AM8106 Juno 106 Filter

AM8106 Prototype

Overview This module is a clone of the Roland Juno 106 Low Pass Filter which is based on the Roland 80017A chip and a discrete 1-pole High Pass Filter circuit (which also has a bass boost setting).  The original Roland 80017A circuit is a classic 4-pole OTA based low pass which can be traced back to the late 1970’s and the Roland 100M and Jupiter 4.

All the IC’s, capacitors and resistors are surface mounted devices, contained in a 46 x 17 mm black encapsulated ceramic PCB. In fact some of the resistors are carbon printed, which means they are rather unreliable and the Roland chip is prone to failure – hence the clone being easily available. The original 80017A contains an IR3109 and 2x BA662 OTA chips, the first controls resonance, the second is used as the final volume VCA. You can see the insides at Obsolete Technology.

The 80017A filter contributes a lot to the warm analog sound of the Roland Juno 106 and makes it a very popular analog Polysynth on the secondhand market. Owners say the filter is softer, and not so aggressive as earlier Roland filters and sounds plastic or sandy. This could be due to the use of SMD ceramic capacitors rather than the THD ceramic capacitors used in earlier designs (Juno and Jupiter).

The Juno 106 also has a 1-pole High Pass filter which does not have a traditional variable frequency control, but instead has 4 fixed filter responses controlled by the on board micro-controller:

  • 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

Quite an interesting set of filters, so I have cloned them both, and made use of the VCA circuit in the 80017A.

Prototype PCB

AM8106 Mark 1 Back in September 2007 I bought a D80017A clone from Jeroen Allaert at Analogue Renaissance with the intention of building a VCF/VCA and HPF module.  During 2008 I prototyped a micro controller design for switching the high pass filter and using the original Juno 106 sliders.

This design evolved into a complete programmable VCF/VCA/ADSR circuit. By late 2009 the AM8106 module had reverted back to a more traditional design with a simple miniature 4-way switch to select HPF settings.

The D80017A is used as a 4-pole LPF with front panels controls for Frequency and Resonance. I have kludged a wire onto onto the SMD Op Amp chip to get a 2-pole response, which is selected by a front panel push button. The VCA level is available externally so a 0 – 10V control voltage such as an ADSR can be patched in. The VCA level pot defaults to a simple VCA gain when no control voltage is applied.

The circuit has Op Amp buffers before and after the D80017A chip for converting the audio signals to the higher levels used in analog modular systems. The control signals are also designed for the same higher voltages levels.

The module has the following front panel controls:

  • Signal Input Level (x2)
  • Low Pass Filter Slope button
  • Frequency
  • Resonance
  • CV Level (x2) – this will; be named KBD and MOD
  • High Pass Filter setting switch

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

  • Signal Inputs (x2)
  • VCF CV Inputs (x2)
  • VCA CV Input
  • Signal Output

Outcomes The prototype PCB’s were tested in early March 2010 and various resistor values altered to produce the right CV response for FREQ and RESONANCE. The filter will self oscillate just like the original. The buffer Op Amps have been set up to give unity gain and to ensure the audio input waveform is not inverted by the signal path through the various chips. The Roland design has been simplified by removing the GAIN trimmer but there are still 6 trimmers!

The front panel is in FracRac format, using a PCB with a printed card overlay. I used this approach to finalise the panel design before having an expensive aluminum version manufactured. I moved on to using 2mm plastic card in the mid 2010’s for panel testing.

2022 Update The availability of both the AS3109 and AS662 OTA chips from Alfa Rpar now means I can recreate the Juno 106 filter accurately, without using old D80017A chips. The AM8106 is back in production with these chips, and SMD filter capacitors to retain the original sound. The filter is 4-pole 24dB/octave, with the high pass filter and bass boost feature retained, but with the final VCA omitted. The new AM8106 module is 10HP wide, and skiff friendly with a depth of 35 mm. Check the webstore for stock.


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