AM8105 Super JX VCF & VCA

IR3R05 Chip

Overview  This module is a clone of the 24dB Low Pass Filter, fixed High Pass Filter and VCA in the Roland Super JX10. The circuit is based around the Roland IR3R05 chip which first appeared in the MKS80 REV5 replacing the IR3109 and CEM3360, and then made its way into the JX-8P, MKS70, JX-10 and Alpha Junos. The filter and VCA was usually followed by a fixed single pole filter in these polyphonic analog synthesizers.

The 14-pin DIL chip contains two 2-pole State Variable Filters configured as a 4-pole Low Pass filter only. The chip has external capacitors with two internal VCA’s; one for voltage controlled Resonance and one as the final VCA in the signal chain – with both Exponential and Linear voltage control.  The IR3R05 does not go into self oscillation at high resonance levels in Roland synthesizers, this is a deliberate design by Roland to stop distortion and to provide a smoother more digital sound.

2022 Update According to the JX-10 Service Manual the final VCA in the IR3R05 has a compressor capability, by using the exponential CV input. The signal is expanded by a M5241 Dual VCA, which also acts as the final stereo level control after the chorus circuit. This is done to improve the S/N ratio in the BBD device used in the chorus. This functionality is not needed in the AM8105 module.

EuroRack Module:   14HP wide
User Manual:          Download PDF

AM8105 Module

Detailed Description The AM8105 module is based around the original IR3R05 as wired up in the JX-10, and the module was released in 2015.  The chip delivers a 4-pole voltage controlled low pass filter, with a smooth and controlled resonance. There is a linear VCA input for connecting an external ADSR, the VCA exponential input is not used and is set at maximum volume. The VCA is followed by a fixed High Pass Filter which has 2x HP modes and a wicked Bass Boost. These modes are selected by a momentary push button, with 4 indicator LED’s.

 The module uses Op Amps to reduce down the signal levels and control voltages to operate the IR3R05, and to boost the circuit back up. A 4013 and 4053 provide the switching functions for the final single pole filter.

The front panel has rotary pots for Frequency Cutoff (FREQ) and two frequency modulation pots; CV1/KYBD has an external input in CV1 but defaults to the Doepfer bus keyboard CV as a keyboard CV. The VCA gain is controlled by the GAIN potentiometer, which doubles as ENV level amount when an ADSR is connected to the ENV input (+10V maximum CV).  A Resonance potentiometer enables increased Q factors into self oscillation, beyond the Roland setting. There are 2x signal inputs with level controls (SIGNALA and SIGNALB).

This module has a really nice smooth and laid back filter with the added benefits of an on board VCA and a fixed filter to cut or boost the output. Just connect one of our dual ADSR’s and a dual VCO and you have a complete synthesizer!

IR3R05 Diagram

IR3R05 Pin Out (see MKS80 Service Manual)

  1. Signal Input: +/- 10mV
  2. Freq Cutoff: -200mV to +10mV
  3. Resonance: 0 to +60mV
  4. VCA Linear: 0 to +900mV
  5. VCA Expo: +110mV to +10mV
  6. Load: 1K5 resistor to Ground
  7. Negative Power: -15V
  8. Signal Output
  9. Ground
  10. C4 – 10nF
  11. C3 – 10nF
  12. C2 – 10nF
  13. C1 – 220pF
  14. Positive Input: +15V

Module Availability  The AM8105 was available in 2014-16 and around 25 were manufactured, followed by a skiff friendly batch in 2019 and 2021, until I ran out of NOS IR3R05 chips. I plan to re-introduce the AM8105 module as a 16HP wide 100M format module. The VCF will use two 12dB SVF filters in series based on the AS3109 chip, and the VCA will use the AS662 and the same Bass Boost/High Pass circuit. The module will be renumbered into the AM812X range.

2023 Update I revisited the IR3R05 chip to understand how to replicate it. The two SVF’s do look to be OTA based, and not biquads, as the capacitors are tied to GND. However the filter capacitors are much larger than in the MKS80 REV4 (at 10nF). The BP and LP modes can be accessed directly from the capacitors, which would be before the internal buffers.

The IR3R05 frequency control voltage is different to the IR3109, 210 mV in range rather than 300 mV. There is also an inverting Op Amp in the CV signal path which the REV4 (IR3109) does not have.  The Q control is the same voltage range, which indicates the same OTA VCA circuit is used (BA662). The closest design is the CEM3350 dual SVF filter chip, which uses the same larger filter capacitors and has Q compensation – which maye the IR3R05 has?

I plan to re-introduce the AM8105 module as a 16HP wide 100M format module. The VCF will use two 12dB SVF filters in low pass mode using the AS3350 chip. The VCA section will use the AS662 OTA and the Bass Boost/High Pass circuit is as used in the JX-8P. The module will be renumbered as the AM8125.


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