Using the Classic VCA
What is a VCA?
The VCA (Voltage controlled Amplifier) is a device used to control the level of one signal by
the application of another. Traditionally, the
controlled
input to the VCA is called the INPUT,
whilst the
controller
input is called the CV, or control voltage. A typical system will have the
input as the audio output from a filter or oscillator, and the CV from an envelope generator.
As the envelope generator’s output voltage rises and falls, so the output of the VCA becomes
louder and softer. The output of the VCA is connected to the OUT A socket on the module.
It should be noted though that the CV input can actually be an audio input, and that the
INPUT can be a control voltage. It is up to you what you put into the module. The
nomenclature refers only to the original and common usage of the input sockets on a VCA
module.
The term
amplifier
is actually slightly different to the one you normally use too. It doesn't
always amplify in so far as it doesn't normally make the input signal bigger. The amplification,
or gain, actually varies from nearly zero, ie. the VCA is closed or off, to about one, or 0dB.
When the gain is one then the output level is the same as the input voltage.
The Oakley Classic VCA
The Oakley Classic VCA features two input signals, IN1 (DC) and IN2 (AC), and each has its
own level control. In this way, the Classic VCA can be used to mix or sum two signals
together before they are processed by the VCA core. One of the inputs, IN1 (DC), is directly
coupled to the VCA core. All signals, CV and audio, connected to this input can therefore be
controlled by the VCA. The other input, IN2 (AC), goes via a capacitor and so is often called
'AC coupled'. The capacitor acts to block very low frequency signals and steady state
voltages. You can think of it as being a high pass filter with a very low cut-off frequency.
The GAIN pot controls the 'initial gain' of the module. This is used to partially open the VCA,
even when there are no other signals applied to either of the two CV inputs. If any positive
CV is applied then this will open the VCA further. While the addition of a negative CV will
actually cause the VCA to close.
Two control voltage inputs are provided, LIN CV and EXP CV. Each has its own level pot
which controls the depth of the effect.
The LIN CV input has a linear response. This means that doubling the CV will double the
output amplitude. In general this is the most useful response for general VCA duties. With the
gain pot at its minimum, and the LIN CV pot at its maximum, +5V at the LIN CV input will
produce a gain of around one.
The EXP CV input has an exponential response. This means that a rising CV will produce a
proportional change in gain measured in decibels. In practice this means that the output signal
appears not to quickly increase in level until the input CV is close to 5V. This affect tends to
produce wonderfully plucky sounds when used with a conventional ADSR as the CV source.
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