D.W. Fearn VT-1, Operating Instructions Manual, Page: 25 / 40

D.W. FEARN
VT-1 & VT-2 Microphone Preamplifers
25
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 conjunction with or as a substitute for the
VT-2 pad should be determined by experimentation. For the cleanest sound it is generally
preferable to pad at the microphone first, then at the VT-2 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-2 input transformer.
Input Lo-Z position
The Lo-Z position should be used only with certain microphones (e.g. the Neumann TLM-
series, some newer Schoeps) that exhibit an extremely low output impedance. These micro-
phones typically feature a transformerless output. In the Lo-Z position, an impedance-match-
ing network is inserted ahead of the input transformer and this optimizes the VT-2 perform-
ance with these microphones. The best position can be determined by listening, although the
difference may be subtle on many mics.
If the 20 dB pad is needed with a very low impedance microphone, simply select the -20 posi-
tion of the INPUT switch. The impedance match will be correct.
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-2 because there is no hard threshold of clipping but rather a comparative-
ly gradual increase in harmonic distortion. Over-driving the VT-2 will increase second har-
monic 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 in professional environments are usually phantom pow-
ered: the microphone electronics 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.
Vacuum tube condenser mics and dynamic microphones do not require this power and the
+48 switch (1) should be turned off when using non-phantom powered mics. Although leav-
ing the +48 on will not damage any properly wired balanced mic, some ribbon mics have
reportedly been damaged when connected or disconnected from phantom powering. Also,
some engineers feel that the performance of some dynamic mics may be subtly degraded
with the phantom power on.