The Benefits Of Balanced Connections; Soft Clipping On/Off; Bridging On/Off; Status Indicator - NAD 218 THX Owner's Manual

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THE BENEFITS OF BALANCED CONNEC-
TIONS
With a conventional (unbalanced) connection,
audio signal current flows from the preamp to the
power amplifier via the cable's center conductor. To
complete the circuit, audio signal current flows back
to the preamp ground via the cable's outer conductor.
The outer conductor also serves as the cable's
shield. When two audio components are connected
together, power-supply noise and "leakage" hum may
also flow on the cable shields, combining with the
return audio current. The resulting distortion and
noise may depend on the orientation of AC power
plugs in their sockets. Designers of some audiophile
cables combat this contamination by leaving the
shield unconnected at one end. Since the shield is
grounded at only one end, the performance of such a
cable may depend on the direction of its connection,
i.e. whether the shield is grounded at the preamp or
at the power amp.
A three-wire balanced connection avoids all of
these uncertainties. The signal "hot" and return cur-
rents are both carried on inner conductors. The sep-
arate cable shield, connected to the amplifier chassis
at both ends, protects the audio signal from all forms
of interference and power-supply noise. The advan-
tage of this approach is particularly evident with long
connecting cables. Therefore, while the 218 THX can
provide excellent sound when used with any pream-
plifier, the best (and most consistent) performance
will be obtained with a preamp that has balanced out-
put wiring.

6. SOFT CLIPPING On/Off.

When an amplifier is overdriven beyond its maxi-
mum power output it normally produces "hard clip-
ping" of the signal with harsh distortion and power-
supply buzz as the output transistors saturate. The
NAD Soft Clipping circuit gently limits the output
waveform and minimizes audible distortion when the
amplifier is overdriven. It should be switched ON
when playing music at very high levels that might
exceed the amplifier's power capacity. For conve-
nience it may be left ON at all times.
7. BRIDGING ON (Mono) / OFF (Stereo).
This switch "bridges" the two channels together,
forming a monophonic amplifier with more than dou-
ble the output power. To convert to bridged opera-
tion, the following procedure should be followed.
(1) Switch OFF the POWER.
NOTE: in the bridged mode the loudspeaker's
impedance 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 a pair of 4-ohm speakers
operated in parallel will resemble 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. In bridged mode you must con-
nect only ONE loudspeaker whose nominal imped-
ance is 8 or higher.
BI-WIRED LOUDSPEAKERS:If you have a loud-
speaker which may be bi-wired and you wish to use
NAD
6
this option in bridged mode, the nominal impedance
of the loudspeaker (when connected conventionally)
must be 8 or higher.
(2) Disconnect any signal cables from the input
jacks. Decide whether this amplifier will be driving
the Left or Right speaker. Connect the corresponding
(left or right) signal cable from your preamplifier to
one of the L input jacks of this amplifier
NOTE: In the bridged mode the amplifier is driven
only through its L (Left) input, even though it may be
connected to the Right speaker. If another NAD 218
THX amplifier in bridged mode is used for the second
stereophonic channel, it also will be driven through its
L input, regardless of whether it is used to drive the
Left or Right loudspeaker.
(3) Disconnect any wires from the SPEAKERS
terminals. Select the wire from the speaker that will
be driven by this bridged amplifier. Connect its "posi-
tive" conductor to the L+ terminal and its "negative"
conductor to the R+ terminal (i.e. the two red termi-
nals). DO NOT connect any wires to the black termi-
nals (L- and R-).
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 electro-
static headphones), nor to anything which shares a
common ground with the amplifier's inputs (such as a
switching comparator or a distortion analyzer).
(4) After the preceding conditions have been satis-
fied, move the Bridging switch to ON (MONO).
Finally turn the Power ON.
(6) To return the amplifier to normal stereo opera-
tion at a later date, first turn off the power. Re-set the
Bridging switch to OFF (STEREO). Restore normal
Left and Right input connections, and re-connect
loudspeaker wires to the speaker terminals as
described above under SPEAKERS.
FRONT PANEL CONTROLS
1. POWER ON/OFF.
Press the Power button to turn on the amplifier.
The green LED glows when the power is on and the
amplifier is ready for use. Press the Power button
again to switch the amplifier off.

2. STATUS INDICATOR.

This multi-color LED indicates the operating status
of the amplifier, as follows.
DARK: Power off. The Power switch may be off,
the AC power cord may be unplugged or not connect-
ed to a live wall outlet, or the internal fuse may have
blown.
GREEN: Power on; the amplifier is ready for use.
RED: Protection mode; the loudspeakers have
been disconnected by an internal relay. This mode is
activated briefly during turn-on and turn-off, to protect
the speakers from low-frequency thumps. At other