D.W. FEARN
VT-12 Microphone Preamplifer
33
Third Stage
The arm of the Gain potentiometer feeds the grid of the third stage (a 12AT7), which also
operates Class A with a gain of approximately 30. This stage is capacitively-coupled to the
grid of the output stage through a polystyrene capacitor.
Output Stage
The output stage operates as a cathode follower, presenting a comparitively low output
impedance (approximately 800 ohms). The cathode output is coupled through a proprietary
polypropylene capacitor to the primary of the output transformer. This is a custom Jensen
transformer and is capable of low distortion output at levels exc24dBm.
VU Meter and Meter Amplifier
A two-stage IC operational amplifier is used to isolate the VU meter from the VT-12 output.
An output sample is derived from the primary of the output transformer. The VU meter
amplifier is constructed on a printed circuit board mounted above the main chassis.
The meter and meter amplifier may be totally disconnected from the circuit by the Meter
switch. The meter calibrationis set with a 20-turn trimmer potentiometer. Reference level (0
VU) can be set from -20 to over +20dBm. The level indictor is a custom true VU meter con-
forming to ASA Standard C16.5-1954.
Power Supplies
Primary power from the AC mains is connected to the VT-12 through a standard IEC power
input connector. The Power switch energizes all four power supplies. A fuse, accessible on the
rear panel, protects the VT-12. The Pilot lamp is a type 1819 bulb, operated far below its rated
voltage 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-12 and has primary taps for 100, 120, or 220-240 volt operation. The VT-12 may be
rewired for different mains voltages.
Summary of Contents for VT-12
Page 1: ...VT 12 Vacuum Tube 70dB Microphone Preamplifier Operating Instructions D W FEARN...
Page 4: ...4...
Page 15: ......
Page 19: ...D W FEARN VT 12 Microphone Preamplifer 22...
Page 27: ...D W FEARN VT 12 Microphone Preamplifer 30...
Page 35: ...D W FEARN VT 12 Microphone Preamplifer 38...
Page 37: ......
