Geosynchronous Synths GS1-F

The GS1-F low pass/high pass filter module is based on Roland’s 4 pole filter design using the BA662 differential op-amp circuit. The classic buttery, self resonating filter sound as present on many of Rolands classic analogue synths of the 70’s and 80’s.

As discussed on the GS1-A page, the BA662 has become almost un-obtanium in the recent years with no NOS supply, only used pulls and sometimes fakes.

The GS1-F is designed to cater for both the OML BA662 clone or the BA6110 which is still available as NOS parts. The space is limited, and as there are four of these ICs in the circuit (one per pole of the filter) the DIY partial kit version come with two different voice boards. Only one of these needs to be built, either the BA662 or BA6110 version which are clearly marked on the underside of the PCBs.

In all testing we couldn’t hear any real difference in either build, and as Geosync managed to get a batch of NOS BA6110 we’ve added those to the partial kit bundles so you can build it either way, with the option to source four of the BA662 clone chips yourself.

Back to the filter, it acts as a mini mixer with three inputs, so you can layer your patch with up to three gain controlled audio sources and then use up to three CV inputs to affect the filter yourself. Envelopes, LFOs, or any such CV can be used. The filter can be calibrated via the frequency and width controls to set the filter cutoff start frequency and resonance width.

The cutoff and resonance pots provide the usual controls over the filter manually, and of course an LFO or other CV modulating the filter will automate this based on the starting position of the pots.

We really love that warm classic Roland analogue filter sound and that’s what drove us to develop this relatively small form factor (8hp) full function filter. Built using the original “tolerant” parts, i.e. 5% carbon film resistors, polystyrene capacitors and a suitable replacement for the also unobtanium NTC thermistor, to make it simple we have also included the styrene caps, a couple of bi-polar electroytics and NTC in the partial kits, as well as the usual rare transistors and other ICs.

The filter also includes a fairly basic high pass filter, this is simply an on/off affair, however you can adjust the degree of filtering by changing a single capacitor, C2. By using values from 1nf to 4.7nf you can increase the filtering, and to some degree alter the type of high pass filter by changing the type of capacitor. Any ceramic or poly will work, but by default we recommend a 2.2nf Nichicon QYX style polyester.