Adcom GFA-555 power amplifier John Atkinson 1990
Introduced in 1985, the original GFA-555 was the subject of a rave review in Vol.8 No.4 from the patrician Anthony H. Cordesman, who felt that its natural presentation of upper-octave detail and its "extraordinarily holographic" imaging might well make it a "legitimate rival" to the Krells and Audio Researches of this world. Three issues later, in Vol.8 No.7, the venerable JGH pretty much agreed that the '555 was something special, rivaling his then reference, the Electron Kinetics Eagle 2.
Personally, I felt the '555's high frequencies to be somewhat emphasized, perhaps even grainy, which for me somewhat offset its superbly defined, massively weighty low frequencies, its excellent dynamics, and its wide, deep, and well-focused soundstage. It would also mean that the '555 required rather more care in the choice of matching source components and loudspeakers than AHC's original review would suggest. Nevertheless, the 200Wpc Adcom, priced at an eminently affordable $600, was superb value and went on to become one of the best-selling amplifiers of all time. Two Stereophile reviewers, TJN and RH, used '555s for much of their listening; and GL was also impressed by its virtues when he reviewed an early 1989 sample in Vol.12 No.12.
At the 1990 SCES, Adcom finally announced a revised version of the '555 to sell for $800, the GFA-555 II. Before continuing the amplifier's inclusion in Stereophile's "Recommended Components," therefore, I thought it a good idea to compare the new with the old (in this case, the same sample we had purchased for GL's 1989 review).
Superficially, the new '555 appears identical to the old. A closer look, however, reveals a greater number of cooling vents in the case and a cooling fan lying just under the top plate. (This fan, a $100 option, is triggered by heatsink temperature and didn't turn on during my auditioning, where average output levels were generally below 8V RMS into the 6 ohm Avalons—just under 11W.) There is now a red LED on the front panel labeled "Thermal Overload," a greater number of fuses between the rear-mounted, curved-edge heatsinks, and the input sockets are now much higher quality and use Teflon insulation. The power transfomer has been increased in size, potted to improve heat transfer, and offers greater regulation. The output circuitry has also been changed, and the LF signal path has been stripped of polarized (electrolytic) capacitors. A new DC-servo circuit minimizes output DC offset.
Both amplifiers are polarity-correct, but the Mk.II version has slightly higher gain—27.75dB vs 26.9dB at 1kHz when loaded by the Avalon speakers. The 0.85dB difference was compensated for during the auditioning. Each amplifier was fed from a Mod Squad Line Drive Deluxe AGT via 1m of AudioQuest Lapis interconnect. Source was exclusively CD from a Meridian 208 CD player/preamp, connected to the Line Drive via 10' of Audio Research interconnect. Loudspeakers were Avalon Eclipses, bi-wired via 8' lengths of Mission spaced-conductor cable. (The fact that the Adcom's output binding posts are placed between the heatsink fins meant that there wasn't sufficient clearance to use the bi-wired sets of AudioQuest Clear that I used for the other follow-up reviews I carried out for this issue.)
The first difference wasn't hard to spot. After three hours' warmup, the heatsinks of the '555 II were appreciably hotter than that of the Mk.I, suggesting a higher degree of output-device bias current. The first track I played was Drew Minter singing Handel's "Va Tacito," from the Harmonia Mundi USA CD Arias for Senesino (HMC 905183). While I felt the older amplifier's high frequencies to be still a little grainy, it drove the Avalon speakers with a degree of authority that was musically satisfying. The voice was a little smaller-sounding than via the Audio Research Classic 60, which has taken pride of place in my system. Nevertheless, the soundstage was well-defined,the low frequencies weighty, and the music communicated well. Switching to the Mk.II '555 revealed an even deeper soundstage—the accompanying French horn could be heard to be set further behind Mr. Minter—while the voice was more robust in its midrange tonality.
Turning to my recording of Anna-Maria Stanczyk playing Chopin on the Stereophile Test CD, the same differences manifest themselves. Via the original '555, the sound of the Steinway was a little lightweight, even "tinkly" in its upper registers, while the Mk.II '555 presented it with what I feel to be a more natural tonal quality. This is not to say the new amplifier is dark-sounding; instead, it has what I feel to be significantly less grainy high frequencies. It's fair to point out, however, that many listeners unfamiliar with the subtleties of live piano sound will prefer the original '555 as being more clear, more vivid.
The new amplifier again threw a better sense of depth than the old. Anna's Steinway was set further behind the plane of the loudspeakers, despite its now having a more robust, more forward midrange tonality.
Both amplifiers sounded pretty well equivalent in the bass with these kinds of music. I therefore reached for something with a bit more low-frequency oomph: Jeff Beck's Guitar Shop album (Epic EK 44313), which has some killer drum sounds. Here, the GFA-555 Mk.II had as good an LF extension apparent, but its upper and midbass registers were, depending on your tastes, better defined/less weighty than its Mk.I incarnation. Both, however, were more than adequate in presenting Terry Bozzio's thunderclap drum sound with more than sufficient weight and dynamic impact.
To sum up, I feel the GFA-555 Mk.II to be significantly more neutral as regards midrange and treble tonal quality than the Mk.I, while preserving its virtues: excellent imaging specificity, a deep, well-defined bass, and a superb sense of dynamics. Currently ranked in Class C of Stereophile's "Recommended Components" listing, the Mk.II '555 could well deserve an even higher rating. I have passed it on, therefore, to Tom Norton for a more detailed evaluation.—John Atkinson