AdcomGFA-555 mkIIusedAdcom GFA-555 II 2 Channel Power AmplifierAdcom GFA-555 II 2 Channel Power Amplifier Note: Separately, we are also selling: Adcom GFP-555 II Preamplifer Adcom GFT-555 II Tuner Adcom GCD-575 CD Player All in mint condition A ...400.00

Adcom GFA-555 II 2 Channel Power Amplifier

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Condition
10/10
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Ships fromSteamboat Springs, CO, 80487
Ships toUnited States
Package dimensions24.0" × 12.0" × 12.0" (40.0 lbs.)
Shipping carriersUSPS, UPS or FedEx
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Original accessoriesManual
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Adcom GFA-555 II 2 Channel Power Amplifier


Note: Separately, we are also selling:
Adcom GFP-555 II Preamplifer  Adcom GFT-555 II Tuner  Adcom GCD-575 CD Player

All in mint condition

A Review and specifications of the Adcom GFA-555 II Power Amplifier:

The GFA-555 remained in the Adcom line until recently, when the company upgraded it to the GFA-555II, which is almost identical to the original but has several new construction and circuit features designed to enhance its performance.

To reduce the amplifier's already low distortion even further, the grounding system for its inputs has been changed to isolate them from the grounded sides of the speaker outputs. The basic circuitry of the amplifier has also been modified slightly to drive reactive loads with lower distortion.

Other differences between the original and new versions of the amplifier involve small details that individually may not make an appreciable change in its performance but collectively constitute a distinct refinement. For example, the gold-plated input phono jacks now have Teflon insulation, and the internal point-to-point wiring uses oxygen-free copper (OFC) wire. A red led on the front of the amplifier glows when the thermal-protection circuit has shut it down, and the DC power-supply fuses, formerly inside the amplifier, are now on the rear so that they can be replaced more easily.

The output-transistor heat sinks that occupy most of the rear apron are now larger, for better cooling, and have rounded corners to make it easier (and safer) to handle the amplifier, which weighs more than 30 pounds. The 700-watt toroidal power transformer has been enlarged, potted, and electrically improved to reduce its operating temperature and provide more reliable operation with difficult, low-impedance loads.

Although the GFA-555II normally runs quite cool under home listening conditions, prolonged operation at high levels into multiple loudspeakers or very low-impedance speakers can raise its temperature to the point where the thermal-protection system shuts it down. To improve reliability under these conditions, Adcom offers an optional fan that comes on when the temperature exceeds 65� C (150� F). The fan speed increases proportionally as the temperature rises.

The eight output transistors in each channel of the GFA-555II operate without current-limiting circuits. The design relies on the DC power-supply fuses to protect the transistors against short circuits and other potential hazards. Each channel is rated to deliver 200 watts to an 8-ohm load or 325 watts into 4 ohms, from 20 to 20,000 Hz, with no more than 0.04 percent total harmonic distortion (THD), and red led indicators on the front panel flash when the instantaneous distortion level exceeds 1 percent. A switch on the rear panel connects the channels for bridged operation to provide a mono output of 600 watts into 8 ohms with less than 0.05 percent distortion.

The Adcom GFA-555II measures 17 inches wide, 11-1/2 inches deep, and 7-3/8 inches high. Rack-mount adaptors are optional. The amplifier is normally finished in black, but a white front panel and white rack adaptors are available on special order at slightly higher cost. Price: black finish, $750; white finish, $800; optional cooling fan, $100; rack-mount adaptors, $20.

Adcom GFA-555II: Lab Tests

With both channels driving 8-ohm loads at 1,000 Hz, the outputs clipped at 288 watts per channel. Into 4 ohms, the power at clipping was 420 watts per channel. Although we did not make clipping measurements into 2 ohms, the dynamic output into a 2-ohm load was 990 watts. Into 8 and 4 ohms, the respective dynamic output readings were 370 and 650 watts.

The input required for a reference output of 1 watt into 8 ohms was 120 millivolts (mv). The A-weighted noise level in one channel was -85.5 dB referred to 1 watt ( - 108.5 dB referred to the rated 200 watts output). The major contribution to this reading, however, was a 120-hz power-supply component at -86 dB and a 240-hz component at -90 dB. The other channel's noise spectrum was similar but lower by 16 to 20 dB over most of the frequency range. No hum or noise was audible in use.

A spectrum analysis of the harmonic components of a 1-watt, 1,000-Hz output into 8 ohms indicated 0.0156 percent THD. At 200 watts output, it was only 0.003 percent. The distortion at 200 watts increased to 0.009 percent at 10,000 Hz and 0.014 percent at 20,000 Hz.

Total harmonic distortion plus noise (THD + N) was between 0.0012 and 0.003 percent at output levels from 1 to 200 watts into 8 ohms and at frequencies from 20 to 2,000 Hz. It reached a maximum of 0.003 to 0.005 percent at about 7,000 Hz and decreased somewhat at higher frequencies. Measurements into 4-ohm loads produced similar readings at power outputs from 200 to 400 watts.

The GFA-555II's frequency response was flat within ±0.05 dB from 10 to 50,000 Hz. Its power bandwidth (maximum power output at a given distortion level over a range of frequencies) was 267 watts into 8 ohms at 0.1 percent THD + N from 20 to 20,000 Hz. Into 4 ohms, it was 520 watts at 0.1 percent distortion from 60 to 12,000 Hz, falling to 500 watts at 20 Hz and rising to 528 watts at 20,000 Hz.

Adcom GFA-555II review: Comments

In our tests it was clear that the Adcom GFA-555II is a formidably powerful amplifier, much more powerful than its 200-watt rating would imply, and that under most conditions of use it has remarkably low distortion even by the standards of today's amplifiers. The top of the cabinet and the heat sinks became quite hot during the preconditioning period and sustained high-power operation, but the thermal-protection circuit shut down the amplifier only once (it returned to operation in about 15 minutes, none the worse for wear).

Another test sample was equipped with the optional cooling fan. The fan was very quiet under the conditions of the test; it did not come on at all until the amplifier was quite warm, and it turned rather slowly during much of the test. We noted, however, that the fan draws cooling air through the amplifier from the bottom to the top, not over the heat-sink fins, which are completely outside the cabinet and apparently not cooled at all by the fan. Still, as the instruction manual notes, the fan is really not needed in a normal home installation.

Used in a music system, the GFA-555II was, in effect, an invisible and inaudible amplifier. By this I mean that it never made its presence known by a hum, click, thump, or any other sound (electrical or acoustic) during operation or when it was turned on and off. Although it is fairly large, it fit unobtrusively on a shelf of the equipment rack, with only a single small, red led indicating that it was "alive." It became no more than faintly warm during our use tests, further emphasizing that the fan would be superfluous for most users.

We also noted that when the amplifier was switched off while music was playing, there was no change in the sound for several seconds, after which it faded out smoothly. This effect is a testimonial to the amplifier's power-supply filter capacitors, which store enough energy to keep it going for a relatively long time (it also implies a considerable power capability at the lowest audio frequencies).

It came as no surprise to find that the GFA-555II had no particular sonic character of its own. Clearly, it is a good choice for driving the most difficult speaker loads, especially those presenting a low or highly reactive load impedance. It is hard to imagine a speaker load that would overtax the GFA-555II in a home system no matter how loud you like your music. In the interests of speaker (and personal!) survival, we never tried to push it to its limits in our listening tests.

I have only one note of caution about installing this amplifier in your system. Be very sure that there is no common ground between the input and output, and follow the warning in the instructions never to make any change of input or output connections while the amplifier is turned on. If these cautions are observed, the Adcom GFA-555II should be effectively "bulletproof."

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