AM1031 Dual State Variable Filter

AM1031 Module

AM1031 Module

Overview The ARP 1031 module was announced back in 1970 but it never made it into production. It was intended to be a dual state-variable filter, with multiple modes. I have recreated the module using a pair of Roland Jupiter 6 multi-mode filters from 1983. The module has two identical SVF’s, each with two filter blocks that can be switched into High Pass or Low Pass mode at 12dB, which results in four filter modes.

AM1031 Module The module uses the same proven schematic as in the AM8060 Mark 3, with 2x IR3109 OTA chips, high quality 1% polystyrene capacitors and upgraded Op Amps (LT1012, OPA2134). The two circuits have been laid out on a 150 x 150 mm green PCB with 40 mil (fat) traces.

The Roland era 1980’s metal resistors have been retained rather than a change to ARP style 5% carbon composite. Modernisation has been limited to modern trimmers and a 10V precision power rail for resonance CV control. The filter resonates smoothly but does not go into self-oscillation, this is the correct Roland original behavior.

The module has the following front panel controls:

  • 2x Filter Cutoff
  • 2x Resonance
  • 2x Audio Inputs
  • 2x Frequency Cutoff CV Inputs
  • 4x Blue rocker switches for filter mode (LP and HP)

There are 10x 3.5mm jack sockets mounted at the base of the front panel:

  • Audio Inputs (x4)
  • VCF CV Inputs (x4)
  • Signal Outputs (x2)

AM1031 PCB

Build Outcomes I had the idea in June 2015 and laid out the PCB in mid August into Eagle CAD, and the prototype PCB populated on 30 August. A front panel was ordered in December but was faulty, so it was not until January 2016 that I had a good panel to wire up the completed module.

The first pair of audio signals are sent through to the second pair of audio inputs as default (normalised), so you instantly get the two filters in parallel. The CV’s are wired the same way.

2019 Update The lettering on the panel quickly rubbed off due to manufacturing defects, and the module was stored. I plan to buy a new panel in 2020 and fit the module into my Tolex cabinet which holds ten modules. I may change the design and use two 1047 style filters and release the IR3109’s, to ensure closer accuracy to the ARP idea.

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