PS Audio P300 Power Plant
I am, of course, talking about the PS Audio P300 Power Plant.
The problem the Power Plant addresses is fundamental: Although audio components use DC internally, their source of power is AC, which is then converted to DC by a component's own power supply. (The exception is battery-operated equipment, which can be made to work well but has practical limitations.) In North America, AC is nominally a 60Hz, 120V sinewave (50Hz/220-240V in some other parts of the world). In practice, it is both less and more than that. Voltage varies, dropping at times to the point where it results in brownouts, or rising to well above the nominal level. (In my home, I've measured AC voltage as high as 129V, and was told by the utility company that they considered this to be within the acceptable range.) Then, superimposed on the 60Hz sinewave are noise, distortion, and voltage spikes. Power supplies in audio equipment are supposed to filter out all this junk, but do so imperfectly; some of the noise and interference gets through to the sensitive audio circuits, where they produce unwanted additions to the signal.
Power-line conditioners (PLCs) are outboard devices designed to clean up the AC and make the job of audio power supplies easier. Many audiophiles have found PLCs to be useful, but these devices also have their detractors, and some equipment manufacturers specifically recommend not using them. The problem is that while PLCs may have a desirable filtering action, they often have undesirable side effects, the most common being a certain "dulling" of the sound and a restriction of dynamics. And, in any case, PLCs deal only with noise and distortion, not voltage variations.
The approach taken by the Power Plant to solve this problem is quite different from that taken by PLCs. Described as a "regenerative AC synthesizer," the Power Plant is essentially an audio power amplifier with a single-frequency output (60Hz, but more on this later) set by a DSP-based sinewave oscillator. The output voltage is regulated so that it's impervious to line-voltage fluctuations. The amplifier is a two-channel class-AB bipolar transistor design using twin transformers. (Early prototypes of the Power Plant used a larger, single transformer, but this was changed for the mechanically quieter twin-transformer design.) Each channel produces 57.5V AC in a balanced configuration, one prong of the AC delivering +57.5V and the other -57.5, the potential difference being the requisite 115V (footnote 2).
The balanced power approach contrasts with the conventional unbalanced AC delivery system, in which one prong is 115V (plus or minus, depending on how it's hooked up at the power distribution box), whereas the other is at ground potential. Balanced power is widely regarded as superior to unbalanced because of its greater immunity to noise and interference. The Power Plant amplifier uses high negative feedback, which can introduce problems if an amplifier has to reproduce the varying frequencies of music. But the Power Plant's only job is to produce a single frequency, and the negative feedback allows it to have the desirable low-impedance output and very low distortion of the AC waveform.
Footnote 1: Note, however, that the Mark Levinson No.33 and No.33H power amplifiers, and No.32 preamplifier, as well as the Linn Lingo turntable power supply, include a dedicated AC regeneration stage as part of the power supply.
Footnote 2: According to Power Plant designer Paul McGowan, 115V was chosen as the approximate middle of the range that American audio equipment is designed to work with. The Power Plant has an internal adjustment for output voltage, and if the unit is to be used for powering electrostatic speakers, McGowan suggests having the factory set the voltage at 125V, which will increase the electrostatics' bias voltage and is said to produce an improvement in the speakers' dynamic ceiling. As this voltage is still within the normal AC range, the speakers should be able to handle it, but I'd want to check with the speaker manufacturer, just to be sure.