Eight years later! And still not quite finished.

OK, so buying an actual 102 took the wind out of my sails a bit, and then there were all the diversions into silly projects like “let’s make a System 700” and “let’s do a Jupiter-4 clone in a rack” which both dragged on forever and are somewhat dead at the moment, but it feels like this is… almost… nearly… there.

The wires poking out of the side hint at the genuine nightmare behind the panel. No, we’re not going to look round the back. At this point Roland were wiring straight to potentiometers, and I’ve done mostly the same. Here’s a view inside the original.

I did make small boards for each set of slide pots so that I could keep the screw fixing to the panel as unobtrusive as possible.

I just about managed to scrape together slide potentiometers of the right (or near enough) values to make this. And the stalks are different lengths and materials. And they don’t even feel that nice – the big one below felt the best but I couldn’t use it in the end, I think the slide cap I was using didn’t fit.

I actually considered going into the 30mm slide potentiometer business – or at least buying whatever minimum amount the manufacturers would sell of each value (seemingly 1000) and flogging the surplus. Thankfully they never got back to me, it would’ve been too far down the rabbit hole.

Anyway, enough complaining about wiring – these are the things I added

• external CV input for the VCA
• white/pink noise (straight from the model 101)
• multiples! 
• one-octave down SH-101-style sub-oscillator normalled to the external input socket
• optional gate boost (the envelope is finicky about the voltage it needs to trigger)
• switch for boosting the otherwise fairly quiet envelope output
• plus/minus one octave switch
• fast/medium/slow LFO ranges
• LFO reset (already there on the model 101 version of the board)
• switch on the S&H clock to the envelope
• envelope fast/normal speed
• extra cv/gate inputs on the far left

There’s probably more I could’ve bunged on it, but I was trying to keep to the look of the 102 as far as I could. Here’s the whole thing propped up a fruit bowl on our kitchen worktop.

I wimped out on the design slightly because I had my doubts that the printing process would be able to reproduce the (grey? green?) original colour scheme. Given that this panel cost £150 back in Nov 2019 from Schaeffer, it was strictly going to be a one-shot thing. I’d hate to think how much it’d cost now.

Now I look at it again, the absence of the block colour gives it a moody sort-of Sys100/SH-09 crossover look.

The only thing that doesn’t work yet is the phones output because I’m missing the Sansui ST-31 transformer, which seems to be hard to get hold of these days, although I’ve seen photos of boxes full of them on the internet, just to taunt me.

Here’s the usual out-of-tune bonky MC-4-driven demo that doesn’t really demo anything, mostly just showing the pulse-width mod/suboscillator and envelope fast/normal switch.

I’ve got a power supply board made up, a clone of the one in the 104, which I actually intended to make but never got round to because of the strange Sanyo TV channel selector switches which it uses. (Also it turns out that the 100M 182 is more fun)

I was going to run the whole thing off a Yamaha PA-20 and keep the transformer outside the box, just to keep it as compact as possible.

And it needs a box making up, and then it’ll be ta-daaa. Hopefully this side of 2030.

Bonus photos ‘cos I was asked: here’s the panel as it arrived, just opened up the package from Schaeffer:

Slightly grim photo of my panel (taken in near darkness, for some reason) balanced on top of my 101, just to show a comparison against the original 102. My version is a bit too wide, I think I must’ve take the full inside width of the original as the graphics edge-to-edge measurement.

I need to try and shift things about to fit things in, maybe on the left of the 102 like in Jack Dangers’ studio; still need that 103 though… (and I’m working on the 100M).

I tried to keep all the slightly idiosyncratic typographical decisions like, for example WAVE(tiny space)FORM and picking out the abbreviations in PULSE WIDTH MOD and VOLTAGE CONTROLLED OSCILLATOR.

Last one; you get the idea.

Here’s my second go at the potentiometer board for my version of the SEM (previous post here).

I’m fairly happy with it, apart from the fact that my additions have pushed it over the original size a bit, meaning I won’t be able put two together in a 19″ box like the ObieRack.

Extra/different stuff:

• second LFO (hence the wonky LM13700)
• plus/minus one volt octave switches for the VCOs (hence a few references and a quad opamp)
• LED indicators for the LFOs (couple of op-amps)
• LED indicator for the second envelope (which is a bit pointless)
• switches for oscillator shapes and modulation sources (due to the general lack of centre-tapped pots)
• external connections routed one side of the board

There’s loads wrong with my paper and cardboard test front panel (it’s too small, for one), but this is some kind of idea of how it might look. The circles around the pots were just to work out maximum sizes for the pots and spacing while I was arranging it.

The attack pot for the first envelope is something like 1mm too far to the right on the board, aargh. Forget doing the board again though.

Initially I was just going to make two, then I ended up making four of them, because why not.

I mean, Mr Splitradix has five of the things, and Vince has bloody ten of them, so just keeping to four seemed fairly (almost) restrained.

This now means I’ve got to build some sort of panning mixer for the four voices. And also a MIDI interface and maybe some sort of flexible cv/gate assigner thing so I can round-robin play and hold the four voices from one or two channels of the MC-4.

And now I think about it a bit more, it’d be a good idea to have some common VCO pitch and cutoff tuning controls as well. And a common LFO. Let’s cover the world in LFOs.

(I hadn’t really thought this whole “let’s make a load of SEMS” thing through, really.)

I’m definitely not doing common envelope controls though. The envelopes on the boards aren’t voltage controllable as they are; it’s said that the FVS used early versions of the CEM3310 for this, and that just sounds like a massive faff. Even more of a faff.

One thing that I absolutely love about the SEM from a construction point-of-view is that the front panel just plugs into the voice board – here’s a photo of one of them balanced in-between some plants on a windowsill:

OK, so it looks strangely wonky in the photo but everything connects up so easily, and it’s just so compact.

I guess this’ll be no surprise to anyone building Eurorack modules, but after having wired up a System 100 model 102 clone all point-to-point, the relative lack of wiring is glorious.

Fuck wiring! It’s no fun!

There was good discussion on the Analogue Heaven mailing list recently (yeah! mailing lists! from the old days!) about the FVS versus the OB-X, and there was some thought that the OB-X sounded somehow nicer because of the OTA used in the audio summer (and the pre-distortion technique they used to counter-intuitively reduce the distortion on the super dynamic signal), so it’s tempting to try and implement that in the output section.

Continuing with the late-70s Japanese synth recreations that I’ll probably never finish – that thing I was working on back in January has grown into this:

Four voices, “compuphonic”. You know. My version has a sawn-off version of the originals’ motherboard in the vague hope that I might be able to fit it in a 4U (or maybe 3U?) box. Maybe in some alternate universe this actually happened and was released with a Roland model number, maybe MKS-4?

So many problems, some of which have been fixed. The chorus sounds a bit too swooshy. The envelopes leak clock noise into the output. The LFO leaks into the audio path, although that one is at least a known problem with the original.

The oscillators required some resistor-twiddling to get the pulse-width wave square. I fret I might have altered the level of the VCO into the filter slightly, I could do with checking further.

I mean, it can sound quite good. With all four voices in unison and chorus on it’s proper shouty raaaargh full-on “I am a synthesiser”.

We’ll get to the chorus later on but here’s a bit of pulse-width single voice then unison:

Also the LFO randomly goes quite fast on that, quite nice for making a horrible racket.

Pile of (not quite finished) boards!

Initially I made voice boards with single row connectors but they were really wobbly. And the voice board was the wrong way round.

I changed the spacing on the card connectors so they’re on a 0.1″ grid, with the initial thought that I would build the motherboard on stripboard, but eventually decided I was done with such masochism for this one.

Predictably then the module controller board and the voice board don’t quite line up in the same way as the original does. I’d re-do the motherboard and the module controller but… it’s a lot of work. Otherwise the voice board and the module controller should be the same as the original, the track layout should be fairly close.

As IR3109s aren’t falling from the sky (give 2020 a chance, though) I went with the rev D voice board, meaning that I had to make loads of BA662 clones. Praise be to openmusiclabs.

Here’s one of the billions of 662 clones/clowns, missing the two transistors and resistors for the buffer ‘cos they weren’t needed in this case:

Maybe if I get round to doing a Promars I’ll try building it with LM13700s in adaptors, using homemade 662s is an exercise in masochism.

The chorus works and sounds ace but for the none-too subtle swooshing from the LFOs. I bought two sets of MN3007 from eBay – inevitably the cheapest pair made my power supply panic, I guess they were MN3207 or something, re-marked.

Here’s a bit of that – listen out for the (stereo) chorus kicking in second time round the bass loop. Later on (at about 39 seconds) when the filter closes you should be able to hear the zapping on the chorus, and some clock noise from the envelopes.

Poking at the output of the BBD with an oscilloscope suggested that the two outputs are at slightly different levels, so maybe I have to balance them out to cancel the swooshiness. Originally the JP-4 didn’t have anything here for trimming the individual outputs from the BBD, but the System-100M delay/chorus module does.

So much dust, it’s been here for a while:

The original used a pair of MN3005s but I adapted the layout for 3007s using the tactics published by fitzgreyve.

The brain of this mess is a dusty Teensy 3.6, which is massive overkill for such a simple synth, but the inbuilt microSD card is handy for saving and loading patches. And the plentiful I/O will be handy when I come to making a front panel for it (In about five years, at this rate).

The DAC is a DAC8562, which has two 16-bit channels. It’s comically tiny, especially next to the through-hole parts on the weird circuit board I had made.

The output from the DAC for the compuphonic settings and the MIDI-to-CV is 5V, giving five octaves of pitch, which matches the four octave keyboard (plus one octave down via the transpose switch) range on the original, but I’m not sure if it’s a bit limiting when it comes to the keyspan.

It does a few different voice modes as per the original which may or may not work reliably: different flavours of unison, round robin, voice per note held down, and some bonus single voice modes to help me tune the individual voices.

This demo uses the unison mode for the bass drone and the round-robin for the wobbly lead that comes in at about 21 seconds, so the notes (with a long release) smear over each other.

Early on I hadn’t realised how much current this thing was drawing – the wires I was using were too thin and causing the pitch to wander about, so tuning was a regular thing. Beefing up the cables has mostly sorted the drifting pitch, but I still think about incorporating some sort of autotune.

One downside with autotune is that I’d lose a bit of pitch range, so I’d definitely need to amplify the voltage from the DAC. Maybe that’s against the spirit of the wobbly original though.

Portamento is done in software as part of MIDI to CV – the original used a 662 for each voice but I’d had enough of making them by this point.

All the compuphonic settings from the original are MIDI-controllable. How will I get the MC-4 to talk to it? I guess I could do with some sort of assignable CV/gate input thing on the Teensy.

Another sort of demo – I was enjoying playing in Ableton for a change and actually being able to play chords. Loads of reverb on the pad, maybe a bit of chorus on the unison bass bit and a single voice for the squeaky acid bit at the end.

There’s something pleasing about the stepped voltages from the DAC, although it was a struggle to get them right.

It didn’t help that my 90s-era Philips oscilloscope died quite early on into this project, and it turns out now that the same model is a hundred quid or so more expensive on eBay. This picture is from my replacement scope, which is a Tektronix 2445 that I’m not so keen on.

I found I had be quite careful in the sequencing and timing of the DAC and S&H multiplexer to get reliable behaviour, and even then I’m not 100% sure I’ve sorted it. The DAC8562 is posh compared to my usual MCP4922, but it still needs at least 10 microseconds to settle down to a consistent voltage. Boo physics.

I’ve got a screen! It’s in colour! Although I’m not sure that’s a good idea, because I’m having to shift more bits out through the SPI interface than I would otherwise with a monochrome display. Here’s my cheesy splash screen.

There’s nothing much actually happening on the screen yet, but here’s a video to show off the patch loading and how the parameter changes are represented on the screen. When it comes to patch 5 I’m changing multiple parameters through MIDI, so predictably enough the screen goes a bit apeshit. (And the camera is a bit out of focus on the last part, soz about that)

The envelope noise is really obvious on patch one. The voice mode display currently doesn’t get updated when the patch changes so that’s all lies. At least the voice allocation numbering is correct.

The random noise burst when it turns on is alarming, but I quite like it. It reminds me of Robotron starting up, or a robot angrily being woken from its slumber. It does need sorting out though, it’s on the list.

I spent way too long on implementing the arpeggiator and got tied up in how it should respond to MIDI clock, so that’s in a half-broken state.

On the arp front I saw a rumour (…in YouTube comments) that the original didn’t do a real random, but that it was actually based on a bunch of numbers stored in ROM. So if you hold the right keys down, eventually you get the “Rio” arp line, god help you.

Presumably that would be in the key assigner board mask ROM, which unfortunately hasn’t been dumped out as far as I can tell. Not that I’m sure I’d be able to do anything with it if I had it.

Next move is to experiment with temporarily taking the voice outputs through a different route to see if we can make the induced clock noise from the envelopes disappear. I suspect it’ll mean cutting tracks on the motherboard – there’s already a suggested fix for the LFO bleed on the originals mentioned elsewhere that I’ll look at as well.

I’d already tried to balance out the chorus swooshiness with a pot and promptly fried one of the BA662s on the output, so that’s next after the envelope noise. Still a long way to go.