I should not be surprised that such knowledge continues to be ignored.

Although traditional solid state amps do have overall lower distortion, the distortion that they **do** have is the higher orders, and higher than you see in most tube amps, hence 'brightness' and 'harshness' attributed to them. If you are not aware of this already, the ear converts all forms of distortion into tonality.

The other complication is Fletcher-Munson, the fact that our ears are tuned to birdsong frequencies (especially at low volume levels) which makes it all that much easier to hear the distortion of solid state amps at low volume levels.

But if the total is low, the components must be even lower (or theoretically at most equal).

@willemj  When you have really low numbers typical of traditional solid state amps, the problem is that the higher ordered harmonics are simply not as low as they should be, while the lower harmonics are indeed suppressed. It cannot be denied that this is a coloration!

This results in the amp being brighter and harsher than real life, even though on the bench the amp measures perfectly flat. Our ears are very sensitive to this distortion spectra, and we've known this for the last 80 years(!) or so (see Radiotron Designer's Handbook, 3rd edition).

I suppose I should not be amazed that basic physiological information about how we hear is ignored on an ongoing basis if the last 8 decades are any indication.

Other than subjective listening, how might a customer objectively derive from specifications alone at which harmonics the distortion is occurring?

That is a good question. You can get some of that from the spectral analysis. this can be tricky- I've seen some of this done on our amps, but the methodology is was incorrect (one speaker terminal got accidentally grounded by the test equipment) so the readings were off. If the measurement technique doesn't introduce problems, this can be one way of sorting things out. 

@atmasphere maybe off topic but do you have a suggestion for a preamp to use with a passlab xa30.5 that will be used with 96db sensitive speakers?

Sure- one of our preamps will do that nicely :) (they're balanced).
The Pass amps are pretty easy to drive though- I would expect it to work with many different preamps.

Which First Watt would you recommend to the owner of high efficiency speakers (101db, 16 Ohm) ? Currently using push-pull tube amp designed (OEM) by Trafomatic Audio for WLM Acoustic back in the days.

A lot depends on your preamp as the single gain stage units need more drive.  I'd be looking at one of the 20 or 25W units since they will be making less power than that into 16 ohms.

FWIW a speaker like that is ideal for tubes. We make a 30-watt/ch OTL that would work great on that.

Also rather interesting that you should point out the relationship distortion has with harmonics and how you consider "weighting".

There is always more to it...

We all know about Fletcher-Munson. The Fletcher-Munson curves show where our ears are most sensitive, which is birdsong frequencies. This is because birds are the first indication of a predator in the vicinity and their calls activate other birds to spread the alarm. It does not matter if you are predator or prey, if you can't hear birdsong you're likely hungry or eaten.

The harmonics of a good number of instruments happen to fall into this region which is up to 7KHz. So the instruments that are making the most energy in music have harmonics that fall in the range to which our ears are most sensitive. This is likely not coincidence, but it presents a daunting challenge to engineers if the goal is to get audio systems to sound like real music and not an audio system! 

Traditional engineering tends to ignore this, although we've known the Fletcher-Munson curves for decades.

One example of this is the use of loop feedback in amp and preamplifier designs. Feedback is known to bifurcate the input signal and thus generate harmonics of its own while also suppressing distortion. For this reason feedback usually contributes to brightness in all amps in which it is used; this brightness is caused by distortion rather than frequency response error. Our ears use the higher ordered harmonics to sense sound pressure, so any addition to these harmonics (or their presence where none was before) is easily heard, despite not being all that easy to measure.

The audio industry is a long ways from acknowledging this fact so we can expect amps to have a tendency to brightness for some time to come. One of the few areas where there are deviations from this norm is in high end audio. Its a weird world...

Given your statement, at what decibel level do you consider the volume to be something "higher" than lower volume? The essence of the question is at what power level do tubes no longer "rule" over SS because of power concerns. Thank you.

Its not so much a decibel thing as it is a distortion thing.

Take a look at the distortion curves of a typical solid state amp. At a certain lower power level, the distortion is at its minimum and as you go to lower levels the distortion can be significantly higher! This is not a given with tube amps, even push-pull amps (our amps are push-pull but distortion linearly decreases to unmeasurable as power is decreased).

So that power level is about 5-7% of full power with most solid state amps. If you've ever heard that bit about the 'first watt' being the most important, that's because its all about distortion (or in this case, the lack of it). BTW, Nelson Pass has a set of solid state amps called the 'First Watt' amps and they are so-named because they diverge from the usual solid state amps in that they too are very low distortion at low power levels. Their designs are also quite minimalist (even simpler than and not unlike tube amps in that regard) although they don't make a lot of power.

If you want good resolution so you hear everything (including good bass) at lower levels, this is what you have to do: tubes or a low powered solid state amp like a First Watt.

Now to the latter portion of the question- even at high power levels, the presentation continues to be all about distortion (or the lack of it). The thing is, the ear weights certain forms of distortion over other forms. What I mean by this is most people understand that the ear hears sound pressure on a logarithmic curve. What is considerably less understood is that the ear also detects harmonics on something that looks like a log curve. It is far more sensitive to higher orders (in particular the odd orders) than it is to the lower orders like the 2nd or 3rd harmonic. For this reason, we tolerate large amounts of lower ordered harmonic easily, but don't tolerate higher ordered harmonics well at all.

Audiophiles have terms for trace amounts of higher ordered harmonics; 'bright' and 'harsh' are common examples. This continues to be the big difference between tubes and solid state with regards to distortion! Now think about that weighting issue I mentioned: essentially, solid state amps have much **greater** amounts of distortion if you weight the distortion according to how the ear/brain system responds to it (all forms of distortion are converted to a tonality of some sort BTW). So a 7th harmonic at 0.005% is a much bigger sin in terms of coloration than a 2nd harmonic at 1%. Its a bit of an inconvenient truth.

Our measurement and testing regime is a good 40-50 years behind our understanding of physiology at this point, which is why there is still a tubes/transistors debate going on in high end audio.

If you are wanting full detail at lower volumes, this is an area where tubes rule. This is because they tend to have a lot lower distortion at lower power levels. Something to think about...