In the audio engineering world, it is widely debated if tube preamplifiers sound better than those designed around the transistor, also known as solid-state amplifiers. These terms refer to the device in the preamplifier’s topology, which is employed to amplify the signal. The common claim regarding the difference between valves and solid-state amplifiers is that valves exhibit “good” even-order harmonic distortion, the same as a musical instrument. It is also believed that solid-state devices exhibit “bad” odd-order harmonic distortion. The fact is that harmonic distortion is generally considered bad and unpleasant to the sound of a preamplifier’s sound and must be below acceptable levels. For instance, a good level would be below .1% or even .01%. This distortion is nearly inaudible to the human ear.
The reality is that bad harmonics are not necessarily even or odd but occur the further you get from the fundamental frequency. The real true advantage of valves over solid-state devices happens at the onset of the distortion of the signal. As a valve is driven, the onset of distortion occurs earlier and gradually, whereas, with transistor devices, it sets in abruptly and harshly. This concept is very similar to the main difference between analog and digital devices and their headroom. It is also known as “soft clipping.” This trait is important when over-driving tubes for guitar amps but is less important in microphone preamplifiers.
Another factor to the myth that tubes sound better than solid-state devices could have been the fact that many transistor designs used Class A/B or B circuit topologies. Older tube amplifiers only utilized class A circuits. Class A circuits do not exhibit the crossover distortion found in A/B and B topologies and therefore are considered more musical and pleasing to the ear. This lack of crossover distortion is particularly important for quiet signals, but that’s another topic. The point is that audio engineers caught on to this and made a move back to recording with early discrete class-A designs like the Quad 8 consoles preamplifier. We know that the Quad 8 was really just an Electrodyne recording console re-branded due to the fact that preamplifiers topology was recreated with discrete circuits instead of integrated circuits, which in turn could be another reason very early solid state designs were shunned.
Tubes and transistors each have their disadvantages and advantages. Transistors are cost–size effective; they do not fail as often and don’t require a warm-up time. In addition, modern advances are currently creating transistors that can exhibit tube-like sounds. The bottom line is the overall topology of the preamplifier circuit is more important than the sound of the amplifying device by itself. In other words, tube vs. transistor is much less important than, say, tube vs. tube or transistor vs. transistor.