Adcom gfa 565 balanced input

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adcom gfa 565 balanced input

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Page 1

GFA-565 BALANCED INPUT OPTION

BALANCED INPUT 0

The BALANCED INPUT is an option which must be installed at the ADCOM plant. If you purchased your GFA-565
without the BALANCED INPUT option, it cannot be retrofitted In the field. The amplifier must be returned to the
factory for its installation should you find the BALANCED INPUT is needed at a later date. The BALANCED INPUT PCB
must be electrically matched to the individual amplifier if its optimal performance is to be achieved; that is, if maximal
common-mode-rejection ratio, lowest distortion, exact level adjustments, etc., are to be attained.

The BALANCED INPUT is intended primarily forusc in professional applications in which balanced linesare the standard
used for interconnection. When the BALANCED INPUToption is installed atourplant,aswitch e is provided which will
permit selection of several operating modes. Because different nominal operating levels are used in balanced-line
operations, three (3) switch-selected sensitivities are available for the BALANCED INPUT:

A. UNBALANCED o
UNBALANCED selects the UNBALANCED INPUT RCA jack 0 directly
below the BALANCED INPUT XLR jack. When the switch is in the
UNBALANCED position, the BALANCED INPUT option is disconnected from
the amplifier.

B. OdBm

When the switch is placed in this position. it disconnects the UNBALANCED
INPUT RCA iack o and connects the BALANCED INPUT 0 to the
amplifier. When the switch is in the OdBm position. the input sensitivity is
nominally the same as the UNBALANCED INPUT RCA jack. That is, for e
1-watt output from the amplifier. into 8 ohms, one needs an input of 130mv
(-15.5dBm) and an input of 2.15V (+8.8dBm) for SOC-watts output into 8
ohms. The nominal OdBm is equivalent to avoltage of0.775 across 600 ohms.

C. +3dBm

This switch position requires an input of 327mV [-7.5dBm) to achieve a
1-watt output into 8 ohms. For SOD-watts output into 8 ohms, the level required
by the BALANCED INPUT 0 will he 5.23v (+16.8¢Bm), The +8dBm line
level is generally used in broadcast applications in which the nominal level of
+8dBm is equivalent to 1 .95 volts across 600 ohms.

D. +14dBm .

In this switch position, the BALANCED INPUT 0 requires 650mV ATI'EtmON .
[-1 .5dBm) to develop a 1 -watt output intoa ohms. For300-wattsoutput into 8 mm""'-""--""
ohms. the BALANCED INPUT will require 10.7 volts l+22.8dBm), Although "-4:

m- -PM

+14dBm is not astandard level, it has been provided to increase the versatility
of the GFA-565 in the maximum number of applications.

GPA-565 HEAR PANEL DIAGRAM

Since many semi-professional mixing boards, as well as certain home preamplifiers, have varying balanced-output
levels to the monitoring amplifier, you should set the switch 9 on the BALANCED INPUT to the position which will
permitthe most desirable playback level. If you are using the BALANCED INPUT with a conventional preamplifier, you
should setthe BALANCED INPUT switch to the position which will allow setting the preamplifier-s volume control as close
to its 12:00 o-clock position as is feasible for a comfortable listening level.

Similarly, the best way to approach the setting of playback levels with a mixing console is to set the BALANCED INPUT
switch (plus any level-setting adjustments provided on the monitor output of the mixing console) such that when the
-slider", or the console-s level control, is close to, or within, the upper-third of its range, the desired playback level is
achieved.

Following the recommendations above will result not only in maximum flexibility of the system, but in the lowest noise
level as well.

The three different sensitivities selected for the BALANCED INPUT are based on actual usage and correct operation of
the systems as well as levels commonly employed in professional applications. These levels are usually referred to the
OdBm standard which is equivalentto1mw into 600 ohms or, more commonly, 0.775V across 600 ohms. This is generally
the rms line-level when a VU meter points to -".0 The recording industry's level has been set at +4dBm which is an rms
level of 1 .23V across 600 ohms. Most broadcast stations use a level of +8dBm (translated to a100% modulation level) and
equivalent to 1.95V across 600 ohms. This level has also been adopted in Europe as DIN level. All these different line
levels appear on transmission lines when the VU meter points to -0" and are the result of different settings within the
electronics of the consoles or "line" amplifiers used: they are nominal or rms levels, not peak. Since live acoustical
music waveforms usually have a crest factorof 20dB, most professional equipmentisdesigned so that its clipping level is
about +24dBrn (some 1 2.25V across 600 ohms]. This provides sufficient headroom during nominal *4dBm applications
without running into clipping,

Page 2

It can be seen then that when a VU meter registers "0", peaks as great as +24dB can be present; this is the reason torthe
three sensitivity settings on the BALANCED INPUT. Unfortunately, further dlscussion otthc above subjectis beyond the
scope of this manual,

The input impedance of the BALANCED INPUT is greaterthan 100,000 ohms and is -bridging": that is, itwill not -load

down- or attest the level. in the slightest degree, of the transmission line to which it is connected. Note also that a
positive-going signal on pin 3 of the XLR connector will appear as a positive-going signal at the output of the amplifier,

BALANCED INPUT OPTION

Frequency Response, 10Hz to 40kHz .............................................................. +0,-0.5dB
THD + Noise @ 2V Out, 20Hz to 20kHz .............................................................. < 0.005%
IM Distortion (SMPTE) @ 2V Out ................................................................... < 0.002%
IM Distortion (CCIF, Any Combination from 4kHz to 20kHz) @ 2V Out ............................. < 0.005%
Common-Mode Reiection. ton: to ZUKHZ ............................................................. > 8503
Input Impedance (Bridging) .................................................................. > 100.000 ohms
Input, Non-inverting, Positive-Going, XLR .............................................................. Pin 3

Input Sensitivity (Selectable, OdBm, +8dBm, +14dBm)
300 watts into 8 ohms (OdBm position) ..
1 watt into 8 ohms (OdBm position) ....
300 watts into 8 ohms (+8dBm position) .
1 watt into 8 ohms (+8dBm position) .....
300 watts into 8 ohms i+14dBm position)
1 watt into 8 ohms (+14dBm position) .....

....................................... +8.8dBm (2.15V)
-15.5dBm(130mV)

.. . +16.8dBm(5.23V)
. -7.5dBm(327mV)
.. +22.BdBm (10.7v
-1.5dBrn (650mV


ADCOM®
11 Elkins Road
East Brunswick, NJ 08816 USA.
Telephone (908) 390-11 30
cvc 1970 Fax (908) 390-9152 Printed in usA.