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.

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

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.