Studio Technologies 68A 2005 - Manual (Page 35)

Model 68A/69A User Guide Issue 2, January 2005
Studio Technologies, Inc. Page 35
for Surround
has a –3 dB point of nominally 100 Hz.
(To be more precise, the math calcula-
tions work out to be 102 Hz.) For some
applications it may be optimal to adjust this
frequency. The Model 68A makes this a
simple task, with no soldering or compli-
cated procedure required.
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 68A printed circuit
board, is used to hold the resistors. As
received from the factory, a 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 fre-
quency: R = 2,250,000 ÷ F, where R
is resistance in ohms and F is frequency
in hertz.
It’s important that any modifi cations be
performed only by a competent technician.
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 printed
circuit board, is used to hold the resistors.
From the factory, two 14.3 k ohm resis-
tors confi gure the fi lter for nominally 5 kHz.
One resistor is inserted into socket pins 1
and 2, the second into pins 3 and 4. To re-
vise the frequency, these can be replaced
with two other resistors, or one 6-pin SIP
resistor 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 to determine the
resistance required for a specifi c fi lter
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 confi gured 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 precise
adjustment of the input circuitry. This was
provided so that level variations in input
sources, the Model 68A’s circuitry, and
the associated power amplifi ers can be
“trimmed out.”
Defi nition of Level—dBu
and dBV
Whenever possible, Studio Technologies
has opted to use the dBu designation as
it seems to be quite rational. Using dBm
was fi ne when all audio line outputs were
terminated with 600 ohm loads. In this way
it was easy to say that 0 dBm is 1 milliwatt
dissipated in the known load (i.e., 0 dBm
across 600 ohms will measure 0.775 V).
In contemporary situations an output is
rarely terminated with 600 ohms; generally
10 k ohms or higher. The dBu designation