Studio Technologies 74 - Manual (Page 39)

Issue 2, January 2006 Model 74/75 User Guide
Page 40 Studio Technologies, Inc.
for Surround
to right; LFE muted. By making simple
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-
fi cation. 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 fi lter associated with the
mono-to-center downmix mode is created
by cascading (connecting in series) a
high-pass and a low-pass active fi lter.
Each fi lter is a Sallen-Key type, with a
12 dB-per-octave response. The high-pass
fi lter has a nominal –3 dB point at 100 Hz;
the low-pass at 5 kHz. For a detailed de-
scription of the fi lters, refer to page 5 of
the Model 74 schematic diagram.
High-Pass Filter
Components were selected at the factory
so that the high-pass fi lter 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 fi lter is
confi gured by means of three resistors,
each identical in value. A 6-position sock-
et, located on the Model 74’s printed circuit
board, is used to hold the resistors. As
received from the factory, one 22 k ohm
6-pin single-inline-package (SIP) resistor
is used to confi gure the fi lter 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 resistors is appropriate.
A simple formula is used to determine
the resistance required for a specifi c fi lter
frequency: R = 2,250,000 ÷ F, where R is
resistance in ohms and F is frequency in
hertz.
Low-Pass Filter
Components were selected at the factory
so that the low-pass fi lter 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 fi lter is
confi gured 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 confi gure the fi lter for nomi-
nally 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 impor-
tant to note that the SIP resistor must be
an isolated-terminal-type, providing three
independent resistors in one assembly.
For details refer to page 5 of the Model 74
schematic diagram.