D.W. Fearn VT15 - Manual (Page 9)

D.W. FEARN
VT-15 Recording Channel
11
History of the VT-15 Vacuum Tube Recording Channel
With the success of the VT-1/VT-2 Vacuum Tube Microphone Preamplifiers, the VT-
4 LC Equalizer, the VT-7 Stereo Compressor, and the VT-I/F and VT-3 Vacuum Tube DIs,
there was a great temptation to combine the features of all of these products into one record-
ing channel.
But I had a problem with the concept. I did not want to be tied into having the com-
pression come before the equalization, or vice-versa, so a prime design goal was to make the
order of the processing switchable. That way the user has a choice of compression before
equalization or equalization before compression. Different circumstances require different
approaches to processing.
Achieving this switching with a minimum of noise and level change was solved quick-
ly, but all the switching in the audio path could result in a degradation of the noise perform-
ance of the VT-15.
By definition, an “all-in-one” box has many compromises over using separate units. I
was not satisfied with the VT-15 until the overall performance was as close as possible to using
a VT-1/VT-7/VT-4. The final result is a box that has about 4dB more noise than the separate
units under most circumstances -- a negligible difference for most applications.
Because the VT-15 is capable of a tremendous amount of gain (about 90dB in the first
prototype), it became a bit tricky to use properly. As long as you were familiar with optimiz-
ing gain structure in a studio environment, it worked great. But not everyone has that back-
ground, and in the heat of a session, it may get neglected. The result could be distortion --
and not the kind you would normally find pleasing. Optimizing the gain structure and head-
room was an ongoing project, complicated by a period of 18 months when we could not keep
up with the orders for our VT-7 Stereo Compressor. In fact, I put the VT-15 project on hold
entirely for six months. When I returned to it over the summer of 2007, the pieces fell into
place and the VT-15 was ready to meet the world.
The mic preamp is identical to the VT-1/VT-2 circuit with one exception: an instru-
ment input has been added. The compressor is identical to one channel of the VT-7 Stereo
Compressor. The equalization is a highly-simplified version of the VT-4 LC Equalizer, with
just one frequency for the low-frequency boost, low-frequency cut, high-frequency boost, and
the high-frequency cut. The chosen frequencies were based on my experience using the VT-
4 and feedback from many users.
Can the VT-15 replace the VT-1/VT-2, VT-7, and VT-4? Not really. The separate units
have features that are just not practical to duplicate in the VT-15. But for many applications,
the VT-15 provides the next best thing.
November 2007.