Mackie SR24•4 Owner's Manual page 53

NOTE: All the
inputs on the SR24•4 are
designed to accept both
balanced and unbalanced
sources.
Note: All the 1 ⁄ " TRS outputs on the
4
SR24•4 implement "impedance balancing".
The tip carries the signal while the ring is tied
to ground through a resistor whose value
mimics the output impedance of the tip cir-
cuit. When you plug a balanced cord into one
of these outputs, both the tip and ring conduc-
tors have equal impedance, significantly
improving the common mode characteristics.
SWITCHED ⁄ " PHONE JACKS
1
4
Switches can be incorporated into
jacks which are activated by inserting the plug.
These switches may open an insert loop in a cir-
cuit, change the input routing of the signal or
serve other functions. The Mackie SR24•4 uses
switches in the channel insert and bus insert
jacks, input jacks and AUX returns. See Special
Mackie Connections farther on. We also use
these switches to ground the line-level inputs
when nothing is plugged into them.
In most cases, the plug must be inserted
fully to activate the switch. Mackie takes
advantage of this in some circuits, specifying
circumstances where you are to insert the plug
only partially. Once again, see Special Mackie
Connections, later in this section.
RCA PLUGS AND JACKS
RCA-type plugs (also known as phono
plugs) and jacks are often used in home stereo
and video equipment and in many other appli-
cations (Figure D). They are unbalanced, and
SLEEVE TIP SLEEVE
Figure D: RCA plug
signal to the center post and the ground
(earth) or shield to the surrounding "basket."
There are Tape In and Tape Out RCA jacks on
the Mackie SR24•4.
UNBALANCING A LINE
1 ⁄ "
4
In most studio, stage and sound reinforce-
ment situations, there is a combination of
balanced and unbalanced inputs and outputs on
the various pieces of equipment. This usually will
not be a problem in making connections.
• When connecting a balanced output to an
unbalanced input, be sure the signal high
(hot) connections are wired to each other,
and that the balanced signal low (cold) goes
to the ground (earth) connection at the
unbalanced input. In most cases, the
balanced ground (earth) will also be
connected to the ground (earth) at the
unbalanced input. If there are hum or radio
frequency ground-loop problems, this
1 ⁄ " phone
4
connection may be left disconnected at the
balanced end.
• When connecting an unbalanced output to a
balanced input, be sure that the signal high
(hot) connections are wired to each other.
The unbalanced ground (earth) connection
should be wired to the low (cold) and the
ground (earth) connections of the balanced
input. If there are ground-loop problems, try
connecting the unbalanced ground (earth)
connection only to the input low (cold)
connection, and leaving the input ground
(earth) connection disconnected.
In some cases, you will have to make up spe-
cial adapters to interconnect your equipment. For
example, you may need a balanced XLR female
connected to an unbalanced
SPECIAL MACKIE CONNECTIONS
The balanced-to-unbalanced connection
has been anticipated in the wiring of the
Mackie SR24•4 jacks. A
electrically
into a
identical to a
TIP
will automatically unbalance the input and
1 ⁄ " TS phone
4
make all the right connections. Conversely, a
plug or jack.
1 ⁄ " TRS plug inserted into a
Connect the
4
input will automatically tie the ring (low or
cold) to ground (earth).
TRS Send/Receive Insert Jacks
The insert jacks on the SR24•4's input
channels 1–20 (1–28 on the SR32•4), the
SUBS and MAIN MIX L/R are the three-
conductor, TRS type
1 ⁄ " TS phone plug.
4
1 ⁄ " TS plug inserted
4
1 ⁄ " TRS balanced input, for example,
4
1 ⁄ " unbalanced
4
1 ⁄ " phone. They are
4
Appendices
51