Adcom Stereo Amplifier GFA 555MS User Manual

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 2.1  
 
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 finest quality film capacitors have been used. Among its other design  
improvements are the following:  
Larger potted transformer provides better overall regulation for driving lower  
impedances at high power levels; also provides greater peak-current capability into  
loudspeaker loads.  
No electrolytic capacitors in the low-frequency signal path or feedback-loop path. Only  
precision non-polarized capacitors are used.  
 
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.  
High-quality, gold-plated brass RCA input jacks with Teflon insulators.  
Thermal overload indicator LED on front panel.  
IMPORTANT NOTICE  
ADCOM PROTECTION PLAN  
(U.S.A ONLY)  
ADCOM offers the enclosed valuable Limited Warranty. Please read the details on the  
Warranty card carefully to understand the extent of the protection offered by the Warranty,  
its reasonable limitations, and what you should do in order to obtain its benefits. Please  
register your warranty online at www.ADCOM.com or www.ADCOM-USA.com  
Be sure to verify that the serial number printed on the rear panel matches the serial number  
on the outer carton. If any number is altered or missing, you should notify us immediately in  
order to insure that you have received a genuine ADCOM product which has not been  
opened, mishandled or tampered with in any way.  
UNPACKING  
Before your GFA-555ms left our plant, it was carefully inspected for physical imperfections  
and tested for all electrical performance parameters as a routine part of ADCOM's  
systematic Quality Control. This, along with full operational and mechanical testing, should  
insure a product flawless in both appearance and performance. After you have unpacked  
the GFA-555ms, inspect it for physical damage. Save the shipping carton and all internal  
packing materials, as they are intended to reduce to a minimum the possibility of  
transportation damage, should the amplifier ever need to be shipped again. In the unlikely  
event damage has occurred, notify your dealer immediately and request the name of the  
carrier so that a written claim to cover shipping damages can be initiated.  
THE RIGHT TO A CLAIM AGAINST A PUBLIC CARRIER CAN BE FORFEITED IF THE  
CARRIER IS NOT NOTIFIED PROMPTLY IN WRITING AND IF THE SHIPPING CARTON  
AND PACKING MATERIALS ARE NOT AVAILABLE FOR INSPECTION BY THE CARRIER.  
SAVE ALL PACKING MATERIALS UNTIL THE CLAIM HAS BEEN SETTLED.  
 
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. You will ensure the amplifier's long-term, trouble-free operation if you  
keep it away from external sources of heat, such as radiators, hot-air ducts or intense direct  
sunlight, and provide reasonable ventilation. The GFA-555ms should never be placed with  
other heat-producing components in a cabinet or enclosure lacking free air flow. You should  
also provide adequate space around the amplifier to insure good air circulation.  
The top, bottom and side panels of the amplifier have been provided with slots to allow  
necessary cooling of the internal components of the amplifier. This GFA-555ms Required  
Four Inches of Clearance on the Top and All Sides. You should ensure that these slots  
are not obstructed in any way.  
We advise that you refrain from stacking other components on top of the GFA-555ms. Not  
only will the heat generated by the amplifier affect the performance of equipment stacked on  
top of the GFA-555ms, but the free flow of air through the ventilating slots provided in the  
chassis of the amplifier may be partially obstructed. If you require that the GFA-555ms be  
mounted in an enclosed cabinet, it is recommended that the rear panel of the cabinet be  
provided with vents or slots at the bottom and top to allow air to circulate freely through the  
cabinet.  
If you observe these recommendations, the GFA-555ms will perform quite reliably in any  
reasonable environment. You should also pay attention to such routine considerations as  
protection from excessive dust and moisture. Occasional vacuuming of accumulated dust on  
the surfaces of the chassis, panels and heatsinks should be all that is required.  
For use in professional installations, the GFA-555ms may be mounted in a standard 19-inch  
rack using the optional RM-7 rack-mount adaptors available through ADCOM dealers. If the  
GFA-555ms is to be mounted on a rack, along with other components which are  
interconnected to the GFA-555ms, the amplifier's chassis must be insulated from the  
metal-rack rails to prevent ground loops, especially if the rack is grounded to "earth", and to  
avoid defeating the audio grounding scheme of the power amplifier (the audio-input grounds  
are isolated from and above the chassis ground). Please consult the instruction sheet  
 
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. Please refer to the  
rear-panel diagram to identify all the connections and their locations.  
NOTE  
WHENEVER CONNECTIONS TO OR FROM THE GFA-555ms ARE BEING MADE, BE  
CERTAIN THAT THE POWER SWITCH ON THE AMPLIFIER IS IN ITS OFF POSITION,  
THE AC LINE CORD OF THE AMPLIFIER IS DISCONNECTED FROM THE AC WALL  
OUTLET AND THAT ALL ASSOCIATED COMPONENTS ARE TURNED OFF.  
RIGHT/LEFT INPUT  
The audio inputs to the GFA-555ms are through two high-quality, gold-plated brass RCA  
jacks using Teflon insulation to minimize high-frequency losses, noise, etc. They will accept  
standard RCA-type plugs, one for each channel, LEFT and RIGHT, usually supplied at the  
ends of interconnecting cables. To insure that the performance designed into the  
GFA-555ms is preserved, you should use the highest quality plugs and cable as are feasible.  
There are many cables which are designed specifically for these applications and your  
ADCOM dealer can be of help in selecting the best cable for your application. Whatever  
cable you finally select, it should have low capacitance. This is particularly important if you  
use a long run between the preamplifier and the amplifier or if your preamplifier has a high  
output impedance. Generally speaking, a cable with a capacitance of around 100pF will  
work well.  
The load impedance which the GFA-555ms inputs present to the source preamplifier is  
100,000 ohms. This load impedance results in minimal amplifier noise and is more than  
adequate for use with any associated source component regardless of its output impedance.  
To preserve the correct stereophonic effects, please be certain to connect the left output of  
the preamplifier to the RCA jack on the GFA-555ms labeled LEFT INPUT and the right  
output of the preamplifier section to the RCA jack labeled RIGHT INPUT.  
RIGHT/LEFT STEREEO SPEAKER OUTPUTS  
The GFA-555ms's connections to the loudspeakers are made through high-grade 5-way,  
gold-plated brass binding-post terminals located on the rear panel. These terminals will  
 
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). The BLACK binding post terminal on the amplifier (labeled "-")  
should be connected to the BLACK loudspeaker terminal (or labeled NEG, "-", C, COM,  
COMMON, G, or GROUND).  
NOTE  
The GFA-555ms is polarity correct; that is, it does not invert "phase". Any positive-going  
signal at its inputs will appear as a positive-going signal at its outputs.  
The RIGHT STEREO OUTPUT should be connected to the right-channel loudspeaker, as  
you face the pair of loudspeakers, and the LEFT STEREO OUTPUT to the left-channel  
loudspeaker.  
Be certain, when the GFA-555ms is used in Its stereophonic mode, the  
STEREO/BRIDGED MONO INPUT switch is in the STEREO position. Otherwise, the  
amplifier will not operate in the stereo mode. You will amplify only the left channel through  
both outputs of the amplifier. For further clarification, please refer to the section  
STEREO/BRIDGED MONO INPUT/OUTPUT.  
In order to insure that connections to the loudspeakers are correct, you must be able to  
identify each wire conductor of the loudspeaker cables at both ends of the cables. This is  
relatively easy to do since most loudspeaker cables consist of two parallel, stranded  
conductors in a flexible insulation, with a coding system for wire identification. Sometimes  
there is a colored "tracer" wrapped around one of the conductors; some cords have one of  
the conductors colored silver and the other copper; some have a "ridge" molded on the  
insulation on one of the conductors, while others are marked with a "+" and/or "_". Your  
ADCOM dealer also sells special loudspeaker interconnecting cables and these are most  
often labeled with respect to polarity.  
Generally speaking, when making connections to the loudspeakers from the amplifier, it is  
very important to use the correct type and size of wire in order to avoid unnecessary loss of  
amplifier power in the cable, reduction of amplifier damping factor (DF) and other  
undesirable conditions. For runs up to 12 feet, ordinary "zip" or lamp cord, made of AWG18  
stranded wire and available in a variety of insulation colors may be used. For runs up to 40  
feet, AWG16 stranded wire should be used to prevent power losses. For lengths over 40 feet  
 
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. For example, if you require a  
run of 35 feet to your loudspeakers and AWG16 wire is not readily available, you can parallel  
two 35-foot lengths of identical AWG18 stranded wire for use with each speaker (you'll  
require a total of four 35-foot lengths in such an instance) and solder the two conductors of  
each wire making up each double cable, at both the speaker and amplifier ends, to insure  
good electrical and mechanical connections of the conductors.  
Regardless of the cables you select to connect your loudspeakers, there are some other  
requirements which you should observe in order to insure maximum performance from your  
amplifier. It is most important that you make certain the wiring you have selected has as  
Iowa capacitance as possible. All amplifiers, particularly wide-bandwidth audio amplifiers,  
are susceptible to the capacitance cables present to their outputs at extremely high  
frequencies. This capacitance, in conjunction with the inductance of the wire itself and the  
reactive load of the loudspeakers, can create anomalies at ultrasonic frequencies which,  
although inaudible, can affect performance in the audible range.  
There are different ways to connect the wiring to the RIGHT/LEFT STEREO SPEAKER  
OUTPUTS. The methods used will depend on the specific type of connectors supplied with  
the loudspeakers, the speaker cables, etc. As a matter of course, we prefer to use double  
banana plugs because it is generally the most secure method of connection. Also, the  
plated-bronze springs of the banana plugs effect a self-cleaning action which insures the  
best contacts between the binding posts and the connectors themselves. There are  
"sockets" provided in the center of the binding posts' studs which permit secure seating of  
the banana plugs. Make certain, however, that the hexagonal head of the binding post is  
securely tightened before inserting the banana plugs firmly into the binding posts' sockets.  
Additionally, when connecting the cables to the amplifier and loudspeakers, it is important  
that you "tin" the wires with good solder (preferably high-silver-content solder) in order to  
minimize contact resistance. Tinning prevents the build-up of surface compounds which  
form with copper wire and which increase its contact resistance. It is partly for this reason  
that double or single banana plugs are preferred. However, make sure that the cable ends  
are tinned before you make the cable connections to the banana plugs. Alternatively, you  
can use "crimped" pins or other lugs to insure lowest contact resistance at the connection to  
the amplifier and loudspeakers.  
 
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. Finger pressure is sufficient and you should not use pliers, or other tools, which  
could damage or over-tighten the binding post assembly. The binding post has been  
designed in such a way that finger pressure is all that is needed to cause a "pinching" action  
among the different metal surfaces to insure proper connection.  
All loudspeaker systems having a nominal impedance down to 2 ohms can be connected to,  
and driven by, the GFA-555ms. The GFA-555ms can drive these low impedances at more  
than adequate power levels with no difficulty. It should be noted here that many loudspeaker  
systems which are rated, nominally, at 4 ohms drop in impedance, in some parts of their  
frequency range, to as low as 2 ohms (and some others to even less than 2 ohms). You will  
not experience difficulties even with these very-low-impedance loads.  
In most applications, you can drive two or more sets of loudspeakers. You should note,  
however, that when loudspeakers are paralleled, the impedance presented to the amplifier is  
lower than the nominal impedance of each loudspeaker. In other words, if you parallel two  
8-ohm sets of loudspeakers, the resultant impedance will be 4 ohms. If you parallel two sets  
of 4-ohm-impedance loudspeakers, the resultant impedance of the load will be 2 ohms. If  
8-ohm and 4-ohm loudspeakers are paralleled, the resultant impedance will be about 2.6  
ohms. In these last two situations, and depending on the lowest impedance of the  
nominally-4-ohm speakers, and when making excessive power demands from the amplifier,  
you may trigger the THERMAL PROTECTION on the amplifier or blow one of the DC RAIL  
FUSES. See their respective sections for more information. In those very rare instances in  
which your particular application causes frequent activation of the THERMAL PROTECTION  
e circuitry, you might consider use of an auxiliary fan to increase amplifier cooling.  
Alternatively, and for convenient switching of multiple sets of speakers with impedance  
protection for the amplifier, you may consider the use of an ADCOM speaker selector. These  
are available from your ADCOM dealer.  
Should you wish to verify that your loudspeakers are in-phase, once connections to the  
amplifier have been made, play a recording of solo voice with single-instrument  
accompaniment, at normal volume, with the preamplifier's mode switch in the mono or A+B  
position. Stand about three feet in front of the loudspeakers and exactly between them. If  
your loudspeakers are in-phase, the voice and accompanying instrument will appear to  
 
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. Depending on the room, the image may appear to be coming from behind you, or  
the sound will seem to surround you, and, as you move left and right from your center  
position, the origin of the sound will seem to change instantaneously.  
Should your connections have resulted in an out-of-phase condition, simply reverse the  
leads on one of the loudspeakers; that is, switch the wire connected to the positive input  
terminal of the loudspeaker to the negative terminal of the loudspeaker and vice-versa.  
Repeat the listening test with the mono signal to make sure you are correct in your initial  
evaluation. If you can now achieve a precise and stable image of the singer and instrument  
between the two speakers, make that connection to the speaker permanent.  
STEREO/BRIDGED MONO INPUT/OUTPUT  
The GFA-555ms can be used as a very powerful mono amplifier to drive 8-ohm impedance  
loudspeakers when in its "bridged" mode. No modification to the amplifier is necessary for  
operation in the bridged mode, nor are any additional accessories required. However, you  
will need two GFA-555ms for stereophonic reproduction, if you are using them in the bridged  
mono mode.  
To set the amplifier in bridged mono operation, flip the STEREO/BRIDGED MONO INPUT.  
switch into the BRIDGED MONO INPUT position. When in the bridged mono mode, input to  
the amplifier is made only through the LEFT INPUT RCA jack. The connection to the RIGHT  
INPUT jack should be removed since the right-channel input portion of the amplifier is  
inoperative.  
Only a single loudspeaker is to be connected to the GFA-555ms when in the bridged mono  
mode. Please note that connections made to the loudspeaker from the GFA-555ms, when  
used in the bridged mono mode, are different from those made when the amplifier is used in  
the stereo mode. The LEFT RED output binding-post terminal (labeled BRIDGED MONO  
OUTPUT "+") should be connected to the loudspeaker input terminal color-coded RED (or  
labeled POSITIVE, "+", POS, 8 OHMS or 4 OHMS). The RIGHT RED output binding post  
terminal on the amplifier (labeled BRIDGED MONO OUTPUT "-") should be connected to  
the BLACK loudspeaker terminal (or labeled NEG, "-", C, COM, COMMON, G, or GROUND).  
All the wiring and phasing recommendations in the section RIGHT/LEFT STEREO  
SPEAKER OUTPUT 8 apply to this connection as well. Please note that if you want to insure  
 
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. Otherwise, you  
may trigger the THERMAL PROTECTION 0 or blow one of the DC RAIL FUSES e. Please  
refer to the section RIGHT/LEFT STEREO SPEAKER OUTPUT for further clarification. A  
little known fact is that when any amplifier is operated in the bridged mode, the load is "split"  
between the two amplifiers in the bridged configuration. Therefore, an 8-ohm loudspeaker  
will be seen by the amplifier as if it were a 4-ohm load; a 4-ohm loudspeaker load will be  
seen by the amplifier as a 2-ohm load.  
NOTE  
If the connections described above are followed exactly, the GFA-555ms will be polarity  
correct, that is, it will not invert "phase". Any positive-going signal at its input will appear  
as a positive-going signal at the loudspeaker.  
DC RAIL FUSES  
The DC RAJ L FUSES provide protection for the output stages and power supply in the  
event of excessive current demands from the amplifier, either long-term or short-term.  
If the amplifier ceases to operate, either on one or both channels, particularly during  
high-level passages, or long-term high-volume playback, and the POWER LED glows while  
the THERMAL PROTECTION LED is out, the chances are that one or both of the DC RAIL  
FUSES on that channel, or both channels, are blown.  
Your ADCOM amplifier is designed to activate its protective devices reliably, particularly  
when the amplifier is carelessly operated well beyond its limitations. Other types of  
protection circuits, beyond the methods used in the GFA-555ms amplifier, such as current  
limiting, etc., usually result in deterioration of the audio performance of the amplifier.  
While the GFA-555ms will operate dependably in every normal situation, no amplifier is  
impervious to abuse. There are conditions which must always be avoided if the amplifier is to  
 
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. Among the undertakings which must be avoided, if damage to the amplifier or to  
the loudspeakers being used is to be prevented, are actions such as connecting the inputs  
or outputs to or from the amplifier while the amplifier is ON, or using what has been  
commonly termed the "thumb test" - that is, touching the center pin of the RCA jack on one  
end of the audio interconnecting cable while the other end is plugged into the amplifier and  
the amplifier is ON.  
NOTE  
In the event that the DC RAIL FUSES need to be replaced, only one of the fuse types listed  
in the table below should be used. Please note that the fuses listed in the table, and their  
time-current blowing points, have been carefully selected and thoroughly tested to deliver  
optimal performance while still accomplishing their protective functions. Replace these  
fuses, individually, only with the specific types listed. DO NOT USE ANY SUBSTITUTE  
FUSES WITH DIFFERENT RATINGS, TIME-CURRENT CURVES OR VALUES. Failure  
to observe this precaution may cause serious damage to the amplifier circuits, MAY  
CREATE A FIRE HAZARD, AND MAY VOID YOUR WARRANTY. For your convenience,  
a replacement set of two of the correct DC RAIL FUSES is supplied with each amplifier to  
facilitate restoration of the amplifier to operation in the event of a blown fuse.  
The GFA-555ms has a massive power supply which remains charged for up to FIVE  
MINUTES after the amplifier is turned off and the POWER LED ceases to glow. It also  
remains energized when the POWER. LED glows even if the THERMAL PROTECTION  
LED is ON. Therefore, you should exercise great caution when connecting arid/or  
disconnecting loudspeakers to or from the RIGHT/LEFT STEREO SPEAKER OUTPUTS.  
Should you inadvertently short out the RED ("+") and BLACK ("-") STEREO SPEAKER  
OUTPUTS, enough power may remain in the power supply to cause sufficient current to  
blow the DC RAIL FUSES on the affected channel. When connecting or disconnecting  
loudspeakers to or from the RIGHT/LEFT STEREO SPEAKER OUTPUTS, always be  
certain to wait at least FIVE MINUTES after turning the amplifier OFF before undertaking  
any such procedure.  
DC RAIL FUSES  
BUSSMAN  
LITTELFUSE  
BEL  
AGC-7/250V  
3AG312007/250V  
3AG 7A/125V  
 
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. To replace the  
fuse-holder cap, once the fuse has been replaced and properly installed in its seat on the  
fuse-holder cap, press lightly inward, once the fuse and cap have been inserted in the  
fuse-holder body, and turn the cap clockwise until it is firmly seated in the fuse-holder body.  
Be certain not to cause cross-threading of the fuse-holder body and cap to prevent  
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 LINE CORD  
The AC cord is a 3-conductor, heavy-duty cable supplied with a heavy-duty, molded 3-prong  
AC plug. It provides the power to operate all the GFA-555ms's circuits. Its plug should be  
inserted in a standard, 120V/60Hz outlet with a minimum capacity of 15amperes and should  
have a dedicated branch circuit. It is not recommended that you connect the GFA-555ms  
into a branch circuit on which other appliances are used. Since conventional household  
branch circuits are often limited to 15amperes, when the GFA-555ms is used in conjunction  
with other appliances on the circuit, you may find the current available for the GFA-555ms is  
insufficient; particularly in systems using very low-impedance, highly reactive loudspeakers.  
NOTE  
The GFA-555ms’s power cord is supplied with a "polarized" AC plug as required by  
UL/CSA standards and the National Electrical Code. To minimize the risk of electrical  
shock, and to insure minimal hum from the system, do not defeat the polarity-insuring  
feature of the plug (one wide blade and one narrow blade). To prevent electrical shock, do  
not use the polarized plug With an extension cord or receptacle, or other outlet, unless the  
blades can be fully inserted to prevent blade exposure.  
AC LINE FUSE  
The AC LINE FUSE protects the electronic circuits of the GFA-555ms. This fuse, normally,  
will blow only if there is an overload within the GFA-555ms. Since this fuse has been  
designed to protect the electronic circuits in the GFA-555ms, it is recommended that it be  
replaced only with one of the fuses listed in the table below. Please note that the fuses listed  
are for operation of the amplifier on 120VAC/60Hz. For the correct fuse values to operate the  
GFA-555ms on other voltages and frequency, please consult the Service Manual for this  
amplifier available from the ADCOM Technical Service Department.  
 
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. If after replacing the fuse, it blows immediately upon  
turning on the amplifier (POWER LED does not glow), a failed electronic component or  
other internal malfunction must be suspected. Make no further attempts at fuse replacement  
or operation of the amplifier. Refer the problem to competent ADCOM-authorized  
service personnel.  
NOTE  
Before checking or replacing a blown fuse, make certain you UNPLUG THE AC LINE  
CORD FROM THE AC WALL OUTLET TO PREVENT POSSIBLE ELECTRICAL  
SHOCK.  
AC LINE FUSES  
BUSSMAN  
LITTELFUSE  
SOC  
ABC-12/250V  
3AB314012/250V  
CES6-12A/125V  
NOTE  
The fuses listed above, and their time-current blowing points, have been carefully selected  
and thoroughly tested to deliver optimal performance while still accomplishing their  
protective functions. Replace the AC LINE FUSE only with one of the fuses listed above.  
DO NOT USE ANY SUBSTITUTE FUSES WITH DIFFERENT RATINGS,  
TIME-CURRENT CURVES OR VALUES. Failure to observe this precaution may cause  
serious damage to the amplifier circuits, MAY CREATE A FIRE HAZARD, AND WILL  
VOID YOUR WARRANTY.  
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. To replace the  
fuse-holder cap, once the fuse has been replaced and properly installed in its seat on the  
fuse-holder cap, press lightly inward, once the fuse and cap have been inserted in the  
fuse-holder body, and turn the cap clockwise until it is firmly seated in the fuse-holder body.  
Be certain not to cause cross-threading of the fuse-holder body and cap to prevent  
 
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.  
POWER LED  
This LED will glow whenever the AC ON/OFF switch is turned on and the GFA-555ms is  
energized. If the AC LINE FUSE blows, the POWER LED will cease to glow.  
The POWER LED indicates that there is AC voltage being fed to the amplifier, but it does not  
signify that all the amplifier's circuits are in operation. If, for example, you have blown one or  
more of the DC RAIL FUSES., the amplifier will not operate - that is, the amplifier will not  
produce any audio signal - even though the POWER LED glows. Similarly, if the THERMAL  
PROTECTION LED glows, the amplifier will not produce sound even though the POWER  
LED may still glow.  
Additionally, the internal power transformer is provided with a thermostat which will interrupt  
power into the transformer if its temperature exceeds 125°C. This high a temperature will  
seldom, if ever, be encountered unless the amplifier is subjected to abnormal conditions,  
such as operation into loads of less than 1 ohm at very high listening levels, etc. If the  
POWER LED does not glow, the THERMAL PROTECTION LED is out and both the DC  
RAIL FUSES and AC LINE FUSE are intact, the indication would be that the thermostat  
within the transformer has opened.  
Once the temperature within the transformer decreases to a normal level, the thermostat will  
reset itself and normal operation will resume. If you are to avoid tripping continually the  
thermostat in the transformer, you must reduce the sound level demands into such low  
impedances, install auxiliary fan cooling on the amplifier, or both.  
For a more detailed description of the operation of the THERMAL PROTECTION LED circuit  
and the DC RAIL FUSES, please refer to their respective sections.  
 
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. Sometimes,  
when the amplifier is in use, the LEDs may occasionally flicker under high-volume listening,  
particularly if you are driving low impedances. This flickering is no cause for concern. The  
LEDs are simply warning you that the amplifier is approaching its maximum power output  
into the specific loudspeakers which you are using. If, however, the INSTANTANEOUS  
DISTORTION ALERT LEDs glow brightly or are on most of the time during playback, you  
are overdriving the amplifier and should lower the volume control to reduce the  
listening-level demands, or you may blow the DC RAIL FUSES, cause the THERMAL  
PROTECTION to be activated or, in extreme cases, damage your loudspeakers.  
THERMAL PROTECTION LED  
The GFA-555ms is provided with a thermal protection circuit which will shut down the  
amplifier if the temperature of either heatsinks reaches 85°C. The THERMAL PROTECTION  
LED will light whenever the thermal protection circuit on either channel, or both channels,  
has been triggered and the amplifier is inoperative. The thermal protection circuitry will  
typically be triggered by very high-power demands into impedances much lower than the  
amplifier is capable of driving at those levels. If the output of either channel of the amplifier  
ceases abruptly, and the THERMAL PROTECTION LED glows, you will know that its  
heatsinks temperature has become unacceptably high and the circuitry is protecting the  
amplification devices. Please note that the POWER LED will remain on and the amplifier will  
still be energized.  
Once the temperature of the heatsinks drops to a safe operating level, the amplifier will  
automatically return to operation.  
If the amplifier ceases to operate and both the POWER LED and the THERMAL  
PROTECTION LED are off, the condition may indicate that the AC LINE FUSE has blown.  
Please refer to the section AC LINE FUSE e for instructions on replacing this fuse.  
 
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.  
CARING FOR YOUR GFA-555ms  
Great care has been taken by ADCOM to assure that your amplifier is as flawless in  
appearance as it is electronically. The front panel is a heavy- gauge, high-grade aluminum  
extrusion carefully finished and anodized for durability. The chassis, top cover and rear  
panel are of heavy-gauge steel, both painted and baked. If the front panel, top or sides  
should become dusty or fingerprinted, they can be cleaned with a soft Iintless cloth, slightly  
dampened with a very mild detergent solution or non-ammonia glass cleaner.  
NOTE  
DO NOT SPRAY OR USE LIQUIDS OF ANY KIND ON THE SURFACES OF THE  
GFA—555se. DO NOT EXPOSE THE AMPLIFIER TO RAIN, WATER OR MOISTURE OF  
ANY KIND.  
SERVICING-North America  
ADCOM has a Technical Service Department to answer questions pertinent to the  
installation and operation of your unit. In the event of difficulty, please contact us for prompt  
advice. If your problem can not be resolved through our combined efforts, we may refer you  
to an authorized repair agency, or authorize return of the unit to our plant. To aid us in  
directing you to a convenient service station, it would be helpful if you indicate which major  
city is accessible to your home.  
Please address mail inquiries to:  
ADCOM-USA/J&B DISTRIBUTION  
PO BOX 54096  
Phone or Fax inquiries:  
Monday through Friday  
9:00AM to 4:00PM Arizona Time  
Phone Number: 480-607-2277  
PHOENIX, AZ 85078  
U.S.A.  
 
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.  
Always ship PREPAID VIA UPS, FDX OR OTHER APPROVED CARRIER. DO NOT SHIP  
VIA PARCEL POST, since the packing was not designed to withstand rough Parcel Post  
handling. FREIGHT COLLECT SHIPMENTS WILL NOT BE ACCEPTED.  
 
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 (SMPTE)  
1 watt to 125 watts into 8 ohms  
1 watt to 200 watts into 4 ohms  
0.009%  
0.009%  
IM Distortion (CCIF, Any Combination from 4kHz to 20kHz)  
125 watts into 8 ohms  
0.002%  
0.003%  
200 watts into 4 ohms  
THD + Noise at 125 Watts into 8 Ohms  
20Hz  
0.004%  
0.003%  
0.006%  
0.020%  
1kHz  
10kHz  
20kHz  
THD + Noise at 200 Watts into 4 Ohms  
20Hz  
0.005%  
0.004%  
0.020%  
0.030%  
1kHz  
10kHz  
20kHz  
IM Distortion, Bridged (SMPTE)  
1 watt to 400 watts into 8 ohms  
0.05%  
 
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%  
0.004%  
0.020%  
0.050%  
1kHz  
10kHz  
20kHz  
Frequency Response @ 1 Watt into 8 Ohms  
10Hz to 20kHz  
+0. -0.25dB  
10 Hz to 100kHz  
1.3dB  
Power Bandwidth (-3dB)  
Dynamic Headroom into 4 Ohms  
Signal-to-Noise Ratio, "A" Weighted  
125 watts into 8 ohms  
102dB  
Gain  
27.2dB  
Input Impedance  
100,000 ohms  
Input Sensitivity  
125 watts into 8 ohms  
1 watt into 8 ohms  
1.38V rms  
123mV rms  
Damping Factor  
20Hz to 20kHz  
>400  
2.3us  
Rise Time  
5kHz,90V peak-to-peak square wave, 20% to 80%  
Semiconductor Complement  
34 transistors, 2 zener diodes,  
13 diodes, 2 ICs, 2 diode bridges  
Power Consumption (Continuous, Both Channels Driven)  
Quiescent  
48VA  
900VA  
420VA  
730VA  
Maximum  
125 watts into 8 ohms  
200 watts into 4 ohms  
 
400 watts into 8 ohms, bridged  
800VA  
GENERAL  
Power  
120VAC/60Hz  
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)  
Chassis Dimensions  
Maximum Dimensions  
Weight  
Weight, Packed  
33 Ibs.(15kgs)  
Specifications subject to change without notice.  
Published by ADCOM LLC Copyright 2011 ADCOM LLC All rights reserved  
ADCOM and the ADCOM logo are registered trademarks of ADCOM LLC  
No part of this manual may be reproduced or electronically transmitted without the express written consent of ADCOM LLC.  
ADCOM LLC shall not be liable for any errors contained herein or for any damages arising out of or related to this document  
or the information contained herein, even if ADCOM LLC has been advised of the possibility of such damages. This  
document is intended for informational and instructional purposes only. ADCOM LLC reserves the right to make changes  
in the specifications and other information contained in this document without prior notification. ADCOM LLC disclaims  
any obligation to update the information contained herein.  
 
 

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