Studio Technologies 220 2008 - Manual (Page 17)

Model 220 User Guide Issue 4, October 2008
Studio Technologies, Inc. Page 17
Microphone Preamplifier Gain
and Phantom Power
Five switches are used to set the gain of
the microphone preamplifier. One switch
is used to select the on/off status of the
phantom power supply.
Microphone Preamplifier Gain
Switches SW1-1 through SW1-5 are used
to select the gain of the microphone
preamplifier. The choices are 20, 30, 40,
50, and 60 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
associated monitor loudspeakers are
temporarily attenuated or muted.
Selecting the correct amount of gain for an
application might take a little experimenta-
tion. The goal is to bring the mic’s signal
up to line level, nominally –2 dBu for the
Model 220’s main output. Operating at this
signal level will help to ensure the delivery
of “clean” audio to the connected device.
The output of the Model 220’s microphone
preamplifier is used by the main output
and, by way of the compressor circuit, the
talkback outputs. So creating a nice “hot”
signal will help maintain audio quality,
specifically the signal-to-noise ratio, when
driving the often-lengthy cable runs.
Unfortunately, there’s no “perfect” gain
setting that this guide can recommend.
The two issues that impact the setting
are output sensitivity of the connected
microphone and the acoustical output
level of the microphone’s user. With
some headset microphones, such as the
Sennheiser HMD25, selecting an initial
setting of 40 dB is appropriate. Users who
speak loudly might need to have the gain
reduced to 30 dB. Quiet users might
need 50 dB of gain.
An LED indicator is provided as an aid
in correctly setting the gain of the micro-
phone preamplifier. Red in color, this LED
is located adjacent to switch assembly 1.
It is visible by observing the bottom of the
Model 220’s enclosure when the security
plate has been removed. Technically, this
red LED lights whenever the compressor
circuitry is controlling the dynamic range
of the signal coming from the microphone
preamplifier. The threshold is set to be 2
dB above the Model 220’s nominal internal
operating level. So a good “rule of thumb”
is to adjust the gain of the microphone
preamplifier so that the compressor active
LED lights (“flashes”) when the connected
microphone is sending signal peaks. Dur-
ing normal operation the LED should not
remain fully lit when audio is present on
the mic input.
It’s important to remember that the com-
pressor active LED is used to assist in
setting the gain to the optimal value. It
Figure 4. Microphone preamplifier gain switch
settings