D.W. FEARN
VT-7 Compression Amplifier
11
History of the VT-7 Compression Amplifier
Compression (or limiting) is sometimes seen as a necessary evil in the recording process.
Often it is used to compensate for less than optimum performance or recording technique.
However, when used appropriately, there is no question that good-sounding compression can
add loudness, power, and improved impact to a recording.
Over many years of recording, I have had the opportunity to use and sometimes own some
of the best compressors in the world. For the D.W. Fearn line of products, a compressor was
obviously important. I began the search for the ultimate compressor design in 1995 and it
wasn’t until 2004 that I developed the sound I was looking for.
There are several ways to achieve compression, and all have their advantages and disadvan-
tages. The earliest, and still one of the best, designs uses the characteristic of vacuum tubes
that allows their gain to be varied by changing the grid bias. Only certain tube types are
appropriate for this (the variable-mu or remote cut-off types), and none are currently being
manufactured. Since I want my products to be used for a long, long time, I could not manu-
facture a product that used tubes that were increasingly rare.
Another excellent approach uses a photoresistor illuminated by a light source to vary the gain
of an amplifier. This method was pioneered by Teletronics/Universal Audio in the LA-2. Many
LA-2s were built over the years, and more recently there have been several new versions of
this classic circuit. The world has a good supply of old and new LA-2-style compressors, and
although I did some experimenting with this circuit, I felt that there were enough of them and
I didn’t need to add another. Also, this design lacks the versatility I thought was necessary.
A more modern approach uses a voltage-controlled amplifier (VCA), which is quite versatile
and can combine several additional functions (de-essing, gating, etc.) in one simple unit.
However, I was never satisfied with the sound of VCA compressors and did not consider that
design for my use.
I experimented with a few off-the-wall approaches, but was not convinced that these designs
would really work to my standards.
Another approach, not often used, utilizes a pulse-width modulator to control the duty-cycle
of a gain reduction element. This approach has several advantages, including the ability to
offer a wide range of adjustment. In the VT-7, a solid-state pulse-width modulator controls a
special shunt element in the audio path to control the gain. Since no audio passes through
the element, there is no added distortion or other degradation of the sound. That’s the sim-
ple part. Much more complex is the surrounding circuitry that provides precision control of
the element in a way that conforms to what I wanted to hear.
Because the element is well-isolated from the vacuum tube circuitry before and after it, the
effect on the audio is entirely linear. It does not introduce any measurable distortion. And it
passes my test for sonic superiority.
The vacuum tube audio path uses circuitry developed for the D.W. Fearn VT-4 LC Equalizer.
In some of the circuit, entirely new designs were needed. The VT-7 uses a combination of
6072A and 6N1P tubes and has the basic sound of all my products.
