PS Audio PMG Signature Power Amp S400 Silver

The PMG Signature voice in a more accessible stereo chassis.

Not every great system has room for two amplifiers. Two boxes, two power cords, two sets of cables — for many listeners that is one requisite too many. The S400 exists for them: the entire PMG Signature experience in a single chassis.
What it is not is a compromise of sonics. This is the full Signature topology built in stereo form — two channels of 200 watts each into 8 ohms, doubling cleanly into 4, drawn from the same state-of-the-art active power supply architecture that drives the M400. The same strikingly low overall feedback. The same bandwidth beyond 500 kHz and slew rate past 200 volts per microsecond — the speed that returns a flawless 10 kHz square wave and lets every transient land exactly when the music intended. The same flat, low distortion across the audible band.
Little in the topology was pared back. One box, one power cord, two channels of genuine reference amplification. Designed by Darren Myers. Engineered by Bob Stadtherr. Hand-built in Boulder, Colorado.

The S400 places two complete PMG Signature channels in a single chassis — and the engineering challenge was making sure neither channel ever knows the other is there. Each channel runs from the same state-of-the-art active power supply architecture as the monoblocks, with the internal layout and isolation engineered so that one channel's demands only minutely disturb the other's signal. Darren Myers's low-feedback, high slew rate topology is carried over without compromise. Hand-assembled in Boulder, Colorado, and brought to perfection by Bob Stadtherr.

The S400 is a single stereo chassis with a single power cord — the Signature experience without two boxes on the floor. Two hundred watts per channel into 8 ohms, doubling into 4, will comfortably drive the great majority of high-quality loudspeakers in rooms of moderate to generous size. It suits the listener who wants genuine reference amplification but values simplicity, space, and a clean installation as much as ultimate output.

Ask an amplifier designer what separates a merely good output stage from a truly transparent one, and the conversation will arrive, quickly, at Class A. In Class A operation every transistor conducts through the entire waveform and never switches off — no crossover point, no handoff between devices, no discontinuity for the music to cross. It is the most linear mode of amplification there is. It is also the most demanding: a Class A stage runs hot and draws power continuously, even in silence. That cost is why genuine Class A is so scarce. It is rare for a high-end amplifier to provide even a single watt of true Class A, and many that claim the term operate purely in it for only a fraction of a watt.
The S400 dedicates 5 watts of pure Class A bias to each of its two channels. In a stereo chassis — where two channels must share one enclosure and one thermal budget — that is a serious commitment. Those 5 watts per channel cover the quiet, low-level passages where the ear is at its most revealing and least forgiving: the inner detail of a recording, the decay of a note into a room, the texture of a voice held softly. It is precisely there, in the delicate material, that crossover artifacts are most audible and most fatiguing — and precisely there that the S400 stays pure. Both channels carry the same operating integrity as a PMG Signature monoblock, in a single chassis that asks for none of the compromises you would expect.

Building a stereo amplifier that truly rivals a pair of monoblocks is harder than it sounds, and the reason comes down to one shared resource: the power supply. In a stereo chassis, both channels draw from it. When the left channel demands a sudden surge of current — a drum strike, a crescendo — that demand pulls on the supply the right channel is using too. On a conventional amplifier the supply voltage dips under the load, and the second channel is dragged a fraction off course. You can measure it as crosstalk; sometimes you can hear it, as a soundstage that narrows when the music turns complex. It is the historical reason monoblocks have long been considered the purer choice.
The S400 closes that gap, and it begins with the power supply itself. A state-of-the-art active power supply delivers far cleaner power, and far greater current on demand, than any traditional transformer-based design — and it holds its output stiff under load. When one channel slams it with a transient, it sags very little, so the other channel sees only the smallest of the disturbance.
The rest is layout. Bob Stadtherr, drawing on decades of amplifier engineering, arranged the S400's grounding, board topology, and mechanical structure so the two channels' currents never intermingle and neither can couple into the other. An active supply that rarely sags, and a layout engineered channel by channel: together they bring the S400 remarkably close to the separation of true monoblocks — 200 watts into 8 ohms per channel, doubling cleanly to 400 into 4, from a single chassis with a single power cord.

Every amplifier colors the music to some degree. The art of high-end design is in reducing that coloration toward nothing — and the temptation, always, is to do it the easy way: wrap the output stage in heavy negative feedback and let the correction loop scrub the distortion away. It works on the test bench. But feedback corrects by reacting, and reaction takes time. With real music the correction is always slightly behind the signal, and the listener pays for it in lost immediacy and a subtle sense that the performance is being reproduced rather than reproduced faithfully.
The S400 follows the same principle as every PMG Signature amplifier: strikingly low overall feedback. Its output stage is engineered to be deeply linear on its own merits, so only a light touch of feedback is required. Distortion stays flat and low across the entire audible band — not scrubbed away after the fact, but largely absent to begin with — and bandwidth reaches well beyond 500 kHz, leaving transients intact and uncolored.
Combined with the S400's construction, the effect is striking: no phase smear, no recovered artifacts masquerading as detail, no soundstage that collapses when the music turns complex. The amplifier disappears. The two loudspeakers in front of you stop sounding like loudspeakers and start sounding like the room the recording was made in. That vanishing act — electronics that refuse to announce themselves — is the whole purpose of the Signature Series, and the S400 performs it in full.