In the "glory days" of HiFi, between the 1980s and 90s, HiFi manufacturers were in a fierce competition to produce audio equipment with the lowest possible THD, a measure of performance that quantifies how much an amplifier alters the sound from its original source. Lower THD was widely marketed as a clear indicator of superior audio quality, pushing manufacturers to innovate aggressively in this area.
To achieve incredibly low THD figures, most engineers relied heavily on negative feedback, a technique in which a portion of the output is fed back into the input to cancel out errors and reduce distortions.
While negative feedback succeeded in reducing THD, it introduced other distortions like Transient Intermodulation Distortion (TIM) and poor slew rates. TIM occurs when an amplifier cannot accurately reproduce the input signal at the speed of transient changes in an audio signal, leading to a loss of detail and clarity. Additionally, excessive negative feedback can slow down the response time of an amplifier (slew rate), affecting its ability to respond quickly to rapid changes in the input signal, which is crucial for performance in a high-end system.
The focus on lowering THD at the expense of other important factors like TIM and slew rate led to a generation of HiFi equipment that, while boasting low distortion figures, did not necessarily deliver the best sound quality in practical terms. In fact, it led to a grouping of equipment that today sounds 2-dimensional and uninteresting—sometimes painful.
We know that THD levels below 0.01% are not audible. What can be audible is the methods and technologies used to get better results.
This doesn't automatically mean products with lower THD are worse. Quite the opposite. What it means is designers must be conscious of side effects. That to strive for one impressive measure must always be tempered with all the other impacts of the design.
And this requires designers to actually listen to what they create.
