D.W. FEARN
VT-12 Microphone Preamplifer
31
5 .
T H E O RY O F O P E R AT I O N
Input section
Microphone level (150-600 ohm source impedance, balanced, -60 dBm nominal) audio
enters through the XLR-3 female INPUT connector to the input transformer. The load
imposed on the microphone is 10k ohms, but varies slightly with frequency but is never lower
than 9k ohms.
In the -20 position, the input passes through a balanced 20 dB pad. This pad is designed to
maintain ap proximately the same load on the microphone as the input transformer.
All switching is through sealed gold-contact instrumentation relays with bifurcated contacts.
Phantom powering
+51 from the phantom power supply is switched on and off by the front panel +48 switch. A
resistor drops the voltage as required depending on the current being drawn by the con -
denser microphone electronics. This makes the phantom power supply universal for most 12
and 48 volt condenser micro phones. The phantom powering resistors are precision matched
to 0.10% or better. They provide exactly equal voltage to pins 2 and 3 respectively of the input
connector. The switching is through sealed gold-contact instrumentation relays with bifur-
cated contacts. In the “Off” position, the phantom power resistors are completely discon-
nected from the microphone connector.
The rear panel ENABLE/DISABLE slide switch can be set to DISABLE to prevent any possi-
bility of 48V phantom power appearing on the microphone input.
Phase reversal switch
In the Normal position, input/output phase (polarity) is maintained (which must be inverted
due to the design of the circuit). The balanced output is polarity reversed in the Reverse posi-
tion. Switching is accomplished with a sealed, gold-contact instrumentation relay with bifur-
cated contacts.
Summary of Contents for VT-12
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