Most modern power amplifier designs have abandoned traditional “linear” power transformer-based supplies in favor of digital switching supplies, shedding about half their weight in the process, getting into the range of 10 kilos or 22 pounds.
Nearly all amps are two “rack spaces” high for a total of 3.5 inches in a standard 19-inch wide equipment rack, with depths ranging from roughly 10 to 20 inches.
Various circuit designs have emerged over the years. Class AB designs provide low distortion at medium efficiency for high fidelity at moderate power levels and are still popular choices for compression drivers.
However, AB uses a full-power DC supply at all levels, generating substantial heat at less than full output. Class D designs employ Pulse Width Modulation, using the input audio signal to switch the output devices at ultrasonic frequencies and then filter those frequencies with a low-pass filter. They’re very efficient, with a slight cost to frequency response, distortion and damping factor.
Class G uses a second output stage that’s turned on when louder input signals require it, while Class H uses a single stage whose DC power supply is controlled by the input audio signal.
Several other classes have been introduced by manufacturers that are proprietary modifications of Class D that address its shortcomings, combining high efficiency with high audio performance.
Meaningful power comparisons are best made with both channels driven to reflect real-world situations. Four-ohm operation is preferred, though most modern designs provide stable operation with two-ohm loads as well.
Subwoofer applications usually require many thousands of watts to drive today’s high-power 18- and 21-inch voicecoils. A rule of thumb is to match amps with at least twice the continuous power rating of the loudspeaker loads they’re driving.