User Guide

Issue฀3,฀November฀2016฀ Model฀74/75฀User฀Guide฀
Page฀42฀ Studio฀Technologies,฀Inc.฀
for Surround
nominal฀for฀approximately฀four฀seconds฀
the฀relays฀are฀allowed฀to฀function฀nor-
mally.฀Whenever฀the฀AC฀mains฀input฀volt-
age฀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฀
flash.
Mono-to-Center฀Polarity
It’s฀worthwhile฀to฀note฀that฀a฀polarity฀in-
version฀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฀Amplifier฀Cable฀Length
There฀are฀no฀hard฀and฀fast฀rules฀defining฀
the฀maximum฀cable฀length฀when฀connect-
ing฀Model฀35฀Talent฀Amplifiers฀to฀the฀Model฀
74฀Central฀Controller.฀The฀maximum฀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฀fac-
tor฀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฀amplifier฀
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฀flow:฀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฀sources฀
connect฀to฀the฀Model฀74.฀They฀then฀route,฀
via฀the฀interface฀cable,฀from฀the฀Model฀74฀
to฀the฀actual฀input฀circuitry,฀located฀in฀the฀
Model฀75.