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H A - 6 A B / H A - 6 A B E M a n u a l
HA-6AB directly without fi rst being preamplifi ed.
The 1/4” phone inputs are stereo jacks (tip-ring-
sleeve) wired for a balanced input. If a mono
phone plug is inserted, the input will automatically
be converted to unbalanced. If using a mono sig-
nal plug into the left ¼” jack the HA-6AB will send
signal to both left and right channels.
The HA-6AB is also equipped with left and right
balanced female XLR connectors in addition to
the standard 1/4” phone jacks. Balanced lines
offer the benefi ts of cancellation of hum, noise,
and RF interference which may be picked up in
the interconnecting cables. For balanced con-
nections, shielded twisted pair cable must be
used. The wiring is as follows: Pin 1-shield; Pin
2-signal positive; and Pin 3-signal negative. The
XLR inputs can not be used in mono like the ¼”
input jacks.
CIRCUIT DESCRIPTION
CAUTION! This section is intended to assist the
professional user with considerable experience
in electronics to achieve a better understand-
ing of the operation of the HA-6AB. Under no
circumstances should persons without electron-
ics troubleshooting experience and training un-
dertake repairs on their own. There are no user
serviceable parts inside the HA-6AB. All problems
should be referred to the factory or to other quali-
fi ed service personnel.
The inputs to the HA-6AB proceeds from the
input connectors to op-amplifi er buffers wired as
differential (balanced) amplifi ers.
The input connectors are stereo 1/ 4” phone jacks.
In the HA-6AB, these will be paralleled by XLR
connectors. If a mono plug is inserted, the ring
connection will be grounded through the body of
the plug, converting the differential amplifi er to a
noninverting amplifi er. After the buffers, the left
and right signals proceed to the two decks of the
stereo Input Level control.
The HA-6AB’s power amplifi cation is derived from
a pair of LM1875 integrated circuits, which have
internal short-circuit and thermal overload pro-
tection. They are set for a voltage gain of 26 dB.
The LM1875’s drive the speaker outputs directly.
The headphone outputs fi rst pass through special
high-power pots wired as volume controls.
The HA-6AB’s power supply provides +/-24 volts
unregulated to the LM1875 stages, which may
drop to +/-20 volts under full load. A network of
dropping resistors and zener diodes provides ±15
volts to the rest of the circuit.
Both the Overload LED’s and the Signal Present
LED’s monitor signal levels at the output (before
the volume controls).
INSTALLATION
The HA-6AB is intended for mounting in
standard 19” equipment racks. Standard racks
come equipped with mounting rails with holes
tapped for 10-32 machine screws. Be sure to
use only 10-32 screws (in particular, avoid 10-
24 screws, which will fi t if forced but will strip
the threads). To avoid marring the panel when
tightening the screws, use nylon washers under
the screw heads.