5 Power amplifiers

5.9 Topologies

Due to the nature of loudspeakers, amplifiers have to be designed so that a loudspeaker can move in (negative) and out (positive) the cabinet. In practise, this means separating the amplifier in two halves: one for each half of this movement. A problem occurs when the two halves have to take over from each other. At near-zero current, both tubes and transistors are non-linear. It means they will not reproduce the signal well (aka "crossover distortion"). This problem returns every half cycle of the waveform, as a waveform crosses zero twice each cycle.

Another problem occurs when the output devices of the amplifier are not fully "on". Because a music signal is of a constantly changing amplitude, this is practically always the case. The connected load of the amplifier receives part of the output voltage, while the output section gets the remainder of the supply voltage. This remainder is converted to waste heat. When an amplifier is working somewhat below its maximum power, more heat than output power is produced, even if the amplifier is theoretically ideal.

There are several ways to address these problems. They're called "classes". Not every amplifier class is suited for audio (there are more purposes for amplifiers). The most important ones are listed below.

  • Class A: Maximum current flows through the output stage at all times. This way the near-zero current is avoided, and thereby crossover distortion eliminated. An unavoidable side-effect is, when no signal is present, power consumption is at maximum, and the amplifier will run hot when no sound is produced. Better still, the amplifier will cool down when operating at moderate to high output power.
  • Class B: The opposite of class A. No current flows through the output stage when in rest. Stand-by power consumption is nearly non-existent, but crossover distortion is eminent, be it acceptible for some applications (like speech or sirens).
  • Class AB: The best of both worlds. A small stand-by current keeps the crossover distortion at a low level, and when silent, power consumption is only a fraction of the maximum power. Nearly all conventional power amps are class AB.
  • Class D: As mentioned above, heat is produced when an amplifier output device is not fully "on". Class D amplifiers use digital technology to rapidly and constantly switch the output devices on and off, effectively avoiding the "in-between" state. By filtering (averaging) the switching frequency out of the output, the intended amplified signal appears on the output. This class is a.k.a. switching amplifiers. It won't be before long when every amplifier uses class D topology ('cept for them good-ol' tube amps, but then again, ya never know). When combined with a switching power supply, instead of a conventional heavy mains transformer, weight, mains power and cooling requirements can be drastically reduced.
  • Class G: This topology uses two sets of output transistors and two supply voltages. One set controls low-to-medium power signals, keeping power consumption and heat at a moderate level. When high power is needed (during signal peaks), the second transistor set takes over and provides the higher voltages, fed by the higer supply voltage. As soon as the peak is over, the first set gets back to work.
  • Class H: Much like class G, this system uses two stages. Only now the supply voltage is temporarily increased (switched) to deal with the peaks. The advantage is: you only need one set of (expensive) output transistors, and the switching can be done by much cheaper electronic switches.

© Joris van den Heuvel 2001-2009