— —
9
Balanced Inputs
A balanced program audio feed to the 708 will use both the
+
and the
–
terminals, plus the associated
G (
ground) terminal for each of the two
stereo channels. Since these are “bridging” (high impedance) inputs,
they provide no termination for an audio processor or other equipment
which feeds the 708. Please feel at liberty to connect a 600-ohm
resistor across the input terminals should you feel this really
necessary. Most professional equipment nowadays features low output
impedances and high input impedances. The concept of 600-ohm “line-
matching” dates from the age of transformer coupling and is rooted in
the mystique of telephone engineering. More often than not, audio line
impedance matching is ignored by today’s enlightened broadcasters
(you) and leading-edge equipment manufacturers (us).
Unbalanced
Inputs
When the generator is fed from a test oscillator or other equipment
with unbalanced outputs, the single center conductor of the shielded
input lead should be connected to the + terminal, and the shield to
G
.
In addition,
a jumper wire should be installed between the – terminal
and
G
.
Input Gain
Range
The 708 can accommodate line-level program inputs with a nominal
“Zero-VU” value between –10dBu and +20dBu. This 30dB range is
divided into two 15dB ranges by internal jumpering.
As shipped, the generator is jumpered for “professional” input levels
b5dBu and +20dBu, corresponding to 100% modulation.
Most processor and STL receiver outputs will fall into this range,
+6dBu, +8dBu and +10dBu being typical levels.
Lower levels, between –10dBu and +5dBu, may be encountered with
feeds via lossy studio-transmitter telephone line circuits. The extra
gain for the low-level range is enabled by changing jumpers beneath
the top cover.
Gain Jumpers
Under the top cover, and just behind the
PROGRAM LINE INPUT
barrier strip, you will find two jumper pin strips, each with a “push-
on” jumper. The strips are identified as
JMP2
and
JMP3
in the circuit
board legend, and each has an
H
and an
L
marking to indicate the
proper jumper placement for high level and for low level inputs,
respectively. Figure 2 illustrates jumpering options.
HIGH LEVEL INPUTS
(+5dBu to +20dBu)
LOW LEVEL INPUTS
(–10dBu to +5dBu)
Figure 2 -
PROGRAM LINE INPUT Range Selection
