This module is a clone of the Low Pass Filter from the legendary Roland Jupiter 8. This is a 2 and 4 pole OTA design that Roland first used in the Jupiter 4 in 1980, with a dedicatde filter chipe (IR3109) replacing a discrete version of the filter which took up more PCB space. The IR3109 chip featured in the next series of Roland polyphonic synthesizer (due to the small space needed for 6 or 8 voices), including the Jupiter 6 and 8, as well as the Juno 6/60. It was also used in the Roland SH101 monosynth, Boss phasers and Roland guitar synths.I have kept the design of the AM8109 to just the LPF rather than replicating the HPF from the Jupiter 8 as well, that's simply because I don't have space on the PCB. The IR3109 chip has a set of 4x OTA filter stages, and an exponential CV generator to control the frequency cut off, as well as a VCA to control resonance.
Description
The first stage was to buy an IR3109 chip (well 2x actually so I could build two identical modules), and read the Jupiter 8 schematics - cool! I designed the circuit using Eagle CAD and laid out the PCB, and managed to fit the design onto a 100 x 80mm PCB.
The core of the filter is based on a single voice of the original Jupiter 8, as is the resonance control circuit using a BA6110 as a VCA. The frequency control circuit is from the SH101 with temperature compensation via a 10K NTC. I have used high quality capacitors for the filter stages and their are high quality Op Amp buffers to translate the signal levels to and from the higher levels used in a modular synthesizers. I have used high quality audio grade Panasonic capacitors in the signal path, and the quality of all these upgrades pays dividends in terms of sound quality - excellent!
There is a simple flip-flop circuit to switch between the 2 and 4 pole modes, a 4013 CMOS chip drives a M5201 switching Op Amp. This is different to the Jupiter 8 design which uses FET's to switch between the poles. A front panel LED shows whether the filter is in 2 or 4 pole mode.
Outcome
The design went together very well, and only a few gain settings in the Op Amps needed a bit of adjustment, and then a perfect sounding low pass filter - with a lot of presence and weight emerged. This is a very smooth sounding filter which breaks into full sine wave oscillation at high resonance settings. The difference between 2 and 4 pole modes is not dramatic - with the 2 pole mode providing a more gradual cutoff slope, but nevertheless worthwhile.
More Information
If you want to build your own Jupiter 8 filter module then visit AMSynths and drop me an email. http://www.amsynths.co.uk/Filter_Bank2/AM8109/am8109.html
PCB's are available as well as EuroRack modules. If you'd like to see and hear the filter, check out the video.
