Adcom GFA-555II

A 200-watt-per-channel MOSFET amplifier that brought high-end power down to an affordable price.

Overview

When Adcom announced the GFA-555II, the company leaned on the reputation of the amplifier it replaced. The original GFA-555 had, not long after its introduction, been widely considered a breakthrough in audio technology. Its performance was rated superior to amplifiers costing two and three times as much, and some critics struggled to name a better-sounding component at any price. That is marketing copy, of course, but a manufacturer only makes that claim when the reviews have already said it first. The II was an evolutionary step, not a clean-sheet redesign. Adcom described it as more stable, with superior heat dissipation and less distortion, and kept the headline rating at 200 watts per channel. HiFi-Stereo (January 1992, p.68) reported the new model as a "new breakthrough in performance and value," rated at 200 watts per channel into 8 ohms.

The 200-watt figure is the whole point of this amplifier. Adcom's own published specifications, to FTC requirements, list continuous power output of 200 watts per channel from 20 Hz to 20 kHz at 8 ohms with total harmonic distortion under 0.04%. That is full rated power across the entire audible band, not a single-frequency hero number. It is the kind of rating that lets an amplifier drive inefficient speakers, the four-ohm loads and low-sensitivity boxes that make lesser receivers choke, without the sound collapsing into compression when the music gets loud.

The circuit is built around MOSFET output devices, and Adcom's advertising made a specific virtue of that choice. The company's pitch, as it ran in period magazines, was that MOSFETs deliver the punch and muscle of solid-state while retaining a remarkable ability to "sound as sweet as tubes." HiFi-Stereo (April 1997, p.5) carried the same language, framing the 555II as the model that set the standard for high-end sound at reasonable cost. Whether the tube comparison holds up is a matter of taste. The design goal is clear enough: an amplifier that stays composed rather than edgy when pushed.

The power supply is where that composure is earned. Adcom built the 555II around what it described as a mighty power supply, sized to meet the enormous demands of the output stages — the reserve capacity that separates an amplifier that merely hits 200 watts on a bench from one that can sustain dynamics without sagging. Protection is deliberately blunt and serviceable: readily accessible, rear-mounted DC power supply fuses guard against very large overloads such as an output short circuit. Rear-panel fuses are an unfashionable solution compared to relay-based protection, but they are honest, cheap to replace, and they tell you exactly what failed.

The 555II's reputation outlasted its production. By the mid-1990s, Adcom was running advertisements announcing the GFA-5500 as the 555II's successor. Those ads read partly like a eulogy. They described the 555II as legendary among audiophiles and serious music listeners, the amplifier that set the standard for high-end sound at reasonable cost and was consistently compared to amplifiers costing two and three times as much. Audio (November 1995, p.11) ran that same successor announcement, noting that while audiophiles might be sad to see the end of the legendary GFA-555II, its replacement was on the way.

A seller advertising a "555" may be describing any of them. Buyers should confirm which revision is actually on the bench before money changes hands.