Geosynchronous Synths GS1-A

The GS1-A and its smaller brother the GS1-A2 are based off the design of Roland’s System 100M series of modular synth modules.

The heart of these 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 BA662 is no longer available, but Open Music Labs created a clone (clown) design that is pin compatible for the original. It uses 9x tiny dual transistors and a couple of resistors and is perfect as a replacement for the original.

The BA6110 IC is still available if you look hard enough, and while not being pin compatible is more or less identical to the BA662 for use in these modules. For this reason, the GS1-A provides footprints for both IC to be used, while the GS1-A2 (smaller) provides a footprint for just the BA6110.

The signal flow of the GS1-A takes several input audio signals (up to three on the 8hp and just two on the 6hp version) and allows you to mix these via the gain controls next to each input jack.

The VCA can be run in either linear mode, or exponential mode by means of the toggle 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 (or two) outputs are provided. Those common to both versions of the model are the L (low) and H (High) outputs. 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 8hp version also includes an M (Middle) output just in case you want a slightly less hot signal.

Available as DIY modules in partial kits with all necessary hard to find parts, transistors, ICs (including the BA6110) or as a pre-built module.