Studio Technologies 78 2004 - Manual (Page 35)

Model 78/79 User Guide Issue 5, January 2004
Studio Technologies, Inc. Page 35
for Surround
Second Stage Low-Pass Filter
After the outputs of the individual low-pass
filters associated with the seven main
input channels (L, R, C, SL, SR, BL, and
BR) are combined (summed), the result is
routed to a second low-pass filter section.
Its –3 dB point is also nominally 80 Hz.
The frequency of this filter can be adjusted
using the same formula and techniques
covered for the low-pass filters associated
with the main input channels. Note that
changing this frequency would impact the
low-pass characteristic of all the main input
channels whose signals have been select-
ed bass management. Careful planning is
critical to reaching the desired loudspeaker
system performance. Be especially cau-
tious should the main input channel’s low-
pass filters be configured for more than
one –3 dB frequency. In this case it may be
better to configure the system to not use
the second stage low-pass filter.
LFE Input Low-Pass Filter
A low-pass filter is associated with the LFE
input channel. This 48 dB/octave filter can
be inserted into the LFE signal path to
simulate the bandwidth restrictions applied
by some digital audio distribution formats.
The low-pass filter section is made up of
four 2
nd -order Sallen-Key circuits that are
cascaded, i.e. connected in series. Com-
ponents were selected at the factory so
that the filter section’s output has its –6 dB
point at nominally 120 Hz. For some ap-
plications it may be desirable to revise the
filter’s cutoff frequency. For example, it’s
possible that the –6 dB point may need to
be configured for 80 Hz to match a future
LFE encoding parameter.
As received from the factory two 8-pin SIP
resistor packages are used to implement
the 120 Hz frequency. Two 8-pin sockets,
located on the printed circuit board, are
used to hold the resistors. To achieve the
120 Hz frequency requires two SIP differ-
ent resistance values. An 8-pin 6.8 k SIP
is “shared” by the first two filters, while an
8.2 k SIP is used by the third and fourth.
For simplicity, eight ¼-watt, 1%-tolerance
resistors of identical value should be used
to implement any revised frequency.
The formula used to determine the resis-
tance required for a specific filter frequency
is: R = 900,000 ÷ F, where R is resistance
in ohms and F is frequency in hertz. As an
example, to revise the low-pass filter for
80 Hz eight 11.3 k (11,300) ohm resistors
should be used.
The process required to actually change
the resistors is basically the same as with
the main channel’s high- and low-pass
sections. The two factory-installed SIP
package resistors should be carefully
removed by using a pair of needle-nosed
pliers. The eight individual 1%-tolerance
resistors must be prepared for insertion.
The first resistor is inserted into socket
pins 1 and 2, the second into pins 3 and 4,
etc. After all the resistors are inserted, en-
sure that they are positioned so that they
stand straight up, safely away from other
components.
Monitor Output Audio Path Anomaly
To control the monitor output level, the
Model 78 uses four digitally controlled ste-
reo level control integrated circuits. These
chips, PGA2310 from Burr-Brown/Texas
Instruments, have many nice performance
characteristics. They exhibit a wide dy-
namic range, low noise, and zero-crossing
switching. Unfortunately, they do exhibit
one minor issue that is worth mentioning.
When changing the output level between
–31.5 and –32.0 dB, or vice versa, it’s