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AM8106 Module - Roland Juno 106 Filter
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 design makes use of the best 80017A clone on the
market, which is available from Analogue Renaissance.
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 high quality polystyrene "Through Hole" capacitors from earlier designs.
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:
Quite an interesting set of filters, so I have cloned them both, and made use of the VCA circuit in the 80017A.
AM8106 The first stage was to buy a D80017A clone (15/09/07) from Jeroen Allaert at Analogue Renaissance. I
then looked through the Juno 106 schematics and designed both the low and high pass filters. During 2008 I prototyped a micro processor 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 been switched 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. This also
enabled the design to be simplified slightly such as removing the GAIN trimmer as we have only one voice.
In the picture you can see the 80017A at the top of the PCB with the kludge wire into the 12dB response. The VCA control
circuit and the output buffer Op Amp are at the bottom of the PCB. The HPF currently sits on a separate PCB. There are a total of 6 trimmers on the prototype, but I have reduced this down a bit for the final production version. The
front panel picture is of the prototype mounted to a PCB front panel. I use this approach to finalise the panel design before having an expensive aluminum version manufactured.
In September 2010 I bought a second hand Roland
80017A chip that has had its black expoxy coating removed and fitted this to by 2nd prototype. Partly because this chip was cheaper than the clone, but also to see whether the response and sound were any different.
Front Panel The front panel is a 3" wide FracRac with Alpha rotary potentiometers, and push fit mixer style knobs.
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