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. However, it is
possible to increase the gain of the VCA over one if the certain requirements are met – see
later for more details.
The signal inputs
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 CV1 input and gain control
The CV1/GAIN pot controls the 'initial gain' of the module when no jack is inserted into CV1.
This is used to open and close the VCA manually, even when there are no other signals
applied to either of the CV inputs. If any positive CV is applied to CV2 then this will open the
VCA further. While the addition of a negative CV will actually cause the VCA to close.
With CV1/GAIN pot turned to its maximum value and no jacks inserted in either CV1 or CV2
then the gain produced by the VCA is 0dB or one.
The OUT B output works in reverse of the OUT A socket. When the GAIN pot is set to its
maximum the output level from OUT B should be at its minimum. However, you should note
that the signal is not completely silent. Unlike OUT A which can be shut off completely when
the VCA is closed, OUT B produces about -50dB of signal cut at its quietest. This is generally
good enough for its intended purposes of ring modulation and auto-panning.
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