Adcom GFA-5500 Amplifier with 200 Watts Per Channel

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The GFA-5500

Note: Many of the design features in the GFA-5500 are virtually identical to the GFA-5800 power amplifier. The primary difference between these amplifiers is power output. The circuit topology and design concepts are virtually identical.

POWER SUPPLY

Completely Separate Secondary Main Power Systems.
The GFA-5500 has a specially wound high power toroidal transformer which completely isolates the two channel secondary systems, not even sharing the ground connection. This eliminates ground loops and greatly reduces the possible interactions between power channels.

High Capacitance Storage
Each channel has nearly 120 Joules of energy storage, adequate for 14 kilowatts of power at 10% ripple. Under normal operation, the power supply ripple voltage is less than 1%.

Separate High Voltage Front End Supplies
The front end circuitry of each channel is supplied power by its own high voltage system - including separate transformer windings, bridge rectifiers and filter capacitors for each channel. This eliminates interaction between the early gain stages of the amplifier and the high power occurring in the output stages. The front end system is operated at a higher voltage than the output stage. This not only improves the distortion characteristic of the front end, but allows the front end to drive the output stage to its supply rails - obtaining every bit of available power to the load.

In Rush Limiting
The power supplies feature inrush limiting which prevents the high current surges which would normally accompany the charge cycle of the transformer and the power supply capacitors on turn-on.

CIRCUITRY

100% MOSFET Construction
All audio elements of the amplifier are Power Mosfets Even the input transistors are power devices, capable of as much as 2 amps of current by themselves. The transconductance characteristic of Mosfets have a number of advantages which are put to good use. Their high input impedance allow for the use of an extremely simple gain path, and their negative thermal characteristics give high thermal stability, allowing high output stage bias, low offset voltages, and very high output current and voltage with reliable operation. The characteristics of the Power Mosfets are sonically superior to the Bipolar transistor, delivering all of the muscle of Bipolar circuitry with the sweeter qualities found in tube equipment.

Ultra Simple Gain Path
Most high end amplifiers have at least 5 successive gain stages associated with their signal path. Some have even more. The GFA-5500 achieves all of its performance with only three. That's two stages less for the signal to travel through, and it makes a difference.

Single Ended Class A Front End Circuitry
The front end circuitry is single ended Class A. Single ended is the purest form of Class A operation, and yields the most pleasing sonic characteristics. It is also the most stable class because the gain devices are biased by constant sources which do not vary their bias under changing temperature or dynamic conditions.

Doubly Regulated Active Current Sources
The current sources in the GFA-5500 are doubly regulated for absolute stability. This stability gives the front end circuitry the ability to ignore supply fluctuations, noise and signals from the other channel (crosstalk). It also provides operation which does not vary with temperature or age. The voltage references driving each source are filtered passively, with different time constants, which allowing for staggered turn-on of the sources, giving low turn-on thump.

Ultra Matched Devices
The output Mosfets are mated to within 2.5% of each other at the bias point, and the input devices are matched to within 0.25%. In the output stage this yields lower distortion and more reliable performance. Tighter tolerances also achieves lower output impedance (higher damping factor) and higher output currents. In the input stage, this yields lower distortion, lower DC offset voltages, and higher thermal stability.

Ultra Low Feedback
The very simple gain path has been configured for very low amounts of feedback. This contributes to the warm and musical quality of the sound and the high stability of the amplifier with a wide variety of loads.

High Power Output Stage
The amplifier uses International Rectifier Hexfets exclusively in all gain stages. The output stage uses the highest Power Mosfet types available. The output stage of each channel is capable of withstanding transients of about 3 kilowatts.

High Current Output
Each channel uses 10 Power Mosfet output devices. These devices are each rated to handle 20 amps of output current. This output capacity is adequate to deliver full power into 2 ohm loads, and the amplifier is completely stable driving 1 ohm or less.

Low Impedance Performance
The amplifier does not produce significantly higher distortion readings into low impedance loads, unlike other amplifiers on the market. Watt for watt, the distortion into 2 ohms is only slightly higher than that into 8 ohms.

Near Class A Bias
Each channel of the amplifier is biased at about 50 watts idle current. This is a lot, and represents near-Class A operation for most listening levels, giving depth and ambiance to its sound. The distortion curve is monotonic, (distortion decreases with level) and is primarily 2nd harmonic at listening levels.

Thermal and Error Warning Indicators
Each channel has a LED to indicate over temperature or excessive distortion.

CONSTRUCTION

Modular Channel Construction
All of the active circuitry for each channel is on a single printed circuit board mounted on the heat sink. This assures short signal paths, accurate wiring placement, and allows for easier servicing. Outboard heatsinks provide the necessary air circulation to allow the output devices to operate within their specified temperature range.

Technical Specifications:
Power output: 200 watts per channel into 8Ω (stereo)
Frequency response: 10 Hz to 20 kHz
Total harmonic distortion: 0.02%
Damping factor: 700
Gain: 29 dB
Input sensitivity: 1.4V
Signal to noise ratio: 100 dB
Dimensions: 7" x 17" x 13.75" (178 x 432 x 349mm)
Weight: 57 lbs (25.9kg)

Our Testing Process for Amplifiers (Solid State) at SkyFi:

We start with a visual inspection of all internal components to make sure there are no signs of stress or aging. Capacitors are checked for telltale sings of bulging or leaking, resistors are checked for signs of overheating or cracking, and transistors are checked for signs of stress or damage.

We then power up the unit carefully and run a simple 1k sine wave while monitoring the output on an oscilloscope for signs of distortion or noise. Finally, we run the amplifier to its full rated power into the appropriate resistive load provided by our Sencore amplifier tester. If there is an issue, it will usually be noticeable at this point. If everything reads clean, we will often test for THD (Total Harmonic Distortion) measured by our Keithley distortion meter, and compare it against the original manufacturers specifications. The amplifier then remains powered on to "burn in" on our test bench for a few hours to make sure there are no heat related issues. If there are balanced inputs available they will be also be tested.

Final testing involves pushing actual music thorough the amplifier. We've learned over time that some issues are only noticeable to a trained ear while listening to familiar source material. Our test bench has reference vintage Kef speakers that we're very familiar with which quickly reveal any discrepancies. Some amplifiers will then move into our listening room where they are tested with our in-house reference speakers of choice.

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