Crown Audio VT-15 5. THEORY OF OPERATION

D.W. FEARN
VT-15 Recording Channel
27
5.
THEORY OF OPERATION
Preamp Section
Input section
Microphone level (150 ohm source impedance, balanced, -50 dBm nominal) or line level
(600-ohm source, +4dBm nominal) audio enters through the XLR-3 female INPUT connec-
tor to the Input selector switch bank. In the 0 position, the input is connected directly to
the input transformer. The load imposed on the microphone is 1500 ohms, but varies slight-
ly 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.
In the Line position, a balanced pad of approximately 50dB attenuation is inserted on the
input. In the Line position, the 48V phantom power and Lo-Z switches are locked-out.
The Mic/Inst switch changes the input from the rear panel XLR Input connector to the one-
quarter inch front panel Inst jack. The audio from the Inst jack is fed directly to the grid of
the first amplifier stage, bypassing all the input switching. The load impedance on the Inst
input is 1 megohm.
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 bifurcat-
ed contacts. In the “Off” position, the phantom power resistors are completely disconnected
from the Input connector. When the VT-15 is in the Line position, the 48V switch is locked
out.
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.
Input transformer
The input transformer is custom-made for D.W. Fearn by Jensen Transformers, Inc. and rep-
resents the state of the art in transformer design. This is the same transformer that we use
on the D.W. Fearn VT-1/VT-2 microphone preamplifiers. It exhibits extremely flat frequency