Studio Technologies 78 2004 - Manual (Page 33)

Model 78/79 User Guide Issue 5, January 2004
Studio Technologies, Inc. Page 33
for Surround
Bass Management—
Advanced Configuration
In this section the configuration of the main
input channel’s high- and low-pass filter
frequencies will be reviewed. In addition,
the secondary low-pass filter frequency
will be discussed. Changes to the filters
can be made if necessary to meet the
needs of the loudspeaker system.
Main Input High-Pass Filters
As previously discussed, each of the seven
main input channels (L, R, C, SL, SR, BL,
and BR) has an independent high-pass
filter section associated with it. Each filter
section is created using a 2
nd -order Sallen-
Key circuit. Components were selected at
the factory so that the filter section’s output
has a –3 dB point of nominally 80 Hz. (To
be more precise, the math calculations
work out to be 83 Hz.) For some applica-
tions it may be optimal to adjust this fre-
quency. The Model 78 makes this a simple
task with no soldering or complicated
procedure required. Note that each of the
seven high-pass filter sections can be in-
dependently configured. This is provided to
meet “real world” monitoring environments.
For example, it’s quite reasonable that the
L, C, and R channels utilize one high-pass
filter frequency, while the SL, SR, BL, and
BR channels use another.
The frequency of each filter section is con-
figured by means of three resistors, each
identical in value. A 6-position socket, lo-
cated on the printed circuit board, is used
to hold the resistors. As shipped from the
factory, a 27 k (27,000) ohm 6-pin single-
inline-package (SIP) resistor is used to
configure the filters for nominally 80 Hz. To
revise the frequency this can be replaced
with a 6-pin SIP resistor, or three ¼-watt,
1%-tolerance resistors. Note that the SIP
resistor used in this application must be
an isolated-terminal-type, providing three
independent resistors in one assembly.
As SIP resistors typically have a tolerance
of 2%, using individual 1%-tolerance,
rather than 5%-tolerance, resistors is
recommended.
A simple formula is used determine the
resistance required for a specific filter
frequency: R = 2,251,000 ÷ F, where R
is resistance in ohms and F is frequency
in hertz. Figure 7 lists several frequencies
that are provided by standard SIP resis-
tors. Using ¼-watt, 1%-tolerance resistors
will allow many other frequencies to be
implemented.
150 Hz: 15 k 68 Hz: 33 k
125 Hz: 18 k 58 Hz: 39 k
113 Hz: 20 k 48 Hz: 47 k
102 Hz: 22 k 40 Hz: 56 k
83 Hz: 27 k 33 Hz: 68 k
Figure 7. High-Pass Filters, Frequency versus
SIP Resistor Value
Once the new SIP or 1%-tolerance resis-
tors have been procured, they should be
double-checked. Use an ohmmeter to con-
firm that the resistance values are correct.
If SIP resistors are being used, check to
ensure that they are isolated-terminal-type.
These simple checks should take only a
short time and will insure that a change
to the filters will achieve the desired audio
performance.
The process required to actually change
the components is simple, although a com-
petent technician must perform the steps.
Following the previously discussed safety
procedures, mains power must be discon-
nected and the Model 78’s cover removed.
The factory-installed 27 k SIP resistors
must be carefully removed by using a pair
of needle-nosed pliers. The SIP resistors