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A bass guitarist’s obsession with audio power amplifiers: an overview of amplifier classes

Audio power amplifiers
Nerdacity

As I posted last time, I’ve been an electronics nerd since the 1960s. This post will give a 10,000′ view of audio power amplifier technology, and how it has changed over the decades.

In my childhood in the 1960s, vacuum tubes were the only practical devices for amplifying sound to high volumes. Early power transistors were too slow and too fragile. The first practical high power amps built from transistors hit the market in the late 1960s, led by the Crown DC300. Transistors, and the circuits built with them, have evolved rapidly since then, and now dominate audio amplification for all but a handful of applications.

There are two types of transistors in widespread use for audio: bipolar transistors and power MOSFETs, both made from silicon. As a general rule, bipolars are used in Class B and its derivatives, while MOSFETs are better at the high speed switching required by Class D. MOSFETs made from gallium nitride (GaN) instead of silicon offer even faster switching and lower losses, but the price is not yet low enough for widespread use.

Irrespective of the device used, the same operating principles apply to all amplifiers. The basis of all amplifier design is simply this: a device, whether a tube or transistor, can be applied as a smoothly controllable (linear) restriction to electrical flow, an on-off switch, or a linear restriction which can be completely turned off.

Knowing this, amplifiers can be divided into several classes, based on a small number of criteria: whether the device operates linearly or as a switch; if used linearly, whether it turns off completely for some fraction of the waveform cycle, and if so, for how much of the cycle. You’ll hear other classes mentioned, but they are all variations on, or combinations of, the classes listed here. A web search should turn up plenty of information on them.

  • Class A is the simplest, and regarded by many as the best sounding class. The amplifying device never fully turns off nor on, always operating in a linear fashion. Preamplifiers generally operate in Class A. For power amplifiers, though, Class A is impractically inefficient. Far more electricity is turned into heat than is sent to the speaker.
  • Class B is a huge step up in efficiency from Class A. Taking advantage of the fact that an audio waveform goes both positive and negative, Class B divides the work among pairs of devices. One device in each pair operates linearly for the positive half of the waveform and turns off for the negative half wave, the other device is linear for the negative half and turns off for the positive half. At a theoretical 70% maximum efficiency, but often much less efficient at lower power levels, Class B still turns a fair bit of electricity into heat. Class B and its derivatives – Class AB, Class G, Class H, et al – made up nearly 100% of all large audio power amps from its invention in the early 20th century until Class D became practical.
  • Class AB is a minor but very common variant of Class B, in which both positive and negative halves are active near zero, to reduce the glitch when one device turns off and the other turns on (known as crossover distortion). This is a difference of degree, rather than a qualitative difference. The increase in power consumption is negligible. Classes AB and B are nearly identical, and in practice, Class AB is the term used most often.
  • In Class C, the amplifying device is in its linear range for less than half of the wave and off for most of it. Useless for audio due to its extreme distortion, Class C is uniquely suited for radio transmitters, whose resonant circuits filter out the distortion quite effectively.
The inner workings of the Sinbosen K4-1400, a modern four-channel Class D amplifier capable of putting out 2200 W per channel.
  • Class D is the most efficient class used in audio. The amplifying device is switched on and off at a very high rate, and the resulting pulse train is run through a filter to smooth it out and turn it into audio. In theory Class D can be 100% efficient – that is, turning all of its input power to useful output – but in practice there are always some modest losses. Full-range Class D amplifiers were not practical until power MOSFETs became widely available. Small Class D amps have become ubiquitous in consumer devices, such as cellphones and car stereos, because they can be made very small, generate little heat, and sip battery power. At all but the lowest power levels, Class D requires careful filtering and shielding to prevent radio frequency interference (RFI). The improved efficiency more than makes up for the added cost. Variants on Class D include Class I, Class S, and Class T. Class D is widely, but inaccurately, referred to as “digital” in product literature.
  • Class H, a variant of Class B (AB), improves efficiency at lower output levels by switching the power supply on the amplifying devices between multiple voltages as needed. The disadvantages are that both the amplifier circuit and its power supply become more complex, and crossover distortion can occur at multiple points in the waveform, not just the zero crossing. Class H dominated the high power end of the audio amplifier market until large Class D amps became practical and reliable, and still makes up a significant fraction of the high power market.
One channel of a Sanway DP10Q, a Class TD amplifier. This one board can produce up to 2500 W!
  • Class TD is a clever but complex variant of Class H, originally invented and patented by Swedish manufacturer Lab Gruppen. When the patent lapsed some years back, a number of Chinese manufacturers started selling large power amps seemingly cloned from the Lab Gruppen designs, in many cases down to the circuit board layout. These clones have become popular among home theater fanatics for driving large subwoofers. (I’ll review one of them later in this series.)

In increasing order of efficiency and complexity, we have Class AB -> Class H -> Class D.

Next: Power amps I have known and loved

Previously: How it started