Sound Theory -

VCA - Voltage Controlled Amplifier

What is a VCA exactly?

A VCA is a voltage controlled amplifier that controls the amount of another signal that is allowed through to the output of the module. The higher the control voltage, the more signal is passed and at some voltage level the entire signal is passed through. 

Some VCAs are called VC Attenuators instead of Amplifiers because they allow the user to reduce the signal level depending on the input CV but don't usually amplify the signal.

A common saying among modular users is -- You can never have too many VCAs!

That is due to the different uses of a VCA in a modular system which include but are not limited to Envelope Shaping, Amplitude Modulation, Tremelo and Volume Control.

Volume Control

  • Run your audio signal through it, then connect the CV input to a mod wheel, foot pedal, or any voltage source you want.

Envelope Shaping

  • You can use the VCA with an Envelope Generator to shape the signal to achieve velocity and further control over your envelope.
  • The voltage sent out by the EG matches the way you want your amplitude to change over time. 
    • Connect the output of the EG into the CV input of your VCA and it will cause the amplitude of your note to fade out like a piano note.

Tremolo

  • Mix a slow sine wave with some DC (to make sure the whole wave stays above 0), then feed them into the CV input of your VCA. 
    • The audio signal will mostly come through to the output because of the DC bias, but you will hear the amplitude get louder and quieter in time with the sine wave you are using to modulate it.
    • Tremolo uses a slow (3Hz, for instance) wave to modulate the amplitude of your audio signal, so you can actually hear the resulting loud/quiet cycles.
     

    Amplitude Modulation

    • If you increase the modulating frequency so that it gets up into the audio range, things start to get interesting.  The modulation is so fast that you are now changing the shape of the original audio signal’s waveform, and new frequencies appear.
    • If we feed a bipolar (swings above and below 0V) 1kHz sine wave into the AUDIO IN jack; this is called the carrier.  If we feed a 500Hz sine wave into the MASTER CV IN jack; this is called the modulator. AM (amplitude modulation) is similar to FM Synthesis in the sense that if operates on the relationship of the modulator and carrier.  
    • If you change one of the input frequencies, you change the relationship between the carrier and modulator and the sidebands appear at different locations.  If you’re listening to the signal as you change the frequency, it can sound like the pitch is going up and down at the same time because the sidebands are moving in different directions (toward or away from the carrier).

    Here are some images to help with further understanding the different uses and functions of a VCA 

     VCA diagram


       

       

       Enjoy your explorations


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