Adcom GFA-555 Power Amplifier Review

The Adcom GFA-555 power amplifier is somewhat unusual in being an affordably priced product that has also received wide acceptance from the “high-end” section of the audio fraternity. The amplifier combines basically conventional circuit design with a number of unusual operating details that set it apart from other high-power amplifiers. It is rated to deliver 200 watts per channel into 8-ohm loads from 20 to 20,000 Hz with no more than 0.09 percent total harmonic distortion, or 325 watts into 4 ohms at 0.25 percent distortion. As a bridged-mono amplifier, it is rated at 600 watts into 8 ohms with less than 0.25 percent distortion.

The Adcom GFA-555 is a full-size, handsomely styled amplifier. Its front panel, decorated with a pattern of horizontal striations, has a large rocker-type power switch and a small red led pilot light at the lower left. At the other end of the panel, two LED’s monitor the left and right channels and flash instantaneously when the distortion in either of them exceeds 1 percent.

Since these overload indicators actually monitor the entire feedback loop, they respond to any form of nonlinear distortion, including transient intermodulation distortion (TIM) or even a DC offset (the amplifier is direct-coupled throughout).

On the rear of the amplifier are the heat-sink fins, heavy-duty insulated binding-post speaker terminals, phono-jack line inputs, and a switch that converts it from a conventional stereo configuration to a bridged-mono amplifier.

The output stage of each channel contains eight high-power transistors capable of delivering more than 20 amperes to low-impedance loads. The operating bias of the driving and output transistors is stabilized against changes in temperature, line voltage, or signal conditions by a newly developed current-feedback technique.

There are no conventional current-limiting protection circuits in the GFA-555. A very large, short-term overload, such as might be caused by an output short-circuit at high power outputs, will blow one of the internal DC power-supply fuses or the 10-ampere line fuse (which is user-replaceable). Sustained high-power operation may eventually increase the heat-sink temperature to 75°C, at which point a thermal circuit breaker shuts off the amplifier. When the amp cools to a safe temperature, operation is automatically restored. The power supply uses a 700-watt toroidal transformer that is said to have many of the advantages of two separate transformers but to be more cost-effective.

The Adcom GFA-555, which weighs about 30 pounds, is 17 inches wide, 11 inches deep, and 7 inches high. Optional rack-mounting extensions and handles add 2 inches to the width and 1-1/4 inches to the depth. The amplifier is available in a black or glossy white finish. Price: $679.95 in black, $749.95 in white. The RM-7 rack adaptors are $19.95 per pair.

Lab Tests

Our standard one-hour preconditioning at one-third rated power left the top of the Adcom GFA-555 very hot to the touch. The amplifier’s output clipped at 242 watts into 8 ohms, 352 watts into 4 ohms, and 465 watts into 2 ohms. In our dynamic power measurements, the clipping level during a 20-millisecond tone burst was 352 watts into 8 ohms, 630 watts into 4 ohms, and 1,120 watts into 2 ohms. We measured the power envelope of the GFA-555 by extending these tone-burst measurements to longer “on” times, up to 300 milliseconds. At the longer burst durations, the maximum output was 65 to 70 percent of the EIA dynamic power output with 8- and 4-ohm loads, and 50 percent of the dynamic power with 2-ohm loads. We also made limited tests of the amplifier in its bridged mode, where the amplifier clipped at 760 watts into 8 ohms and 900 watts into 4 ohms.

The 1,000-Hz distortion was nearly constant over the full power output range of the amplifier, and only slightly higher with 2- and 4-ohm loads than with 8 ohms. The 8-ohm readings were between 0.02 and 0.04 percent from 1 watt to over 200 watts output. Driving 4 ohms, the distortion was between 0.038 and 0.07 percent from 1 watt to more than 300 watts. The 2-ohm measurements ranged from 0.07 to 0.16 percent from 1 watt to 450 watts per channel. Over the entire audio frequency range, the distortion was nearly the same at rated power, halfpower, and one-tenth power. It was between 0.02 and 0.04 percent from 20 to 10,000 Hz and reached a maximum of 0.07 percent at 20,000 Hz and 200 watts output.

The distortion indicators on the front panel operated with exceptional effectiveness and accuracy. They lit precisely at the point where the waveform began to show signs of clipping.

The GFA-555 required an input of 0.13 volt for a 1-watt reference output, and the A-weighted output noise was 101.5 dB below 1 watt. The slew factor was 3, with a 200-watt sine-wave output assuming a triangular shape at about 60,000 Hz. The reactive load factor was 1.02 dB, and the amplifier was stable with simulated reactive speaker loads. Its frequency response, flat within 0.1 dB throughout the audio range, was down 1.2 dB at 5 Hz and 3 dB at 140,000 Hz. Although Adcom says that the GFA-555 has no capacitors in its signal path, the infrasonic rolloff we found suggests that there is at least an input blocking capacitor.

Comments

The Adcom GFA-555 demonstrated its indestructibility during our tests. Starting from a very hot top surface after the one-hour preconditioning period, it became even hotter as we drove low-impedance loads to high levels. Finally, with an output of hundreds of watts into 2-ohm loads, it simply shut down. Since the front-panel pilot light was extinguished, we thought at first that the 10-ampere line fuse had blown. When it proved to be intact, we waited with as much patience as we could muster for the amplifier to return to life, which it did in about half an hour. The thermal circuit breaker operated several other times during our tests, but this minor annoyance was certainly preferable to the alternative of blown output transistors! In the bridged mode, the line fuse finally blew out at 900 watts into 4 ohms.

Clearly, the Adcom GFA-555 is a muscular component that is thoroughly protected against most of the accidents that can befall a high-power amplifier. Although there were frequent thermal shut-downs during some of our bench tests, the amplifier ran almost perfectly cool during our listening tests. Adcom stresses the 60,000 microfarads of power-supply filter capacitance and suggests shutting off the AC power to the amplifier while it is playing as a demonstration of the power supply’s energy-storage capability. Depending on the playing volume and the nature of the program, we found that it played without audible change for 12 to 15 seconds after being switched off. We were impressed.

Another noteworthy characteristic of this amplifier is the absence of relays—it turns on and off in total silence, with no thumps in the speakers or audible sounds from relay operation. And we certainly appreciated the absence of a fan when we were listening to the amplifier.

It is difficult to comment on the sound of the GFA-555. It is obviously a superb amplifier by any objective standard. As we have found, A/B comparisons with other amplifiers are fruitless, and none of the speakers we had on hand presented the kind of load that would reveal the special qualities of an amplifier like the GFA-555. Adcom says that it will drive the 1-ohm impedance of an Apogee speaker, and everything we learned about it makes that seem like a reasonable claim.

The GFA-555 is not only one of the smoothest, quietest, and nicest-performing amplifiers we have used, but it also happens to be one of the most attractive. Our test sample had the optional white panel finish, which deserves to be given a highly visible position in any music room. Other Adcom components are also available in white—could this herald a trend away from the all-black styling that seems to dominate the audio industry?

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