Friday, June 28, 2013

Ambika or Juno?

One thing that comes up from time to time over at the Mutable Instruments forum is the IR3109 filter board. If you do not know, this (discontinued) board used the famous IR3109 filter chip that's found in the Juno 6/60/106, SH-101, JX3P, Jupiter 6/8 and some of the later Jupiter 4's.  To say the least, a very popular and recognizable filter.  Since Mutable Instruments discontinued the filter board (mainly because the IR3109 is no longer produced and should be used to fix the synths they belong to), there have been discussions on wanting to remake the boards.  The schematics and board layouts have since been deleted and all traces removed, but every so often someone comes along looking for one.

Now, the standard Shruthi-1 kit comes with the SMR-4 filter board, which, when you look at the schematic, is basically the same design as the IR3109 chip!  There are some differences, but for the most part they are the same.  There is a mod that can be done (a couple resistor changes and a couple diodes added) that gives it an almost identical sound to the IR3109.  Actually, there is a new replacement for the Juno 106 80017A (which was a board that had an SMT version of the IR3109 and some other stuff encased in black epoxy) which uses the same 31700 Op Amp!

But, the problem is, when someone hears the IR3109, it's typically in a polyphonic scenario.  Well, I just finished a build for a customer that has 6 SMR-4 voice cards!  I decided to set it up like a Juno and see exactly how close it is.

The settings:

Osc 1: Saw
Osc 2: Off
Sub Osc: about half way
LFO 1 Triangle (sloow) > Osc 1 (very little)
Filter Cutoff about half
Filter Resonance about half
Filter Cutoff > Envelope 1 about 2/3's

Then I run it through a chorus pedal to give it that "Juno" sound.  Finally a little reverb.

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