AM1031 Dual Multi Purpose Filter


AM1031 Eurorack

2024 Update I revisited the ARP 2500 in the Spring of 2024 and designed a set of ten “lost modules” that ARP mentioned in 1970 but never manufactured. These Eurorack modules are commercially available from the AMSynths webstore here. The ARP 1031 Multipurpose Filter module is included in this release.

I initially recreated the module in 5U format using a pair of Roland Jupiter 6 multi-mode filters in 2015. 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. Ten years later in 2024 I did some proper research and discovered what the ARP 1031 module was probably based on.

AM1031 5U Module

AM1031 Module (2015) The module was based two AM8060 circuits with 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.

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. The lettering on the panel quickly rubbed off due to manufacturing defects, and the module was scrapped in 2019, releasing the IR3109 chips for AM8109 modules.

Hand Drawn Schematic

AM1031 Module (2024) Nearly 10 years I carried out some proper research into what the design of the ARP 1031 module could be. The starting point was the knowledge that Dennis Colin was the electronics engineer on the 2500 (and 2600) projects, and would have been the designer of the 1031 if it had made it to prototype stage.

The 1031 could be similar to the SEM SVF, which Dennis designed for Oberheim in 1974, or the ARP 4023 module of 1972 (not sure who designed this – but Dennis was at ARP at the time). However none of these designs have the five filter modes mentioned in the 1970 Tonus Catalog; LP, HP, BP, Notch and Peak.

Dennis created two state variable filters at Aries in 1974/75, which were called Multipurpose filters, which is the same name as the ARP 1031. They have a Peak mode, which is the only place I have found this idea; it is an inverse Notch modeines HP, LP and BP outputs. The first Aries 327 module from 1975 uses CA3080 chips, LM301 IC’s with JFET buffers, and voltage controlled resonance. The circuit also includes dynamic resonance control to avoid the filter breaking up as it gets to self oscillation. This filter is more refined than the 1047 which simply reduces the input level and switches it to the other side of the resonance OTA (LIMIT function).

Outcomes This is the design I have re-engineered as the AMSynths AM1031, as a single (rather than dual) Multipurpose Filter in Eurorack. It sounds amazing but is a bit of a bear to set up with 7 trimmers! The module was tested in April 2024 with 15V and 12V power rails and very few changes were needed, as the trimmers take up the “slack”. The 1031 worked first time, and is a credit to Dennis Colin’s skills. The module was reworked into SMD components and went into production in May 2024. More details are here.

Copyright AMSynths 2024