Studio Technologies 742A - Manual (Page 26)

Issue 1, July 2012 Model 742A User Guide
Page 26 Studio Technologies, Inc.
Audio Mixer
with the cover off, it is located on the far
right side of the printed circuit board,
adjacent to the DC power input 4-pin
XLR connector. It is labeled FREQ.
Be careful not to adjust the trim pot that
is labeled LEVEL. This adjusts the output
level of the oscillator to give precisely
–2 dBu to the main audio buses when the
front-panel tone button is enabled. Should
this pot have to be readjusted the proce-
dure is quite simple: measure from pin
1 (common) to pin 2 (audio) of the 3-pin
header labeled P5 FACTORY and adjust
the level trim pot to give –2 dBu. The mea-
surement equipment must have a high
input impedance (>2 k ohms) so as not
to load the oscillator’s output.
Model 742A Feature Set
The Model 742A was carefully designed
with simplicity of operation in mind. Only
the required features and functions were
included. The goal was reliable audio
performance with little risk that operator
confusion or errors would occur. Protect-
ing the audio signals associated with live
news “shots” was foremost on the list of
requirements. The Model 742A’s design
team spent lots of time making lists of
what features could be included, only to
be told by field personnel that everything
that wasn’t absolutely required should
stay out! Here’s a review of several com-
mon features that weren’t included and
the reasons why:
• Including high-pass (low-cut) filters on
the mic/line inputs was a possibility. But
in the “heat of battle” it’s unlikely that
an operator will have the time to en-
able them. Also, these functions require
another four front-panel switches for
high-pass enable/disable selection.
More switches can lead to more
operator error! If wind noise is an
issue, microphone selection (or adding
a windscreen) will often resolve the
issue. Alternately, personnel “down
stream” in an editing suite or at a live
audio console can typically apply a
high-pass filter as required.
• Allowing an operator front-panel ad-
justment of the compressor functions
was certainly a possibility. But frankly,
it would guarantee that audio perfor-
mance would often be compromised.
The Model 742A’s compressor circuits
were optimized for the unit’s intended
applications. The compression ratio,
nominally 5:1, is very effective in con-
trolling the dynamic range of voice
signals, the primary application for the
unit. The attack and release times were
also carefully selected for overall excel-
lent performance. A typical user is not
in a position to, “on the fly,” improve
compressor performance. In addition,
no operator-accessible compressor
on/off switches are provided. If at the
time of installation the “powers that be”
make the decision that dynamic range
control is desired, the internal jumpers
are configured accordingly. From then
on “hands off” operation will take place.
Definition of Level
Studio Technologies has opted to use
the dBu designation. Using dBm was fine
when all audio outputs were terminated
with 600 ohm loads. In this way it was
easy to say that “0 dB” was 1 milliwatt
dissipated in the known load (i.e., 0 dBm
across 600 ohms will measure 0.775 V).
Today, an output is rarely terminated with
600 ohms; generally 10 k ohms or higher.