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).

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.

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.

Peak Selector PCB

The Peak Selector is effectively just a comparator. It is a simple interface, with an input, an output, an LED, and a pot to control threshold.  It is interesting in that it doesn't use an OpAmp comparator, but is all transistors, caps, and other discrete goodies.  I'm still trying to figure out exactly how it works!  The output is a trigger (in my case, an S-Trigger).

Despite being a relatively simple concept, historically, not a lot of modulars have included comparators as distinct modules.  In brief, it will output a high signal (or here, a trigger) while the input signal exceeds a given threshold.  It sounds simple, but has a lot of possible uses... creating "digital noise" by feeding it white noise, generating a trigger sequence sync'd to an LFO, starting an event (envelope, sequencer, etc.) when an incoming signal reaches a particular level, etc.  It would even act as an (expensive) V-Trig to S-Trig converter!

This was a simple board to populate.  It calls for a 1.2u electrolytic capacitor, but since those are unavailable, I've used a tantalum.  The tantalum caps are slightly more expensive, but since there aren't many needed in the whole system, it isn't a big deal.

I've also entirely omitted the circuitry for the V-Trigger outputs (three resistors, a transistor, and the associated MTA header).  They're clearly marked on the PCB. If you were to add the circuitry here, you'd get +12V on the S-Trig output while at rest, which probably isn't desirable if you plan to OR various triggers.

Voltage Controlled Oscillator PCB

And, finally, the Voltage Controlled Oscillator PCB!  (Two of them, actually!)  The is the bigger "A" model oscillator in the Synthasystem. Feature-wise it is a pretty typical oscillator and has sine, saw, triangle, and pulse outputs. Each output has an attenuator, which is a bit atypical. There's both coarse and fine controls, along with exponential (1V/Oct) inputs, one of which is attenuated. In addition, there's a sync/reset input, pulse width control, and a pulse width modulation input.

A rather big board, for sure!  Lots of transistors and trimpots.  There's a few more "gotchas" with this one than the other builds.  First, note that I haven't installed the ICs yet: there's 4 LM741 OpAmps, and one THAT340 for the exponential converter.  There will also be a 1k thermistor/tempco sitting over the THAT340, but I don't want to instal that until everything else is ready.  Also, there's pads for two SSM2210s, but I've used unmatched transistor pairs: apparently they should be fine. (Note that later Synthasystems even used unmatched pairs for the critical exponential converter!)

There's a few 0.1% tolerance resistors that you can see at the far end of the picture... there's two for summing the VC input, and four more around the expo converter.  You can also see the styrene capacitor. The pad spacing on this board revision is pretty small, so I've mounted it with one lead coming down from the top, along the side.  Hopefully this will be fine!  (It looked cleaner than trying to position it some other way.)

You'll notice space for a trimmer up front there: it is actually supposed to be empty. There's two spots next to each other, for the triangle shaper. You're supposed to use the right one, and jumper a pair of (marked) pads on the left one. If the oscillator doesn't perform well, then you're supposed to swap the trimpot and the jumper. Hopefully that won't be necessary!

There's a couple other trimpots that have been omitted too.  One is the 1V/Oct calibration, which I'm instead moving to the front panel as one of the panel-mount trimpots (there's an MTA header for it, so no flying wires from pads in the middle of the PCB).

The other is for the scale of the 3rd VC input. There's a few options for this: I've opted to use a 200k pot on the front panel (it is a feature I like), but you could use the trimpot, the panel-mount trimpot (again, there's an MTA header), or make it a fixed scale!  There's an associated scaling resistor... for my 200k pot, I'm using a 22k resistor (as suggested in the BOM) - it would be the same for any equivalent trimpot. If you're using a fixed scale, choose the resistor and then jumper two of the marked pads under the trimpot.

Voltage Controlled Filter PCB

The voltage controlled filter! As I mentioned before, this uses a different topology than the Steiner Synthacon filter. I'm no expert in filters (trying to learn!), but apparently it has some unique elements. Also, it will operate in low-pass, band-pass, or high-pass modes (there's a switch on the front panel).

Note the 0.1% tolerance input summing resistors. I've also used a 200k trimpot instead of 250k, as on the VCA.  1.2u electrolytic caps are unavailable, so I've used tantalum ones here instead: a little pricier, but not a big deal for just three of them.  And, as before, I've used the discrete transistor pairs: a totally discrete VCF!

VC Trigger Generator PCB

The VC Trigger Generator is a bit like an LFO, I suppose, but for generating triggers (these are the S-Triggers I was writing about earlier).  It has pads to allow for V-Triggers too; I've installed all of those parts, for completeness (they may help for any debugging, too!), but I'll be using the S-Triggers.

Yep, that's a LOT of transistors! 17 of them! (And only 2 are for the V-Trigger add-ons.)  Two SSM2210s have been replaced with discrete pairs. Note also the 2N4891 on the left (the little tin can)... hard to find, but I got 10 from Nikko in the UK. That should be plenty for what I may eventually build: they're only used here, the Sequencer, and the S&H.

There's a 2M resistor printed on the board; it should be 100k. It is well-documented on the schematic, etc., but just make sure you take note of it.

Another resistor is listed as "SELECT" - this is the resistor for the LED. I put a 1k resistor in there, which should be fine for the yellow LED I plan on using (Mouser 78-TLHY5401). There's not many LEDs in the Synthasystem: just here, the Peak Selector, and the Sequencer... this is the only one where you need to select a resistor value, though. In case you don't remember:

R_LED = (V_cc - V_f) / I_f , where:
V_cc is the source voltage (here, per the schematic, +12V)
V_f is the forward voltage drop of the LED (look this up on the datasheet or Mouser page, 2.4V for mine)
I_f is the forward current of the LED (look this up on the datasheet or Mouser page, 10mA for mine)

So mine calculates out as (12 - 2.4) / .01 = 9.6 / .01 = 960R, and I rounded up to 1k.

VCA/Mixer PCB

The Synthasystem VCA doubles as a mixer with its 3 inputs, which is a nice feature. It is a relatively easy build.  This is one of the modules that uses the external power regulation PCB: it connects to the board via the 5-pin MTA-100 header on the right.

The two sets of input summing resistors are all 0.1% tolerance. The BOM calls for the CV Reject trimmer to be 250k, but I've used a 200k as there's no 250k with this footprint. It should work fine, and David's Synthasystem pages mention this. The final thing to note is that I've used a pair of discrete BC500B transistors, rather than a SSM2210 chip. The pads are labeled to allow for discrete transistors; just be sure to check the datasheet to position them correctly.

Originally I planned to use the SSM2210s, but at $10 each (and given the number in the system), it didn't seem too practical. Plus, apparently they aren't really crucial either; in some later Synthasystems, even the transistors in the VCOs weren't matched.  I will still use a THAT340 chip in the VCO exponential converter (which is temperature controlled too), but I think that's the only place I'll use a matched pair.

HF Correction PCB

The HF Correction PCB is a small add-on PCB for the VCO. It basically just helps improve the oscillator's tracking at high frequencies.

It is clearly not a very exciting board, but obviously it isn't supposed to be.  Two of the connections on the MTA go to different points on the VCO board. The third is ground, which I'll probably just tie to something on the power regulation board.

Anyhow, I just ordered panels for the Voltage Follower, Sample & Hold, and Trigger Generator. They should arrive by the end of next week. Hopefully everything is fine for my banana jack conversions!!

Sample & Hold PCB

The Sample & Hold is another dual module in the Synthasystem.  While it doesn't have a built-in clock (it requires external triggers), each S&H circuit has its own random voltage generator, which is a nice feature. Any external source can also be used.

As you can see, this uses the rather rare 2N4891 UJTs ($10 for each of those tiny metal cans!!), though some other UJTs may work. Take note, too, that good quality caps are needed here - I've used styrene.

Since I'm using banana jacks, this module becomes a bit more complicated.  The unit is designed for switching jacks on the signal inputs: when no plug is inserted, the input is the internal noise generator.  I'll just be adding a couple switches to the panel to take care of it.  Besides the switches, signal input, trigger input, and output, each S&H unit also has an output level pot.

Voltage Follower PCB

The Voltage Follower is probably the most simple module in the Synthasystem.  In fact, it is actually two units in one module. "Voltage follower" is yet another name for slew, lag, glide, portamento, etc.  The power regulation is all on the PCB.

As before, I've omitted the Doepfer power header and LEDs... I won't mention this in the future.  I also haven't installed the LM741s or the heatsinks. The panel components are pretty simple: a pot and two jacks (input/output) for each of the two followers in the unit.

While I'm at it, I'll mention that all of the boards are set up for using MTA100 connectors. I'm comfortable with these, having used them on my Dotcom system.  The "T"-handle tool is a bit of a pain, but it has grown on me. The MTAs are probably a little bit overkill for a banana jack system (and some of that panel wiring can be fun), but it certainly can't hurt.

Power Regulation PCBs

The Synthasystem runs off of a rather odd power configuration: +12V/-10V. Thankfully, David has made provisions on his PCBs for regulators, so you can run the system off of +/-12V or +/-15V, and each module will take care of its own power requirements. (Both MOTM/Blacet and Doepfer power headers are available too.)  There's a pair of trimpots to set each voltage correctly.

On the smaller modules the power regulation is directly on the PCB.  Most of the larger modules, however, use a generic power regulation PCB.  I'd recommend labeling the PCBs for what module they're used with - there's a couple of resistors and caps which change (or are omitted) depending on the power filtering requirements of the module.

It is a really simple build, and the power regulation sections on the smaller boards are similar.  I'll be adding heatsinks to the LM317 and LM337 later. You'll note that I've omitted the Doepfer power header, and two LEDs.  The LEDs are only really there to give some load if you're testing and configuring it without any external load.  Actually, I intended to add them, but I goofed and forgot them in my giant Mouser order; I installed the associated resistors in case I ever decide to add them.