(Variable) Noise Generator complete!

This morning I completed my second noise module.  The one I did previously had a switch to go between white and pink nose, this one uses a pot as a cross-fader between the two. The PCB and all components are the same, it is just a matter of connecting a pot to one header, or a switch to another.

It has been a while since I used my other noise module, so I can't do an A/B comparison quite yet.  But, yes, it does sound noisy. ;-)  I also just used one transistor, and didn't try a few and choose my favorite.  (I thought about it, but oh well!)  The sound is interesting with the cross-fader.  Turning it to the right is a bit like lowering the frequency on an LPF (with white noise input), but not quite.  It gives some interesting flavors of noise at middle settings.

My Input Amplifier and Tuner/Monitor are so close to being finished, but I'm cursed with missing about $0.80 worth of panel parts.  I also have boards to build some more of other modules, but those are competing with my 3-panel Best Of CGS build... most likely I'll jump back and forth.

Voltage Processor/Mixer complete!

Wow, has it seriously been 4 months since I last updated this blog?!  Things have been a bit of a roller-coaster here, but I'm finally getting back into this project.

Today I finished assembling the Voltage Processor/Mixer module.

It is a fairly straight-forward build, though with a number of jacks.  (Nothing nearly as complex as the Selective Inverter, however!)  I tested it and everything works great - the attenuated inputs amplify as well, which is a nice plus!  As with the S.I., the offset voltage is bipolar and doesn't center at zero (due to the +12V/-10V supply), so just bear that in mind with testing so that you don't confuse yourself like I initially did with the S.I.  (There's a reason why the switch is there!)  It should be interesting to play around with this one, with the various inversions and summings.

In the meantime, I'm finishing up a few other modules (Noise module with white/pink pot, Input Amplifier, and Tuner/Monitor - just waiting on a few parts), and should have PCBs in soon for some more VCOs, VCAs and VCFs.  In addition, I'm also getting everything together to (finally!) get everything together in a cabinet... the small rack I have has just been more of a pain than anything, and I really never got much set up in it!

Sequencer complete!

The sequencer is now ready!!  It is quite a dense and heavy beast...

Oops, there's a few bits of wire insulation that got stuck to the knobs. ;-)

As I mentioned, I didn't solder in the UJT initially, but just set it in there, in case there was any issue with it at high frequencies.  Tested it, and it worked fine on the first try!  (Well, I had two switches upside down, but it only took a couple minutes to fix those.)  So, I soldered in the UJT from the top, and just left the legs hanging down... no big deal.

I've mostly just played around with the clocking mechanism, but everything seems to work fine elsewhere too and it is really interesting.  The RYM (3rd row control of timing) is fun, as are the different modes. 

Now, if only the screws to mount these modules to the rails would finally arrive here(?!?!), I could get everything working together!

Sequencer Output PCB

And here's the second PCB in the Sequencer!  All of the stage pots, output attenuators, and LEDs are board-mounted, which will make wiring a whole lot simpler.

The pots & LEDs get installed on the other side (but soldered on this side).  The jacks & LED lenses had already been installed on the panel.  I've only added the standoffs at this point.  I also had to use diagonal cutters to clip the (plastic) pot shafts down to a reasonable length - a bit of a pain, as my large diagonal cutters are MIA.

Anyhow for the pots I plugged them in and soldered the wiper pins only, and make sure they were flush with the surface.  I then slipped the LED in, but didn't solder them, and then shoved it into the panel.  A little bit of juggling, but the pots all went in and I put the nuts on a few, and then pushed the LEDs into the lenses.  I then soldered everything in, and started putting the knobs on the front.

Sequencer Logic PCB

The Sequencer is composed of two PCBs: the logic board, and the output board.  The logic board contains all of the clocking mechanism and other logic (number of steps, etc.).  The original boards only supported S-Triggers, but the current revision support both S-Triggers and V-Triggers.

It is a pretty full-featured sequencer, especially for the time, and has a fair amount in common with the Moog 960 - at least its features are more similar to that than the Buchla and Serge sequencers.

There's a lot of features to try and explain, and naming them off here in a couple of paragraphs here isn't going to be too useful.  But basically it is an 8x3 sequencer (i.e. 8 stages, 3 rows) with a built-in clock (w/voltage control). Other features include 16-step sequences (from rows 1 & 2), row output attenuators, stage trigger outputs, third row control of timing, and a bunch of other control options (stop/loop, reset, max stages, etc.)  David's Synthasystem website has a more detailed overview of the individual controls, and I'll hopefully make a demo that shows them too.

I've omitted all of the V-Trigger components - hopefully I got it all right!  I was tracing things based on the original schematic, etc.  I've used discrete transistors in place of the SSM2210.  I've also left off the UJT, which is the core of the oscillator.  Once I have it put together, I will test different UJTs and select one with the most appropriate range (there can be a variability in UJTs).  This PCB will be connected to the output board with a ribbon cable (note the 16-pin header).

Selective Inverter complete!

And now, the Selective Inverter is finished!

It is a really crowded build.   Getting the banana jacks on there was tough, as the nut drivers didn't cut it - I had to use pliers to get the nuts tightened.  (Of course, that's what I get for having all those multed jacks in a banana system!  But, I don't mind.) 

One thing that I didn't realize from the description, but suspected from the schematic, was that in the fixed mode the "B" output is always inverted (i.e. the trigger inputs do nothing).  Also, I was really confused at first while testing it, until I remembered that the offset voltage is bipolar (-10V to +12V).  Anyhow, I'm very much looking forward to trying this out with some "real-world" applications!

Input Amplifier PCB

A fairly obvious module, the Input Amplifier is used to bring external signals into the Synthasystem and pull them up to appropriate levels for the modular synth.  The module offers a few different input jacks (mine will probably be TS, banana, and RCA), although they are all multed into the same input (so, only apply one signal to the three inputs!).  There's a pot to adjust gain, a switch to set gain to high or low, and a switch to enable RIAA equalization.

(Sorry about the picture quality - I got a new phone and indoor close-ups are kinda blurry.)

It is a really simple build.  The current PCB revision has a couple additions that Nyle and David have made to the input.  Originally the input jacks were connected directly to the opamp, but now there's a capacitor to AC couple the input, and a pull-down resistor to hold the input to 0V when no signal is present.

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!

Switching Things Up

I switched out my Electroswitch rotary switches on the VCF with some Alpha ones.  They have a much nicer feel & response (at least to me).

Since they don't have a way to set the max number of positions, I wired one up for band-pass, the 6 to the left as low-pass, and the 6 to the right as high-pass.  (The final position at the bottom I left disconnected.)  I also used liquid electrical tape since heat-shrink wouldn't work so great for this.  Didn't turn out super pretty, but it does the job (and the heat-shrink is pretty much overkill, but I am doing it anyways!).

Oh, and here's a bit of a preview of the Triple EG!

This was a total beast to build!  It is a dense and heavy module!  Can't wait to test it (I was missing some MTA connectors for power, but they should be here Wednesday.)

Phase Shifter complete!

And the phase shifter is now complete, too!

A fair amount of op-amps in there!  It is a 4-stage, 720-degree phaser. There's a wet/dry knob to control a mix of the original signal & the phase shifted signal, which is ideal for effects.  I only tested it lightly, but it seemed that the frequency knob didn't do much above 2:00 - perhaps it was just due to the setting of the original waveform, as it clearly did a lot to the signal before it got to 2:00!  There's a VC input for this too, but I haven't played with it yet - since it is summed with the frequency knob, however, it should work just fine.

Ring Modulator complete!

And, the Ring Modulator has also been completed!

This uses an MC1495 IC (four quadrant multiplier), which is certainly a more frequently encountered solution (with a variety of different chips & circuits) than the diode "ring" design.

It has a couple of trimpots, for the Signal Null and Carrier Null... they're pretty simple to set.  Obviously the one is on the front panel, but I'm not sure why, i.e. if it needs more frequent adjustment or what.  There's also a pot to adjust the mix of original signal to modulated signal, which could prove interesting to play with.

While I'm at it, let me admonish the reader to watch out for those blue plastic panel-mount trimpot assemblies.  The threads are very prone to being stripped, and, as they're plastic (and have some flashing), they're quite easy to strip if you're not careful (and sometimes even if you are careful... oops!).

And finally, there's a Mult/Square switch.  In the multiply position, it acts as a standard ring modulator.  However, in the squared position it sends the signal input to the carrier input too... honestly, I'm not sure why this wasn't originally implemented with a normalled jack (as they're used in other situations in the Synthasystem), but no matter - for me it'll save a banana cable!  Theoretically I think it is supposed to double the frequency, but instead it just seems to change the signal a bit.  I honestly haven't played around with this a whole lot (mostly just setting the trimmers), so I'm sure there's quite a bit to discover.

Peak Selector complete!

Here's the completed Peak Selector module.

Another fairly simple build. I now feel a lot more comfortable wiring up LEDs. This is basically just a comparator that outputs an S-Trigger (and lights an LED) whenever the input exceeds the set threshold.  However, it isn't done with OpAmps, so (to me) it is a bit more confusing, circuit-wise.  It has a minimum on-time, which seems useful, but means that if you want to feed it noise, you'll have to do so through a sample & hold or something - otherwise the output will be on all the time.

In the meantime, I also ordered a panel for the Triple EG yesterday... I tried to order all the panel parts too, but I'm afraid that I forgot some of the MTA connectors for the power - aaah!!  There's always one thing I forget whenever I place a Mouser order...

Noise Generator complete!

Here's the completed Noise Generator module!

Simple panel assembly; just a pair of jacks, a level pot, and a white/pink switch.  The next one I do will have the pot to go between white and pink.  Sounds great!  And works like a charm.

Rack-Mount Power Supply

Unrelated to the Synthasystem, I just had a bit of an "accident" with some liquid electrical tape. Oops! What a mess!  Anyhow, ... here's the rack-mount power supply I've been working on.  If my Synthasystem reaches the size that I secretly hope it will, I'll have four of these.  But for now, one will be fine!

Sorry for the horrible pics!  This is using a PowerOne HAA15-0.8-AG PSU, an MOTM-960 distro-board, a few banana jacks (for a common ground), and Schurter plug/switch/fuse/etc. (KD13.1101.151, 4303.2906, and 0859.0074)  If only these things were more constantly in-stock at Mouser... ugh.

Anyhow, I made a full 3U high panel to hold all of these, based on David Ingebretsen's PSU panels. This uses the same PSU and switch/plug/fuse panel, but not the Blacet distro boards.  I've added 3 banana-jacks for ground, attached to the distro-board.  This is essential for many banana-based systems: individual modules aren't connected to a common ground via the PSU (i.e. different/multiple PSUs) and the path-cables (being bananas) don't include ground. So, there's a need to connect the ground of all power supplies.

Most of the connections are pretty self-explanatory... power supply common output goes to ground on the distro board, and then get brought out to the banana jacks.  +/- outputs on the PSU go to their respective terminals on the distro board as well.  The PSU inputs are all documented in the datasheet that comes with the PowerOne supply, and then the earth connection from the plug unit gets bolted to the PSU.  Since I want it to run +/- 15V, I cut the two jumpers that are marked on the PSU PCB.  Plugged it in and the house didn't burn down!  Measured the outputs and they are +15.02V and -15.02V - yay!

In the meantime I've also finished the Noise, Peak Selector, Ring Modulator, and Phase Shifter modules, and will post those soon.  I still need to order the panels for the Triple EG and Frequency Divider... unfortunately, I'm still working out an ideal rotary switch (need for the Frequency Divider).

ENVDT PCB

And here's the third board from the Triple EG, the ENVDT.  Its envelope is a bit more complicated than the others; it has the attack/delay switch like the 2nd EG, but it is more of an Attack/Release envelope (it lacks the "Duration Level", i.e. Sustain of the others).  The duration time can be controlled either by the incoming trigger's length, or a pot.

Though more complex than the ENVDL boards, it is a bit more straightforward since there's only one variant.  On both this and the ENVDL boards, I've used tantalum caps in place of the impossible-to-find 1.2u and 3.9u electrolytics.  Also, on this board, I went ahead and used the modified values for R22 and R28, as described in the build docs.

ENVDL PCBs

The Synthasystem's Triple Envelope Generator uses three PCBs (not counting the power regulation one).  The first two EGs use these ENVDL boards, while the third uses the ENVDT board.

I'll get into the specifics of the EG in more detail at a later time (i.e. when I finish the module), but basically, the first two EGs are nearly ADSR, although Decay and Release are controlled by the same knob.  With a short trigger input, they act as AD envelopes.  Both of them also have an overall level control, Damping and "Quathin" switches (for slightly different behaviors), and switches to combine their output with the third EG.

The second EG also features a switch to change the Attack control to be a delay control (with a near instantaneous attack).

Here's the two boards.  As you can see, I've marked them "1" and "2", and crossed out the parts to be omitted for the two variants.  Perhaps it isn't super pretty, but I find it useful when stuffing them, and checking them over later.  One of the MTA connectors is for a connection to the ENVDT board for the switchable combined 1+3 and 2+3 EG outputs.

I'll cover the ENVDT board tomorrow.  In the meantime, I finished up my power supply over the weekend (just a couple tweaks before I present it here), and the panels for the Noise, Phase Shifter, Ring Modulator, and Peak Selector are scheduled to arrive on Wednesday!

Frequency Divider PCB

The Synthasystem Frequency Divider is a rather unique and interesting module.  It produces four user-selectable divisions of an input signal and sends them to a mixer with level controls for each division, and a master level control.  Each divider outputs a square wave, so you could use it for audio and CV signals, creating suboctaves, staircases, etc.  And it will work as a clock divider too, with alternate inputs and outputs for S-Triggers.

This sort of module was pretty rare for modulars in the 1970s. In fact, as far as I can tell, the only other manufacturer to come close was Polyfusion, with their model 2090 Octave Divider.  I don't know much about the 2090, but I've seen a schematic and it is based around the 4024 ripple counter, and has seven fixed division outputs; I'm not even sure if it was intended more for CV or audio. There was also a frequency divider from Blacet, but I'm not sure what features were on the original (or even what it looked like). John Blacet had an article describing a simple 4024-based frequency divider in Vol. 1 Issue 3 of Synapse in 1976; I'm guessing his own was an expanded version, and perhaps only available as a kit. Other than those, I am not aware of anything from the 1970s (Moog, Buchla, Aries, ARP, E-μ, Serge, etc.) that did any sort of clock divisions or suboctaves (besides using a sequential switch for basic clock divisions).

Anyhow, enough archaeological discussions!

Yeah, that's a lot of parts!  No CMOS here, just a ton of transistors and diodes.  I've omitted three resistors and a transistor at the bottom center - they're for the the S-Trig to V-Trig converter on the output, and I've omitted it for the same reason discussed previously regarding the Peak Selector (i.e. to keep the output floating when off).

I'm holding off on ordering the panels until I get a couple of Alpha rotary switches from Mouser.  I'd like something a bit different from the Electroswitch ones, so I want to check these out before I get the panel made.

Ring Modulator PCB

Another "effects" module, the Synthasystem Ring Mod features a wet-dry crossfade like the Phase Shifter. Like many modular ring modulators, it isn't a "true" ring modular: there's no diode ring.  Instead, it uses an MC1495 Four Quadrant Multiplier IC. This is, in fact, the only "specialty" chip found within the entire Synthasystem! All the other ICs are either purely opamps or matched transistor pairs.

Besides the wet-dry crossfade, this has another interesting feature: there's a switch (Multiply/Square) to send the input signal to the carrier input as well.  When this "square" mode is enabled, it theoretically doubles the frequency, but apparently does some more interesting distortion too (especially for audio signals).

I still need to install the chips, but it is another straightforward build.  There's a 1.2u electrolytic capacitor which (due to unavailability) I've replaced with a tantalum cap.  There's also two trimpots (besides the power ones): the Signal Null (on the PCB), and the Carrier Null, which is a panel-mounted trimpot like on the oscillators.

I've also just ordered the panels and panel parts for this, the Phase Shifter, the Peak Selector, and the Noise.

Phase Shifter PCB

Design-wise, and to the best of my understanding, the Steiner Phase Shifter is a fairly straight-forward four-stage phaser (180 deg. per stage). As an added bonus, it features voltage control, and a wet-dry crossfade.

Lots of diodes and op-amps (LM741s to be inserted later)! It is interesting to me how some of the Synthasystem designs utilize opamps, while many don't use them at all. They are certainly a feature of the "modern" modular synth (whatever that means), particularly for input and output buffers.  Presumably their availability was limited when Nyle was designing the main/core modules, but were more available when he was designing others. But, I don't know for sure!

Anyhow, I'm really looking forward to hearing this in action!

Noise PCB

Here's the Noise PCB. It is pretty simple, both in parts & features. It is basically a white noise generator, with a filter for pink noise output, and an output attenuator. There's actually two possible front-panel configurations, as there's only a single output: one with a white/pink selector switch, and another with a pot for a continuous cross-fade between white & pink noise.

I think I'll build this one with a switch; I plan to build at least one of each, though.

(While the PCB pics like this with flash are definitely less blurry, the transistors and some other parts get a bit lost. So, I'll generally use the non-flash ones. Sorry!)

There's one optional resistor, which I've omitted. And I've also omitted the trimpot, and replaced it with a jumper. Both of these can be seen in the pic & are described in the build docs (and PCB notes).

I'm thinking that I possibly should've put a socket in for the reverse-biased NPN transistor which provides the white noise. I've never built a noise generator before, but I know that different individual transistors can provide different responses, and you sometimes have to choose the "right one". Hopefully, however, it won't be a problem and I'll get a good output from this board.

It's A Sine!

Finally got my second VCO producing a sine wave this evening!  Since both oscillators used stuff from the same batch of components, I measured the outputs of the voltage dividing trimpots (Sine Shape and Sine Shape Bias) on the working oscillator, and dialed them in to the same thing on the other oscillator.  (All the other waveforms were nearly exactly the same.)  And, guess what? It worked! Just a tiny bit of tweaking, and I had it looking pretty nice.

So, meet the four waveforms!

As you can see, there's a bit of a spike at the top of the triangle.  It is the same for both modules; I'm hoping to eliminate it through further trimming, but it isn't problematic... the triangle sounds good. 

I still need to get the range and 1V/Oct fully calibrated, but I have it pretty close for just one pass at trimming them.

By the way, here's the problematic waveform I was getting off of the sine output for a long time:

It is not the same "bad sine" as described in the build docs, which is a sine with distorted peaks and large Vpp. This one has too small of a peak-to-peak voltage, and this is the best I could get it (it was a triangle at some settings). Anyhow, if you get this waveform, your oscillator isn't build wrong, but you need to keep on trimming!