HIGH CURRENT POWER AMPLIFIER GFA-555ms The contents of this manual are subject to change. Please check our website www.ADCOM.com or www.ADCOM-USA.com for the most current version. Copyright © 2011 ADCOM LLC Rev 2.
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.
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.
Addition of a servo circuit minimizes DC-offset voltage at the loudspeaker outputs; this insures that there is no woofer "biasing" with attendant low-frequency distortion. Larger heatsinks for each channel for greater heat dissipation and better temperature and dynamic bias tracking. Greater thermal stability is thereby insured. More cooling vents on top cover and chassis for greater cooling efficiency and cooler operation into lower impedances.
INSTALLING THE GFA-555ms The GFA-555ms is equipped with large heatsinks to dissipate the heat generated by the output power transistors. Although during normal home operation the heatsinks will become just warm to the touch, there are instances during high-level playback into low impedance speakers which will cause the heatsinks to become much warmer than normal. Therefore, adequate air circulation MUST be made available to ensure proper heat dissipation from the heatsinks.
packed with the optional rack-mount adaptors for more information. CONNECTING THE GFA-555ms The optimal performance of the GFA-555ms will ultimately depend on the care with which you perform the connections to the amplifier, the preamplifier and the loudspeakers. All the input- and output-signal connections should be made only with high-quality, low-loss cables following the recommendations made in the individual sections below.
accommodate either bare wire, tinned wire, terminal pins, spade lugs, or "banana plugs", both single and dual. The output terminals are color-coded RED and BLACK to indicate polarity. To insure correct stereo phasing, you must connect the RED output terminal (labeled "+") to the loudspeaker input terminal color-coded RED (or labeled POSITIVE, "+", POS,8OHMS or 4 OHMS).
and not exceeding 60 feet, useAWG14stranded wire only. Runs exceeding 60 feet require the use of heavier conductors such asAWG12stranded wire. If you find it difficult to obtain the correct-size wire for your specific connecting length, you can parallel two runs of the next smaller gauge of wire to keep wire resistance at a minimum.
If you prefer to use other methods of connection, unscrew the insulated, hexagonal head of the binding post until the hole in the binding-post stud is accessible. You can then insert the bare or tinned wire, or terminal pin, through the hole. You can also use the many varieties of spade lugs available by simply placing the tines of the spade lug onto the binding post stud. Turn the insulated hex head of the binding post clockwise until the wire or connector is firmly secured.
originate from a point directly in front of you and exact localization will be fairly easy. If you now move a foot or two to the left and the right of your previous position, the singer and instrumentalist will still appear to come from a point directly in front of you. If your loudspeakers are out-of-phase, the image of the performers will be imprecise and difficult to pinpoint.
correct stereo phasing with optimal bass response, you must observe these connections precisely. Although the GFA-555ms can generate a substantially greater amount of power in the bridged mono mode than when it is in its normal stereo mode, it requires the use of loudspeakers the nominal impedance of which does not drop below 4 ohms. It is not recommended that the GFA-555ms be used in the bridged mono mode into loudspeakers, or multiple loudspeaker loads, which drop in value substantially below 4 ohms.
operate reliably and if triggering of protective devices is to be avoided. The preceding is particularly true of amplifiers which have extremely wide audio bandwidth, such as ADCOM amplifiers.
To remove a blown or suspect fuse from its fuse holder, use only a number 2 Phillips screwdriver to prevent damage to the fuse holder. Simply press lightly on the fuse-holder cap and turn counterclockwise. The cap will "pop" out after several turns.
Whenever the POWER switch on the front panel is turned on and the amplifier is energized, the POWER LED will glow. If turning on the amplifier does not cause the POWER LED to glow, it may be an indication that the AC LINE FUSE is blown. Unplug the AC LINE CORD from the AC wall outlet and turn the POWER0 switch off and check the fuse. If the fuse is blown, replace it with one of the fuses listed in the table below, plug the amplifier into its AC-wall outlet and turn on the amplifier.
damaging the fuse holder. DO NOT FORCE THE FUSE-HOLDER CAP INTO THE THREADS. Seating of the cap in the fuse-holder body should be easily accomplished without excessive force. AC ON/OFF SWITCH The AC ON/OFF switch controls power to the power transformer and circuits of the GFA-555ms. Whenever the GFA-555ms is energized, the red POWER LED will glow. Push the top of the rocker switch to energize the GFA-555ms. Push the bottom of the rocker switch to turn the unit off.
INSTANTANEOUS DISTORTION ALERT LEDs The INSTANTANEOUS DISTORTION ALERT circuit is a unique ADCOM distortion-detection system which reads all forms of non-linear distortion such as THD, 1M, slew-induced, "clipping", etc. The INSTANTANEOUS DISTORTION ALERT LEDs will light when distortion reaches 1% regardless of the impedance or the phase angle of the current voltage and the reactance of the loudspeakers which the amplifier is driving.
NOTE ACTIVATION OF THE THERMAL PROTECTION CIRCUITRY IN THE GFA-555ms IS AN INDICATION THAT THE AMPLIFIER HAS BEEN OVERDRIVEN OR THAT THE LOAD THE LOUDSPEAKERS ARE PRESENTING TO THE AMPLIFIER IS UNREASONABLY LOW. IF YOU WISH TO PREVENT RECURRENT ACTIVATION OF THE THERMAL PROTECTION CIRCUITRY, YOU MUST REDUCE THE VOLUME LEVEL DEMANDS, RECTIFY THE LOAD-IMPEDANCE CONDITION WHICH MAY BE CAUSING ACTIVATION OF THIS CIRCUITRY, INSTALL AN AUXILIARY FAN OPTION OR, IN EXTREME CASES, UNDERTAKE ALL THREE.
When calling or writing about your GFA-555ms, be sure to note and refer to its model and serial numbers as well as the date of purchase and the ADCOM authorized dealer from whom it was purchased. In the event the unit must be returned for service, you will be instructed as to the proper procedure when you call or write. UNDER NO CIRCUMSTANCES SHOULD YOUR UNIT BE SHIPPED TO US WITHOUT PRIOR AUTHORIZATION, OR PACKED IN OTHER THAN ITS ORIGINAL CARTON AND FILLERS.
GFA-555ms SPECIFICATIONS Power Rating (To FTC Requirements) 125 watts continuous average power into 8 ohms at any frequency between 20Hz and 20kHz with both channels driven at less than 0.04% THD. 200 watts continuous average power into 4 ohms at any frequency between 20Hz and 20kHz with both channels driven at less than 0.05% THD. 400 watts continuous average power into 8 ohms at any frequency between 20Hz and 20kHz at less than 0.09% THD, bridged.
IM Distortion, Bridged (CCIF, Any Combination from 4kHz to 20kHz) 400 watts into 8 ohms ≦0.005% THD + Noise at 400 Watts into 8 Ohms, Bridged 20Hz 0.004% 1kHz 10kHz 0.004% 0.020% 20kHz 0.050% Frequency Response @ 1 Watt into 8 Ohms 10Hz to 20kHz +0. -0.25dB 10 Hz to 100kHz Power Bandwidth (-3dB) 1.3dB Dynamic Headroom into 4 Ohms Signal-to-Noise Ratio, "A" Weighted 125 watts into 8 ohms ≧102dB 27.2dB Gain 100,000 ohms Input Impedance Input Sensitivity 125 watts into 8 ohms 1.
400 watts into 8 ohms, bridged 800VA GENERAL Power 120VAC/60Hz Chassis Dimensions Maximum Dimensions 5-1/4" (134mm) x 16-15/16" (430mm) x 12-1/8" (308mm) 6” (152mm) x 17" (432mm) x 12-1/2" (318mm) 28.6lbs.(13kgs) 33 Ibs.(15kgs) Weight Weight, Packed Specifications subject to change without notice.
