If an amplifier is to accurately provide a large current to its load, that is to say a loudspeaker, there are two essential prerequisites: the power supply must be capable of delivering ample energy, and the high-current output stage must be designed with low impedance so that its operation is not adversely affected by the load. The latter requirement is especially important, because the impedance of a loudspeaker fluctuates considerably, depending on the frequency of the signal. Only an output stage with very low impedance will be able to assure accurate music reproduction under these demanding conditions.

To fulfill these demands, the P-550 employs multi-emitter type power transistors specially developed for high-power audio applications. These advanced devices have wide and flat frequency response, and their current amplification linearity of forward-current transfer ratio as well as their switching characteristics are excellent. With these transistors arranged in a 10-parallel push-pull configuration, the P-550 achieves an impressive power output rating of 270 watts into 8 ohms per channel. By using bridged mode, the P-550 can be turned into a monaural power amplifier with an output of 840 watts into 8 ohms.
Another design highlight of theP-550 is the "current negative feedback" principle. As opposed to conventional voltage NFB designs, there is virtually no phase shift in the upper frequency range, and frequency response does not change when gain is altered. This new type of circuit therefore provides ideal amplification characteristics, combining operation stability with excellent frequency response. Phase compensation can be kept at a minimum, since only moderate amounts of NFB are required. This assures outstanding transient response, and the advantages are clearly audible as superb sonic realism and extraordinary detail resolution.
The basic source of energy for an amplifier is of course the power transformer and the filtering capacitors. Consequently, the sonic end result depends to a considerable degree on how much of a performance margin is built into the power supply. The P-550 makes absolutely no compromises in this regard, featuring a large toroidal power transformer and enormous filtering capacity. And to maintain absolute sonic purity, all vital parts where the audio signal passes, such as the traces on the printed circuit boards and input and output terminals are gold-plated.
In the original Accuphase tradition, theP-550 features a champagne gold colored front panel made from heavy, 15-mm thick extruded aluminum with a hand-brushed finish. Its simple and uncluttered design gives the amplifier an elegant and sophisticated visual appeal.