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.