Studio Technologies 740 - Manual (Page 23)

Model 740 User Guide Issue 3, May 2004
Studio Technologies, Inc. Page 23
Model 740
microphone selection (or adding a
windscreen) will resolve the issue.
Alternately, personnel “down stream”
in an editing suite or at a live audio
console can apply a high-pass filter.
• Allowing operator adjustment of the
compressor function was certainly a
possibility. But frankly, it would guar-
antee that often audio performance
would be compromised. The Model
740’s compressor circuit was optimized
for the unit’s intended application. The
compression ratio (nominally 5:1) is
very effective in controlling the dynamic
range of voice signals, the primary
application for the unit. In addition,
no operator-accessible compressor
on/off switch was provided. If at the
time of installation the “powers that be”
decide that dynamic range control is
desired, the internal jumper is config-
ured accordingly. From then on “hands
off” operation will take place.
• The Model 740 has a monaural audio
mixing bus. Certainly not a sexy choice,
but it meets the real needs of most ENG
applications where rarely, if ever, are
two discrete audio paths transmitted
simultaneously. Other audio mixers that
support dual-channel or stereo opera-
tion are often either hard to use, poorly
implemented, or both. For applications
that require separate audio buses there
is no reason why two Model 740s can’t
be installed together. (Or Studio Tech-
nologies’ technically rich Model 750
Audio Mixer could be used. In just two
rack spaces, two stereo or four monau-
ral mixes can be created.)
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.
The dBu designation is better because
it refers to dB referenced to 0.775 V, with
load impedance not a factor. This takes
into account the contemporary audio
scene where most equipment has a low
output source impedance and a high
input impedance.
Mic/Line Switching
As previously discussed, the six input
circuits can be selected for compatibility
with microphone or line-level signals. For
compatibility with line-level signals a 43 dB
balanced attenuator (“pad”) is connected
between each input connector and its as-
sociated microphone preamplifier circuit.
Front-panel pushbutton switches are used
to select whether the pads are bypassed
(mic input setting) or active (line input set-
ting). While the switches are conveniently
located on the front panel, audio does not
actually pass through them. Instead, the
audio signals are routed through sealed,
bifurcated contact electromechanical
relays. These subminiature relays are
located very close to the input connectors,
minimizing the chance of hum or noise
pickup and helping to maintain good com-
mon-mode rejection. The relays should
prove to be very reliable, being designed
for telecom use where typical operating
life is in the millions of cycles.