Selective Inverter PCB

The Selective Inverter is one of the more unique Synthasystem modules.  There's really two parts to it: the voltage processor, and the inverter.  The voltage processor is actually quite similar to the Voltage Processor/Mixer but simplified a bit.  There's three pairs of inputs (each pair having one attenuated and one non-attenuated) and an offset voltage (again, -10/+12V range).  One of the input pairs is inverted, the other two are not.  In a pinch you can use this module as a voltage processor, and the "A" output will always be the voltage processor output.

The "B" output, though, is where the magic happens.  When the mode switch is set to "Fixed" the "B" output will be the inverse of "A", and any trigger signals will be ignored.  When the mode switch is set to "Select" and there's no trigger input (via the push-button switch, the jack, or the pedal input), the "B" output will be the same as the "A" output.  However, apply a trigger and "B" will be the inverse of "A" with respect the voltage when the trigger was applied.  If you're as confused as I was originally, let's try it with an example.  Say we have a repeating stair-case sequence: 0V, 1V, 2V, 3V, 4V, 5V, 0V, ...  Without a trigger, the A and B outputs will be the same.  But, imagine a trigger is applied when the sequence is at 2V: the "A" output will continue on its merry way with nothing changing, but the "B" output will now be: 4V, 3V, 2V, 1V, 0V, -1V, 4V, ... (with the two sequences intersecting at 2V).

There's three sets of resistors that need to be closely matched; I've used 47k 0.1% tolerance resistors.  The 0.15u cap should ideally be polystyrene, but this is impossible to find, so I've used polypropylene.  It also uses one CA3140 - just remember that it is there, so you don't accidentally throw another LM741 in there.  Note too that R15 (one of the summing resistors) should be 22k, not 22R (it is a typo on the PCB).

Also, I've omitted the V-Trigger components, and will do so on future modules.  I'm far enough along in this project where I can't see myself switching away from S-Triggers, so there's no need to waste all those transistors and other components.

Voltage Processor/Mixer PCB

The Voltage Processor/Mixer is basically what the name implies: a module for processing and mixing control voltages.  Like many modulars from the 70s it has options that allow the voltages to be combined in slightly unusual ways - or at least unusual compared to many of the more straightforward modules available today.  Buchla had its model 257 Voltage Processor which included various scaling, inverting, and other features.  Aries had its AR-323 Dual Mixer which allowed for various inversions, and adding & subtracting the two mixer outputs.  Serge probably had the most straightforward of the voltage processors, which featured bipolar attenuators on the inputs, mixed them, and allowed the addition of an offset voltage - most modern CV processors are similar to this.  Of course, other unusual things can be done, but most often in conjunction with other modules.

The Synthasystem Voltage Processor/Mixer has three pairs of inputs, and two outputs.  Each of the input pairs features one attenuated input, and one unattenuated input.  The pairs are applied differently (inverted or non-inverted) to the two outputs based on their naming (A+ B-, A- B+, A+ B+), and an offset voltage can be applied to both outputs as well.  Note that the offset voltage ranges from -10V to +12V - an extremely wide range!  If you're not building this into a Synthasystem, but as a stand-alone module for a Eurorack or other system, you may consider adding a couple of resistors to cut that voltage range down.

It is a pretty straight-forward circuit, using four inverting op-amps to sum everything at different points.  A few of the resistors need to be 0.1% tolerance, and those are the black ones seen in the picture.  Some of them need to be 47k, while the summing & feedback resistors on IC1 can be anything from 10k-47k (of course, though, all the same!).  I chose 47k, since I was already buying a bunch of them.

Frequency Divider complete!

And now the Frequency Divider is done! Sorry the pics aren't so great, but the sun never came out today...

As you can see, I've used some washers as spacers on the rotary switches - otherwise they protrude too far.  I've used some rather long standoffs to accommodate the Alpha rotary switches.

I haven't played around with it too much, and even then only with the oscilloscope, but it seems like a really interesting module which can perform not only suboctave divisions and clock divisions, but can also mix said suboctaves and produce staircase style sequences like the CGS Suboscillator/Harmonic Sequencer and the Blacet Binary Zone.

In other news, I've got panels here for both the Selective Inverter and Sequencer, and the boards are almost complete, but unfortunately I am missing one capacitor for each!  Hopefully I'll have those shortly, and get them finished up.

Triple Envelope Generator complete!

Sorry, it has been quite some weeks since I last updated this blog!  I've been on jury duty for a major trial, with no real end in sight.  Trying to juggle that with work is a bit of a headache, to say the least...

At least I've managed to work on my Synthasystem a bit during this time, including completing the Triple EG.

Clearly, there is a ton of wiring on this module!  Four PCBs and 28 panel components, of which most are pots and switches.  Initially EGs 1 & 3 worked fine, but EG 2 would only produce a gate output (i.e. Attack/Decay had no effect), but after some debugging I figured out that the pad on the 3.9u tantalum cap got lifted or something - I put a quick jumper wire across to fix that; not the prettiest fix, but it certainly works!

It runs on just the +12V rail, and (as the name suggests) contains 3 envelope generators.  The first two are quite similar: something of a cross between an ADSR and an AD/AR envelope, with some interesting modes.  The second one also allows the Attack knob to act as a delay.  The third EG is more of an AD/AR envelope, with a "hold" time (similar to the Korg MS-10, etc.) that can be used instead of the input trigger's duration.  The third envelope can also be summed into either of the first two envelopes.

I like the fact that the envelopes are much simpler to control, compared to traditional ADSRs, which I sometimes find to be a bit of a pain to adjust on the fly.  I'm hoping that these will fit my needs a lot better (and I think they will).

I'm hoping to get everything set up fine in my rack soon... I really just need to make some power cables, and get some screws to install the modules.  I hope to make some demos of this (and other modules) soon after!