ADCOM GFA 5500

The GFA-5500 is equipped with a specially wound high-power toroidal transformer that completely isolates the two secondary channel systems, not even sharing the ground connection. This eliminates ground loops and significantly reduces potential interactions between the power channels.

Each channel's front end is powered by its own high-voltage system, including separate transformer windings, bridge rectifiers, and filter capacitors for each channel. This eliminates interaction between the amplifier’s early gain stages and the high power present in the output stages. The front-end system operates at a higher voltage than the output stage, improving the distortion characteristic of the front end and allowing the output stage to be driven to its power supply rails, utilizing all available power to the load.

The power supplies feature inrush limiting, which prevents the high current surges that would normally accompany the transformer and power supply capacitors' charge cycle upon power-up.

All audio elements of the amplifier are power MOSFETs. Even the input transistors are power devices capable of handling up to 2 amps of current each. The transconductance characteristics of MOSFETs provide numerous advantages that are well utilized. Their high input impedance allows for an extremely simple gain path, and their negative thermal characteristics ensure high thermal stability, allowing for high bias in the output stage, low offset voltages, and very high output current and voltage with reliable operation. The characteristics of power MOSFETs are superior to bipolar transistors, delivering all the muscle of bipolar circuitry with the smoother qualities found in tube equipment.

The front-end section is single-ended Class A. The single-ended configuration is the purest form of Class A operation and provides the most pleasing sonic characteristics. It is also the most stable class, as the gain devices are biased by constant sources that do not vary their bias with changes in temperature or dynamic conditions.

The current sources in the GFA-5500 are doubly regulated for absolute stability. This stability allows the front-end circuitry to ignore power fluctuations, noise, and signals from the other channel (crosstalk). It also ensures operation that does not vary with temperature or age. The voltage references driving each source are passively filtered, with different time constants, allowing staggered turn-on of the sources, minimizing turn-on thump.

The output MOSFETs are matched to within 2.5% at the bias point, while the input devices are matched to within 0.25%. In the output stage, this results in lower distortion and more reliable performance. Tighter tolerances also achieve lower output impedance (higher damping factor) and higher output currents. In the input stage, this results in lower distortion, lower DC offset voltages, and greater thermal stability.

Ultra Low Feedback

The very simple gain path has been designed with extremely low feedback. This contributes to the warm and musical quality of the sound and the amplifier's high stability with a wide variety of loads.

High Power Output Stage

The amplifier exclusively uses International Rectifier Hexfets in all gain stages. The output stage uses the most advanced power MOSFETs available. Each channel’s output stage is capable of withstanding transients of about 3 kilowatts. Each channel uses 10 power MOSFET output devices. These devices are each rated to handle 20 amps of output current. This output capacity is sufficient to deliver full power into 2-ohm loads, and the amplifier remains completely stable even when driving 1-ohm or lower loads.

All active circuitry for each channel is mounted on a single printed circuit board attached to the heat sink. This ensures short signal paths, accurate wiring placement, and easier maintenance. External heatsinks provide the necessary airflow to allow the output devices to operate within their specified temperature range.

Technical Specifications:

• Output Power: 200 watts per channel into 8Ω (stereo)
• Frequency Response: 10Hz to 20kHz
• THD: 0.02%
• Damping Factor: 700
• Gain: 29 dB
• Input Sensitivity: 1.4V
• S/N Ratio: 100dB
• Dimensions: 178 x 432 x 349mm
• Weight: 25.9kg