To understand the dichotomy in tube versus solid state one must start forty or more years ago. Back then virtually all solid state was lean, trebly, and hard sounding. Tube equipment was warm and rolled off the treble and was wooly in the bass.
This statement is false. 65 years ago Harmon Kardon made the Citation 2 and a few years later the Citation 5, both of which had 100KHz response. The ’softness’ in the high end of tubes is correct. The brightness of solid state isn't- and is caused by distortion.
Generally speaking there has been a failure in the industry to recognize that the ear converts distortion of any kind into tonality: hence the 2nd harmonic imparts ’warmth’ and unmasked higher ordered harmonics as often experienced in solid state gear imparts harshness and brightness. The other inconvenient truth about the ear is that it uses the higher ordered harmonics to sense sound pressure.
These harmonics also have a lot to say about how instruments sound; if additional harmonics are present the ear will perceive things as being brighter than real life.
Tube amps generally have more higher ordered harmonic content than solid state amps do, but because of the ear’s masking principle (where a louder sound masks the presence of a quieter sound) their prodigious 2nd and 3rd harmonic mask the higher orders, allowing the amp to sound nice and smooth.
In case its not obvious the primary sonic differences between tubes and solid state, their ’sonic signature’, is really their ’distortion signature’.
If you could build a solid state amp with the same distortion signature as a tube amp, it would sound like a tube amp. That sort of thing is really hard to do with conventional A/AB solid state amps. But apparently it is possible with class D, since in some class D amps the things that cause distortion tend to create lower ordered harmonics (not talking about all class D amps, as they seem to have more variability in sound than tube amps do).
FWIW dept.: triode tubes are the most linear amplification devices known. So you might wonder what it is that tube amps tend to have more distortion. The answer is that tube amps tend to run less feedback as they have less overall gain, being a far simpler design. If you were able to have enough gain and therefore be able to run enough feedback the tube amp would sound just like a solid state amp.
Feedback is playing an enormous role!! One thing that limits feedback in conventional solid state amps and all tube amps is something called ’phase margin’ where if too much feedback is applied, at some frequency due to phase shift in the circuit the feedback is positive rather than negative. If this frequency is exceeded (for example due to harmonics caused by distortion) the phase margin is exceeded and so the amp goes into oscillation.
Another problem faced by all tube amps and most solid state amps is that of having enough gain and bandwidth to support the feedback. If it does not, feedback decreases with frequency at what is effectively a turnover point, usually at a higher frequency. This causes distortion to rise with frequency which is a recipe for harshness and brightness.
In addition, feedback is usually applied to a non-linear point in the amplifier, possibly the cathode of the input tube or the base of a transistor paired with an input transistor. This means the feedback signal is distorted before it can do its job, and so will add distortion of its own. It is for this reason the feedback has got a bad rap in high end audio.
Class D does offer some ways around these issues. Because it is very easy to get a high gain/bandwidth product, feedback can be supported at all audio frequencies, preventing brightness and harshness. If the amp is of the ’self-oscillating’ variety, you can add so much feedback that the amp goes into oscillation as soon as its turned on. The oscillation is then used as the switching frequency. By this method you can run quite a lot more feedback without it causing so many problems per conventional designs.
As a result you can have a class D amp that is as smooth as any tube amp, but with the greater neutrality (otherwise, without the brightness and harshness) of traditional solid state- best of both worlds.
