D.W. Fearn VT-2 - Manual (Page 31)

D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
31
5 .
T H E O RY O F O P E R AT I O N
Input section
Microphone level (150 ohm source impedance, balanced, -50 dBm nominal) audio enters
through the XLR-3 female INPUT connector to the three-position Input selector switch. In
the 0 position, the input is connected directly to the input transformer. The load imposed on
the microphone is 1500 ohms, but varies slightly with frequency but is never lower than 1100
ohms.
In the -20 position, the input passes through a balanced 20 dB pad. This pad is designed to
maintain approximately 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 microphones. 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.
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.