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D.W. FEARN
VT-15 Recording Channel
29
VU Meter and Meter Amplifier
A two-stage IC operational amplifier is used to isolate the VU meter from the VT-15 output.
An output sample is derived from the primary of the output transformer. The meter calibra-
tion is 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 conforming to ASA Standard
C16.5-1954.
Power Supplies
Primary power from the AC mains is connected to the VT-15 through a standard IEC power
input connector. A rear-panel voltage selector slide switch selects either 120 or 220V opera-
tion (the switch is labeled “115/230”). The VT-15 can also operate on 100V power with an
internal wiring change. The mains fuse is slow-blow type 3AG, rated at 2A for 100/120V
operation and 1A for 230V operation. The VT-15 can operate on either 50 or 60Hz power.
The Power switch energizes all power supplies. 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-15.
Filament Supply
The power transformer output is rectified by a bridge rectifier and filtered before being reg-
ulated by a three-terminal regulator. This supply also powers all the switching relays in the
VT-15, as well as the pilot lamp and VU meter lamps.
+/- 15V Supply
The solid-state control circuitry for the VT-15 compressor requires regulated +15 and -15V
supplies.
B+ Supply
Seven separate regulated voltages are required for the plates of the VT-15. 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.
+48 Supply
The regulator circuitry is on the power supply printed circuit board. The actual output of this
supply is 51 volts, which is reduced to 48 volts through the decoupling resistor at the micro-
phone input (see Phantom Power, above).