AM 8121 Module - Roland 100M LPF + HPF

Overview This module is a clone of the Voltage Controlled Filter from the legendary Roland 100M analog modular synthesizer. This VCF appeared in two formats:

• Dual LPF in the 110 module (no HPF)
• LPF and a HPF in the 121 module

It is this later circuit I have cloned, with a simple non-voltage controlled single pole 6dB High Pass Filter in front of a fully voltage controlled 4-pole 24dB Low Pass Filter with resonance into full self oscillation.

This is the "classic" Roland 4-pole OTA based Low Pass Filter, first developed for the Jupiter 4 and SH-7 in 1978 using discrete components. It makes use of OTA chips such as the CA3080 or BA662 and produces a nice warm sound with a well controlled resonance. The design was gradually miniaturized, first as the IR3109 VCF chip in the Jupiter 8 (AM8109) and later as the D80017 VCF and VCA chip in the Juno 106 (AM8106).

AM8121 Module The AM circuit is a careful replication of the original Roland 100M design with the HP and LP filters in series. The OTA chips are BA662's in the original, but I have used the BA6110 chip which is still available at a low price. I have modified the CV circuit to make use of easily available 1K tempcos and to use USA transistors (2N3904/06).

I have used 1% polystyrene capacitors for the filter stages and upgraded the audio path capacitors as well. The Op Amp used in the High Pass filter has been upgraded from a 4558 to a TL071, and the CV summing Op Amp has been replaced with a low offset OP177 Op Amp. The 2SK30 FET transistors after each OTA filter pole have been replaced with easy to locate 2N5485 FET's. I have not utilised the BA6110 buffer, although some other Roland filters used this approach.
I usually fit a FINE frequency pot to the LPF's I build, but on this module space is tight and I have gone for 3x CV's including a KBD pot which defaults to the internal keyboard note CV and just 2x audio signal inputs. This leaves room for the miniature 4-way toggle switch which controls the HPF frequency.

The circuit was designed using Eagle CAD and laid out onto a 80mm x100mm PCB, which is the standard AM format for FracRacs. The module has the following front panel controls:

• Signal Input Level (x2)
• Frequency
• Resonance
• KBD Level - CV from keyboard note value
• CV Level (x2)
• HP filter settings (four) on a slide switch

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

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

Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, with 15mm diameter Mixer knobs. I have used a simple piece of Veroboard to mount the 4-way slide switch attached to the right hand pots.
 

Roland 121 VCF