AM8125 VCF, HPF & VCA (100M)


Overview The AM8125 is a re-imagined Roland 100M 120 module, with a replica of the Roland JX8-P (and MKS-70) filters and  linear VCA. It is a reworking of the AM8105 Super JX module, that utilised the now rare IR3R05 chip with some additional resonance.

EuroRack Module: 16HP wide
User Manual:           AM8125 User Manual

Module Description  The AM8125 use the an AS3350 chip to provide two state variable low pass filters, with optional Q compensation. The original Roland circuit used the IR3R05 VCF and  VCA chip which first appeared in the MKS80 REV5, and the last generation of Roland analog polysynths (JX-8P, MKS70, MKS50, Alpha Junos). Roland had used the IR3109 chip in SVF form in the REV4, and the IR3R05 is a reworking of this design to reduce three 3 chips to one. The AS3350 chip has been chosen as it very close in topology to the IR3R05, with no requirement for external feedback components.

The VCF has both 12dB and 24dB outputs and a slide switch to modify the Q compensation. When switched to the ON position the signal input goes into both the variable and fixed filter inputs.  The high pass filter has four positions;

  • 0 = bass boost by a shelving EQ with 3dB boost at 70Hz
  • 1 = no filtering
  • 2 = 6dB high pass at 225 Hz cutoff
  • 3 = 6dB high pass at 700 Hz cutoff

This is the same HP filter design that originated in the Juno 106, with slightly less gain on the bass boost. A momentary switch cycles through the four modes with a 7 segment LED displaying the filter position (0,1,2,3).

The VCA uses one side of a AS3381 dual VCA chip, and requires very few additional components. The VCA is AC coupled and designed for audio signals.

The module is 16HP wide and has two PCB’s. The front PCB holds the jack sockets and potentiometers, with SMD Op Amps providing input signal and CV mixing. The main PCB has the combined  24 pin VCF and VCA chip. The PCB is 79mm wide and contains a lot of circuits, so 1/8W THD resistors have been used in the LED drivers. The output of the VCF is normalised into the first signal input of the VCA and the HPF follows the linear VCA.

The physical design is similar to the Behringer System 100 in terms of layout and the use of tall trimmers for signal and CV adjustment. LED slide potentiometers are used for the primary filter controls (FREQ, RESONANCE) and a rotary control for VCA GAIN.

Front Panel The AM8125 has a powder coated front panel with white silk screen printing. The basic panel and holes was drawn with Front Panel Designer and exported as a DXF file. This was then imported into Illustrator and used as a background layer to position and draw the lettering. The finished drawing was exported as a 300 dpi PDF and sent for manufacturing along with the DXF file and the choice of panel colour (Pantone Mid Grey 5U).

Module Outcome & Availability  The AM8125 was developed in the spring of 2023 and production modules with grey powder coated panels will be available in mid 2023.

 

 

 

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