Adcom 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
Copyright © 2011 ADCOM LLC
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 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
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