Class A or Class D solid-state amplifiers (modern designs)

Actually, the 70s were the Spec Wars.  Objectivism ruled and high negative feedback gave vanishingly low THD and IM numbers.  However, the resulting sound quality got worse, not better.  TIM distortion was discovered, feedback was dialed back, and the importance of listening was once again proved.

Anyone remember the Leach amp, designed by Marshall Leach?  Another young amp designer to emerge at this time was Nelson Pass.  His Threshold CAS amps were the result of a design exercise in minimizing the need for feedback by optimizing the circuit's inherent linearity.  While he was already producing STASIS amps, the CAS line brought his work into a more affordable range.  In many respects, they're the progenitors of First Watt.

@atmasphere 

My comment was about amp history.  I don't know when the cascode amp paper was written, but the Threshold CAS-1 & CAS-2 amps date from the early 80s.  I don't know the history of class D amps, but am not surprised a paper likely written ~40 years ago about audio amplification didn't include class D.

I do have a question for you, if you don't mind my asking.  As we all get older, we're discovering that parts inside our stereos age (and worse) as well.  What have you been able to glean about the long term reliability of class D amps, what goes wrong, and what is the result?  I know it's probably a lot of the same parts, but they're being used differently.

@atmasphere

"Lowering distortion in power circuits without compromising their transient response remains a primary problem for designers of audio power amplifiers. Until fairly recently, the favorite technique for removing distortion components in linear amplifiers was to cascade many gain stages to form a circuit having enormous amounts of gain and then using negative feedback to control the system and correct for the many errors introduced by this large number of components.

While the sum of these components’ distortions may cause large complex nonlinearities, the correspondingly large amounts of feedback applied are generally more than equal to the task of cleaning up the performance with only one trade-off—the high frequency performance of the system. Because each amplifying device also contributes its own high frequency roll-off, and because the sum of many of these roll-offs creates a complex, multi-pole phase lag, a system using large amounts of negative feedback tends to be unstable at high frequencies, resulting in phenomena popularly referred to as Transient Intermodulation Distortion (TIM). As this phenomena has been well described elsewhere, it will be sufficient here to point out that two solutions to TIM problems exist. The first solution is to not require any high frequency performance of the circuit, that is, not to feed it high frequency signals it cannot handle. While this solution works very well for many operational amplifier applications requiring only low frequency performance, it is judged to be unacceptable in high-fidelity applications where frequency response is required beyond 100 kiloHertz. Although human hearing is generally very poor above 20,000 Hertz, ultrasonic frequency roll-offs produce phase and amplitude effects in the audible region; for example, a single pole (6dB/octave) roll-off at 30 kHz produces about 9 phase lag and 0.5 dB loss at 10 kHz. The effects may be subtle, but their audibility is undesirable in a piece of equipment whose performance is judged by its neutrality.

Because of this bandwidth requirement, designers of state-of-the-art amplifiers are turning to the other solution; simple circuits having few amplifying devices and relatively low open loop gain. The simplicity and low gain allows the circuitry to respond to signals very quickly, thus eliminating transient problems, but it does so at the expense of higher harmonic and intermodulation distortions."

Nelson Pass, from "Cascode Amp Design"