Studio Technologies 65 - Manual (Page 19)

Model 65 User Guide Issue 4, January 2004
Studio Technologies, Inc. Page 19
for Surround
There might be a case where it is desirable
to have one of the five main inputs not
assigned to a subwoofer output. While we
can’t really think of an application like this,
there is no technical problem in having no
routing selected for a main input channel.
Leaving jumper positions unused should not
pose a problem.
Advanced Configuration
In this section the configuration of the main
input’s high- and low-pass filter frequencies
will be reviewed. Changes will be made if
necessary to meet the needs of the monitor
system. The frequency of the low-pass filter
associated with the LFE input will be
reviewed. Again, changes will be made
if required.
Main Input High-Pass Filters
As previously discussed, each of the five
main input channels (L, C, R, LS, and RS)
has a high-pass filter section associated
with it. Each filter section is created by
cascading (connecting in series) two identi-
cal 2
nd -order Sallen-Key filter circuits. Com-
ponents were selected at the factory so that
the filter section’s output has a –6dB point
of nominally 80Hz. (To be more precise, the
math calculations work out to be 83Hz.) For
some applications it may be optimal to
adjust this frequency. The Model 65 makes
this a simple task, with no soldering or
complicated procedure required. Note that
each of the five high-pass filter sections can
be independently configured. This is pro-
vided to meet “real world” monitoring envi-
ronments. For example, it’s quite reason-
able that the L, C, and R channels utilize one
frequency, while the LS and RS channels
use another.
The frequency of each filter section is con-
figured by means of six resistors, each
identical in value. Two 6-position sockets,
located on the printed circuit board, are
used to hold the resistors. As received from
the factory, two 27k (27,000) ohm 6-pin
single-inline-package (SIP) resistors are
used to configure the filters for nominally
80Hz. To revise the frequency these can be
replaced with two 6-pin SIP resistors, or six
individual ¼-watt, 1%-tolerance resistors.
The SIP resistors must be isolated-terminal-
type, providing three independent resistors
in one assembly. As the SIP resistors have
a tolerance of 2%, using ¼-watt 1%-toler-
ance, rather than 5%-tolerance, resistors is
appropriate.
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 resistors.
Using ¼-watt, 1%-tolerance resistors will
allow many other frequencies to be imple-
mented.
Once the new SIP or 1%-tolerance resistors
have been procured, they should be double-
checked. Use an ohmmeter to confirm that
the resistance value is correct. If SIP resis-
tors 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.
150Hz: 15k 68Hz: 33k
125Hz: 18k 58Hz: 39k
113Hz: 20k 48Hz: 47k
102Hz: 22k 40Hz: 56k
83Hz: 27k 33Hz: 68k
Figure 7. High-Pass Filters, Frequency versus
SIP Resistor Value