Using the Classic VCA
W at is a VCA?
The VCA (Voltage controlled Ampli ier) is a device used to control the level o one signal by
the application o 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 rom a ilter or oscillator, and the CV rom an envelope generator.
As the envelope generator’s output voltage rises and alls, so the output o the VCA becomes
louder and so ter. The output o 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 re ers only to the original and common usage o the input sockets on a VCA
module.
The term amplifier is actually slightly di erent to the one you normally use too. It doesn't
always ampli y in so ar as it doesn't normally make the input signal bigger. The ampli ication,
or gain, actually varies rom nearly zero, ie. the VCA is closed or o , 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 o the VCA over one i the certain requirements are met – see
later or more details.
T e signal inputs
The Oakley Classic VCA eatures 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 be ore they are processed by the VCA core. One o the inputs, IN1 (DC), is directly
coupled to the VCA core. All signals, CV and audio, connected to this input can there ore be
controlled by the VCA. The other input, IN2 (AC), goes via a capacitor and so is o ten called
'AC coupled'. The capacitor acts to block very low requency signals and steady state
voltages. You can think o it as being a high pass ilter with a very low cut-o requency.
T e CV1 input and gain control
The CV1/GAIN pot controls the 'initial gain' o 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 o the CV inputs. I any positive CV is applied to CV2 then this will open the
VCA urther. While the addition o 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 o the OUT A socket. When the GAIN pot is set to its
maximum the output level rom 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 o completely when
the VCA is closed, OUT B produces about -50dB o signal cut at its quietest. This is generally
good enough or its intended purposes o ring modulation and auto-panning.
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