Until fairly recently, the audio power amplifier was almost as synonymous for excessive heat and weight as for its primary application. This reputation was the result of early inefficient output stages.
Early designs dissipated most of the power drawn from the wall as heat, which then had to be sucked away from internal electronics with large metal heat sinks.
Amplifiers have never enjoyed a reputation for being particularly sexy, being largely black boxes sporting a couple of big rotary dials and an LED or two, and mounted in racks that are usually concealed from audience view.
But now we see a different story. The brutes of the past have been tamed, exceedingly efficient and delivering far more audio power with far less heat and weight.
Amplifier design topology has evolved from Class A to several advanced topologies:
Class D uses an on-off switching method for its transistors called Pulse Width Modulation. Because its output devices are either on or off, the efficiency of the amplifier it greatly increased, as very little power is lost to heat. Class D designs do this without jeopardizing the integrity of the audio waveform by switching positive and negative output transistors on and off many times per waveform cycle.
It’s analog but similar in theory to digital sampling, where a 44.1 kHz sampling rate is used to accurately capture a 20 kHz signal. This rapid switching creates a square wave that is then low-pass filtered to recreate the audio waveform.
Class G takes a post-AB Class signal and switches it between two power supplies rather than switching between positive and negative transistors in the amplifier’s output stage. One supply is for softer output levels and the other for louder. Efficiency improves as the power amplifier draws full power from the wall when higher a level of amplification is needed.
Class H also works on the two-power supply output concept but is a bit more precise in terms of what AC power it draws, and when. Instead of simply switching between lower and higher voltage power supplies, the second power supply’s voltage level is controlled via the audio input signal. If the signal increases, so does the power supply’s voltage.
Class I, a topology design patented by Crown, is an advancement of the Class D switch-mode design. The risk in the purely Class D design is the potential for distortion in the moment that one transistor turns off and another turns on. If both transistors are momentarily off during the switch, audible distortion results. If both transistors are momentarily on during the switch, excess current can cause physical damage to the circuitry.
The big factor in the continued evolution of the power amplifier is multitasking, with most premium units now outfitted some kind of signal processing control, external or internal, providing EQ, filter, delay, compression, limiting, routing and monitoring in real time.
Let’s take a look at a wide range of recently debuted amplifier models and technologies. (Click on slideshow directly below.)