Crown Audio manual History of the VT-15 Vacuum Tube Recording Channel

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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.

VT-15 Recording Channel D.W. FEARN