AM8180 Roland MKS-80 Filters

AM8180 Mockup

Overview  This module is a clone of the 24dB Low Pass Filter, High Pass Filter and Bass Boost in the Roland MKS-80 REV4 from 1984. The original circuit is based on a Roland IR3109 chip configured as two cascaded 12dB state variable filters. This design first appeared in the Jupiter 6 and included high and band pass modes. In the MKS-80 only the 24dB mode is implemented, presumably because Roland wanted to emulate the Jupiter 8 filter chain.

The LPF is followed by a variable single pole high pass filter and an “always on” bass boost. These are unique designs for the MKS-80 based on low noise DBX1252 VCA’s. This design was not repeated but it was simplified in the Super JX and MKS-70. The REV5 uses the same HPF and Bass Boost circuits as the REV4, the only change is to use the VCA in the IR3R05 rather than a separate DBX1252 VCA.

The dual state variable filters do not go into self oscillation at high resonance, and this is a deliberate design by Roland to stop distortion and to provide a smoother more digital sound. The REV4 design at high resonance does result in clipping and this was reduced down in the REV5.

EuroRack Module:   12HP wide
User Manual:              Download PDF

AMSynths Design Fitting the three filters into one 12HP module is tricky and necessitated using SMD versions of the Op Amps and narrow PCB traces. The modules also draws more current thanks to 8x Op Amps and three custom chips. The core of the module is the Rpar AS3109 chip for the 24dB LPF and a second one for resonance control. They are configured just like the original, including ceramic MLCC capacitors in the filter core, which have been matched to 1% to improve the sound quality. The low pass filter does not self oscillate at high Q values (just like the original) and voltage controlled resonance has been added with external CV control.

The low pass filter is followed by a variable 6dB high pass filter that uses a TL082, a DBX1252 VCA and half of a M5218L. I have used a THAT2181 VCA chip to avoid using old DBX1252 chips (it has the same CV to dB mapping) and modernised the Op Amps (TL072, RC4580). The high pass filter is described in a THAT application note, and I have adjusted various resistor values to run the circuit from 12V power rails rather than the  7.3V in the MKS-80. I am fortunate to own a MKS-80 (Rev 5), so I can compare the high pass filter with my replica.

The high pass filter is followed by a 6dB bass boost circuit that uses an Op Amp to increase the bass response (centered at 267Hz), see the graph.

The front panel has slide potentiometers for HPF cutoff, LPF cutoff, frequency modulation (MOD and ENV) and resonance modulation (RES). There is an external pitch CV input and rotary level pot (KYBD), this is NOT connected to the Doepfer bus (as its gives unpredictable results). There are rotary pots for audio input mix (MIX) and resonance (RES)

This module has a really nice smooth and laid back sound, with the added benefits of nicely designed HPF and the “always on” bass boost.

Outcomes & Availability  The AM8180 started development in May 2022 with an initial prototype. Some minor errors were corrected and the low pass filter was up and working. The high pass filter needed more attention, as the initial resistor values did not proceed sufficient cutoff. A spectrum analyzer plugin was used to check the bass boost was operating properly.

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