Here are some possible reasons. There are differing theories, thoughts; apparently, some people honestly can not hear the difference. But, some can.
The purpose and goal of semiconductors was always primarily cheapness (to make and operate) and also high-frequency operation — sound quality was never a goal or purpose.
If but at a microscopic level, the semiconductor atomic lattice inherently imparts a ‘digitizing’ quality having discrete steps of valence charge (theoretically). The charges do not move smoothly through analog transitions but rather step or lurch into and out of comparatively rigid positions.
The narrow and shallow channels of infused impurity/dopant deposition inherently favor high-frequency operation.
What is the chief complaint of “transistorized” sound? Harsh and unnatural-sounding high-end. Is such a consequently expected match merely a total coincidence?
Moreover, any collision into supply rails is almost always made audibly far worse by the extremely high (and insurmountably time/phase-lagging) negative-feedback present to tame semiconductive amplification, notorious for its proclivity of collapse into uncontrolled oscillation.
From 21:30 into video
“Calvin Fuller … pioneered the use of diffusion…to impregnate a very narrow layer [of dopant] on the surface of silicon and germanium wafers … By making very narrow layers you [can] get the very high frequencies. The frequency [attainable] is roughly inversely proportional to the thickness of the base layer…a micron or two [microns 10^-6] thick…[was already possible] in 1955.” Atom thickness is generally thought to be 10^-10, four orders of magnitude less, and the above-all-else effort to attain toward this narrowness/shallowness has never abated.
https://www.youtube.com/watch?v=WpyUf3kmtkg
Semiconductors, when used in audio circuits, can introduce certain distortions and artifacts that may be perceived as unpleasant or undesirable in audio signals. Here are some reasons why semiconductors can sound unpleasant when used at audio frequencies:
1. **Nonlinearities**: Semiconductors exhibit nonlinear behavior, meaning that their output is not directly proportional to their input. In audio circuits, nonlinearities can introduce harmonic distortion, which adds additional frequencies to the original signal. While some degree of harmonic distortion can enhance the perceived warmth or richness of audio signals, excessive distortion can sound harsh or unpleasant.
2. **Transient Response**: Semiconductors may have slower transient response compared to other electronic components, such as vacuum tubes. This slower response time can affect the accuracy and fidelity of fast-changing audio signals, leading to smearing or blurring of transients, such as sharp attacks or percussive sounds.
3. **High-Frequency Roll-Off**: Some semiconductor devices, such as operational amplifiers (op-amps), may exhibit high-frequency roll-off characteristics, meaning that they attenuate high-frequency components of audio signals. This can result in a loss of detail and clarity in the higher frequency range, making the audio sound dull or muffled.
4. **Noise**: Semiconductors can introduce noise into audio signals due to thermal fluctuations, electronic interference, or other factors. While some amount of noise is inevitable in electronic circuits, excessive noise can degrade the signal-to-noise ratio and reduce the overall quality of the audio.
5. **Biasing and Operating Conditions**: The performance of semiconductor devices in audio circuits can be sensitive to biasing and operating conditions, such as voltage levels, temperature, and impedance matching. Improper biasing or operating conditions can lead to distortion, instability, or other undesirable effects in the audio signal.
6. **Quality of Components and Circuit Design**: The quality of semiconductor components, as well as the design of the audio circuit itself, can significantly impact the sound quality. High-quality components and well-designed circuits can minimize distortions and artifacts, resulting in cleaner, more natural-sounding audio.
Overall, while semiconductors are widely used in audio equipment due to their cost-effectiveness, versatility, and reliability, their inherent characteristics can introduce certain sonic characteristics that may be perceived as unpleasant or unnatural in audio signals. However, with careful selection of components and thoughtful circuit design, it is possible to mitigate these effects and achieve high-quality audio reproduction using semiconductor devices.