Adcom GFP-345 User Manual

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HIGH DEFINITION

PREAMPLIFIER

GFP-345

INTRODUCTION

Piease read thoroughly these operating instructions forthe G FP-345 before connecting or attempting to operate this unit.
For your own benefit, foHow alt the instructions in this manual and save it tor future reference.

The ADCOM GFP-345 is a result of a thorough examination and réévaluation of all factors affecting the sonic performance
of preamplifiers. A great deal of care was taken to insure that the audio performance of the GFP-345 could be maintained
for years to come by the selection of top-quality components and circuit topologies. The result is a preamplifier which is
as musical-sounding as, and which preserves the dynamic contrasts of, an original performance.

The installation and operation of the GFP-346 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 GFP-345, Please read it carefully to fully comprehend all its features and functions
and thereby derive the maximum performance from its use in your system.

FEATURES GFP-345

The GFP-345 is modeled after the highly innovative internal designs and circuit topologies of both the "Reference
Standard” GFP-565 and GFP-555D preamplifiers — accordingly, it is setting new standards for products in its price
category.

Many of the most significant performance characteristics of the GFP-565 and GFP-555D have been built into the GFP-345.
The power supply and low impedance outputs were designed to preserve music dynamics. The amplification stages
employ direct-coupled, custom-designed, linear-gain amplifiers operating in full Class A to insure the minimum
distortion and low-noise required for digital-source material. Buffered tape outputs insure input-signal integrity, and
preclude interference with the main-source signal in the preamplifier's circuits when digital or cassette tape recorders are
connected (even when these are not operating).

The optional phono preamplifier {available as a plug-in board: the PHO-802A) delivers the quietest obtainable

performance with unmatched record-equalization accuracy and lowest distortion. Operating in full class A, its
unrestricted dynamics and musical accuracy are delivered at surprisingly modest cost.

The GFP-345 contains the highest-quality components; never before available in a component of such moderate cost.

This should insure stable, dependable operation and long, trouble-free life.

• Low-impedance power supply for best regulation and optimal circuit performance. This type of design insures that
music dynamics are delivered through the preamplifier without compression.

• Thick power-supply traces on printed circuit board for minimal losses and improved voltage stability. This
technique insures constant voltage to all the circuitry, regardless of volume or dynamic demands of the music.

• All capacitors are state-of-the-art electrolytics and film capacitors with very low ESR and unsurpassed sonic purity.
This design feature provides an audible tenefit, the lowest possible distortion of music signals.

• Overdesigned mu-metal-$hielded power transformer for better regulation. This transformer design eliminates stray
hum fields for lowest noise and provides undiminished voltage to circuits for best performance under any demands
for level.

• Precision, programmable regulators for stabilized, constant DC voltages, regardless of load. This type of regulator
insures stable operation of all circuits regardless of the dynamic demands of the music being played.

• Amplifier stages use ADCOM premium-grade ICs selected for their linear operation and sonic quality. These linear
amplifiers insure extremely low noise, low distortion and low DC offset for best performance with either set of
preamp outputs.

• Amplifiers are operated in pure Class-A mode to insure lowest distortion regardless of the signal level or setting of
the level control.

• Entire signal path is direct-coupled from input to output. Direct coupling minimizes the number of components
signal must go through, preventing degradation of the music.

• Two sets of outputs: OUTPUTS LAB are direct-coupled. They provide a signal in the most straightforward, simplest

path. OUTPUTS NORM are capacitor-coupled using precision, high-grade film capacitors for power amps which
can not accept ultra-wide bandwidth inputs.

•

• Low-output-impedance {100 ohms) from both sets of outputs to reduce interaction of connecting cables and other
equipment and preserve music dynamic contrasts.

• Five sets of inputs, front panel controlled, including TAPE IN; six sets if the EXTERNAL PROCESSOR IN jacks are
used also as an auxiliary input.

• Low-impedance tape outputs (100 ohms) to perfectly interface with any type of tape recorder. This technique allows
the preamp to mate with any type recorder, including DAT and DCC, and insures the best recording characteristics
independently of the cable used and the toad impedance of the tape recorder inputs.

• Buffered tape outputs to preserve input-source-signal integrity and preclude "loading" of the signal-source
components and distortion. Buffering prevents the degradation of the source signal caused by most tape recorders
when they are connected to the tape outputs. The buffers insure that the integrity of the internal signal path will not
be affected by the tape recorder used.

• Front-panel-switchable processor loop. Shortens the signal path by switching the processor in and out of the circuit
directly on the main circuit board. Allows the processor to be removed totally from the circuit when not needed.

• All high-grade construction parts including 1%, Roederstein metal-film resistors and all metallized-film, precision
capacitors throughout. These components helps the GFP-345 achieve lower noise levels and lower signal
degradation than preamps selling for many times its price.

• Extremely-low-noise, optional phono preamplifier board (PHO-802A) with virtually zero RIAA error, optimized for

high-output moving-coil and moving-magnet cartridges plus direct-coupled signal output for maximum bass
impact and dynamic response. These features will provide the flattest, lowest-noise, lowest-distortion response
from a phono cartridge and musically satisfying reproduction from any vinyl record.

• Low-impedance, RIAA-equalization network. This unique network insures the lowest achievable noise level in a

phono preamp.

UNPACKING THE GFP-345

Before each GFP-345 left the factory, it was carefully inspected for physical imperfections and electrical performance as
a routine part of AOCOM’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 GFP-345, inspect it for
physical damage. Save the shipping carton and all packing materials as they are intended to reduce to a minimum the
possibility of transportation damage, should the product ever need to be shipped again. In the unlikely ëyent damage has
occurred, notify your dealer immediately and request the name of the carrier so that a written claim to cover shipping
damage 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 GFP-345

Although the GFP-345 does not generate much heat, you will help insure its long-term, trouble-free operation if you keep
it away from external sources of heat, such as radiators or hot-air ducts, and provide reasonable ventilation. The GFP-345
should never be placed with other heat-producing components in a cabinet or enclosure lacking free air flow. If placed
near or above any heat-producing component, such as a power amplifier, please allow a minimum of three inches between
the components.

For use in professional installations, the GFP-345 may be mounted in a standard 19-inch rack using the optional RM-3
rack-mount adaptors available through ACCOM dealers.

CONNECTING THE GFP-345

The performance of the GFP-345 will depend on the quality of the interconnection of both the preamplifier and its

associated equipment. All the input- and output-signal connections should be made only with high-quality, low-loss
audio cables. LEFT CHANNEL and RIGHT CHANNEL inputs and outputs are cleaHy labelled on the rear panel.

NOTE

WHENEVER CONNECTIONS TO OB FROM THE GFP-345 ARE BEING MADE, BE CERTAIN ALL
ASSOCIATED COMPONENTS ARE TURNED OFF.

GND O

Ground connection to chassis is provided primarily to connect phonograph (turntable) ground terminals. Most turntables
are supplied with a separate ground wire to help eliminate hum. Connect the separate ground wjre, if supplied with your
turntable, to the GND O terminal. The GND O terminal can also be used to connect the chassis of the GFP-345 directly
to the center screw of your AC wait outlet or other good "earth” connection such as a water pipe or radiator. This earth
connection may be useful in areas of high RF interference, and may help to minimize noise particularly when playing low­outpuHevel phonograph cartridges (see section PHONO (PHO-802A OPTION) 0 following).

NOTE

Although the GND O connection is probably the best place at which to connect your system to a good earth
ground, it should be pointed out that only one connection to an earth ground is to be made iri any system.
Otherwise what is commonly known as a “ground-loop" will be caused and hum and noise may increase
sufficiently to become a nuisance. In any system, only one ground connection can be made to earth. If one of
your components is supplied with a three-prong AC connector which is plugged into the AC wall outlet, earth
connection will be made through this plug and additional ground connections are not recommended. Similarly,
if the system’s units are mounted in a rack and ground connection between chassis is made through the rack
mounting, special techniques must be undertaken to minimize ground-loops and subsequent noise, A detailed

discussion of ground-loops with methods for their elimination is beyond the scope of this manual.

AUX1 О

This set of input jacks is for use with a high-level signal source, such as a tape player, the audio signal from a video-disc

player, or video-cassette recorder (VCR). The load impedance of this input is approximately 22,000 ohms. This input can
also be converted for use with an analog disc turntable if the optional PHO-802A board is installed onto the main PCB of
the GFP-345. This is a procedure best left to your dealer, or ADCOM authorized service personnel. It is not recommended
you undertake the installation of the PHO-802A board within the GFP-345 yourself.

PHONO (PHO-802A OPTION) О

To use an analog disc turntable with the GFP-345 requires installation of the optional РНО-в02А phono preamp board

onto the main PCB of the GFP-345. Once the PHO-802A board is installed, the input jacks marked AUX 1 © are then

usable with any high-output moving-coil or moving-magnet phono cartridge. These two standard RCA jacks will accept

all common RCA plugs, one for each channel, left and the right, usually supplied at the ends of the turntable’s cables.

LEFT CHANNEL and RIGHT CHANNEL inputs are clearly labelled and their routing, even when the PHO-802A board is
installed, will remain left and right as indicated. Determine the exact color coding or markings on your turntable's cables

for left and right channels in order to insure the correct connections. The PHO-802A circuit is designed to accept the

signal from any high-output moving-coil, moving-magnet, induced-magnet, moving-iron or variable-reluctance car

tridge, the output from which is rated at 2.2 mV/cm or higher. The PHO-802A-input electrical impedance characteristics

are the standard 47,000 ohms with lOOpF shunt capacitance. ADCOM high-output moving-coil cartridges, for example,
are ideally suited for use with this phono preamplifier. Low-output moving-coil cartridges which normally require a pre­preamplifier (sometimes referred to as a "head-amp”), or step-up transformer must have these auxiliary devices

connected l>efore the RCA jacks feeding the input to the PHO-802A board. A pre-preamplifier, or step-up transformer
provides the additional gain required by very-low-output moving- coil cartridges to operate with a standard preamplifier
circuit. A pre-amplifier, or step-up transformer, is not necessary and must not be used with high-output moving-coil
cartridges or moving-magnet, induced-magnet, etc. cartridges. If you are uncertain as to which type of cartridge you will
be using, please consult the instruction manual or specification sheet which wés included with your cartridge, or contact

the dealer from whom you purchased the cartridge, or the cartridge manufacturer.

Some turntables are supplied with a separate ground wire, which must be connected to ground, to reduce hum. If the
turntable is provided with this separate ground wire simply connect it to the GND О screw on the rear panel of the

GFP-345 directly to the left of the AUX 1 inputs (now converted to PHONO),
There are simplified instructions on the main PCB oftheGFP-345to facilitate installation of thePHO-802A board. Detailed

instructions are also supplied in the section INSTALLING THE PHO-802A BOARD IN THE GFP-345 at the end of this
manual.