Studio Technologies 68A 2001 - Manual (Page 35)

Model 68A/69A User Guide Issue 1, March 2001
Studio Technologies, Inc. Page 35
for Surround
and a low-pass active filter. Each filter is
a Sallen-Key type, with a 12dB-per-octave
response. The high-pass filter has a nomi-
nal –3dB point at 100Hz; the low-pass at
5kHz. For a detailed description of the
filters, refer to page 5 of the Model 68A
schematic diagram. It is available upon
request from [email protected].
High-Pass Filter
Components were selected at the factory
so that the high-pass filter section’s output
has a –3dB point of nominally 100Hz. (To
be more precise, the math calculations
work out to be 102Hz.) For some applica-
tions it may be optimal to adjust this fre-
quency. The Model 68A makes this a
simple task, with no soldering or compli-
cated procedure required.
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 68A printed
circuit board, is used to hold the resistors.
As received from the factory, a 22k ohm
6-pin single-inline-package (SIP) resistor
is used to configure the filter for nominally
100Hz. 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 determine the
resistance required for a specific filter
frequency: R = 2,250,000 ÷ F, where R
is resistance in ohms and F is frequency
in hertz.
It’s important that any modifications be
performed only by a competent technician.
Low-Pass Filter
Components were selected at the factory
so that the low-pass filter section’s output
has a –3dB point of nominally 5kHz. (To
be more precise, the math calculations
work out to be 5.25kHz.)
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 printed
circuit board, is used to hold the resistors.
From the factory, two 14.3k ohm resistors
configure the filter for nominally 5kHz. 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 resis-
tor package. It’s important to note that the
SIP resistor must be an isolated-terminal-
type, providing three independent resistors
in one assembly.
A simple formula is used 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.
Monitor Output Gain
Structure
The Model 68A Central Controller’s moni-
tor outputs are configured for unity gain.
When the rotary level control on the Model
69A Control Console is set for maximum
level (fully clockwise) the output level is
essentially the same as the input level.
The input trim potentiometers allow pre-
cise adjustment of the input circuitry. This
was provided so that level variations in
input sources, the Model 68A’s circuitry,
and the associated power amplifiers can
be “trimmed out.”