Adcom GFA-7000 Power Amplifier Review

Performance and installation requirements for home-theater systems vary as widely as those for conventional stereo systems. An A/V installation can be as simple as a basic receiver with a built-in Dolby Pro Logic decoder and four channels of amplification plus five modest speakers and a TV. Properly installed, such a system can provide, at minimum cost, a reasonable simulation of the dynamic and spatial characteristics incorporated in the soundtracks of many movies and other video programs.

Unfortunately, such a basic home-theater installation is likely to be as far from realizing the full potential of a movie soundtrack as a department-store “shelf” system is from a serious high-fidelity music system costing many times as much.

Most people are familiar with the THX logo displayed at many topnotch movie theaters. The THX trademark is licensed by its creator, Lucasfilm, to theater installations that meet a number of critical requirements, primarily dealing with audio performance. In addition to audio quality, the THX movie-theater program also covers things like the spatial qualities of the complete installation.

In recent years THX has also become a factor on the home-theater scene. Speakers, amplifiers, surround-sound processors, and other components bearing the THX logo are available from many companies. In order for a manufacturer to display the THX logo on its products, they must meet rigorous requirements imposed by Lucasfilm dealing with such matters as the frequency-response and dispersion characteristics of loudspeakers, the power capabilities of amplifiers, the performance of Pro Logic decoders, and even the design of interconnecting cables and connectors. The goal of these requirements is to insure that the spatial, spectral, and dynamic properties of film soundtracks mixed for large theaters are preserved in the smaller home-theater environment.

The Adcom GFA-7000 is a five-channel THX-certified power amplifier whose construction and performance dramatically demonstrate some of the differences between ordinary stereo amplifiers and those designed to deliver the impact of a movie soundtrack in your home.

The GFA-7000 is rated to deliver up to 130 watts into an 8-ohm load or 200 watts into 4 ohms in each of its five identical channels, with less than 0.05 percent total harmonic distortion. That makes it suitable for a discrete 5.1-channel Dolby Digital (AC-3) system, in which the “.1” channel reproduces only low frequencies and would normally be powered by the amplifier built into the subwoofer. These ratings apply to continuous (and simultaneous) operation at any frequency from 20 Hz to 20 kHz.

As might be expected from an amplifier with such capabilities, the GFA-7000 is quite large, though not unreasonably so considering its power ratings. It is effectively protected against the damage risks that might be expected in normal home use. If any individual channel overheats, the amplifier’s thermal-protection circuit will shut it down and a red LED on the front panel will light up until the temperature drops to a safe level; the other channels continue to operate normally. If the temperature of the massive power transformer exceeds 257° F, a built-in thermostat shuts down the entire amplifier (it automatically returns to service when the temperature falls to a safe value). Adcom says that this condition is unlikely to occur unless the amplifier is operated for extended periods into loads of 3 ohms or less at very high listening levels.

In addition to a large pushbutton power switch and the thermal-protection indicators, the front panel of the GFA-7000 contains a row of five red distortion-alert LED indicators. These light up when the distortion in the corresponding channel reaches 1 percent and serve as a warning to reduce the playback volume.

The GFA-7000, despite its enormous power capability (totaling 1,000 watts continuous into 4 ohms) and imposing bulk, is relatively light at 41 pounds. It is also unusual among its peers in not being fan-cooled. Each channel is constructed as a separate module containing input and output connectors, a heat sink, and power-supply components, including 24,000 microfarads of filter capacitance, which can greatly improve dynamic-head-room performance. The top and bottom of the cabinet are extensively perforated and should provide effective cooling under almost any reasonable operating condition.

The power transformer, a high-efficiency toroidal design, occupies much of the forward part of the GFA-7000.

It’s positioned so that its considerable weight balances that of the amplifier section, making the component relatively easy to handle.

The input and output connectors are in a row across the top rear of the cabinet (integrated with the individual amplifier modules). The signal inputs are gold-plated phono jacks, and the speaker outputs are insulated dual binding posts that accept lugs, wires, or banana plugs. The rear panel also contains the line cord, a 15-ampere AC fuse, and a 25-pin computer-type connector that makes connecting the amplifier to a THX processor a singlecable affair. An optional rack-mount adaptor is also available, and a 230-volt version of the amplifier is available for the European market.

The logistics of using a large number of high-wattage load resistors, which would become very hot when dissipating a total of a kilowatt or so (to say nothing of taking up more room than our test bench provides), limited us to driving only two channels of the GFA-7000 at a time, into either 8-ohm or 4-ohm loads, during our lab tests.

The performance of the amplifier modules was superb, confirming the manufacturer’s claims and closely matching the test data on the review sample that was supplied to us by the manufacturer. Not only did the test unit easily surpass its ratings, but its exterior never became unreasonably hot to the touch, not even directly over the heat sink of the driven channel. Also, although it would probably have little effect on the ultimate sound quality, the amplifier’s harmonic distortion (the rated maximum is 0.05 percent) rarely reached 0.01 percent under any conditions and was typically in the vicinity of 0.002 percent or less at any usable power and frequency.

The Adcom GFA-7000 should be a star performer in any serious home-theater installation, especially one set up to reproduce Dolby Digital soundtracks, which are available now on some laserdiscs and will be standard on the 5-inch DVD movie discs expected to be introduced in the coming months. Too often we find that very powerful amplifiers must be handled with some delicacy to avoid an expensive mishap. Judging from how easily the GFA-7000 dealt with overloads and occasional ventures into hard clipping, and how close to perfect its electrical performance was, its price tag seems like a downright bargain!

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