D.W. Fearn VT-4 Vacuum Tube LC Equalizer 21
5 .
T H E O RY O F O P E R A T I O N
Input section
Line level (600 ohm source impedance, balanced, +4 dBm nominal) audio enters
through the XLR-3 female INPUT connector to the seven-position Input level control.
The input is balanced-bridging, with an impedance of 40k ohms. The input transformer
is made by Jensen Transformers Inc. The input switch is a passive pad on the primary of
input transformer. A two-stage Class-A triode amplifer utilizing 6N1P vacuum tubes pro-
vides isolation from the input of the VT-4 to the passive LC equalization circuitry.
Equalization section
All equalization is accomplished with passive LC (inductor/capacitor) circuitry.
The inductors are custom-made for the VT-4 by Jensen Transformers Inc. All capcitors
are polystyrene or polypropylene types. The rotary switches are semi-sealed, silver-con-
tact types selected for quiet operation and long life.
Output section
The output amplifier is similar to the Input section, using a two-stage, Class-A 6N1P
triode amplifier with a cathode-follower output. The output is tranformer-balanced, using
a custom-made Jensen transformer. The output is optimized for feeding bridging inputs,
which are standard in the recording industry. Either a balanced or unbalanced source may
be fed.
Power Supplies
Both the filament and B+ power supplies are solid-state and fully regulated. The fila-
ments are operated on regulated DC. A custom-made toroidal power transformer supplies
the proper voltages and has primary windings for 100, 110-130, or 220-240 VAC. The
power switch, mains voltage selector (115/230VAC), and line fuse are on the back panel.
