D.W. Fearn VT-7 - Manual (Page 21)

D.W. FEARN
VT-7 Compression Amplifier
26
Second stage
The output of the first stage is coupled to the grid of the second stage through a polypropy-
lene capacitor. This stage operates as a Class A voltage amplifier with a gain of approximate-
ly 20.
Output Stage
The output stage operates as a cathode follower, followed by a custom output transformer
built for D.W. Fearn by Jensen Transformers, Inc. The output impedance is 115 ohms.
Meter and Meter Amplifier
Two-stage IC operational amplifiers are used to isolate the VU meters from the VT-7 outputs.
An output sample is derived from the primary of the output transformer. The VU meter
amplifiers are constructed on the printed circuit board that also contains the Link/Link HPF
circuitry. It is mounted above the main chassis. The meter calibrations set with a 20-turn
trimmer potentiometer. Reference level (0 VU) can be set from -20 to over +20dBm.
The meter can monitor gain reduction in the “GR” position. The meter is driven by a preci-
sion circuit that provides exact tracking with the actual amount of gain reduction. The meter
“0” adjustments are on the PWM boards.
The level indictor is a custom true VU meter conforming to ASA Standard C16.5-1954.
Power Supplies
Primary power from the AC mains is connected to the VT-7 through a standard IEC power
input connector with an integrated power switch. The Power switch energizes all five power
supplies. A mains voltage selector switch on the back panel allows the primary voltage of the
VT-7 to be easily changed in the field.
A fuse protects the VT-7. The Pilot lamp is a type 1819 bulb, operated far below its rated volt-
age of 28. The life of the bulb is lengthened, and the light output is more compatible with
other modern studio equipment. The power transformer is a toroidal unit custom-made for
the VT-7 and has primary taps for 100, 120, or 220-240 volt operation.
Filament Supply
The power transformer output is rectified by a bridge rectifier and filtered before being reg-
ulated, The output of this supply is 12 volts.
Low-voltage Supplies
This supply provides +15, -15, and +5 volts for the solid-state circuitry in the Pulse Width
Modulator.
B+ Supply
Eight separate regulated voltages are required for the plates of the VT-7. The B+ is filtered
with long-life, low-leakage, computer-grade filter capacitors before being regulated and exten-
sively bypassed and decoupled. The negative side of the supply is grounded.