A-131 Exponential Voltage Controlled Amplifier

The A-131 module is the “sibling module” of the A-130 VCA. In contrast to this, it works with an exponential characteristic, but is otherwise identical.

This makes it rather unsuitable for amplifying control voltages, but in most cases it is very suitable for amplifying audio signals.

User interface


  1. CV1: Control voltage input for modulating gain (without attenuator).
  2. CV2: Control voltage input for modulating the gain (with attenuator “CV2”).
  3. Audio In 1: Input for audio signals (theoretically also for control voltages), which are amplified according to the control voltage at “CV1” and “CV2”.
  4. Audio In 2: Like “Audio In 1”, an additional input for audio signals that are amplified according to the control voltage at “CV1” and “CV2”. The module has a small “internal mixer” – luxurious!


  1. Audio Out: Output for the amplified signal from “Audio In 1” and “Audio In 2”.


  1. Gain: Manual controller for the gain (setting a continuous level).
  2. CV2: Attenuator for the control voltage input “CV2”.
  3. In1: Attenuator for the audio input “Audio In 1”.
  4. In2: Attenuator for the audio input “Audio In 2”.
  5. Out: Attenuator for the “Audio Out” sum output.

A slightly “coloring” VCA

At high input levels or large gain factors (control voltage), the A-131 VCA produces slight distortion (“clipping”). So you’re not dealing with a “hi-fi” amplifier that performs 1:1 amplification under all conditions. The A-131 is a “coloring” amplifier that changes the signal in borderline areas.

You can like that or not: the results don’t sound bad at all, they mostly sound a bit “retro” – but if the sonic ideal is a “hi-end laboratory instrument”, then that will be less suitable.

“Thickening” with an additional sine in the VCA

A “conservative” synthesizer patch sums multiple VCOs in a mixer, sends the result into a filter and then into the VCA. Through the two inputs of the A-131, a sine wave can also be branched off from a VCO and fed unfiltered into the sum signal. In this way, bass can be emphasized even further.

The A-131 VCA as the final (and slightly coloring the sound) element in a standard synthesizer.

Technical specifications

Width8 HP
Depth45 mm
Power requirements20 mA (+12V) / -10 mA (-12V)