Hello and thank you for using the Solar Gravity Vactrol Response VCA for Eurorack. We hope you will
find its sounds most pleasing!
The Solar Gravity is an analog vactrol-based VCA designed to bring the lovely response of optocouplers
into the realm of clean, performance-based Eurorack VCAs. Starting with this simple concept, the Solar
Gravity utilizes some neat active circuitry to significantly reduce the bleed inherent in vactrol-based VCAs
and to provide the user with lots of control over the level, direction, and response of incoming control
voltage.
Though it’s primarily designed for use with audio signals, the Solar Gravity can also be used to control the
level of CV such as envelopes or stepped voltages with a unique vactrol response through its DC-coupled
input.
Technical Specifications (Eurorack standard)
Width: 4hp
Depth: 28mm
Peak Current Draw: 16mA @ +12V, 18mA @ -12V
Input Impedance (All Inputs): 100kΩ
Output Impedance: 1kΩ
AC Input Cutoff Frequency (-3dB High Pass): 5 Hz approx.
1. Connecting your Solar Gravity
Connect a 10-to-16 pin ribbon power cable to the header at the top of the Solar Gravity pcb. The -12V/red
stripe side of the connector must be aligned with white stripe silkscreen indicator labeled “-12V” for proper
use. This module is reverse diode protected, but it is still not recommended that you plug it in backwards
to see what happens.
2. Theory of Operation
The Solar Gravity is an active adaptation of the Tidbit Audio Vactrol VCA. It uses the same vactrol as
found in the Tidbit, but uses op-amps to provide various CV processing options and simultaneous AC-
and DC- coupled inputs, to mitigate the bleed at low level settings often inherent in vactrol-based circuits,
and to match the voltage level of the input relatively closely.
In short, the Solar Gravity is a fun, useful and great-sounding VCA that harnesses the pleasing behavior
of vactrols to provide interesting contours for your audio and CV signals, and there are a lot of fun ways to
use it! We hope you enjoy it.
3. Summary of Functions
Offset: Controls the initial level of the VCA by increasing the brightness of the LED controlling the vactrol
as the knob moves clockwise. With standard calibration, moving the Offset knob fully clockwise produces
unity-gain amplification.
CV Level: Bipolar control (aka attenuverter) for processing the level of the incoming CV signal influencing
the vactrol. A setting close to the middle will have little to no influence on the vactrol’s LED and
consequently the level of the VCA, while settings further clockwise will have a
