D.W. Fearn VT-4 Vacuum Tube LC Equalizer 13
HISTORY OF THE VT-4
Throughout my career in recording, there have always been a few equalizers that stood out as being
exceptional. Among my favorites are the 1970s-era Neve input-strip eqs, and the Trident CB9066 para-
metric. I began my equalizer development project by first building a series of test circuits, using all
the various tone-modification techniques. After listening to a wide variety of equalization circuits, it
was obvious to me that the passive inductor-capacitor (LC) circuit was the one that sounded the most
musical and natural to me.
In thinking about how I use equalization, I realized that having simultaneous boost and cut at the low
and end frequencies was often very useful. For mid-frequencies, I found that I always cut, usually
around 400 Hz, and never had any reason to boost in that range. If I were using a parametric equal-
izer, I invariably tended to use the low-Q (broadest) settings, and if I had a choice between shelving or
peaking on the high and low end, I almost always prefered the shelving curve.
So the VT-4 was designed around those preferences -- low-Q curves, shelving, with simultaneous boost
and cut, mid-range cut but not boost, and using passive LC circuitry.
The amplification stages would be vacuum tube, and since the VT-1/VT-2 mic preamps have had such
a gratifying acceptance in the world of recording, it was important to preserve the same sonic charac-
teristics that distinquished the preamps. I decided to try the Svetlana 6N1P dual triode, and was pleas-
antly surprised to find that it is a wonderful-sounding tube, with many of the same sonic characteris-
tics as the 6072A used in my preamps. The active tube circuitry fell into place with relatively little
effort. Now it was time to make the equalization circuitry work the way I wanted.
To start, I used the filter design tables developed by Bell Labs in the 1930s. That got the project off
the ground and it was starting to sound pretty good. For several months, I listened to a variety of music
through a prototype equalizer while I was working on other things, and gradually narrowed-in on what
sounded really good and what didn’t. I would frequently have a box of capacitors and clip leads next
to the prototype and often clipped-in a different value here or there and continued listening.
Eventually, the final frequencies, curves, control operation, etc. was determined. To this day, I have
only a vague idea of what the actual curves look like. Equalizers, like all audio equipment, should
please your ears, not your test equipment.
My experience with Jensen Transformers Inc. was so positive that I knew from the beginning that I
would utilize their products. The first couple of prototypes used inductors that I wound myself, but for
production units more-consistent inductors would be necessary. Jensen agreed to manufacture the
necessary inductors to my specifications, and the quality of the parts is astounding.
