NAD 2155 - Bridging; AC Line Cord; AC Outlets; Speakers A

cause internal fuses to blow in order to protect the amplifier.
if this occurs, return the amplifier to your dealer for service.
4. BRIDGING
This switch “bridges” the two power amplifier channels
to form a monophonic amplifier with more than double the
output power. To convert to bridged operation, the following
procedure should be followed.
(1) Switch OFF the POWER.
(2) Be sure that the IMPEDANCE switch is set to 40
(NORMAL). If it is at 8O (HIGH), re-set it to 4Q (NORMAL)
and secure the bracket to prevent the Impedance switch
from being moved accidentally.
NOTE: In the bridged.mode the loudspeaker’s imped-
ance is effectively halved as “seen” by the amplifier. An
8-ohm load looks like 4 ohms, a 4-ohm load looks like 2
ohms, and pairs of 4-ohm speakers operated in parallel will
look like a 1-ohm load. Driving paralleled low-impedance
speakers to high levels will cause the amplifier to overheat
and shut down, or may cause internal fuses to blow in order
to protect the amplifier. For best results the bridging mode
should be used with a single 4-ohm or 8-ohm speaker in
each channel.
(3) Disconnect any signal cable from the Left-channel
MAIN-IN jack. In the bridged mode the amplifier is driven
only through its Right-channel input jack, even though it
may be connected to the “Left” speaker. If another NAD
amplifier in bridged mode is used for the second stereo-
phonic channel, it too will be driven through its Right input
jack, regardless of whether it is used to drive the Left or
Right loudspeaker.
(4) Disconnect any speaker wires from both the SPEAK-
ERS A and SPEAKERS B terminals. Separate the conduc-
tors in the cable from the speaker that is to be driven by this
bridged amplifier. Connect the “positive” conductor to the
R-+ terminal and the “negative” conductor to the L+ terminal
(i.e. to the two red terminals in the “A’ group). DO NOT
connect any wires to the black terminals (R—.and L—).
If you want to drive two speakers in parallel, connect the
second speaker's leads to the red (R+ and L+) terminalis in
the “B” group. Do not connect any wires to the black (R—
and L—) terminals.
CAUTION: In the bridged mode the speaker wires
must be “floating” with respect to the circuit ground. Do NOT
connect the speaker wires to anything that shares a common
ground between stereo channels (such as a speaker switch
or an adapter for electrostatic headphones), nor to anything
which shares a common ground with the amplifier’s inputs
(such as a switching comparator or a distortion analyzer).
(5) After the preceding conditions have been satisfied,
re-set the Bridging switch. It is held in place by a plastic
bracket and a screw in a slot. Use a small screwdriver to
loosen the bracket screw, turning it about a half-turn counter-
clockwise; then slide the switch to ON (MONO). The bracket
will move with the switch. Re-tighten the screw to secure the
switch in its new position. Finally, turn the power on.
(6) To return the amplifier to normal stereo operation at a
later date, first turn off the power. Loosen the bracket screw,
re-set the Bridging switch to OFF (STEREO), and tighten
the bracket screw to prevent the switch from being moved
accidentally. Restore normal Left and Right input connec-
tions, and re-connect loudspeaker wires to the speaker
terminals as described below under SPEAKERS A and
SPEAKERS B.
5. AC LINE CORD
Plug the AC line cord into a “live” wall socket.
6. AC OUTLETS
The AC power line cords of other stereo compo-
nents may be plugged into these accessory outlets. The
SWITCHED outlet is intended for all-electronic products
(e.g., a tuner, equalizer, or other signal processor), and will
be switched on and off by the amplifier’s main POWER
button. The UNSWITCHED outlet should be used to power
products involving mechanical operations (e.g., a turntable
or tape deck); such products should be switched on and off
with their own power switches.
The UNSWITCHED outlet can also be used to power
any device containing a clock timer, or a digital tuner that
requires uninterrupted AC power to maintain station tuning
information stored in its memory.
7. SPEAKERS A
If the wiring to each loudspeaker will not be longer than
about 6 meters (20 feet), then connections should be made
using 18-gauge wire such as common lamp cord (“zip” cord),
available from hardware and electrical-supply stores in either
white, black, or brown insulation. If the wiring to the speakers
will be longer than about 6 meters, heavier 16-gauge or
14-gauge wire is preferred. Heavy-duty wiring is especially
desirable if you are using speakers of low impedance or two
pairs of speakers wired in parallel.
This amplifier is equipped with special high-current
speaker terminals to handle the highest peak power
levels that may occur in the “bridged” mode or with low-
impedance loudspeakers.
To make connections, separate the two conductors of
the cord, strip off about a half-inch (1 cm) of insulation from
each, and in each conductor twist together the exposed wire
strands. Unscrew the red or black cap on the terminal, in
order to expose the small opening in the base of the ter-
minal. Insert the bared wire into the hole, and screw the cap
down tight until it grasps the wire and holds it securely.
Repeat for each conductor, connecting the wires from
the left-channel speaker to the (L+) and (L—) terminals and
the wires from the right-channel speaker to the (R+) and
(R—) terminals in the “A’ group. Check to be sure that no
loose strand of wire is touching any adjacent terminal or the
amplifier chassis.
Phasing. Stereo speakers should operate in phase
with each other in order to yield a good stereo image and
to reinforce rather than cancel each other’s output at low
frequencies. If your speakers are easily moved, their phasing
can easily be checked. Make the connections to both speak-
ers, place the speakers face-to-face only a few inches apart,
play some music, and listen. Then swap the connection of
the two wires at the back of ONE of the speakers, and listen
again. The connection which produces the fullest, boomiest
bass output is the correct one. Connect the wires securely to
the speaker terminals, being careful not to leave any loose
strands of wire that might touch the wrong terminal and
create a partial short-circuit; then move the speakers to
their intended locations.
If the speakers cannot easily be set face-to-face, then
phasing must rely on the “polarity” of the connecting wires.
Note that the SPEAKERS terminals on the amplifier are color
coded: in each channel the red terminal has positive “+”
polarity and the black terminal is negative “—”. The terminals
at the rear of the speakers are also marked for polarity, either
via red and black connectors or by labels: “+”, 1, or 8 ohms
for positive, “—”, 0, or G for negative. As a general rule the
positive (red) terminal on the amplifier is to be connected to
the positive terminal of the speaker, in each channel.