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THE BENEFITS OF BALANCED CONNEC-this option in bridged mode, the nominal impedance
TIONS of the loudspeaker (when connected conventionally)
With a conventional (unbalanced) connection,must be 8 or higher.
audio signal current flows from the preamp to the(2) Disconnect any signal cables from the input
power amplifier via the cableOs center conductor. Tojacks. Decide whether this amplifier will be driving
complete the circuit, audio signal current flows backthe Left or Right speaker. Connect the corresponding
to the preamp ground via the cableOs outer conductor.(left or right) signal cable from your preamplifier to
The outer conductor also serves as the cableOsone of the L input jacks of this amplifier
shield. When two audio components are connectedNOTE: In the bridged mode the amplifier is driven
together, power-supply noise and Oleakage� hum may
also flow on the cable shields, combining with theonly through its L (Left) input, even though it may be
connected to the Right speaker. If another NAD 218
return audio current. The resulting distortion andTHX amplifier in bridged mode is used for the second
noise may depend on the orientation of AC powerstereophonic channel, it also will be driven through its
plugs in their sockets. Designers of some audiophile
cables combat this contamination by leaving theL input, regardless of whether it is used to drive the
Left or Right loudspeaker.
shield unconnected at one end. Since the shield is(3) Disconnect any wires from the SPEAKERS
grounded at only one end, the performance of such a
cable may depend on the direction of its connection,terminals. Select the wire from the speaker that will
i.e. whether the shield is grounded at the preamp orbe driven by this bridged amplifier. Connect its Oposi-
tive� conductor to the L+ terminal and its Onegative�
at the power amp. conductor to the R+ terminal (i.e. the two red termi-
A three-wire balanced connection avoids all of
nals). DO NOT connect any wires to the black termi-
these uncertainties. The signal Ohot� and return cur-nals (L- and R-).
rents are both carried on inner conductors. The sep-CAUTION: In the bridged mode the speaker wires
arate cable shield, connected to the amplifier chassis
at both ends, protects the audio signal from all formsmust be Ofloating� with respect to the circuit ground.
Do NOT connect the speaker wires to anything that
of interference and power-supply noise. The advan-shares a common ground between stereo channels
tage of this approach is particularly evident with long(such as a speaker switch or an adapter for electro-
connecting cables. Therefore, while the 218 THX can
provide excellent sound when used with any pream-static headphones), nor to anything which shares a
common ground with the amplifierOs inputs (such as a
plifier, the best (and most consistent) performanceswitching comparator or a distortion analyzer).
will be obtained with a preamp that has balanced out-
put wiring. (4) After the preceding conditions have been satis-
fied, move the Bridging switch to ON (MONO).
Finally turn the Power ON.
6. SOFT CLIPPING On/Off.
When an amplifier is overdriven beyond its maxi-(6) To return the amplifier to normal stereo opera-
tion at a later date, first turn off the power. Re-set the
mum power output it normally produces Ohard clip-Bridging switch to OFF (STEREO). Restore normal
ping� of the signal with harsh distortion and power-
supply buzz as the output transistors saturate. TheLeft and Right input connections, and re-connect
NAD Soft Clipping circuit gently limits the outputloudspeaker wires to the speaker terminals as
described above under SPEAKERS.
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 amplifierOs power capacity. For conve-
nience it may be left ON at all times. FRONT PANEL CONTROLS
1. POWER ON/OFF.
7. BRIDGING ON (Mono) / OFF (Stereo).
This switch Obridges� the two channels together,Press the Power button to turn on the amplifier.
forming a monophonic amplifier with more than dou-The green LED glows when the power is on and the
amplifier is ready for use. Press the Power button
ble the output power. To convert to bridged opera-again to switch the amplifier off.
tion, the following procedure should be followed.
(1) Switch OFF the POWER.
NOTE: in the bridged mode the loudspeakerOs2. STATUS INDICATOR.
impedance is effectively halved as Oseen� by theThis multi-color LED indicates the operating status
amplifier. An 8-ohm load looks like 4 ohms, a 4-ohmof the amplifier, as follows.
load looks like 2 ohms, and a pair of 4-ohm speakersDARK: Power off. The Power switch may be off,
operated in parallel will resemble a 1-ohm load.the AC power cord may be unplugged or not connect-
Driving paralleled low-impedance speakers to highed to a live wall outlet, or the internal fuse may have
levels will cause the amplifier to overheat and shutblown.
down, or may cause internal fuses to blow in order toGREEN: Power on; the amplifier is ready for use.
protect the amplifier. In bridged mode you must con-
nect only ONE loudspeaker whose nominal imped-RED: Protection mode; the loudspeakers have
ance is 8 or higher.been disconnected by an internal relay. This mode is
activated briefly during turn-on and turn-off, to protect
BI-WIRED LOUDSPEAKERS:If you have a loud-the speakers from low-frequency thumps. At other
speaker which may be bi-wired and you wish to use
NA D
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ACEC
