Studio Technologies 74 - Manual (Page 41)

Issue 2, January 2006 Model 74/75 User Guide
Page 42 Studio Technologies, Inc.
for Surround
connection of AC mains power electro-
mechanical relays maintain a short circuit
condition on the monitor outputs. If the
AC mains input voltage exceeds 90%
of nominal for approximately four sec-
onds the relays are allowed to function
normally. Whenever the AC mains input
voltage drops below approximately 85% of
nominal the relays immediately go to their
mute state. The Model 74 will stay in this
low-voltage state unit the AC mains volt-
age has again exceed 90% of nominal for
approximately four seconds. Any time the
Model 74 is in the low-voltage state the
power LED, located on the front panel, will
fl ash.
Mono-to-Center Polarity
It’s worthwhile to note that a polarity inver-
sion takes place whenever the stereo to
mono downmix’s mono-to-center func-
tion is active. By design, the input signals
experience a 180-degree inversion in the
Model 74’s mono-to-center combining
circuitry. This should not cause a problem
as monitoring audio signals over a single
loudspeaker is considered polarity insensi-
tive. Only during electrical testing would
this condition be detected.
Talent Amplifi er Cable Length
There are no hard and fast rules defi ning
the maximum cable length when con-
necting Model 35 Talent Amplifi ers to the
Model 74 Central Controller. The maxi-
mum cable length is directly related to the
amount of resistance in the connecting
cable; the lower the resistance per foot
(or meter), the longer the cable can be.
(Although cable capacitance affects high
frequency performance, resistance is the
limiting factor is this case.)
To lay out the facts in grammar-school
story problem format: for correct operation,
a Model 35 needs to see at least 20 volts
DC between pins 1 and 2 of their input
connector. The Model 74’s talent amplifi er
output voltage across pins 1 and 2 is 27
volts DC, with a maximum current draw of
200 milliamperes. This difference between
the voltage supplied and the voltage re-
quired results in a maximum voltage drop
of 7 volts over the interconnecting cables.
Since cable is rated in ohms per 1000
feet (or ohms per 1000 meters), you need
to know what the maximum cable resis-
tance is. This can be easily calculated by
dividing the maximum voltage drop by the
maximum current fl ow: 7 volts divided by
200 milliamperes (0.2 A) = 35 ohms. For
example, a standard 20 AWG microphone
cable is Belden 8412, which has 10.9
ohms resistance per conductor per 1000
feet. Since we’re using two conductors to
carry the signal (pins 1 and 2) you’d get
21.8 ohms per 1000 feet of microphone
cable. With our 35 ohm maximum resis-
tance you’d be able to use 1600 feet (492
meters) of this cable.
By using the numbers provided you can
select a cable, and its maximum length,
for your application.
Model 74 to Model 75
Connections
Figure 11 gives a detailed view of
the signals that connect between the
Model 74 Central Controller and the Model
75 Control Console. The Model 74 pro-
vides +15 volt DC power for the Model 75.
The Model 75 generates MIDI system-
exclusive messages and sends them to
the Model 74. From the outside world,
the four hard-wired remote control input