Studio Technologies 220 2004 - Manual (Page 16)

Model 220 User Guide Issue 2, August 2004
Studio Technologies, Inc. Page 17
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
doesn’t necessarily indicate that the main
output’s signal is being compressed. Un-
less specifically configured to do other-
wise, the output of the compressor is only
used for the talkback outputs.
It’s expected that the 20 and 60 dB gain
settings will not often be used. But there
are always exceptions and that’s why they
were included. It’s possible that with a
very “hot” microphone, such as a phan-
tom-powered condenser-type, 20 dB of
gain could be correct. It’s also possible
that a microphone with a very low-level
output, such as a ribbon-type, would need
60 dB of gain. But in general, the 30, 40,
and 50 dB gain settings will serve most
applications.
Note that if no gain switch is set to its ac-
tive (on) position the preamplifier will oper-
ate at unity (0 dB) gain. In this mode the
preamplifier remains stable, but is intend-
ed for use only during factory testing. A
valid exception would be where a line-level
signal is connected to the microphone
input. This could occur with a special
Model 220 application. But with a micro-
phone connected as the input source,
one should never use the 0 dB setting.
The issue is that with no gain added to the
microphone input signal, the relative noise
floor on the main and talkback outputs
will be much too high. These outputs are
designed for handling line-level signals,
expecting to receive the output of the mic
preamplifier. In conclusion, the 0 dB gain
setting doesn’t highlight a problem, but
simply reflects the unit’s gain structure.
Phantom Power On/Off
The Model 220 can provide 48 volt phan-
tom power to the microphone input.
Switch SW1-8 controls whether or not
phantom power is active. By phantom
power’s very nature it could be left applied
to the microphone input at all times. But
generally people prefer to turn it off unless
required for a specific microphone.
Figure 5. Phantom power switch settings