AM8109 Module - Roland Jupiter 8 Low Pass Filter

Overview 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, 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. It was eventually replaced in 1984 with the 80017A chip which contains the VCF (as a SMD IR3109 chip) and a VCA, and implemented in the Roland Juno 106. See AM8106 module.

Here is what Roland had to say: The IR3109 chip contains four variable transconductance amplifiers designed for VCF application in electronic musical instruments. The device is equipped with for high impedance buffers and anti-log circuitry which controls conductances of four amplifiers.

The circuit is similar in concept to the SSM2040 and CEM3320 filter chips which are of the same time period, and used in the first analog polyphonic synthesizers.

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.

AM8109 Module 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 1% polystyrene 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. I cloned the idea and scavenger some M5201 chips from a Korg DSS-1 Sampler.

Trimmers have been added to enable Frequency cutoff to be set, as well as the onset of oscillation as the Resonance pot is turned up. Overall 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. The CV inputs do not have a V/OCT trimmer, but could be added to an updated design when I would replace the flip flop circuit with a simple locking DPDT push button.

Whilst some analog geeks may wonder at the use of an IR3109 chip outside a vintage synthesizer, these two AM8109 modules sound fantastic and the PCB can be used by any one with a spare chip. The design has also worked well as a test bed for circuitry that is used in the AM8106 which uses an easily obtainable 80017 clone chip.

The module has the following front panel controls:

• Signal Input Level (x3)
• Frequency
• Resonance
• CV Level (x2)
• Momentary Push Button: selects 2 or 4 pole LPF
• LED for filter mode

There are six 3.5 mm jack sockets mounted on the left hand side of the panel, these are:

• Signal Inputs (x3)
• CV Inputs (x2)
• Signal Output

Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, and push fit mixer style knobs. The panel photo on the right is a prototype PCB and card panel, prior to getting an aluminum panel made.

Roland Jupiter 8