Price: black finish, $750 white finish, $800 optional cooling fan, $100 rack-mount adaptors, $20. 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. The Adcom GFA-555II measures 17 inches wide, 11-1/2 inches deep, and 7-3/8 inches high. 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. 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. The design relies on the DC power-supply fuses to protect the transistors against short circuits and other potential hazards. The eight output transistors in each channel of the GFA-555II operate without current-limiting circuits. The fan speed increases proportionally as the temperature rises. To improve reliability under these conditions, Adcom offers an optional fan that comes on when the temperature exceeds 65� C (150� F).
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.Īlthough 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. 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. 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. 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. 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. The basic circuitry of the amplifier has also been modified slightly to drive reactive loads with lower distortion. 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 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.
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