VCA

CV Input Controls the amplitude of the output signal linearly. 0V means 0% (no output). In VCA + mode, 10V means 100%, while in VCA ± and Ring modes, 5V is 100% and -5V is 100% with inverted polarity.
Bias Knob Voltage added to the CV inputs.
Vel Input Modulates the CV and Bias voltage with an exponential curve (0V=0dB).
Mode Button Switches between three modes of operation. VCA + is a standard two-quadrant VCA, VCA ± is a four-quadrant VCA that can invert the polarity of the input signal, and Ring is an audio-rate four-quadrant multiplier for ring modulation effects.
VCA Inputs Signal that will be amplified or attenuated by the CV signals and the Bias control (stereo).
VCA Outputs Output signal (stereo).

Overview ⚓︎

The VCA (Voltage Controlled Amplifier) modulates the amplitude of the signal on the In jack with a gain signal on one of the CV inputs, and routes the modulated signal to the Out jack.

In simpler terms, you can imagine the VCA as a volume knob between the In and Out jacks. The signal on the CV input changes the knob’s position: 0V is the minimum, 10V is full scale.

The input and output come in pairs for stereo processing. In a mono patch, the convention is to use the left channel (top jack).

By default, this module acts as a linear unipolar VCA, but it can also be used as a bipolar VCA (four quadrant multiplier) or as an audio-rate ring modulator.

The top section with CV, Bias and Vel is for amplitude control. A gate or envelope signal will usually be connected into the first CV input.
The Mode section in the middle lets you switch between the three operation modes.
The bottom section with the In and Out jacks is for signal input and output.

Tutorial ⚓︎

Learn how to use a VCA for volume and modulation control in this tutorial (the video shows the Multiphonics CV‑2 VCA):

https://www.applied-acoustics.com/l/multiphonics-cv-2-vca-tutorial/

Usage ⚓︎

Unless otherwise specified, the descriptions in this section are valid when the CV attenuators are fully clockwise (100%), the Bias is fully counterclockwise (0V) and nothing is connected into the Vel input.

The most basic application of a VCA is to let a sound play when a note is playing. If you are not familiar with modular synthesis and want to get an idea of what a VCA does, try building this patch:

A Classic VCO always produces sound. By routing it through a VCA, the sound will be blocked until a positive voltage is applied on one of the CV inputs. When no note is playing, the Keyboard gate signal is at 0V, and the VCA blocks the sound. When you play a note, the gate goes up to 10V, and the VCA lets the sound through.

By connecting the gate into an ADSR and using the ADSR to control the VCA, the volume would change more smoothly.

In Depth ⚓︎

VCA Modes ⚓︎

This VCA offers three modes selectable from the Mode section.

Math geeks might want to take a look at the Technical Notes section below for the exacts formulas used in the VCA.

VCA + (Unipolar)

Unipolar VCA mode.

CV input is expected to be between 0V (no output) to 10V (full output, unity gain). CV inputs below 0V are ignored. Minimal slew-limiting is applied to the input for a smoother response when the CV input has sharp edges.

VCA ± (Bipolar)

Bipolar VCA mode.

CV input is expected to be between -5V and 5V. At 5V, the output signal will be the same as the input signal (unity gain). At -5V, the output signal will be at unity gain, but its polarity will be inverted relative to the input. At 0V there will be no output. Minimal slew-limiting is applied to the input for a smoother response when the CV input has sharp edges.

In this mode, inputs below -5V or above 5V will boost the input signal amplitude. For example, a CV of 10V will add 6dB to the input signal. If you are using VCA + mode with an envelope and the output is not loud enough, you can try switching to VCA ± for an extra boost.

Ring

Ring modulation mode.

It works exactly like VCA ± mode, but with no slew-limiting, so audio-rate signals can be connected to the CV inputs. This often produces sounds that can be described as thin or metallic.

The lack of slew-limiting in that mode mean that CV-rate signals, such as those produced by LFOs or ADSRs, will not work optimally and might introduce a subtle high-frequency buzz.

Amplitude Control ⚓︎

Output amplitude is controlled at the top of the module from three different sources: CV, Bias and Vel.

CV

The two CV inputs control the amount of amplification or attenuation applied to the input signal. They each have an attenuator, and their voltages are summed.

A typical patch will only use one CV input, often from an ADSR or Keyboard Gate.

As described in the previous section, unity gain is at 10V in VCA + mode, and 5V in VCA ± and Ring mode.

Bias

The Bias knob is used to add a fixed voltage to the CV inputs.

If nothing is connected to the CV inputs, the Bias knob will act as a volume knob.

Connecting a LFO in the CV input (with some attenuation) and adding bias to bring it above 0V is a simple way to create a tremolo effect, as shown in this example.

Vel

This exponential input is designed to be connected to a Keyboard Vel output. The exact formula used to apply the exponential curve is described in Multiphonics Concepts—Velocity.

However, in a patch where the volume is controlled by an envelope generator (such as the ADSR module), it is often better to use the Vel input on the envelope generator rather than on the VCA.

Note that the Vel input is applied to the voltage from the CV inputs and Bias knob, not to the input signal. This means that the VCA will not produce sound from the Vel input alone; it also needs a CV input or some bias.

That being said, with a judicious use of the Bias control, the Vel input can be used to turn this module into an exponential VCA. By setting the Bias to 0.01V (using the Shift key for fine-tuning) and the Vel attenuator to 100%, a 10V signal on Vel will produce unity gain, and a 0V signal will produce -60dB of attenuation. Between 0V and 10V, the VCA will follow an exponential curve of 6dB per volt.

Technical Notes ⚓︎

Internally, the gain is limited to ×50 and the output is limited to ±100V to avoid feedback loops that could crash the synth.

In VCA + mode, negative CV inputs are clipped to 0V and the formula for the output is:

out=in×0.1×(cv1×cv1_att+cv2×cv2_att+bias)×2^vel×vel_att

In VCA ± and Ring modes, the formula for the output is:

out=in×0.2×(cv1×cv1_att+cv2×cv2_att+bias)×2^vel×vel_att

In these formulas, in is the voltage on the In jack, cv1 and cv2 are the CV input voltages, cv1_att and cv2_att are the CV attenuator knob values, bias is the voltage set by the Bias knob, vel is the Vel (Exp) input voltage, and vel_att is the Vel (Exp) attenuator knob value.