Description

The GS1-A is based off the design of Roland’s System 100M series. The heart of this modules is the BA662 differential op-amp that Roland used in just about everything in the late 70’s and into the 80’s.

The signal flow of the GS1-A takes up to three input audio signals  and allows you to mix these via the gain controls next to each input jack. The VCA can be run in either linear or exponential mode by means of the toggle selector switch. Next to this is an “initial GAIN” control. This sets the gain of the incoming CV modulation sources allowing you to boost the CV if necessary.

Speaking of CV, the module again provides either three or two CV modulation inputs to trigger/effect the VCA itself. Most normally a gate or envelope would be applied, but of course with a modular synth any suitable CV input can be used. Each input has its own CV attenuation control. Remember that the initial GAIN has a general influence over these as well.

The rear of the module has 3 trimmers that can be used to adjust the overall drive of the VCA (adjust so at peaks the red LED is just showing, or overdrive if thats what you are after!) and the other two set the level of the linear and exponential modes accordingly. Calibration details are provided in the build guide.

Finally, three outputs are provided. Essentially these can be thought of as line level (L) and euro level (H) – so you can use this VCA as an output module to get a euro signal down to line level for use outside of your Euro setup. The M (Middle) output is slightly less hot than the H output.

Inspired by the BA662 IC used in classic Roland analog synths, the Ge0sync (GS) series 1 is a DIY option to get small hp equivalents to some of the Roland System 100M into your Eurorack. Making use of the BA662 clone available from Open Music Labs (OML) or in some cases the almost equivalent BA6110 IC which is not yet un-obtainium. The GS series 1 provides everything you need to build a classic analogue synth voice including utility and modulation modules including VCO, VCF, VCA, ENV and LFO. The VCO make use of the CEM3340 VCO IC as many of the original parts used in the 100M VCO are long since discontinued. The series also adds a simple mixer than can average or sum the inputs (so works for both CV and audio). All partial kits include a bonus passive multiple panel/pcb set.

GS1-A Features: 

• Audio Inputs: 3x each with gain pot
• CV Inputs: 3x each with attenuation pot
• Initial GAIN pot for CV inputs
• Linear / Exponential toggle switch
• Audio Outputs: 3x (line, medium, euro)
• Bonus passive multiple panel/pcb set included
• Width: 8HP

This Panel & PCB set includes the following rare parts (Green in BOM):

• 4 x 2SA1015GR
• 6 x 2SC1815GR
• 1 x 2SK30A-GR
• 1 x BA6110
• 1 x RC4558
• 2 x 10uF Bipolar electrolytic capacitors

GS1-L – BOM & Build Guide