User Guide

Issue฀3,฀November฀2016฀ Model฀74/75฀User฀Guide฀
Page฀40฀ Studio฀Technologies,฀Inc.฀
for Surround
resistor฀changes฀the฀level฀changes฀can฀be฀
easily฀revised.฀In฀addition,฀provision฀has฀
been฀made฀to฀allow฀the฀LFE฀signal฀to฀be฀
routed฀to฀the฀left฀and฀right฀channels฀during฀
downmix.
While฀no฀soldering฀is฀required,฀a฀compe-
tent฀technician฀is฀required฀to฀perform฀any฀
changes฀to฀the฀downmix฀function.฀This฀will฀
help฀to฀ensure฀a฀safe฀and฀successful฀modi-
fication.฀Page฀5฀of฀the฀Model฀74฀schematic฀
contains฀the฀circuitry฀that฀implements฀the฀
downmix฀function.฀Please฀contact฀the฀fac-
tory฀for฀a฀copy฀of฀the฀schematic.
Bandpass฀Filter
The฀bandpass฀filter฀associated฀with฀the฀
mono-to-center฀downmix฀mode฀is฀created฀
by฀cascading฀(connecting฀in฀series)฀a฀฀
high-pass฀and฀a฀low-pass฀active฀filter.฀
Each฀filter฀is฀a฀Sallen-Key฀type,฀with฀a฀฀
12฀dB-per-octave฀response.฀The฀high-pass฀
filter฀has฀a฀nominal฀–3฀dB฀point฀at฀100฀Hz;฀
the฀low-pass฀at฀5฀kHz.฀For฀a฀detailed฀de-
scription฀of฀the฀filters,฀refer฀to฀page฀5฀of฀฀
the฀Model฀74฀schematic฀diagram.
High-Pass฀Filter
Components฀were฀selected฀at฀the฀factory฀
so฀that฀the฀high-pass฀filter฀section’s฀output฀
has฀a฀–3฀dB฀point฀of฀nominally฀100฀Hz.฀(To฀
be฀more฀precise,฀the฀math฀calculations฀
work฀out฀to฀be฀102฀Hz.)฀For฀some฀applica-
tions฀it฀may฀be฀optimal฀to฀adjust฀this฀fre-
quency.฀The฀Model฀74฀makes฀this฀a฀simple฀
task,฀with฀no฀soldering฀or฀complicated฀
procedures฀required.฀Refer฀to฀page฀5฀of฀
the฀schematic฀for฀details.
The฀frequency฀of฀the฀high-pass฀filter฀is฀
configured฀by฀means฀of฀three฀resistors,฀
each฀identical฀in฀value.฀A฀6-position฀socket,฀
located฀on฀the฀Model฀74’s฀printed฀circuit฀
board,฀is฀used฀to฀hold฀the฀resistors.฀As฀re-
ceived฀from฀the฀factory,฀one฀22฀k฀ohm฀6-pin฀
single-inline-package฀(SIP)฀resistor฀is฀used฀
to฀configure฀the฀filter฀for฀nominally฀100฀Hz.฀
To฀revise฀the฀high-pass฀frequency฀this฀SIP฀
resistor฀can฀be฀replaced.฀The฀SIP฀resistor฀
must฀be฀an฀isolated-terminal-type,฀providing฀
three฀independent฀resistors฀in฀one฀assembly.฀
Alternately,฀using฀¼-watt฀1%-tolerance฀resis-
tors฀is฀appropriate.
A฀simple฀formula฀is฀used฀to฀determine฀the฀
resistance฀required฀for฀a฀specific฀filter฀fre-
quency:฀R฀=฀2,250,000฀÷฀F,฀where฀R฀is฀resis-
tance฀in฀ohms฀and฀F฀is฀frequency฀in฀hertz.
Low-Pass฀Filter
Components฀were฀selected฀at฀the฀factory฀฀
so฀that฀the฀low-pass฀filter฀section’s฀output฀
has฀a฀–3฀dB฀point฀of฀nominally฀5฀kHz.฀(To฀฀
be฀more฀precise,฀the฀math฀calculations฀work฀
out฀to฀be฀5.25฀kHz.)
The฀frequency฀of฀the฀low-pass฀filter฀is฀฀
configured฀by฀means฀of฀two฀¼-watt,฀1%-
tolerance฀resistors,฀each฀identical฀in฀value.฀
One฀6-pin฀socket,฀located฀on฀the฀Model฀74’s฀
printed฀circuit฀board,฀is฀used฀to฀hold฀the฀
resistors.฀From฀the฀factory,฀two฀14.3฀k฀ohm฀
resistors฀configure฀the฀filter฀for฀nominally฀5฀
kHz.฀One฀resistor฀is฀inserted฀into฀socket฀pins฀
1฀and฀2,฀the฀second฀into฀pins฀3฀and฀4.฀To฀
revise฀the฀frequency,฀these฀can฀be฀replaced฀
with฀two฀other฀resistors,฀or฀one฀6-pin฀SIP฀฀
resistor฀package.฀It’s฀important฀to฀note฀that฀
the฀SIP฀resistor฀must฀be฀an฀isolated-termi-
nal-type,฀providing฀three฀independent฀resis-
tors฀in฀one฀assembly.฀For฀details฀refer฀to฀
page฀5฀of฀the฀Model฀74฀schematic฀diagram.
A฀simple฀formula฀is฀used฀to฀determine฀฀
the฀resistance฀required฀for฀a฀specific฀filter฀
frequency:฀R฀=฀75,000,000฀÷฀F,฀where฀R฀is฀
resistance฀in฀ohms฀and฀F฀is฀frequency฀in฀
hertz.