D.W. Fearn VT-1 Betriebsanleitung PDF herunterladen (Seite 33)

D.W. FEARN

VT-1 &

VT-2 Microphone Preamplifers

Third Stage

The ar

m of the Attenuation potentiometer feeds the grid of the third stage (a 6072A), which

also operates Class A wit

h a g

ain of approximately 30. This stage is capacitively-coupled to

the grid of the output stage through a polystyrene capacitor.

Output St

age

The output st

e 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

transf

mer and is capable of low distortion output at levels exceeding +24dBm.

Meter and Meter Amplifier

A two-stage IC operational amplifier is used to isolate the VU meter from the VT-2 output. An

output sample is derived from th secondary of the output transformer. The VU meter ampli-

fier and -

2 volt regulator are 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 fr

om -20 to o

ver +20dBm. The level indictor is a custom true VU meter con-

forming to ASA Standard C16.5-1954.

ower Supplies

Primary power from the AC mains is connected to the VT-1/VT-2 through a standard IEC

power input connector. The Power switch energizes all four power supplies. A fuse, accessi-

ble on t

he r

ear panel, protects the VT-1/VT-2. The Pilot lamp is a type 1819 bulb, operated

ar below its rated voltage of 28. The life of the bulb is lengthened, and the light output is

mor

e compatible with other modern studio equipment. The power transformer is a toroidal

unit custom-made for the VT-1/VT-2 and has primary taps for 100, 120, or 220-240 volt oper-

ation. The VT-2 may be rewired for different mains voltages.

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D.W. Fearn VT-1 Betriebsanleitung Seite 33