Jupiter 8 Low Pass Filter - AM8109

Introduction
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.