A single-ended stage has a single output device (or combination of devices eg a Darlington or Sziklai pair) which handles all of the signal. As mentioned, preamp gain stages and the front ends of power amplifiers are routinely single-ended "pure" Class A, and because the signal levels are at small fractions of a watt, the efficiency of the circuit is not important. The "purity" of Class A designs has been at issue in the last few years. The single-ended (SE) power output stage is the simplest form of power amplifier. Single-ended amps are usually fairly low power (less than 15W say) and are invariably cathode-biased. only a single Tube, power BJT or power MOSFET at a high bias current does the final amplification, making it the.