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!

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.

VC Oscillator complete!

I've now completed one of my two initial VCOs.

As you can see, there's a lot of wiring! I've added the HF compensation board, which gets wired down to three points on the main PCB. It is indicated on the schematic, and some good quality photos that David has on his site. (Of course, ground can go to many places, but I just chose one that was easy.)

The calibration is pretty extensive for this module. First the waveforms need to be calibrated: first the saw, followed by the pulse, triangle, and sine.  Then the 1V/Oct tracking, and then the frequency range.  This is documented on the build pages.  I've done all on this module, but will go over it again just to get things as precise as possible - I just did a quick setup of the tracking, for instance.  One thing to note on the waveform calibration is that if you see truncated waveforms, be sure to check the DC offset for that waveform.

Unfortunately, my second VCO isn't going so well, at least where the sine wave is concerned.  There's two settings of the trimmer where you get a sine wave, and only one is correct.  (The incorrect one has a very high point-to-point voltage!)  I've managed to get to both of them, but haven't managed to get the trimmer settings right.  (Or there's something wrong!)  A bit frustrating staring at a scope and holding the module awkwardly and turning the trimpots.  (I really ought to power this on my bench, rather than out of my Dotcom modular...)

Once I have it working, though, I'll share any further tips... plus oscilloscope shots!  Hopefully I'll have some time this weekend, though all of Sunday I'll be up in Anaheim at NAMM.

In the meantime, I've been working on the power supply & distribution panel, which I'll share shortly.  Additionally, I've ordered PCBs for the Noise, Peak Selector, Ring Modulator, Phase Shifter, Triple EG, and Frequency Dividers.  All the board parts are on-hand, so putting those together should go rather smoothly!

VC Filter complete!

And now, the VC Filter is also completed!

Like the VCA, this also has a CV Reject trimpot. I ended up having to adjust it the same way - fully CW - to minimize (but not eliminate) VC bleed into the output.  I did a few audio tests, and it sounds really nice!

I used one of the Electroswitch rotary switches, and I must confess that I don't really like it... it feels very "mushy" to turn - you don't get a solid "click".  I'll want to find something else for future modules, and eventually replace the one here.

VCA/Mixer complete!

The VCA/Mixer module is now finished!

Aside from the pull-down resistors previously mentioned, it was pretty easy to assemble to the panel.  There's only one trimmer on this module, which is the CV Reject for cutting down on CV bleeding to the output. I sent the VC inputs an audio signal from a VCO, and could eliminate most, but not all, bleed, by turning it fully CW. Everything seemed to work fine in testing.  Note that this uses linear pots, so you may not get the typical response you may expect from VCAs that use log pots.

VCA/Mixer Wiring

The VCA/Mixer is one of the few modules that uses switching jacks, and thus needs special consideration when being converted to banana jacks.  The only other modules that use switching jacks are the Triple EG (and only for chaining trigger inputs - easy to remove the normalled jacks and stack patch cables), and the Sample & Hold (which I've already taken care of with switches to go between the internal & an external source).

On the VCA, switching jacks are used for both the signal and VC inputs.  The switches are connected to ground, to provide 0V at those inputs when no jack is inserted. Obviously switches could be added to the panel, but that would overkill for something like a VCA (and a pain to use!).  I'm actually not certain how critical these switching jacks are, but I figured I should try and implement it as best as possible.

Although I've seen this method elsewhere, the first time I saw it was on Ken Stone's CGS Digital Noise schematic: there's a CV input where the jack is normalled to +15V.  Clearly, this would pose a problem for banana jacks, but the solution was to put a 100k pull-up resistor between the jack and +15V, and then connect the jack directly to the input on the PCB. The theory is that when there's no input, the +15V flows through the resistor to the input. But when a signal is inserted, its impedance is lower than the 100k resistor, and overrides the +15V signal.  I've seen it elsewhere since, but usually just for clock signals.

Anyhow, I've done something similar here, with a pull-down resistor to 0V on these inputs.

Hopefully it will work, and not distort the signals in any way!  If it does cause any problem, though, I can just snip it off - at least for the VC inputs, I can't see why a floating input would cause an issue (it isn't much different than any other modules here.)

More panels arrive!

Yesterday I got the rest of the panels for this batch of modules!

I've already put down all the jacks and standoffs, and everything fits just fine.  I also got the panel for the rack-mounted power supply I'm setting up - more on that later.

Unfortunately my order of knobs and additional heatshrink from Small Bear is MIA(?!), so I was only able to wire up the VCF and one of the VCOs. The great news is that the VCO works!  I haven't done the range and V/Oct calibration, but that shouldn't be an issue, as all the other calibration went fine.  I haven't tested the VCF with any input, but I verified that it does self-oscillate at high resonance, so something is definitely working!

I'm going to wait until the modules are all done with knobs to post pics, but in the meantime, here's a sneak-peek at the VCO wiring.

As you might imagine, I'm really thrilled with how well this project is going so far. Thanks to Nyle for his great designs, and to David for creating these PCBs, his excellent instructions, and help!

In anticipation of having the VCA and the other VCO completed sometime next week, I'll be ordering a stack of new boards tomorrow!

VCF Cap Replacement

Well, I got all my panel parts, but it looks like my panels for the rest of this first batch of modules won't be here until Tuesday at the soonest! Bummer... I was spoiled by having a coupon code for the 3-day service from Front Panel Express last time!

Anyhow, as I mentioned before I wasn't too happy with these ceramic capacitors (MLCCs) that I'd picked up.  They seem to be working fine on the modules I've completed, but there's three .0022u caps on the VCF that are ceramic, but that could (potentially) benefit from better caps.  So, I decided to replace them with polyester metallized film capacitors.

Those feel a lot nicer!  I'm not a big fan of using my hand-held desoldering pump, but it does the job... honestly, it works fastest on single-sided boards, but on some of those it also has a tendency to lift the traces.  At least it isn't something I have to use often!  I'll continue using MLCCs for the decoupling and other non-critical caps, but I'll switch to a different make for future boards. (Not gonna swap out the other caps on my current boards, though!)

Well, there's not much I can do until those panels arrive, so I think I'll kick back with a glass of Racer 5 IPA tonight, and try (once again) to get my YuSynth Quadrature LFO working in my Dotcom system this weekend...

Voltage Follower All Done!

And the last of my first 3 modules to be completed is the Voltage Follower.

Simple circuit and simple interface.  I tested both circuits using the S&H as input, and it worked fine.  This is an exponential slew (or lag, portamento, etc.).  I don't know why the term "Voltage Follower" was used for that, as I've only seen it used before for a unity-gain buffer. Though, I guess this has four voltage followers in it (more OpAmp buffers than in any other Synthasystem module, I believe).

I haven't tried to see what the max lag time is here, since it is obviously a bit tough using random input from the S&H, combined with the exponential response.

Sample & Hold Complete!

Although I finished this (and the Voltage Follower) before I got the VC Trigger Generator working, I decided to wait until the latter was functioning before I posted these.  Here's my completed banana-fied S&H!

Things are a bit crowded on the front panel with the switches, but I figure that the switches & knobs on the S&H aren't used a whole lot once a patch is set up.  (At least based on my experience with other S&H units.)  I took off the large tabbed washers from the NKK switches, because they protruded just slightly from the edges.  Obviously I could've ground them down, or just made sure to put the S&H in the cabinet first, but they aren't critical, so it didn't seem worth it.

No calibration is required on this module, and both circuits worked just fine.  I've tried triggering it from the VC Trigger Generator, and even just a banana cable connected to ground.  The grounded banana was a bit wonky, since the input isn't debounced, and you'll accidentally trigger it 2-3 times - but it worked!  I initially tried testing it with my DMM, but the output wasn't steady at all (I'm not sure why) - everything showed up fine in the oscilloscope, though.

You'll notice on this (and the VC Trigger Generator) some of the MTA headers are unused - those are for the V-Triggers that I'm not using.

The boards are designed to be used with switching jacks for the inputs, with the switch lug attached to the internal "noise" generator.  I've used a SPDT switch instead, with common attached to the signal input, and the ends attached to the internal "noise" generator and the jack, respectively.  I didn't try it with an external signal, but considering it works from an internal source, there shouldn't be a problem.

I've called it the "internal 'noise' generator" because (to my surprise) it isn't actually a noise generator!  Instead, it is actually a ramp oscillator (based off of a UJT) running at approximately 12kHz. It is going fast enough where you'll get something that is effectively random. There's two of these on-board, with one for each S&H, which is nice, so you won't have the output correlated between the units.

VC Trigger Generator: Done!

Success at last! I got the VC Trigger Generator working, finally.

The culprit ended up being the UJT. However, given that this was a $10 part, I wanted to make certain that it was at fault before replacing it!  David Ingebretsen helped me with understanding and debugging this circuit - thanks so much!  I learned a lot in doing it too.  Lots of measuring voltages at various points, particularly transistor bases.  Eventually figured out which blocks were working, and which weren't. Narrowed it down to the UJT, popped it out, put a new one in, and presto!!

Calibration was a bit of a pain, due to the fact that I'd moved the trimpots all around, but it was all set after about 5-10 minutes in front of the oscilloscope.  The Duration/Width pot was working the opposite way of how I'd expected it to work, with 10 being the smallest pulse, and 0 being the widest. Apparently this is how David's works as well, it is how it is noted in the schematic, and Nyle didn't notice anything odd with David's unit... so presumably it is how it is supposed to be?  Anyhow, it seems a bit odd to me, so I switched the MTA and reversed it.

Another oddity is the fact that at rest it outputs approx 10V, rather than floating - this is due to the way that the LED is set up. On its own this isn't a problem, but I am a bit concerned about OR-ing trigger signals from these modules.  It doesn't look like it should be an issue with the other trigger generators (i.e. the Sequencer and Peak Selector).

Also, note that I didn't screw the PCB down all the way onto the top two standoffs - the pushbutton switches are rather large, and push against the board.  If I were to do it over, I'd just put the wires going to the pushbuttons going off to the side, rather than straight out.

It really has a lot of range - I didn't measure it, but with the range knob at 12:00 the pulses have a period of 85ms, and from that it goes really fast, and really slow.  I didn't try the 3 inputs yet, but given the circuit and that everything else works, those should work without issue.

A great way to start the new year by getting this working!