D.W. FEARN
VT-7 Compression Amplifier
20
CONTROLS (see Figure 2.)
Threshold
The Threshold control adjusts the point where compression begins. With the con-
trol all the way down, there is no compression and the VT-7 operates as a straight amplifier.
As the control is turned clockwise, the amount of compression increases. This can be moni-
tored on the VU meter (in the GR position), or by ear.
Gain
Whenever compression is applied, the signal is reduced in level by the amount of gain
reduction. The Gain control allows you to make up the lost gain as necessary. At the full
counter-clockwise position, the gain of the VT-7 (with no compression) is about -3dB. The
maximum gain available is about 15dB.
Attack
This control adjusts the time it takes the VT-7 gain reduction circuitry to react to a
signal. It might seem that the faster the attack time, the better, but very fast attack times will
result in significant distortion on material with a lot of low frequency content (this is true of
all compressors). As the control is turned clockwise, the attack time becomes longer. Often
with percussive sounds it is advantageous to have fairly long attack times to allow the initial
transient of the sound to pass through the VT-7 without gain reduction. Experiment with the
Attack control on percussive material to see how it changes the sound. Generally speaking,
a fast attack time is best when complete control of the maximum instantaneous level is
required. This might be desirable to protect a digital input from overload. However, a more
natural sound is usually obtained with a slightly slower attack time. Long attack times can be
useful as an effect, adding power to percussive instruments.
Release
The Release control adjusts how long it takes for the gain to return to normal after
a sound ceases (or drops in level). Fast release times add more energy to the sound, but can
add distortion to low frequency sounds. (This is true of all compressors.) A fast release time
adds density to the sound, often with the compression becoming more obvious. Long release
times make the compression less obvious and more natural, but can “punch holes” in the
lower level audio under certain conditions of high percussive levels.
Harder/Softer
This control adjusts the nature of the compression curve. Toward the
“Harder” end of the control, the compression ratio is higher and the levels more tightly con-
trolled. At the “Softer” end, the compression ratio is lower and the compression is more gen-
tle. This is not merely a ratio control, however, as other parameters also change as the con-
trol is adjusted. More than any other control, this one needs to be adjusted by ear. This con-
trol interacts with the Threshold, Attack, and Release controls, so after changing the
Softer/Harder control, you should experiment with others to obtain the effect you desire. Its
technical operation is complicated to explain, so it is much easier to understand it simply by
experimenting.
Separate/Link/Link HPF
This is a three-position switch that links the two channels togeth-
er for stereo operation. In the Separate position, the two channels operate completely inde-
pendently, and can be used on totally different sources without any interaction. In the Link
position, the two channels track each other precisely, so that the stereo image will remain
constant even if one channel has much more level than the other. In the Link HPF position,
a high-pass (low cut) filter is inserted in the sidechain. This makes the compression less bass-
sensitive and may be useful on mixes that have a very heavy bass content. It will reduce
“pumping” of the mix from bass or bass drum hits, and increase the amount of low-end in the
mix. The HPF operates on both channels, in a linked configuration. The roll-off of bass sen-
sitivity is a very gentle curve (about 6dB per octave) and has little effect above 150Hz.
