Studio Technologies 240 - Manual (Page 14)

Issue 1, August 2013 Model 240 User Guide
Page 14 Studio Technologies, Inc.
Microphone Preamplifier Gain
Five switches, SW1-1 through SW1-5, are
used to select the gain of the microphone
preamplifier. The choices are 35, 40, 45,
50, and 55 dB. Only one switch should
be enabled at a time. There’s no problem
changing the gain setting while the unit is
operating. Audio clicks or pops might occur
during gain transitions, but this shouldn’t
be a major issue as long as any associated
monitor loudspeakers are temporarily at-
tenuated or muted.
Selecting the correct amount of gain for an
application might require some experimen-
tation. The goal is to bring the microphone’s
signal up to the nominal line level, which
is 0 dBu, on the Model 240’s main outputs.
Operating at this signal level will help to
ensure the delivery of “clean” audio to the
connected devices. The output of the Model
240’s microphone preamplifier is used, by
way of the compressor circuit, by the four
main output functions. So creating a nice
“hot” signal will help maintain audio quality,
specifically the signal-to-noise ratio, when
driving lengthy cable runs. It will also
ensure that, if selected, audio from a line
input (assigned as a program source) or
talk audio routed to the intercom interface
will be at the optimal level.
Unfortunately, there’s no “perfect” gain set-
ting that this guide can recommend. The
two issues that impact gain selection are
output sensitivity of the connected micro-
phone and the amount of acoustical output
from the microphone’s user. With some
headset microphones, such as the one
associated with the beyerdynamic DT109,
selecting an initial gain setting of 45 dB is
appropriate. Users who speak loudly might
need to have the gain reduced to the 40 or
35 dB setting. Users with quiet voices might
need to select 50 or 55 dB of gain.
An LED indicator is provided as an aid
in optimally setting the gain of the micro-
phone preamplifier. Red in color, this LED
is located adjacent to switch assembly
SW1. It is visible by observing the bottom
of the Model 240’s enclosure when the se-
curity plate has been removed. Technically,
this red LED lights whenever the com-
pressor circuitry is actively controlling the
dynamic range of the signal coming from
the microphone preamplifier. The threshold
is set to be 2 dB above the Model 240’s
nominal internal operating level. So a good
“rule of thumb” is to adjust the gain of the
microphone preamplifier so that the com-
pressor active LED lights (“flashes”) when
the connected headset’s microphone is
sending signal peaks. During normal op-
eration the LED should never remain fully
lit when typical audio signals are present
on the connected headset’s microphone.
Note that if no gain switch is set to its ac-
tive (on) position the preamplifier will oper-
ate at unity (0 dB) gain. This is provided for
compatibility when line-level signals need
to be connected to the microphone input.
But when a microphone is serving as the
Figure 5. Microphone preamplifier gain settings