Adcom GFP-345 - Manual (Page 4)

WHENEVER CONNECTIONS TO OB FROM THE GFP-345 ARE BEING MADE, BE CERTAIN ALL
ASSOCIATED COMPONENTS ARE TURNED OFF.
NOTE
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 low-
outpuHevel 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 pre-
preamplifier (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.