D.W. Fearn VT-12, Руководство по эксплуатации, Страница: 22 / 37

D.W. FEARN
VT-12 Microphone Preamplifer
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Power switch and indicator
Primary power is applied to the VT-12 circuits when the Power switch (8) is in the up posi-
tion. The amber pilot lamp (7) indicates that the unit is on. It takes about twenty sec onds for
the preamp to start working, but it is suggested that you turn on the power at least five min-
utes prior to use. The tubes are often noisy until all the internal elements reach a stable oper-
ating temperature.
Input “0” position.
In the “0” position, microphone audio is connected directly to the input transformer. This
provides the proper amplification for most ribbon microphones, and will be used in many
micing situa tions. In the “0” position, the VT-12 can accept up to about a -30 dBm input sig-
nal at 20 cps with full gain (70dB) without an increase in distortion.
Input “-20” position.
In the “-20” position, a pad is inserted between the input connector and the input
transformer. This position would be used when the level is too high for the “0” position. On
condenser microphones that have a switchable pad, it will usually be necessary to use a -10
or -20 dB pad in the mic when recording very high sound levels to prevent overload of the
microphone electronics. Whether this is used in conjunc tion with or as a substitute for the
VT-12 pad should be determined by experimentation. For the cleanest sound it is generally
preferable to pad at the microphone first, then at the VT-12 if necessary. The sound of some
microphones will change slightly from the “0” to “-20” position. This is a function of the
interaction between the microphone transformer and the VT-12 input transformer.
Keep in mind that many musical instruments and vocalists produce peaks that are as much
as 20 dB above the average level. Often the “peak clipping” that occurs is of such short dura-
tion that it is not easily noticed and may be acceptable in many situations. This is particular-
ly true with the VT-12 because there is no hard threshold of clipping but rather a compara-
tively gradual increase in harmonic distortion. Over-driving the VT-12 will increase second
harmonic content, adding fullness and richness to the sound. Use your ears to determine the
best position for the Input control.
+48 Volt Switch (1)
Solid-state condenser microphones, and active ribbon microphones in professional environ -
ments are usually phantom powered: the microphone electron ics are DC powered through
the audio cable. This is accom plished by feeding the positive side of a 48 volt power supply
to both pin 2 and 3 of the input connector (through precision matched resistors), and the
negative to pin 1 (ground). The DC voltage is recovered at the microphone with negligible
effect on the audio signal.