Front Panel Controls - NAD 218 THX Manuel D'installation

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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

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.