VCA

VCAs

1. CV Inputs Control the amplitude of the output signal linearly; both inputs are summed. Connect this to an envelope or gate signal. 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.
2. Bias Knob Adds a constant voltage to the CV inputs.
3. Exp Mod Input Applies an exponential amplitude modulation to the CV or Bias. Modulation depth can be adjusted with the knob. Expects a signal ranging from -5V to +5V.
4. Vel Mod Input Applies an amplitude modulation to the CV or Bias input following a curve optimized for the Keyboard module Vel output. Modulation depth can be adjusted with the knob. Expects a signal ranging from -5V to +5V.
5. 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.
6. VCA Inputs Signal that will be amplified or attenuated by the CV signals and the Bias control (stereo).
7. 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 mutes the signal, 10V lets it pass through at 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, Exp Mod and Vel Mod 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 modes of operation.
• The bottom section with the I/O 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‑1 VCA, but the same principles apply in Multiphonics CV‑3.

Watch the video tutorial here.

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 Exp Mod or Vel Mod inputs.

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 Keyboard 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 (default).

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 CV input for a smoother response when the CV signal 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 CV input for a smoother response when the CV signal 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 Ring mode mean that CV-rate signals, such as those produced by LFOs or ADSRs, will not work optimally when connected to the CV inputs and might introduce a subtle high-frequency buzz.

Amplitude Control ⚓︎

Output amplitude is controlled by the sum of the CV signals and the Bias knob. If Bias is at 0V and no voltage is present on the CV inputs, the VCA will not let any signal through.

The sum of the CV signals and the Bias can be further modulated by the Exp Mod and Vel Mod modulation inputs.

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 occurs at 10V in VCA + mode, and 5V in VCA ± and Ring modes.

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.

To create a tremolo effect, connect an LFO to the CV input (with some attenuation) and adjust the Bias knob to bring the signal above 0V, as shown in this example.

Exp Mod

This input applies an exponential modulation to the amplitude, and is designed for voltages ranging from -5V to +5V, though it can also work with signals outside this range. 0V represents 0 dB (unity gain).

When the attenuverter is set to 100%:

• Each increase by 1V will double the amplitude (about +6 dB)
• Each reduction by 1V will halve the amplitude (about -6 dB)

Even though the VCA module is a linear VCA, a judicious use of the Bias control and the Exp Mod input can turn it into an exponential VCA. Set the Bias knob to 0.01V (using the Shift key for fine-tuning), adjust the Exp Mod attenuverter to 100%, and leave the CV inputs unconnected. Instead, connect your envelope signal to the Exp Mod input. The VCA will attenuate the signal by -60 dB when the envelope signal is at 0V and will be near unity gain when the envelope reaches 10V. Between 0V and 10V, the VCA will follow an exponential curve of about 6 dB per volt.

Vel Mod

This input is designed to be connected to a Keyboard Vel output. See Understanding Signals—Velocity for more information about velocity signals. The expected input range is -5V to +5V: 0V applies no modulation, negative voltages reduce the amplitude, and positive voltages increase it.

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^exp×exp_att×velocity

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

out=in×0.2×(cv1×cv1_att+cv2×cv2_att+bias)×2^exp×exp_att×velocity

Where:

• 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,
• exp is the Exp Mod input voltage,
• exp_att is the Exp Mod attenuverter knob value,
• velocity is the velocity modulation computed from the Vel Mod input and attenuverter knob according to the rules described in Understanding Signals—Velocity.