are present on these terminals when the amplifier is
producing maximum power. After you have connect-
ed the speaker cables, bend the protective cover
down against the rear panel and fasten it in place. It
is particularly important that curious children and
small pets be prevented from touching the terminals.
Phasing. Stereo speakers must operate "in phase"
with each other in order to produce a focused stereo
image and to reinforce rather than cancel each
other's output at low frequencies. An in-phase con-
nection is assured if the red (positive) terminal on the
amplifier is connected to the red (positive) terminal on
the loudspeaker, in each channel.
If your speakers are easily moved, their phasing
can easily be checked. Make the connections to both
speakers, 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 connec-
tion 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 ter-
minal 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 con-
necting wires. The speaker terminals on the amplifier
are identified as red (+) and black (-) in each channel.
The terminals at the rear of the speakers are also
marked for polarity, either via red and black connec-
tors or by labels: "+", 1, or 8 ohms for positive, "-", 0,
or G for negative. The red (+) terminal on the amplifi-
er should be connected to the red (positive) terminal
of the speaker, in each channel.
To facilitate this, the two conductors comprising the
speaker wire in each channel are different, either in
the color of the wire itself (copper vs. silver) or in the
presence of a small ridge or rib pattern on the insula-
tion of one conductor. Use this pattern to establish
consistent wiring to both speakers of a stereo pair.
Thus if you connect the copper colored wire (or
ribbed insulation) to the (+) amplifier terminal in the
Left channel, do the same in the Right channel. At
the other end of the wire, if you connect the copper
colored wire (or the ribbed insulation) to the red or
positive terminal on the left-channel speaker, do the
same at the right-channel speaker.
3. LEFT CHANNEL INPUTS (Balanced,
Before making or changing input connections to the
amplifier, make certain that the Power is OFF.
The 218 THX amplifier is equipped with two input
connectors for each channel. The RCA phono jack is
a conventional "unbalanced" input. The three-hole
XLR socket is a professional "balanced" input. You
may use either type of input, but not both.
If your preamplifier has only conventional outputs
with RCA phono jacks, connect an audio connecting
cable from the left channel output of the preamp to
the left channel UNBALanced input of the 218 THX.
Set the BALANCE switch to UNBAL.
If your preamplifier has balanced XLR outputs, con-
nect a three-conductor cable from your left-channel
preamp output to the left-channel XLR input on the
218 THX, and set the BALANCE switch to BAL. If
your audio dealer does not have the appropriate
cables, purchase balanced "microphone" cables from
a shop that sells professional recording equipment.
The end of the cable that has a "male" XLR plug (with
three metal pins) should be connected to the 218
THX amplifier. The end of the cable that has a
"female" XLR socket (with three holes) should be
connected to your preamp.
An XLR plug is "keyed" so that it fits into the socket
only one way. If there is a set-screw in the barrel of
the plug, align it with the top of the connector. Push
the plug fully into the XLR socket until it latches in
The three pins of an XLR-type ("Cannon") connec-
tor are numbered. Pin 2 is the signal "hot" connec-
tion; in the 218 THX, Pin 2 is connected directly to the
center pin of the unbalanced RCA phono jack. Pin 3
is the signal return (signal ground) connection. Pin 1
is the chassis earth (ground), to which the shield of a
balanced-wire cable is connected.
UNPLUGGING. The XLR socket has a latching
feature that prevents the connector from being pulled
out by accident. Before disconnecting an input cable,
turn off the Power. Use one hand to press the latch-
ing tab above the XLR socket while using the other
hand to pull the XLR plug out.
4. RIGHT CHANNEL INPUTS (Balanced,
Make connections to the right-channel input in the
same way that you did for the left channel.
5. INPUT SELECT (Balanced/Unbalanced).
Set this switch to match your selection of input
connector. Set to UNBAL if you have connected a
cable from your preamp to the RCA phono input
jacks. Set the switch to BAL if you are making con-
nections to the balanced XLR inputs.
Normally the choice of input connector is deter-
mined by the output connectors on your preamp. If
your preamp has balanced outputs, use three-con-
ductor cables equipped with XLR connectors. If your
preamp has only "unbalanced" connections with RCA
phono jacks, use the corresponding inputs on the 218
It is possible to connect an "unbalanced" source
(such as a CD player) to the XLR inputs, by means of
adapter cable in which the audio-return wire (Pin 3)
and the cable shield (Pin 1) are both connected to the
collar of the RCA phono plug at the source end of the
cable. With such a connection you may wish to
experiment with the setting of the BAL/UNBAL switch.
In the UNBAL position Pins 3 and 1 are short-circuit-
ed together at the amplifier input, effectively convert-
ing a three-wire cable into a conventional two-con-