AM8122 Dual SVF (100M)
AM8122 Module
Overview The original Roland 100M (1978) has only one type of voltage controlled filter, 24dB low pass. This rather limits the range of sounds that can be created with the synthesizer and it is strange that Roland never ported the 703 2-pole OTA SVF into the 100M format. The AM8122 closes this gap and provides two 2-pole State Variable Filters in one 16HP module.
AMSynths Design The AS3320 chip is used to create the SVF, rather than the CA3080 OTA in the original 703 module. This is partly down to the limited PCB space for two filters and partly because the 3320 chip in SVF mode sounds very good, think Oberheim OBX. An alternative approach would be to build a dual Jupiter 6 SVF with AS3109’s, but this is tricky to achieve given the limited PCB space and we already produce a Jupiter 6 module – the AM8060 module.
The AM8122 circuit reuses the 2-pole SVF in the AM8325 and adds the BP mode and voltage controlled resonance. The third CV input is used as the Resonance CV input. The front panel design follows the Behringer 121 module with signal and CV input trimmers, frequency and resonance on 30mm sliders and green and red indicator LED’s.
Each filter has two outputs, with a slide switch selecting which filter output goes to which jack socket. The upper jack socket (OUT) can be set to either LP, BP or HP and the lower jack socket (REV OUT) follows the reverse (HP, BP, LP). This approach ensure the LED’s track the filter mode selected by the switch, whilst still providing individual outputs of each filter mode. The three signal inputs of the second filter are normalised to the outputs of the first filter (HP, BP and LP), so that the filter modes can be mixed together and filtered again!
Front Panels The panels use the same manufacturing process as the earlier AMSynths 100Mmodules, with a white silk screen over a dusty grey powder coated panel.
Outcome & Availability The module and panel were designed in December 2022. In 1Q23 the prototype PCB’s were debugged, with the revised LED drive and increased filter output levels so they are all the same (10V p-t-p). Production modules will be availble in 3Q23. The AM8122 will be the first AMSynths module to use SMD manufacturing processes (JLCPCB) for the PCB’s.
