D.W. FEARN
VT-12 Microphone Preamplifer
27
VU Meter and VU Meter ON/OFF (5 & 6)
The VU meter (6) measures the output level directly across the primary of the output trans-
former, through an isola tion amplifier. It is calibrated so that a +4 dBm output will indicate 0
on the meter. This is the standard “0 VU” level for all professional audio recording equipment
built since the early 1970s. “0 VU” on the VT-12 should result in “0 VU” on a properly
aligned recorder. (This reference level can be changed; see the Maintenance Section.) This
is a true VU meter, and conforms to ASA Standard C16.5-1954.
In some applications, or while testing the VT-12, the output level may be considerably high-
er than 0 VU. To prevent damage to the meter, it should be turned off under these conditions,
using the Meter switch (5). This switch has no other effect on the operation of the VT-12.
Occasional meter pinning will not damage the meter.
Bench Test
If desired, test the VT-12 before installation. The source generator should be set to -60 dBm,
150 ohms impedance, balanced, and the output should feed a balanced bridging input of the
audio analyzer. Measured bandwidth should be 22 cps to 22 kc to obtain the same readings
as the factory test results. Compare your measurements with the test data supplied with VT-
12. Keep the results for comparison in future maintenance tests.
Initial Set-Up
The VT-12 should be installed as detailed in the Installation section. With the outputs con-
nected to an appropriate destination (typically to audio recorder inputs), configure the stu-
dio to monitor the VT-12 output. Apply power and wait about twenty seconds for the tube fil-
aments to get up to temper ature. Check for hum, buzz, or other noise. For the first few min-
utes after a cold start it is not unusual for the VT-12 to produce hiss, pops, and microphonic
“clanks” as the internal elements of the tubes expand from the heat. Correct any ground loop
problems before proceeding.
Summary of Contents for VT-12
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