The ramblings of an IT Professional, Long Distance Runner, Creative, and Maker of Stuff.
Modular Synthesizer Project
Part 5 - VCA
29th October 2020
 2 min read

This is the second module that uses the LM13700 OTA. In this module we are using the IC to attenuate the input signal.

This module is built from the schematic by Kristian Blasol. Its a dual channel VCA which uses both 'sides' of the LM13700 as independent VCA's. Its a lovely one-chip-wonder and super effective.


A Voltage Controlled Amplifier (VCA) is exactly that; rather than having a volume knob to turn manually, the voltage is controlled from a CV input. Again rather like the VCF, the CV input often comes from an envelope generator so that other modules can modulate the sound.

In simple terms controlling the volume of the audio signal from a envelope generator means that we can control the shape of sound in terms of volume; the time / rate that the sound increases from quiet and loud (the attack), then the time / rate that the sound decreases from loud to quiet. Sharper attack means punchier transients (eg bass), longer attacks mean a more gradual swell, like a string. Likewise with the release; a sort release means the sound is deadened abruptally (again, great for bass) or with a long release, the sound fades off gradually, making it ideal for pads and more melodic sounds.

In terms of controls, this is a dual channel VCA, so 2x independent VCA's that operate as follows:

  • Signal in - The input audio signal to be affected / attenuated.
  • Signal out - The resulting effected / attenuated signal.
  • CV in - Input CV to control the volume / attenuation. From a envelope generator, for example.
  • Offset - Allows fine tuning of the output signal's zero-crossing. An ideal signal should equally span +ve and -ve across 0v. If this doesn't happen, the VCA either doesn't close completely or the output volume is reduced. Nice article on offset voltages

Front panel
Front panel

The video does a better job of demonstrating the impact of the filter in the signal chain.

More blog posts in this series: