The one key advantage of class A topology is high fidelity, ie the best sonic performance.
Basics
Class A amplifiers deliver the smoothest, most euphonic sonic performance and the lowest objectionable distortion. That’s why class A is used in high-end designs despite its shortcomings and why the very best amplifiers are class A, where cost is no object. There is no disputing this, nobody with any education or understanding of amplifier design would argue this and so that’s an important point to note.
But Mike, I heard a cheap Chinese class-A tube amplifier and it sounded like sh%t!
Typical enquirer
I said that class A is the best design where cost is no option. This immediately excludes the myriad of cheap, low-quality designs, poorly built and filled with bad parts flooding the market. These sound bad, no matter what amplifier class they are, because garbage is garbage.
Compromises
There are no free lunches though and, as per my discussions on class AB, class B and class D, each mode has its advantages and drawbacks. Class A amplifiers draw and dissipate their maximum rated power, whether playing music or just idling. Like a car engine running at the redline, things run hot and wear out faster at full power, there is no avoiding this.
This is all well and good where the required power output is very small, such as a headphone amplifier or preamplifier like my Accuphase C-280V, which operates in class A, but at low power. Dissipating such low power is relatively easy. Big amplifiers running in class A have to be large and very substantially built to cope with this constant high power demand, 100% duty cycle and high heat dissipation.
As a result, class A amplifiers are big, heavy and need excellent ventilation. This means that some environments won’t be well suited to class A amplifiers. My power amplifier delivers only about 10 Watts per channel of class A and the Liquid Audio listening room gets a bit warm in summer, let me tell you! Good A/C is essential when running class A amplification.
Bottom Line
With these overbuilt designs come advantages though, like massive power supplies and premium, heavy-duty parts that can cope with the load. These build factors also deliver sonic advantages of their own, so class-A amplifiers tend to just be better all around.
So, the disadvantages of class-A amplifiers such as high cost, constant high power dissipation, electricity bills, carbon footprint, size and weight must also be taken into account. Class A amplifiers have the highest cost per Watt, so cheap consumer designs avoid the use of class A.
For the best sound, whether it be using tubes or transistors, class A is the gold standard.
