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manual abstract
Copyright © 2011 ADCOM LLC Rev 2.1 INTRODUCTION Please read thoroughly these operating instructions for the GFA-555ms before connecting or attempting to operate it. The installation and operation of the GFA-555ms are described in the following pages. We sincerely hope you will value and enjoy the considerable attention we have given its design and construction. This manual has been written to help you understand the correct operation of the GFA-555ms. Please read it carefully to fully comprehend all its features and functions and thereby derive maximum performance from its use in your system. It is a good idea to keep this manual handy for future reference. PREFACE WARNING The GFA-555ms is a very powerful amplifier capable of delivering very large peaks into low impedances, exceeding 80 amperes per channel. Therefore, be certain to exercise extreme caution when making connections to and from the amplifier. Always make certain that the amplifier is disconnected from the AC outlet, and its large filter capacitors are discharged. Please note that the power supply capacitors can take more than five minutes to discharge. Failure to observe this precaution may result in damage to the loudspeakers and/or blowing of the amplifier's DC RAIL FUSES, conditions which are NOT covered by the warranty. The GFA-555ms amplifier can drive a very broad range of loudspeakers. including those with very low impedances, at sustained high-power levels, even when the impedances are highly reactive. Great care was taken to insure that distortion in the amplifier would remain extremely low, particularly when driving these highly reactive loads in which the phase angles of the voltage and the current are substantially different. A little known fact is that amplifiers which measure extremely well on the test bench into a resistive load may not develop the same amount of power into a loudspeaker. Depending on the amplifier's design, sometimes as little as one third of the power which the amplifier develops into a resistor is delivered to a speaker load. Also, the distortion level measured when the amplifier is driving a resistive load is degraded. in some cases quite substantially, when driving the loudspeaker. It is, however, very difficult to measure the performance of an amplifier into a loudspeaker. One reason why this information is not widely known is because during such high-power tests, destruction of the loudspeaker is often the result. In order to determine the performance of the GFA-555ms, ADCOM developed a "computer model" of a "difficult" loudspeaker load. A computer model "mimics" the reactance of the very difficult speaker and duplicates the actual phase angles of the voltage and current throughout the entire audio range. In this manner, high-power testing of amplifier parameters could be undertaken without repetitive destruction of the loudspeaker load. To achieve the above, eight sets of specially selected TO-3-type, metal-cased bipolar output transistors are used in the GFA-555ms in a triple-Darlington configuration of its driver and output stages. The safe operating area of these transistors, along with their circuit topology, insures very efficient power delivery to reactive loads, regardless of the phase angle of voltage and current. In addition, no protection or current-limiting circuitry is used in the GFA-555ms which could degrade the Signal. Electronic protection, or current-limiting circuitry, would impede the delivery of large amounts of current (power) to low-impedance, highly reactive loads, thereby constricting dynamics and deteriorating sonically the performance of the amplifier. Therefore, only fuses on the DC rails are used for protection. The power supply in the GFA-555ms has enormous reserve power capability with an extremely large transformer feeding a storage bank of 40,000uF of filter capacitance. The transformer itself was designed for extremely good regulation, insuring stable voltages regardless of the power demands from the amplifier. Its higher efficiency was insured by using a toroidal design. Additionally, thermal and dynamic tracking of the bias for the triple-Darlington driver and output stages is provided to insure that the amplifier operates in its optimal range regardless of the length of time and the level of operation. The GFA-555ms's circuitry is direct-coupled internally to minimize the sonic deterioration which capacitors, when used between stages of amplification, can cause. As a result, the GFA-555ms can amplify DC and has no limit as to low-frequency or peak-bass energy delivery. In particular, the driver stages are designed to provide all the current required by the output devices without limiting, compressing, or otherwise distorting the signal coming into the power amplifier. All internal point-to-point wiring is OFC and the highest-grade parts, consistent with their application and voltage requirement, have been used in its construction. In especially critical circuits, the f...